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W3C
Cascading Style Sheets, level 2 revision 1
CSS 2.1 Specification
W3C Working Draft 28 January 2003
This version:
http://www.w3.org/TR/2003/WD-CSS21-20030128
Latest version:
http://www.w3.org/TR/CSS21
Previous version:
http://www.w3.org/TR/2002/WD-CSS21-20020802
Editors:
Bert Bos <bert @w3.org>
Tantek Çelik <tantekc @microsoft.com>
Ian Hickson <ian @hixie.ch>
Håkon Wium Lie <howcome @opera.com>
This document is also available in these non-normative formats: plain
text, gzip'ed tar file, zip file, gzip'ed PostScript, PDF. See also
translations.
Copyright © 2003 W3C^® (MIT, ERCIM, Keio), All Rights Reserved. W3C
liability, trademark, document use and software licensing rules apply.
_________________________________________________________________
Abstract
This specification defines Cascading Style Sheets, level 2 revision 1
(CSS 2.1). CSS 2.1 is a style sheet language that allows authors and
users to attach style (e.g., fonts, spacing, and aural cues) to
structured documents (e.g., HTML documents and XML applications). By
separating the presentation style of documents from the content of
documents, CSS 2.1 simplifies Web authoring and site maintenance.
CSS 2.1 builds on CSS2 [CSS2] which builds on CSS1 [CSS1]. It supports
media-specific style sheets so that authors may tailor the
presentation of their documents to visual browsers, aural devices,
printers, braille devices, handheld devices, etc. It also supports
content positioning, table layout, features for internationalization
and some properties related to user interface.
CSS 2.1 corrects a few errors in CSS2 (the most important being a new
definition of the height/width of absolutely positioned elements, more
influence for HTML's "style" attribute and a new calculation of the
'clip' property).
CSS 2.1 corrects a few errors in CSS2 (the most important being a new
definition of the height/width of absolutely positioned elements, more
influence for HTML's "style" attribute and a new calculation of the
'clip' property), and adds a few highly requested features which have
already been widely implemented. But most of all CSS 2.1 represents a
"snapshot" of CSS usage: it consists of all CSS features that were
implemented interoperably at the date of publication.
Status of this document
This document is produced by the CSS working group (part of the Style
Activity, see summary).
The CSS WG is aware that this draft does not yet address all the
comments raised on the previous draft. The next draft should follow in
a few weeks and is expected to be the final one.
The (archived) public mailing list www-style@w3.org (see instructions)
is preferred for discussion of this and other drafts in the Style
area. When commenting on this draft, please put the text "CSS21" in
the subject, preferably like this: "[CSS21] <summary of comment>"
For this specification to exit the CR stage, the following conditions
shall be met:
1. There must be at least two interoperable implementations
implementing 'all' the features. An implementation can implement a
superset of the features and claim conformance to the profile. For
the purposes of this criterion, we define the following terms:
feature
An individual test case in the test suite.
interoperable
passing the respective test case(s) in the CSS test
suite, or, if the implementation is not a web browser, an
equivalent test. Every relevant test in the test suite
should have an equivalent test created if such a UA is to
be used to claim interoperability. In addition if such a
UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those
equivalent tests in the same way for the purpose of
interoperability. The equivalent tests must be made
publicly available for the purposes of peer review.
implementation
a user agent which:
1. implements the feature.
2. is available (i.e. publicly downloadable or available
through some other public point of sale mechanism). This
is the "show me" requirement.
3. is shipping (i.e. development, private or unofficial
versions are insufficient).
4. is not experimental (i.e. is intended for a wide
audience and could be used on a daily basis.)
2. A minimum of sixth months of the CR period must have elapsed. This
is to ensure that enough time is given for any remaining major
errors to be caught.
3. Features may/will be dropped if two or more interoperable
implementations are not found by the end of the CR period.
4. Features may/will also be dropped if adequate/sufficient (by
judgment of CSS WG) tests have not been produced for those
feature(s) by the end of the CR period.
This is a public W3C Working Draft for review by W3C Members and other
interested parties. It is a draft document and may be updated,
replaced, or obsoleted by other documents at any time. It is
inappropriate to use W3C Working Drafts as reference material or to
cite them as other than "work in progress."
Patent disclosures relevant to CSS may be found on the Working Group's
public patent disclosure page.
A list of current W3C Recommendations and other technical documents
can be found at http://www.w3.org/TR.
Quick Table of Contents
* 1 About the CSS 2.1 Specification
* 2 Introduction to CSS 2.1
* 3 Conformance: Requirements and Recommendations
* 4 Syntax and basic data types
* 5 Selectors
* 6 Assigning property values, Cascading, and Inheritance
* 7 Media types
* 8 Box model
* 9 Visual formatting model
* 10 Visual formatting model details
* 11 Visual effects
* 12 Generated content, automatic numbering, and lists
* 13 Paged media
* 14 Colors and Backgrounds
* 15 Fonts
* 16 Text
* 17 Tables
* 18 User interface
* Appendix A. Aural style sheets
* Appendix B. Bibliography
* Appendix C. Changes
* Appendix D. A sample style sheet for HTML 4.0
* Appendix E. Property index
* Appendix F. Index
* Appendix G. Grammar of CSS 2.1
Full Table of Contents
* 1 About the CSS 2.1 Specification
+ 1.1 CSS 2.1 vs CSS 2
+ 1.2 Reading the specification
+ 1.3 How the specification is organized
+ 1.4 Conventions
o 1.4.1 Document language elements and attributes
o 1.4.2 CSS property definitions
# Value
# Initial
# Applies to
# Inherited
# Percentage values
# Media groups
# Computed value
o 1.4.3 Shorthand properties
o 1.4.4 Notes and examples
o 1.4.5 Images and long descriptions
+ 1.5 Acknowledgments
+ 1.6 Copyright Notice
* 2 Introduction to CSS 2.1
+ 2.1 A brief CSS 2.1 tutorial for HTML
+ 2.2 A brief CSS 2.1 tutorial for XML
+ 2.3 The CSS 2.1 processing model
o 2.3.1 The canvas
o 2.3.2 CSS 2.1 addressing model
+ 2.4 CSS design principles
* 3 Conformance: Requirements and Recommendations
+ 3.1 Definitions
+ 3.2 Conformance
+ 3.3 Error conditions
+ 3.4 The text/css content type
* 4 Syntax and basic data types
+ 4.1 Syntax
o 4.1.1 Tokenization
o 4.1.2 Keywords
o 4.1.3 Characters and case
o 4.1.4 Statements
o 4.1.5 At-rules
o 4.1.6 Blocks
o 4.1.7 Rule sets, declaration blocks, and selectors
o 4.1.8 Declarations and properties
o 4.1.9 Comments
+ 4.2 Rules for handling parsing errors
+ 4.3 Values
o 4.3.1 Integers and real numbers
o 4.3.2 Lengths
o 4.3.3 Percentages
o 4.3.4 URL + URN = URI
o 4.3.5 Counters
o 4.3.6 Colors
o 4.3.7 Strings
o 4.3.8 Unsupported Values
+ 4.4 CSS document representation
o 4.4.1 Referring to characters not represented in a
character encoding
* 5 Selectors
+ 5.1 Pattern matching
+ 5.2 Selector syntax
o 5.2.1 Grouping
+ 5.3 Universal selector
+ 5.4 Type selectors
+ 5.5 Descendant selectors
+ 5.6 Child selectors
+ 5.7 Adjacent sibling selectors
+ 5.8 Attribute selectors
o 5.8.1 Matching attributes and attribute values
o 5.8.2 Default attribute values in DTDs
o 5.8.3 Class selectors
+ 5.9 ID selectors
+ 5.10 Pseudo-elements and pseudo-classes
+ 5.11 Pseudo-classes
o 5.11.1 :first-child pseudo-class
o 5.11.2 The link pseudo-classes: :link and :visited
o 5.11.3 The dynamic pseudo-classes: :hover, :active, and
:focus
o 5.11.4 The language pseudo-class: :lang
+ 5.12 Pseudo-elements
o 5.12.1 The :first-line pseudo-element
o 5.12.2 The :first-letter pseudo-element
o 5.12.3 The :before and :after pseudo-elements
* 6 Assigning property values, Cascading, and Inheritance
+ 6.1 Specified, computed, and actual values
o 6.1.1 Specified values
o 6.1.2 Computed values
o 6.1.3 Actual values
+ 6.2 Inheritance
o 6.2.1 The 'inherit' value
+ 6.3 The @import rule
+ 6.4 The cascade
o 6.4.1 Cascading order
o 6.4.2 !important rules
o 6.4.3 Calculating a selector's specificity
o 6.4.4 Precedence of non-CSS presentational hints
* 7 Media types
+ 7.1 Introduction to media types
+ 7.2 Specifying media-dependent style sheets
o 7.2.1 The @media rule
+ 7.3 Recognized media types
o 7.3.1 Media groups
* 8 Box model
+ 8.1 Box dimensions
+ 8.2 Example of margins, padding, and borders
+ 8.3 Margin properties: 'margin-top', 'margin-right',
'margin-bottom', 'margin-left', and 'margin'
o 8.3.1 Collapsing margins
+ 8.4 Padding properties: 'padding-top', 'padding-right',
'padding-bottom', 'padding-left', and 'padding'
+ 8.5 Border properties
o 8.5.1 Border width: 'border-top-width',
'border-right-width', 'border-bottom-width',
'border-left-width', and 'border-width'
o 8.5.2 Border color: 'border-top-color',
'border-right-color', 'border-bottom-color',
'border-left-color', and 'border-color'
o 8.5.3 Border style: 'border-top-style',
'border-right-style', 'border-bottom-style',
'border-left-style', and 'border-style'
o 8.5.4 Border shorthand properties: 'border-top',
'border-bottom', 'border-right', 'border-left', and
'border'
o 8.5.5 The border model for inline elements
* 9 Visual formatting model
+ 9.1 Introduction to the visual formatting model
o 9.1.1 The viewport
o 9.1.2 Containing blocks
+ 9.2 Controlling box generation
o 9.2.1 Block-level elements and block boxes
# Anonymous block boxes
o 9.2.2 Inline-level elements and inline boxes
# Anonymous inline boxes
o 9.2.3 Run-in boxes
o 9.2.4 The 'display' property
+ 9.3 Positioning schemes
o 9.3.1 Choosing a positioning scheme: 'position' property
o 9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'
+ 9.4 Normal flow
o 9.4.1 Block formatting context
o 9.4.2 Inline formatting context
o 9.4.3 Relative positioning
+ 9.5 Floats
o 9.5.1 Positioning the float: the 'float' property
o 9.5.2 Controlling flow next to floats: the 'clear'
property
+ 9.6 Absolute positioning
o 9.6.1 Fixed positioning
+ 9.7 Relationships between 'display', 'position', and 'float'
+ 9.8 Comparison of normal flow, floats, and absolute
positioning
o 9.8.1 Normal flow
o 9.8.2 Relative positioning
o 9.8.3 Floating a box
o 9.8.4 Absolute positioning
+ 9.9 Layered presentation
o 9.9.1 Specifying the stack level: the 'z-index' property
+ 9.10 Text direction: the 'direction' and 'unicode-bidi'
properties
* 10 Visual formatting model details
+ 10.1 Definition of "containing block"
+ 10.2 Content width: the 'width' property
+ 10.3 Computing widths and margins
o 10.3.1 Inline, non-replaced elements
o 10.3.2 Inline, replaced elements
o 10.3.3 Block-level, non-replaced elements in normal flow
o 10.3.4 Block-level, replaced elements in normal flow
o 10.3.5 Floating, non-replaced elements
o 10.3.6 Floating, replaced elements
o 10.3.7 Absolutely positioned, non-replaced elements
o 10.3.8 Absolutely positioned, replaced elements
o 10.3.9 'Inline-block', non-replaced elements in normal
flow
o 10.3.10 'Inline-block', replaced elements in normal flow
+ 10.4 Minimum and maximum widths: 'min-width' and 'max-width'
+ 10.5 Content height: the 'height' property
+ 10.6 Computing heights and margins
o 10.6.1 Inline, non-replaced elements
o 10.6.2 Inline replaced elements, block-level replaced
elements in normal flow, 'inline-block' replaced
elements in normal flow and floating replaced elements
o 10.6.3 Block-level and 'inline-block', non-replaced
elements in normal flow and floating, non-replaced
elements
o 10.6.4 Absolutely positioned, non-replaced elements
o 10.6.5 Absolutely positioned, replaced elements
+ 10.7 Minimum and maximum heights: 'min-height' and
'max-height'
+ 10.8 Line height calculations: the 'line-height' and
'vertical-align' properties
o 10.8.1 Leading and half-leading
* 11 Visual effects
+ 11.1 Overflow and clipping
o 11.1.1 Overflow: the 'overflow' property
o 11.1.2 Clipping: the 'clip' property
+ 11.2 Visibility: the 'visibility' property
* 12 Generated content, automatic numbering, and lists
+ 12.1 The :before and :after pseudo-elements
+ 12.2 The 'content' property
+ 12.3 Interaction of :before and :after with 'run-in' elements
+ 12.4 Quotation marks
o 12.4.1 Specifying quotes with the 'quotes' property
o 12.4.2 Inserting quotes with the 'content' property
+ 12.5 Automatic counters and numbering
o 12.5.1 Nested counters and scope
o 12.5.2 Counter styles
o 12.5.3 Counters in elements with 'display: none'
+ 12.6 Lists
o 12.6.1 Lists: the 'list-style-type', 'list-style-image',
'list-style-position', and 'list-style' properties
* 13 Paged media
+ 13.1 Introduction to paged media
+ 13.2 Page boxes: the @page rule
o 13.2.1 Page margins
# Rendering page boxes that do not fit a target sheet
# Positioning the page box on the sheet
o 13.2.2 Page selectors: selecting left, right, and first
pages
o 13.2.3 Content outside the page box
+ 13.3 Page breaks
o 13.3.1 Page break properties: 'page-break-before',
'page-break-after', 'page-break-inside'
o 13.3.2 Breaks inside elements: 'orphans', 'widows'
o 13.3.3 Allowed page breaks
o 13.3.4 Forced page breaks
o 13.3.5 "Best" page breaks
+ 13.4 Cascading in the page context
* 14 Colors and Backgrounds
+ 14.1 Foreground color: the 'color' property
+ 14.2 The background
o 14.2.1 Background properties: 'background-color',
'background-image', 'background-repeat',
'background-attachment', 'background-position', and
'background'
+ 14.3 Gamma correction
* 15 Fonts
+ 15.1 Introduction
+ 15.2 Font matching algorithm
+ 15.3 Font family: the 'font-family' property
+ 15.4 Font styling: the 'font-style' property
+ 15.5 Small-caps: the 'font-variant' property
+ 15.6 Font boldness: the 'font-weight' property
+ 15.7 Font size: the 'font-size' property
+ 15.8 Shorthand font property: the 'font' property
* 16 Text
+ 16.1 Indentation: the 'text-indent' property
+ 16.2 Alignment: the 'text-align' property
+ 16.3 Decoration
o 16.3.1 Underlining, overlining, striking, and blinking:
the 'text-decoration' property
+ 16.4 Letter and word spacing: the 'letter-spacing' and
'word-spacing' properties
+ 16.5 Capitalization: the 'text-transform' property
+ 16.6 Whitespace: the 'white-space' property
o 16.6.1 The 'white-space' processing model
* 17 Tables
+ 17.1 Introduction to tables
+ 17.2 The CSS table model
o 17.2.1 Anonymous table objects
+ 17.3 Column selectors
+ 17.4 Tables in the visual formatting model
o 17.4.1 Caption position and alignment
+ 17.5 Visual layout of table contents
o 17.5.1 Table layers and transparency
o 17.5.2 Table width algorithms: the 'table-layout'
property
# Fixed table layout
# Automatic table layout
o 17.5.3 Table height algorithms
o 17.5.4 Horizontal alignment in a column
o 17.5.5 Dynamic row and column effects
+ 17.6 Borders
o 17.6.1 The separated borders model
# Borders and Backgrounds around empty cells: the
'empty-cells' property
o 17.6.2 The collapsing border model
# Border conflict resolution
o 17.6.3 Border styles
* 18 User interface
+ 18.1 Cursors: the 'cursor' property
+ 18.2 User preferences for colors
+ 18.3 User preferences for fonts
+ 18.4 Dynamic outlines: the 'outline' property
o 18.4.1 Outlines and the focus
+ 18.5 Magnification
* Appendix A. Aural style sheets
+ A.1 The media type 'aural'
+ A.2 Introduction to aural style sheets
o A.2.1 Angles
o A.2.2 Times
o A.2.3 Frequencies
+ A.3 Volume properties: 'volume'
+ A.4 Speaking properties: 'speak'
+ A.5 Pause properties: 'pause-before', 'pause-after', and
'pause'
+ A.6 Cue properties: 'cue-before', 'cue-after', and 'cue'
+ A.7 Mixing properties: 'play-during'
+ A.8 Spatial properties: 'azimuth' and 'elevation'
+ A.9 Voice characteristic properties: 'speech-rate',
'voice-family', 'pitch', 'pitch-range', 'stress', and
'richness'
+ A.10 Speech properties: 'speak-punctuation' and
'speak-numeral'
+ A.11 Audio rendering of tables
o A.11.1 Speaking headers: the 'speak-header' property
+ A.12 Sample style sheet for HTML
+ A.13 Emacspeak
* Appendix B. Bibliography
+ B.1 Normative references
+ B.2 Informative references
* Appendix C. Changes
+ C.1 Additional property values
o C.1.1 Section 4.3.5 Colors
o C.1.2 Section 9.2.4 The 'display' property
o C.1.3 Section 18.1 Cursors: the 'cursor' property
o C.1.4 Section 16.6 Whitespace: the 'white-space'
property
+ C.2 Changes
o C.2.1 Section 3.2 Conformance
o C.2.2 Section 6.4.3 Calculating a selector's specificity
o C.2.3 Section 6.4.4 Precedence of non-CSS presentational
hints
o C.2.4 Chapter 9 Visual formatting model
o C.2.5 Section 10.3.7 Absolutely positioned, non-replaced
elements
o C.2.6 Section 10.6.4 Absolutely positioned, non-replaced
elements
o C.2.7 Section 11.1.2 Clipping: the 'clip' property
o C.2.8 17.4.1 Caption position and alignment
o C.2.9 17.5.4 Horizontal alignment in a column
o C.2.10 Section 17.6 Borders
o C.2.11 Chapter 12 Generated content, automatic
numbering, and lists
o C.2.12 Section 12.2 The 'content' property
o C.2.13 Chapter 13 Paged media
o C.2.14 Chapter 15 Fonts
o C.2.15 Chapter 16 Text
o C.2.16 Appendix A. Aural style sheets
o C.2.17 Other
+ C.3 Errors
o C.3.1 Shorthand properties
o C.3.2 Section 4.1.1 (and G2)
o C.3.3 4.1.3 Characters and case
o C.3.4 Section 4.3 (Double sign problem)
o C.3.5 Section 4.3.2 Lengths
o C.3.6 Section 4.3.6
o C.3.7 5.10 Pseudo-elements and pseudo-classes
o C.3.8 8.2 Example of margins, padding, and borders
o C.3.9 Section 8.5.2 Border color: 'border-top-color',
'border-right-color', 'border-bottom-color',
'border-left-color', and 'border-color'
o C.3.10 Section 8.4 Padding properties
o C.3.11 8.5.3 Border style
o C.3.12 Section 8.5.4 Border shorthand properties:
'border-top', 'border-bottom', 'border-right',
'border-left', and 'border'
o C.3.13 8.5.4 Border shorthand properties: 'border-top',
'border-bottom', 'border-right', 'border-left', and
'border'
o C.3.14 Section 9.3.1
o C.3.15 Section 9.3.2
o C.3.16 Section 9.4.3
o C.3.17 Section 9.7 Relationships between 'display',
'position', and 'float'
o C.3.18 Section 10.3.2 Inline, replaced elements (and
10.3.4, 10.3.6, and 10.3.8)
o C.3.19 Section 10.3.3
o C.3.20 Section 10.6.2 Inline, replaced elements ... (and
10.6.5)
o C.3.21 Section 10.6.3
o C.3.22 Section 11.1.1
o C.3.23 11.2 Visibility: the 'visibility' property
o C.3.24 12.6.2 Lists
o C.3.25 Section 15.2.6
o C.3.26 Section 15.5
o C.3.27 Section 16.6 Whitespace: the 'white-space'
property
o C.3.28 Section 17.2 The CSS table model
o C.3.29 17.2.1 Anonymous table objects
o C.3.30 17.5 Visual layout of table contents
o C.3.31 17.5 Visual layout of table contents
o C.3.32 Section 17.5.1 Table layers and transparency
o C.3.33 Section 17.6.1 The separated borders model
o C.3.34 Appendix D.2 Lexical scanner
+ C.4 Clarifications
o C.4.1 2.2 A brief CSS2 tutorial for XML
o C.4.2 Section 4.1.1
o C.4.3 Section 5.5
o C.4.4 Section 5.9 ID selectors
o C.4.5 Section 5.12.1 The :first-line pseudo-element
o C.4.6 Section 6.2.1
o C.4.7 6.4 The Cascade
o C.4.8 Section 6.4.3 Calculating a selector's specificity
o C.4.9 Section 7.3 Recognized media types
o C.4.10 Section 8.1
o C.4.11 Section 8.3.1
o C.4.12 Section 9.4.2
o C.4.13 Section 9.4.3
o C.4.14 Section 9.10
o C.4.15 10.3.3 Block-level, non-replaced elements in
normal flow
o C.4.16 Section 10.5 Content height: the 'height'
property
o C.4.17 Section 10.8.1
o C.4.18 Section 11.1
o C.4.19 Section 11.1.1
o C.4.20 Section 11.1.2
o C.4.21 12.1 The :before and :after pseudo-elements
o C.4.22 Section 12.4.2 Inserting quotes with the
'content' property
o C.4.23 Lists 12.6.2
o C.4.24 14.2 The background
o C.4.25 14.2.1 Background properties
o C.4.26 Section 16.1
o C.4.27 16.2 Alignment: the 'text-align' property
o C.4.28 Section 17.5.1 Table layers and transparency
o C.4.29 Section 17.5.2 Table width algorithms
o C.4.30 17.6.1 The separated borders model
o C.4.31 Borders around empty cells: the 'empty-cells'
property
o C.4.32 Section 17.6.2 The collapsing borders model
o C.4.33 Section 18.2
o C.4.34 Section A.3
o C.4.35 Appendix G.2 Lexical scanner
o C.4.36 Appendix E. References
* Appendix D. A sample style sheet for HTML 4.0
* Appendix E. Property index
* Appendix F. Index
* Appendix G. Grammar of CSS 2.1
+ G.1 Grammar
+ G.2 Lexical scanner
+ G.3 Comparison of tokenization in CSS 2.1 and CSS1
_________________________________________________________________
1 About the CSS 2.1 Specification
Contents
* 1.1 CSS 2.1 vs CSS 2
* 1.2 Reading the specification
* 1.3 How the specification is organized
* 1.4 Conventions
+ 1.4.1 Document language elements and attributes
+ 1.4.2 CSS property definitions
o Value
o Initial
o Applies to
o Inherited
o Percentage values
o Media groups
o Computed value
+ 1.4.3 Shorthand properties
+ 1.4.4 Notes and examples
+ 1.4.5 Images and long descriptions
* 1.5 Acknowledgments
* 1.6 Copyright Notice
1.1 CSS 2.1 vs CSS 2
The CSS community has gained significant experience with the CSS2
specification since it became a recommendation in 1998. Errors in the
CSS2 specification have subsequently been corrected via the
publication of various errata, but there has not yet been an
opportunity for the specification to be changed based on experience
gained.
While many of these issues will be addressed by the upcoming CSS3
specifications, the current state of affairs hinders the
implementation and interoperability of CSS2. The CSS 2.1 specification
attempts to address this situation by:
* Maintaining compatibility with those portions of CSS2 that are
widely accepted and implemented.
* Incorporating all published CSS2 errata.
* Where implementations overwhelmingly differ from the CSS2
specification, modifying the specification to be in accordance
with generally accepted practice.
* Removing all CSS2 features which, by virtue of not having been
implemented, have been rejected by the CSS community.
* Removing CSS2 features that will be obsoleted by CSS3, thus
encouraging adoption of the proposed CSS3 features in their place.
* Adding a (very) small number of new property values, when
implementation experience has shown that they are needed for
implementing CSS2.
Thus, while it is not the case that a CSS2 stylesheet is necessarily
forwards-compatible with CSS 2.1, it is the case that a stylesheet
restricting itself to CSS 2.1 features is more likely to find a
compliant user agent today and to preserve forwards compatibility in
the future. While breaking forward compatibility is not desirable, we
believe the advantages to the revisions in CSS 2.1 are worthwhile.
1.2 Reading the specification
This specification has been written with two types of readers in mind:
CSS authors and CSS implementors. We hope the specification will
provide authors with the tools they need to write efficient,
attractive, and accessible documents, without overexposing them to
CSS's implementation details. Implementors, however, should find all
they need to build conforming user agents. The specification begins
with a general presentation of CSS and becomes more and more technical
and specific towards the end. For quick access to information, a
general table of contents, specific tables of contents at the
beginning of each section, and an index provide easy navigation, in
both the electronic and printed versions.
The specification has been written with two modes of presentation in
mind: electronic and printed. Although the two presentations will no
doubt be similar, readers will find some differences. For example,
links will not work in the printed version (obviously), and page
numbers will not appear in the electronic version. In case of a
discrepancy, the electronic version is considered the authoritative
version of the document.
1.3 How the specification is organized
The specification is organized into the following sections:
Section 2: An introduction to CSS2.1
The introduction includes a brief tutorial on CSS2.1 and a
discussion of design principles behind CSS2.1.
Sections 3 - 20: CSS 2.1 reference manual.
The bulk of the reference manual consists of the CSS 2.1
language reference. This reference defines what may go into a
CSS 2.1 style sheet (syntax, properties, property values) and
how user agents must interpret these style sheets in order to
claim conformance.
Appendixes:
Appendixes contain information about aural properties
(non-normative), a sample style sheet for HTML 4.0, changes
from CSS2, the grammar of CSS 2.1, a list of normative and
informative references, and two indexes: one for properties and
one general index.
1.4 Conventions
1.4.1 Document language elements and attributes
* CSS property, descriptor, and pseudo-class names are delimited by
single quotes.
* CSS values are delimited by single quotes.
* Document language element names are in uppercase letters.
* Document language attribute names are in lowercase letters and
delimited by double quotes.
1.4.2 CSS property definitions
Each CSS property definition begins with a summary of key information
that resembles the following:
'property-name'
Value: legal values & syntax
Initial: initial value
Applies to: elements this property applies to
Inherited: whether the property is inherited
Percentages: how percentage values are interpreted
Media: which media groups the property applies to
Computed value: how to compute the computed value
Value
This part specifies the set of valid values for the property whose
name is 'property-name'. Value types may be designated in several
ways:
1. keyword values (e.g., auto, disc, etc.)
2. basic data types, which appear between "<" and ">" (e.g.,
<length>, <percentage>, etc.). In the electronic version of the
document, each instance of a basic data type links to its
definition.
3. types that have the same range of values as a property bearing the
same name (e.g., <'border-width'> <'background-attachment'>,
etc.). In this case, the type name is the property name (complete
with quotes) between "<" and ">" (e.g., <'border-width'>). Such a
type does not include the value 'inherit'. In the electronic
version of the document, each instance of this type of
non-terminal links to the corresponding property definition.
4. non-terminals that do not share the same name as a property. In
this case, the non-terminal name appears between "<" and ">", as
in <border-width>. Notice the distinction between <border-width>
and <'border-width'>; the latter is defined in terms of the
former. The definition of a non-terminal is located near its first
appearance in the specification. In the electronic version of the
document, each instance of this type of value links to the
corresponding value definition.
Other words in these definitions are keywords that must appear
literally, without quotes (e.g., red). The slash (/) and the comma (,)
must also appear literally.
Values may be arranged as follows:
* Several juxtaposed words mean that all of them must occur, in the
given order.
* A bar (|) separates two or more alternatives: exactly one of them
must occur.
* A double bar (||) separates two or more options: one or more of
them must occur, in any order.
* Brackets ([ ]) are for grouping.
Juxtaposition is stronger than the double bar, and the double bar is
stronger than the bar. Thus, the following lines are equivalent:
a b | c || d e
[ a b ] | [ c || [ d e ]]
Every type, keyword, or bracketed group may be followed by one of the
following modifiers:
* An asterisk (*) indicates that the preceding type, word, or group
occurs zero or more times.
* A plus (+) indicates that the preceding type, word, or group
occurs one or more times.
* A question mark (?) indicates that the preceding type, word, or
group is optional.
* A pair of numbers in curly braces ({A,B}) indicates that the
preceding type, word, or group occurs at least A and at most B
times.
The following examples illustrate different value types:
Value: N | NW | NE
Value: [ <length> | thick | thin ]{1,4}
Value: [<family-name> , ]* <family-name>
Value: <uri>? <color> [ / <color> ]?
Value: <uri> || <color>
Initial
This part specifies the property's initial value. If the property is
inherited, this is the value that is given to the root element of the
document tree. Please consult the section on the cascade for
information about the interaction between style sheet-specified,
inherited, and initial values.
Applies to
This part lists the elements to which the property applies. All
elements are considered to have all properties, but some properties
have no rendering effect on some types of elements. For example,
'border-spacing' only affects table elements.
Inherited
This part indicates whether the value of the property is inherited
from an ancestor element. Please consult the section on the cascade
for information about the interaction between style sheet-specified,
inherited, and initial values.
Percentage values
This part indicates how percentages should be interpreted, if they
occur in the value of the property. If "N/A" appears here, it means
that the property does not accept percentages as values.
Media groups
This part indicates the media groups to which the property applies.
Information about media groups is non-normative.
Computed value
This part describes the computed value for the property. See the
section on computed values for how this definition is used.
1.4.3 Shorthand properties
Some properties are shorthand properties, meaning that they allow
authors to specify the values of several properties with a single
property.
For instance, the 'font' property is a shorthand property for setting
'font-style', 'font-variant', 'font-weight', 'font-size',
'line-height', and 'font-family' all at once.
When values are omitted from a shorthand form, each "missing" property
is assigned its initial value (see the section on the cascade).
Example(s):
The multiple style rules of this example:
h1 {
font-weight: bold;
font-size: 12pt;
line-height: 14pt;
font-family: Helvetica;
font-variant: normal;
font-style: normal;
}
may be rewritten with a single shorthand property:
h1 { font: bold 12pt/14pt Helvetica }
In this example, 'font-variant', and 'font-style' take their initial
values.
1.4.4 Notes and examples
All examples that illustrate illegal usage are clearly marked as
"ILLEGAL EXAMPLE".
All HTML examples conform to the HTML 4.0 strict DTD (defined in
[HTML40]) unless otherwise indicated by a document type declaration.
All notes are informative only.
Examples and notes are marked within the source HTML for the
specification and CSS1 user agents will render them specially.
1.4.5 Images and long descriptions
Most images in the electronic version of this specification are
accompanied by "long descriptions" of what they represent. A link to
the long description is denoted by a "[D]" to the right of the image.
Images and long descriptions are informative only.
1.5 Acknowledgments
CSS 2.1 is based on CSS2. See the acknowledgments section of CSS2 for
the people that contributed to CSS2.
Fantasai for clarifications related to backgrounds on table elements
(17.5.1 and 17.6.1) and other ambiguities.
Kevin Smith for pointing out a mistake in the "Applies to" line for
'float'.
...
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_________________________________________________________________
2 Introduction to CSS 2.1
Contents
* 2.1 A brief CSS 2.1 tutorial for HTML
* 2.2 A brief CSS 2.1 tutorial for XML
* 2.3 The CSS 2.1 processing model
+ 2.3.1 The canvas
+ 2.3.2 CSS 2.1 addressing model
* 2.4 CSS design principles
2.1 A brief CSS 2.1 tutorial for HTML
In this tutorial, we show how easy it can be to design simple style
sheets. For this tutorial, you will need to know a little HTML (see
[HTML40]) and some basic desktop publishing terminology.
We begin with a small HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
To set the text color of the H1 elements to red, you can write the
following CSS rules:
h1 { color: red }
A CSS rule consists of two main parts: selector ('h1') and declaration
('color: red'). In HTML, element names are case-insensitive so 'h1'
works just as well as 'H1'. The declaration has two parts: property
('color') and value ('red'). While the example above tries to
influence only one of the properties needed for rendering an HTML
document, it qualifies as a style sheet on its own. Combined with
other style sheets (one fundamental feature of CSS is that style
sheets are combined) it will determine the final presentation of the
document.
The HTML 4.0 specification defines how style sheet rules may be
specified for HTML documents: either within the HTML document, or via
an external style sheet. To put the style sheet into the document, use
the STYLE element:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
h1 { color: red }
</STYLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
For maximum flexibility, we recommend that authors specify external
style sheets; they may be changed without modifying the source HTML
document, and they may be shared among several documents. To link to
an external style sheet, you can use the LINK element:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<LINK rel="stylesheet" href="bach.css" type="text/css">
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
The LINK element specifies:
* the type of link: to a "stylesheet".
* the location of the style sheet via the "href" attribute.
* the type of style sheet being linked: "text/css".
To show the close relationship between a style sheet and the
structured markup, we continue to use the STYLE element in this
tutorial. Let's add more colors:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
body { color: black; background: white }
h1 { color: red; background: white }
</STYLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
The style sheet now contains four rules: the first two set the color
and background of the BODY element (it's a good idea to set the text
color and background color together), while the last two set the color
and the background of the H1 element. Since no color has been
specified for the P element, it will inherit the color from its parent
element, namely BODY. The H1 element is also a child element of BODY
but the second rule overrides the inherited value. In CSS there are
often such conflicts between different values, and this specification
describes how to resolve them.
CSS 2.1 has more than 90 properties, including 'color'. Let's look at
some of the others:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
body {
font-family: "Gill Sans", sans-serif;
font-size: 12pt;
margin: 3em;
}
</STYLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
The first thing to notice is that several declarations are grouped
within a block enclosed by curly braces ({...}), and separated by
semicolons, though the last declaration may also be followed by a
semicolon.
The first declaration on the BODY element sets the font family to
"Gill Sans". If that font isn't available, the user agent (often
referred to as a "browser") will use the 'sans-serif' font family
which is one of five generic font families which all users agents
know. Child elements of BODY will inherit the value of the
'font-family' property.
The second declaration sets the font size of the BODY element to 12
points. The "point" unit is commonly used in print-based typography to
indicate font sizes and other length values. It's an example of an
absolute unit which does not scale relative to the environment.
The third declaration uses a relative unit which scales with regard to
its surroundings. The "em" unit refers to the font size of the
element. In this case the result is that the margins around the BODY
element are three times wider than the font size.
2.2 A brief CSS 2.1 tutorial for XML
CSS can be used with any structured document format, for example with
applications of the eXtensible Markup Language [XML10]. In fact, XML
depends more on style sheets than HTML, since authors can make up
their own elements that user agents don't know how to display.
Here is a simple XML fragment:
<ARTICLE>
<HEADLINE>Fredrick the Great meets Bach</HEADLINE>
<AUTHOR>Johann Nikolaus Forkel</AUTHOR>
<PARA>
One evening, just as he was getting his
<INSTRUMENT>flute</INSTRUMENT> ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
</PARA>
</ARTICLE>
To display this fragment in a document-like fashion, we must first
declare which elements are inline-level (i.e., do not cause line
breaks) and which are block-level (i.e., cause line breaks).
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
The first rule declares INSTRUMENT to be inline and the second rule,
with its comma-separated list of selectors, declares all the other
elements to be block-level. Element names in XML are case-sensitive,
so a selector written in lowercase (e.g. 'instrument') is different
from uppercase (e.g. 'INSTRUMENT').
One way of linking a style sheet to an XML document is to use a
processing instruction:
<?xml-stylesheet type="text/css" href="bach.css"?>
<ARTICLE>
<HEADLINE>Fredrick the Great meets Bach</HEADLINE>
<AUTHOR>Johann Nikolaus Forkel</AUTHOR>
<PARA>
One evening, just as he was getting his
<INSTRUMENT>flute</INSTRUMENT> ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
</PARA>
</ARTICLE>
A visual user agent could format the above example as:
Example rendering
Notice that the word "flute" remains within the paragraph since it is
the content of the inline element INSTRUMENT.
Still, the text isn't formatted the way you would expect. For example,
the headline font size should be larger than then the rest of the
text, and you may want to display the author's name in italic:
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
HEADLINE { font-size: 1.3em }
AUTHOR { font-style: italic }
ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }
A visual user agent could format the above example as:
Example rendering
Adding more rules to the style sheet will allow you to further
describe the presentation of the document.
2.3 The CSS 2.1 processing model
This section presents one possible model of how user agents that
support CSS work. This is only a conceptual model; real
implementations may vary.
In this model, a user agent processes a source by going through the
following steps:
1. Parse the source document and create a document tree.
2. Identify the target media type.
3. Retrieve all style sheets associated with the document that are
specified for the target media type.
4. Annotate every element of the document tree by assigning a single
value to every property that is applicable to the target media
type. Properties are assigned values according to the mechanisms
described in the section on cascading and inheritance.
Part of the calculation of values depends on the formatting
algorithm appropriate for the target media type. For example, if
the target medium is the screen, user agents apply the visual
formatting model.
5. From the annotated document tree, generate a formatting structure.
Often, the formatting structure closely resembles the document
tree, but it may also differ significantly, notably when authors
make use of pseudo-elements and generated content. First, the
formatting structure need not be "tree-shaped" at all -- the
nature of the structure depends on the implementation. Second, the
formatting structure may contain more or less information than the
document tree. For instance, if an element in the document tree
has a value of 'none' for the 'display' property, that element
will generate nothing in the formatting structure. A list element,
on the other hand, may generate more information in the formatting
structure: the list element's content and list style information
(e.g., a bullet image).
Note that the CSS user agent does not alter the document tree
during this phase. In particular, content generated due to style
sheets is not fed back to the document language processor (e.g.,
for reparsing).
6. Transfer the formatting structure to the target medium (e.g.,
print the results, display them on the screen, render them as
speech, etc.).
Step 1 lies outside the scope of this specification (see, for example,
[DOM]).
Steps 2-5 are addressed by the bulk of this specification.
Step 6 lies outside the scope of this specification.
2.3.1 The canvas
For all media, the term canvas describes "the space where the
formatting structure is rendered." The canvas is infinite for each
dimension of the space, but rendering generally occurs within a finite
region of the canvas, established by the user agent according to the
target medium. For instance, user agents rendering to a screen
generally impose a minimum width and choose an initial width based on
the dimensions of the viewport. User agents rendering to a page
generally impose width and height constraints. Aural user agents may
impose limits in audio space, but not in time.
2.3.2 CSS 2.1 addressing model
CSS 2.1 selectors and properties allow style sheets to refer to the
following parts of a document or user agent:
* Elements in the document tree and certain relationships between
them (see the section on selectors).
* Attributes of elements in the document tree, and values of those
attributes (see the section on attribute selectors).
* Some parts of element content (see the :first-line and
:first-letter pseudo-elements.
* Elements of the document tree when they are in a certain state
(see the section on pseudo-classes).
* Some aspects of the canvas where the document will be rendered.
* Some system information (see the section on user interface).
2.4 CSS design principles
CSS 2.1, as CSS2 and CSS1 before it, is based on a set of design
principles:
* Forward and backward compatibility. CSS 2.1 user agents will be
able to understand CSS1 style sheets. CSS1 user agents will be
able to read CSS 2.1 style sheets and discard parts they don't
understand. Also, user agents with no CSS support will be able to
display style-enhanced documents. Of course, the stylistic
enhancements made possible by CSS will not be rendered, but all
content will be presented.
* Complementary to structured documents. Style sheets complement
structured documents (e.g., HTML and XML applications), providing
stylistic information for the marked-up text. It should be easy to
change the style sheet with little or no impact on the markup.
* Vendor, platform, and device independence. Style sheets enable
documents to remain vendor, platform, and device independent.
Style sheets themselves are also vendor and platform independent,
but CSS 2.1 allows you to target a style sheet for a group of
devices (e.g., printers).
* Maintainability. By pointing to style sheets from documents,
webmasters can simplify site maintenance and retain consistent
look and feel throughout the site. For example, if the
organization's background color changes, only one file needs to be
changed.
* Simplicity. CSS is a simple style language which is human readable
and writable. The CSS properties are kept independent of each
other to the largest extent possible and there is generally only
one way to achieve a certain effect.
* Network performance. CSS provides for compact encodings of how to
present content. Compared to images or audio files, which are
often used by authors to achieve certain rendering effects, style
sheets most often decrease the content size. Also, fewer network
connections have to be opened which further increases network
performance.
* Flexibility. CSS can be applied to content in several ways. The
key feature is the ability to cascade style information specified
in the default (user agent) style sheet, user style sheets, linked
style sheets, the document head, and in attributes for the
elements forming the document body.
* Richness. Providing authors with a rich set of rendering effects
increases the richness of the Web as a medium of expression.
Designers have been longing for functionality commonly found in
desktop publishing and slide-show applications. Some of the
requested rendering effects conflict with device independence, but
CSS 2.1 goes a long way toward granting designers their requests.
* Alternative language bindings. The set of CSS properties described
in this specification form a consistent formatting model for
visual and aural presentations. This formatting model can be
accessed through the CSS language, but bindings to other languages
are also possible. For example, a JavaScript program may
dynamically change the value of a certain element's 'color'
property.
* Accessibility. Several CSS features will make the Web more
accessible to users with disabilities:
+ Properties to control font appearance allow authors to
eliminate inaccessible bit-mapped text images.
+ Positioning properties allow authors to eliminate mark-up
tricks (e.g., invisible images) to force layout.
+ The semantics of !important rules mean that users with
particular presentation requirements can override the
author's style sheets.
+ The 'inherit' value for all properties improves cascading
generality and allows for easier and more consistent style
tuning.
+ Improved media support, including media groups and the
braille, embossed, and tty media types, will allow users and
authors to tailor pages to those devices.
Note. For more information about designing accessible documents
using CSS and HTML, see [WAI-PAGEAUTH].
_________________________________________________________________
_________________________________________________________________
3 Conformance: Requirements and Recommendations
Contents
* 3.1 Definitions
* 3.2 Conformance
* 3.3 Error conditions
* 3.4 The text/css content type
3.1 Definitions
In this section, we begin the formal specification of CSS 2.1,
starting with the contract between authors, users, and implementors.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 (see
[RFC2119]). However, for readability, these words do not appear in all
uppercase letters in this specification.
At times, this specification recommends good practice for authors and
user agents. These recommendations are not normative and conformance
with this specification does not depend on their realization. These
recommendations contain the expression "We recommend ...", "This
specification recommends ...", or some similar wording.
Style sheet
A set of statements that specify presentation of a document.
Style sheets may have three different origins: author, user,
and user agent. The interaction of these sources is described
in the section on cascading and inheritance.
Valid style sheet
The validity of a style sheet depends on the level of CSS used
for the style sheet. All valid CSS1 style sheets are valid
CSS 2.1 style sheets, but some changes from CSS1 mean that a
few CSS1 style sheets will have slightly different semantics in
CSS 2.1. Some features in CSS2 are not part of CSS 2.1, so not
all CSS2 style sheets are valid CSS 2.1 style sheets.
A valid CSS 2.1 style sheet must be written according to the
grammar of CSS 2.1. Furthermore, it must contain only at-rules,
property names, and property values defined in this
specification. An illegal (invalid) at-rule, property name, or
property value is one that is not valid.
Source document
The document to which one or more style sheets apply. This is
encoded in some language that represents the document as a tree
of elements. Each element consists of a name that identifies
the type of element, optionally a number of attributes, and a
(possibly empty) content.
Document language
The encoding language of the source document (e.g., HTML, XHTML
or SVG). CSS is used to describe the presentation of document
languages and CSS does not change the underlying semantics of
the document languages.
Element
(An SGML term, see [ISO8879].) The primary syntactic constructs
of the document language. Most CSS style sheet rules use the
names of these elements (such as P, TABLE, and OL in HTML) to
specify how the elements should be rendered.
Replaced element
An element for which the CSS formatter knows only the intrinsic
dimensions. In HTML, IMG and OBJECT elements can be replaced
elements. For example, the content of the IMG element is often
replaced by the image that the "src" attribute designates.
Intrinsic dimensions
The width and height as defined by the element itself, not
imposed by the surroundings. CSS does not define how the
intrinsic dimensions are found. In CSS 2.1 it is assumed that
all replaced elements, and only replaced elements, come with
intrinsic dimensions.
Attribute
A value associated with an element, consisting of a name, and
an associated (textual) value.
Content
The content associated with an element in the source document.
Some elements have no content, in which case they are called
empty. The content of an element may include text, and it may
include a number of sub-elements, in which case the element is
called the parent of those sub-elements.
Ignore
This term has three slightly different meanings this
specification. First, a CSS parser must follow certain rules
when it discovers unknown or illegal syntax in a style sheet.
The parser must then ignore certain parts of the style sheets.
The exact rules for what parts must be ignored is given in
these section: Declarations and properties, Rules for handling
parsing errors, Unsupported Values, or may be explained in the
text where the term "ignore" appears. Second, a user agent may
(and, in some cases must) disregard certain properties or
values in the style sheet even if the syntax is legal. For
example, table-column-group elements cannot have borders around
them, so the border properties must be ignored.
Rendered content
The content of an element after the rendering that applies to
it according to the relevant style sheets has been applied. The
rendered content of a replaced element comes from outside the
source document. Rendered content may also be alternate text
for an element (e.g., the value of the XHTML "alt" attribute),
and may include items inserted implicitly or explicitly by the
style sheet, such as bullets, numbering, etc.
Document tree
The tree of elements encoded in the source document. Each
element in this tree has exactly one parent, with the exception
of the root element, which has none.
Child
An element A is called the child of element B if and only if B
is the parent of A.
Descendant
An element A is called a descendant of an element B, if either
(1) A is a child of B, or (2) A is the child of some element C
that is a descendant of B.
Ancestor
An element A is called an ancestor of an element B, if and only
if B is a descendant of A.
Sibling
An element A is called a sibling of an element B, if and only
if B and A share the same parent element. Element A is a
preceding sibling if it comes before B in the document tree.
Element B is a following sibling if it comes after A in the
document tree.
Preceding element
An element A is called a preceding element of an element B, if
and only if (1) A is an ancestor of B or (2) A is a preceding
sibling of B.
Following element
An element A is called a following element of an element B, if
and only if B is a preceding element of A.
Author
An author is a person who writes documents and associated style
sheets. An authoring tool generates documents and associated
style sheets.
User
A user is a person who interacts with a user agent to view,
hear, or otherwise use a document and its associated style
sheet. The user may provide a personal style sheet that encodes
personal preferences.
User agent (UA)
A user agent is any program that interprets a document written
in the document language and applies associated style sheets
according to the terms of this specification. A user agent may
display a document, read it aloud, cause it to be printed,
convert it to another format, etc.
An HTML user agent is one that supports the HTML 2.x, HTML 3.x,
or HTML 4.x specifications. A user agent that supports XHTML
[XHTML], but not HTML (as listed in the previous sentence) is
not considered an HTML user agent for the purpose of
conformance with this specification.
Here is an example of a source document written in HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<TITLE>My home page</TITLE>
<BODY>
<H1>My home page</H1>
<P>Welcome to my home page! Let me tell you about my favorite
composers:
<UL>
<LI> Elvis Costello
<LI> Johannes Brahms
<LI> Georges Brassens
</UL>
</BODY>
</HTML>
This results in the following tree:
Sample document tree
According to the definition of HTML 4.0, HEAD elements will be
inferred during parsing and become part of the document tree even if
the "head" tags are not in the document source. Similarly, the parser
knows where the P and LI elements end, even though there are no </p>
and </li> tags in the source.
Documents written in XHTML (and other XML-based languages) behave
differently: there are no inferred elements and all elements must have
end tags.
3.2 Conformance
This section defines conformance with the CSS 2.1 specification only.
There may be other levels of CSS in the future that may require a user
agent to implement a different set of features in order to conform.
In general, the following points must be observed by a user agent
claiming conformance to this specification:
1. It must support one or more of the CSS 2.1 media types.
2. For each source document, it must attempt to retrieve all
associated style sheets that are appropriate for the supported
media types. If it cannot retrieve all associated style sheets
(for instance, because of network errors), it must display the
document using those it can retrieve.
3. It must parse the style sheets according to this specification. In
particular, it must recognize all at-rules, blocks, declarations,
and selectors (see the grammar of CSS 2.1). If a user agent
encounters a property that applies for a supported media type, the
user agent must parse the value according to the property
definition. This means that the user agent must accept all valid
values and must ignore declarations with invalid values. User
agents must ignore rules that apply to unsupported media types.
4. For each element in a document tree, it must assign a value for
every applicable property according to the property's definition
and the rules of cascading and inheritance.
5. If the source document comes with alternate style sheet sets (such
as with the "alternate" keyword in HTML 4.0 [HTML40]), the UA must
allow the user to select one from among these sets and apply the
selected one.
Not every user agent must observe every point, however:
* An application that reads style sheets without rendering any
content (e.g., a CSS 2.1 validator) must respect points 1-3.
* An authoring tool is only required to output valid style sheets
* A user agent that renders a document with associated style sheets
must respect points 1-5 and render the document according to the
media-specific requirements set forth in this specification.
Values may be approximated when required by the user agent.
The inability of a user agent to implement part of this specification
due to the limitations of a particular device (e.g., a user agent
cannot render colors on a monochrome monitor or page) does not imply
non-conformance.
UAs must allow users to specify a file that contains the user style
sheet. UAs that run on devices without any means of writing or
specifying files are exempted from this requirement. Additionally, UAs
may offer other means to specify user preferences, for example through
a GUI.
3.3 Error conditions
In general, this document does not specify error handling behavior for
user agents (e.g., how they behave when they cannot find a resource
designated by a URI).
However, user agents must observe the rules for handling parsing
errors.
Since user agents may vary in how they handle error conditions,
authors and users must not rely on specific error recovery behavior.
3.4 The text/css content type
CSS style sheets that exist in separate files are sent over the
Internet as a sequence of bytes accompanied by encoding information.
The structure of the transmission, termed a message entity, is defined
by RFC 2045 and RFC 2068 (see [RFC2045] and [RFC2068]). A message
entity with a content type of "text/css" represents an independent CSS
document. The "text/css" content type has been registered by RFC 2318
([RFC2318]).
_________________________________________________________________
_________________________________________________________________
4 Syntax and basic data types
Contents
* 4.1 Syntax
+ 4.1.1 Tokenization
+ 4.1.2 Keywords
+ 4.1.3 Characters and case
+ 4.1.4 Statements
+ 4.1.5 At-rules
+ 4.1.6 Blocks
+ 4.1.7 Rule sets, declaration blocks, and selectors
+ 4.1.8 Declarations and properties
+ 4.1.9 Comments
* 4.2 Rules for handling parsing errors
* 4.3 Values
+ 4.3.1 Integers and real numbers
+ 4.3.2 Lengths
+ 4.3.3 Percentages
+ 4.3.4 URL + URN = URI
+ 4.3.5 Counters
+ 4.3.6 Colors
+ 4.3.7 Strings
+ 4.3.8 Unsupported Values
* 4.4 CSS document representation
+ 4.4.1 Referring to characters not represented in a character
encoding
4.1 Syntax
This section describes a grammar (and forward-compatible parsing
rules) common to any version of CSS (including CSS 2.1). Future
versions of CSS will adhere to this core syntax, although they may add
additional syntactic constraints.
These descriptions are normative. They are also complemented by the
normative grammar rules presented in Appendix D.
4.1.1 Tokenization
All levels of CSS -- level 1, level 2, and any future levels -- use
the same core syntax. This allows UAs to parse (though not completely
understand) style sheets written in levels of CSS that didn't exist at
the time the UAs were created. Designers can use this feature to
create style sheets that work with older user agents, while also
exercising the possibilities of the latest levels of CSS.
At the lexical level, CSS style sheets consist of a sequence of
tokens. The list of tokens for CSS 2.1 is as follows. The definitions
use Lex-style regular expressions. Octal codes refer to ISO 10646
([ISO10646]). As in Lex, in case of multiple matches, the longest
match determines the token.
Token Definition
_________________________________________________________________
IDENT {ident}
ATKEYWORD @{ident}
STRING {string}
HASH #{name}
NUMBER {num}
PERCENTAGE {num}%
DIMENSION {num}{ident}
URI url\({w}{string}{w}\)
|url\({w}([!#$%&*-~]|{nonascii}|{escape})*{w}\)
UNICODE-RANGE U\+[0-9A-F?]{1,6}(-[0-9A-F]{1,6})?
CDO <!--
CDC -->
; ;
{ \{
} \}
( \(
) \)
[ \[
] \]
S [ \t\r\n\f]+
COMMENT \/\*[^*]*\*+([^/*][^*]*\*+)*\/
FUNCTION {ident}\(
INCLUDES ~=
DASHMATCH |=
DELIM any other character not matched by the above rules, and neither
a single nor a double quote
The macros in curly braces ({}) above are defined as follows:
Macro Definition
_________________________________________________________________
ident {nmstart}{nmchar}*
name {nmchar}+
nmstart [_a-zA-Z]|{nonascii}|{escape}
nonascii [^\0-\177]
unicode \\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])?
escape {unicode}|\\[ -~\200-\4177777]
nmchar [_a-zA-Z0-9-]|{nonascii}|{escape}
num [0-9]+|[0-9]*\.[0-9]+
string {string1}|{string2}
string1 \"([\t !#$%&(-~]|\\{nl}|\'|{nonascii}|{escape})*\"
string2 \'([\t !#$%&(-~]|\\{nl}|\"|{nonascii}|{escape})*\'
nl \n|\r\n|\r|\f
w [ \t\r\n\f]*
Below is the core syntax for CSS. The sections that follow describe
how to use it. Appendix D describes a more restrictive grammar that is
closer to the CSS level 2 language.
stylesheet : [ CDO | CDC | S | statement ]*;
statement : ruleset | at-rule;
at-rule : ATKEYWORD S* any* [ block | ';' S* ];
block : '{' S* [ any | block | ATKEYWORD S* | ';' ]* '}' S*;
ruleset : selector? '{' S* declaration? [ ';' S* declaration? ]* '}' S*;
selector : any+;
declaration : property ':' S* value;
property : IDENT S*;
value : [ any | block | ATKEYWORD S* ]+;
any : [ IDENT | NUMBER | PERCENTAGE | DIMENSION | STRING
| DELIM | URI | HASH | UNICODE-RANGE | INCLUDES
| FUNCTION any* ')' | DASHMATCH | '(' any* ')' | '[' any* ']' ] S
*;
COMMENT tokens do not occur in the grammar (to keep it readable), but
any number of these tokens may appear anywhere between other tokens.
The token S in the grammar above stands for whitespace. Only the
characters "space" (Unicode code 32), "tab" (9), "line feed" (10),
"carriage return" (13), and "form feed" (12) can occur in whitespace.
Other space-like characters, such as "em-space" (8195) and
"ideographic space" (12288), are never part of whitespace.
4.1.2 Keywords
Keywords have the form of identifiers. Keywords must not be placed
between quotes ("..." or '...'). Thus,
red
is a keyword, but
"red"
is not. (It is a string.) Other illegal examples:
Illegal example(s):
width: "auto";
border: "none";
background: "red";
4.1.3 Characters and case
The following rules always hold:
* All CSS style sheets are case-insensitive, except for parts that
are not under the control of CSS. For example, the
case-sensitivity of values of the HTML attributes "id" and
"class", of font names, and of URIs lies outside the scope of this
specification. Note in particular that element names are
case-insensitive in HTML, but case-sensitive in XML.
* In CSS 2.1, identifiers (including element names, classes, and IDs
in selectors) can contain only the characters [A-Za-z0-9] and ISO
10646 characters 161 and higher, plus the hyphen (-) and the
underscore (_); they cannot start with a hyphen or a digit. They
can also contain escaped characters and any ISO 10646 character as
a numeric code (see next item). For instance, the identifier
"B&W?" may be written as "B\&W\?" or "B\26 W\3F".
Note that Unicode is code-by-code equivalent to ISO 10646 (see
[UNICODE] and [ISO10646]).
* In CSS 2.1, a backslash (\) character indicates three types of
character escapes.
First, inside a string, a backslash followed by a newline is
ignored (i.e., the string is deemed not to contain either the
backslash or the newline).
Second, it cancels the meaning of special CSS characters. Any
character (except a hexadecimal digit) can be escaped with a
backslash to remove its special meaning. For example, "\"" is a
string consisting of one double quote. Style sheet preprocessors
must not remove these backslashes from a style sheet since that
would change the style sheet's meaning.
Third, backslash escapes allow authors to refer to characters they
can't easily put in a document. In this case, the backslash is
followed by at most six hexadecimal digits (0..9A..F), which stand
for the ISO 10646 ([ISO10646]) character with that number. If a
character in the range [0-9a-zA-Z] follows the hexadecimal number,
the end of the number needs to be made clear. There are two ways
to do that:
1. with a space (or other whitespace character): "\26 B" ("&B").
In this case, user agents should treat a "CR/LF" pair (13/10)
as a single whitespace character.
2. by providing exactly 6 hexadecimal digits: "\000026B" ("&B")
In fact, these two methods may be combined. Only one whitespace
character is ignored after a hexadecimal escape. Note that this
means that a "real" space after the escape sequence must itself
either be escaped or doubled.
* Backslash escapes are always considered to be part of an
identifier or a string (i.e., "\7B" is not punctuation, even
though "{" is, and "\32" is allowed at the start of a class name,
even though "2" is not).
4.1.4 Statements
A CSS style sheet, for any version of CSS, consists of a list of
statements (see the grammar above). There are two kinds of statements:
at-rules and rule sets. There may be whitespace around the statements.
In this specification, the expressions "immediately before" or
"immediately after" mean with no intervening whitespace or comments.
4.1.5 At-rules
At-rules start with an at-keyword, an '@' character followed
immediately by an identifier (for example, '@import', '@page').
An at-rule consists of everything up to and including the next
semicolon (;) or the next block, whichever comes first. A CSS user
agent that encounters an unrecognized at-rule must ignore the whole of
the at-rule and continue parsing after it.
CSS 2.1 user agents must ignore any '@import' rule that occurs inside
a block or that doesn't precede all rule sets.
Illegal example(s):
Assume, for example, that a CSS 2.1 parser encounters this style
sheet:
@import "subs.css";
h1 { color: blue }
@import "list.css";
The second '@import' is illegal according to CSS2.1. The CSS 2.1
parser ignores the whole at-rule, effectively reducing the style sheet
to:
@import "subs.css";
h1 { color: blue }
Illegal example(s):
In the following example, the second '@import' rule is invalid, since
it occurs inside a '@media' block.
@import "subs.css";
@media print {
@import "print-main.css";
body { font-size: 10pt }
}
h1 {color: blue }
4.1.6 Blocks
A block starts with a left curly brace ({) and ends with the matching
right curly brace (}). In between there may be any characters, except
that parentheses (( )), brackets ([ ]) and braces ({ }) must always
occur in matching pairs and may be nested. Single (') and double
quotes (") must also occur in matching pairs, and characters between
them are parsed as a string. See Tokenization above for the definition
of a string.
Illegal example(s):
Here is an example of a block. Note that the right brace between the
double quotes does not match the opening brace of the block, and that
the second single quote is an escaped character, and thus doesn't
match the first single quote:
{ causta: "}" + ({7} * '\'') }
Note that the above rule is not valid CSS 2.1, but it is still a block
as defined above.
4.1.7 Rule sets, declaration blocks, and selectors
A rule set (also called "rule") consists of a selector followed by a
declaration block.
A declaration-block (also called a {}-block in the following text)
starts with a left curly brace ({) and ends with the matching right
curly brace (}). In between there must be a list of zero or more
semicolon-separated (;) declarations.
The selector (see also the section on selectors) consists of
everything up to (but not including) the first left curly brace ({). A
selector always goes together with a {}-block. When a user agent can't
parse the selector (i.e., it is not valid CSS 2.1), it must ignore the
{}-block as well.
CSS 2.1 gives a special meaning to the comma (,) in selectors.
However, since it is not known if the comma may acquire other meanings
in future versions of CSS, the whole statement should be ignored if
there is an error anywhere in the selector, even though the rest of
the selector may look reasonable in CSS 2.1.
Illegal example(s):
For example, since the "&" is not a valid token in a CSS 2.1 selector,
a CSS 2.1 user agent must ignore the whole second line, and not set
the color of H3 to red:
h1, h2 {color: green }
h3, h4 & h5 {color: red }
h6 {color: black }
Example(s):
Here is a more complex example. The first two pairs of curly braces
are inside a string, and do not mark the end of the selector. This is
a valid CSS 2.1 statement.
p[example="public class foo\
{\
private int x;\
\
foo(int x) {\
this.x = x;\
}\
\
}"] { color: red }
4.1.8 Declarations and properties
A declaration is either empty or consists of a property, followed by a
colon (:), followed by a value. Around each of these there may be
whitespace.
Because of the way selectors work, multiple declarations for the same
selector may be organized into semicolon (;) separated groups.
Example(s):
Thus, the following rules:
h1 { font-weight: bold }
h1 { font-size: 12px }
h1 { line-height: 14px }
h1 { font-family: Helvetica }
h1 { font-variant: normal }
h1 { font-style: normal }
are equivalent to:
h1 {
font-weight: bold;
font-size: 12px;
line-height: 14px;
font-family: Helvetica;
font-variant: normal;
font-style: normal
}
A property is an identifier. Any character may occur in the value.
Parentheses ("( )"), brackets ("[ ]"), braces ("{ }"), single quotes
(') and double quotes (") must come in matching pairs, and semicolons
not in strings must be escaped. Parentheses, brackets, and braces may
be nested. Inside the quotes, characters are parsed as a string.
The syntax of values is specified separately for each property, but in
any case, values are built from identifiers, strings, numbers,
lengths, percentages, URIs, and colors.
A user agent must ignore a declaration with an invalid property name
or an invalid value. Every CSS 2.1 property has its own syntactic and
semantic restrictions on the values it accepts.
Illegal example(s):
For example, assume a CSS 2.1 parser encounters this style sheet:
h1 { color: red; font-style: 12pt } /* Invalid value: 12pt */
p { color: blue; font-vendor: any; /* Invalid prop.: font-vendor */
font-variant: small-caps }
em em { font-style: normal }
The second declaration on the first line has an invalid value '12pt'.
The second declaration on the second line contains an undefined
property 'font-vendor'. The CSS 2.1 parser will ignore these
declarations, effectively reducing the style sheet to:
h1 { color: red; }
p { color: blue; font-variant: small-caps }
em em { font-style: normal }
4.1.9 Comments
Comments begin with the characters "/*" and end with the characters
"*/". They may occur anywhere between tokens, and their contents have
no influence on the rendering. Comments may not be nested.
CSS also allows the SGML comment delimiters ("<!--" and "-->") in
certain places, but they do not delimit CSS comments. They are
permitted so that style rules appearing in an HTML source document (in
the STYLE element) may be hidden from pre-HTML 3.2 user agents. See
the HTML 4.0 specification ([HTML40]) for more information.
4.2 Rules for handling parsing errors
In some cases, user agents must ignore part of an illegal style sheet.
This specification defines ignore to mean that the user agent parses
the illegal part (in order to find its beginning and end), but
otherwise acts as if it had not been there.
To ensure that new properties and new values for existing properties
can be added in the future, user agents are required to obey the
following rules when they encounter the following scenarios:
* Unknown properties. User agents must ignore a declaration with an
unknown property. For example, if the style sheet is:
h1 { color: red; rotation: 70minutes }
the user agent will treat this as if the style sheet had been
h1 { color: red }
* Illegal values. User agents must ignore a declaration with an
illegal value. For example:
img { float: left } /* correct CSS 2.1 */
img { float: left here } /* "here" is not a value of 'float' */
img { background: "red" } /* keywords cannot be quoted */
img { border-width: 3 } /* a unit must be specified for length values */
A CSS 2.1 parser would honor the first rule and ignore the rest,
as if the style sheet had been:
img { float: left }
img { }
img { }
img { }
A user agent conforming to a future CSS specification may accept
one or more of the other rules as well.
* Invalid at-keywords. User agents must ignore an invalid at-keyword
together with everything following it, up to and including the
next semicolon (;) or block ({...}), whichever comes first. For
example, consider the following:
@three-dee {
@background-lighting {
azimuth: 30deg;
elevation: 190deg;
}
h1 { color: red }
}
h1 { color: blue }
The '@three-dee' at-rule is not part of CSS 2.1. Therefore, the
whole at-rule (up to, and including, the third right curly brace)
is ignored. A CSS 2.1 user agent ignores it, effectively reducing
the style sheet to:
h1 { color: blue }
4.3 Values
4.3.1 Integers and real numbers
Some value types may have integer values (denoted by <integer>) or
real number values (denoted by <number>). Real numbers and integers
are specified in decimal notation only. An <integer> consists of one
or more digits "0" to "9". A <number> can either be an <integer>, or
it can be zero or more digits followed by a dot (.) followed by one or
more digits. Both integers and real numbers may be preceded by a "-"
or "+" to indicate the sign.
Note that many properties that allow an integer or real number as a
value actually restrict the value to some range, often to a
non-negative value.
4.3.2 Lengths
Lengths refer to horizontal or vertical measurements.
The format of a length value (denoted by <length> in this
specification) is a <number> (with or without a decimal point)
immediately followed by a unit identifier (e.g., px, em, etc.). After
a zero length, the unit identifier is optional.
Some properties allow negative length values, but this may complicate
the formatting model and there may be implementation-specific limits.
If a negative length value cannot be supported, it should be converted
to the nearest value that can be supported.
There are two types of length units: relative and absolute. Relative
length units specify a length relative to another length property.
Style sheets that use relative units will more easily scale from one
medium to another (e.g., from a computer display to a laser printer).
Relative units are:
* em: the 'font-size' of the relevant font
* ex: the 'x-height' of the relevant font
* px: pixels, relative to the viewing device
Example(s):
h1 { margin: 0.5em } /* em */
h1 { margin: 1ex } /* ex */
p { font-size: 12px } /* px */
The 'em' unit is equal to the computed value of the 'font-size'
property of the element on which it is used. The exception is when
'em' occurs in the value of the 'font-size' property itself, in which
case it refers to the font size of the parent element. It may be used
for vertical or horizontal measurement. (This unit is also sometimes
called the quad-width in typographic texts.)
The 'ex' unit is defined by the font's 'x-height'. The x-height is so
called because it is often equal to the height of the lowercase "x".
However, an 'ex' is defined even for fonts that don't contain an "x".
Example(s):
The rule:
h1 { line-height: 1.2em }
means that the line height of "h1" elements will be 20% greater than
the font size of the "h1" elements. On the other hand:
h1 { font-size: 1.2em }
means that the font-size of "h1" elements will be 20% greater than the
font size inherited by "h1" elements.
When specified for the root of the document tree (e.g., "HTML" in
HTML), 'em' and 'ex' refer to the property's initial value.
Pixel units are relative to the resolution of the viewing device,
i.e., most often a computer display. If the pixel density of the
output device is very different from that of a typical computer
display, the user agent should rescale pixel values. It is recommended
that the reference pixel be the visual angle of one pixel on a device
with a pixel density of 96dpi and a distance from the reader of an
arm's length. For a nominal arm's length of 28 inches, the visual
angle is therefore about 0.0213 degrees.
For reading at arm's length, 1px thus corresponds to about 0.26 mm
(1/96 inch). When printed on a laser printer, meant for reading at a
little less than arm's length (55 cm, 21 inches), 1px is about
0.20 mm. On a 300 dots-per-inch (dpi) printer, that may be rounded up
to 3 dots (0.25 mm); on a 600 dpi printer, it can be rounded to 5
dots.
The two images below illustrate the effect of viewing distance on the
size of a pixel and the effect of a device's resolution. In the first
image, a reading distance of 71 cm (28 inch) results in a px of
0.26 mm, while a reading distance of 3.5 m (12 feet) requires a px of
1.3 mm.
Showing that pixels must become larger if the viewing distance
increases
In the second image, an area of 1px by 1px is covered by a single dot
in a low-resolution device (a computer screen), while the same area is
covered by 16 dots in a higher resolution device (such as a 400 dpi
laser printer).
Showing that more device pixels (dots) are needed to cover a 1px by
1px area on a high-resolution device than on a low-res one
Child elements do not inherit the relative values specified for their
parent; they (generally) inherit the computed values.
Example(s):
In the following rules, the computed 'text-indent' value of "h1"
elements will be 36px, not 45px, if "h1" is a child of the "body"
element.
body {
font-size: 12px;
text-indent: 3em; /* i.e., 36px */
}
h1 { font-size: 15px }
Absolute length units are only useful when the physical properties of
the output medium are known. The absolute units are:
* in: inches -- 1 inch is equal to 2.54 centimeters.
* cm: centimeters
* mm: millimeters
* pt: points -- the points used by CSS 2.1 are equal to 1/72th of an
inch.
* pc: picas -- 1 pica is equal to 12 points.
Example(s):
h1 { margin: 0.5in } /* inches */
h2 { line-height: 3cm } /* centimeters */
h3 { word-spacing: 4mm } /* millimeters */
h4 { font-size: 12pt } /* points */
h4 { font-size: 1pc } /* picas */
In cases where the computed length cannot be supported, user agents
must approximate it in the actual value.
4.3.3 Percentages
The format of a percentage value (denoted by <percentage> in this
specification) is a <number> immediately followed by '%'.
Percentage values are always relative to another value, for example a
length. Each property that allows percentages also defines the value
to which the percentage refers. The value may be that of another
property for the same element, a property for an ancestor element, or
a value of the formatting context (e.g., the width of a containing
block). When a percentage value is set for a property of the root
element and the percentage is defined as referring to the inherited
value of some property, the resultant value is the percentage times
the initial value of that property.
Example(s):
Since child elements (generally) inherit the computed values of their
parent, in the following example, the children of the P element will
inherit a value of 12px for 'line-height', not the percentage value
(120%):
p { font-size: 10px }
p { line-height: 120% } /* 120% of 'font-size' */
4.3.4 URL + URN = URI
URLs (Uniform Resource Locators, see [RFC1738] and [RFC1808]) provide
the address of a resource on the Web. Another way of identifying
resources is called URN (Uniform Resource Name). Together they are
called URIs (Uniform Resource Identifiers, see [URI]). This
specification uses the term URI.
URI values in this specification are denoted by <uri>. The functional
notation used to designate URIs in property values is "url()", as in:
Example(s):
body { background: url("http://www.example.com/pinkish.png") }
The format of a URI value is 'url(' followed by optional whitespace
followed by an optional single quote (') or double quote (") character
followed by the URI itself, followed by an optional single quote (')
or double quote (") character followed by optional whitespace followed
by ')'. The two quote characters must be the same.
Example(s):
An example without quotes:
li { list-style: url(http://www.example.com/redball.png) disc }
Parentheses, commas, whitespace characters, single quotes (') and
double quotes (") appearing in a URI must be escaped with a backslash:
'\(', '\)', '\,'.
Depending on the type of URI, it might also be possible to write the
above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as
described in [URI].
In order to create modular style sheets that are not dependent on the
absolute location of a resource, authors may use relative URIs.
Relative URIs (as defined in [RFC1808]) are resolved to full URIs
using a base URI. RFC 1808, section 3, defines the normative algorithm
for this process. For CSS style sheets, the base URI is that of the
style sheet, not that of the source document.
Example(s):
For example, suppose the following rule:
body { background: url("yellow") }
is located in a style sheet designated by the URI:
http://www.example.org/style/basic.css
The background of the source document's BODY will be tiled with
whatever image is described by the resource designated by the URI
http://www.example.org/style/yellow
User agents may vary in how they handle URIs that designate
unavailable or inapplicable resources.
4.3.5 Counters
Counters are denoted by identifiers (see the 'counter-increment' and
'counter-reset' properties). To refer to the value of a counter, the
notation 'counter(<identifier>)' or 'counter(<identifier>,
<list-style-type>)' is used. The default style is 'decimal'.
To refer to a sequence of nested counters of the same name, the
notation is 'counters(<identifier>, <string>)' or
'counters(<identifier>, <string>, <list-style-type>)'. See "Nested
counters and scope" in the chapter on generated content.
In CSS2, the values of counters can only be referred to from the
'content' property. Note that 'none' is a possible <list-style-type>:
'counter(x, none)' yields an empty string.
Example(s):
Here is a style sheet that numbers paragraphs (P) for each chapter
(H1). The paragraphs are numbered with roman numerals, followed by a
period and a space:
P {counter-increment: par-num}
H1 {counter-reset: par-num}
P:before {content: counter(par-num, upper-roman) ". "}
Counters that are not in the scope of any 'counter-reset', are assumed
to have been reset to 0 by a 'counter-reset' on the root element.
4.3.6 Colors
A <color> is either a keyword or a numerical RGB specification.
The list of keyword color names is: aqua, black, blue, fuchsia, gray,
green, lime, maroon, navy, olive, orange, purple, red, silver, teal,
white, and yellow. These 17 colors have the following values:
maroon #800000 red #ff0000 orange #ffA500 yellow #ffff00 olive #808000
purple #800080 fuchsia #ff00ff white #ffffff lime #00ff00 green
#008000
navy #000080 blue #0000ff aqua #00ffff teal #008080
black #000000 silver #c0c0c0 gray #808080
In addition to these color keywords, users may specify keywords that
correspond to the colors used by certain objects in the user's
environment. Please consult the section on system colors for more
information.
Example(s):
body {color: black; background: white }
h1 { color: maroon }
h2 { color: olive }
The RGB color model is used in numerical color specifications. These
examples all specify the same color:
Example(s):
em { color: #f00 } /* #rgb */
em { color: #ff0000 } /* #rrggbb */
em { color: rgb(255,0,0) }
em { color: rgb(100%, 0%, 0%) }
The format of an RGB value in hexadecimal notation is a '#'
immediately followed by either three or six hexadecimal characters.
The three-digit RGB notation (#rgb) is converted into six-digit form
(#rrggbb) by replicating digits, not by adding zeros. For example,
#fb0 expands to #ffbb00. This ensures that white (#ffffff) can be
specified with the short notation (#fff) and removes any dependencies
on the color depth of the display.
The format of an RGB value in the functional notation is 'rgb('
followed by a comma-separated list of three numerical values (either
three integer values or three percentage values) followed by ')'. The
integer value 255 corresponds to 100%, and to F or FF in the
hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF.
Whitespace characters are allowed around the numerical values.
All RGB colors are specified in the sRGB color space (see [SRGB]).
User agents may vary in the fidelity with which they represent these
colors, but using sRGB provides an unambiguous and objectively
measurable definition of what the color should be, which can be
related to international standards (see [COLORIMETRY]).
Conforming user agents may limit their color-displaying efforts to
performing a gamma-correction on them. sRGB specifies a display gamma
of 2.2 under specified viewing conditions. User agents should adjust
the colors given in CSS such that, in combination with an output
device's "natural" display gamma, an effective display gamma of 2.2 is
produced. See the section on gamma correction for further details.
Note that only colors specified in CSS are affected; e.g., images are
expected to carry their own color information.
Values outside the device gamut should be clipped: the red, green, and
blue values must be changed to fall within the range supported by the
device. For a typical CRT monitor, whose device gamut is the same as
sRGB, the four rules below are equivalent:
Example(s):
em { color: rgb(255,0,0) } /* integer range 0 - 255 */
em { color: rgb(300,0,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(255,-10,0) } /* clipped to rgb(255,0,0) */
em { color: rgb(110%, 0%, 0%) } /* clipped to rgb(100%,0%,0%) */
Other devices, such as printers, have different gamuts than sRGB; some
colors outside the 0..255 sRGB range will be representable (inside the
device gamut), while other colors inside the 0..255 sRGB range will be
outside the device gamut and will thus be clipped.
4.3.7 Strings
Strings can either be written with double quotes or with single
quotes. Double quotes cannot occur inside double quotes, unless
escaped (as '\"' or as '\22'). Analogously for single quotes ("\'" or
"\27").
Example(s):
"this is a 'string'"
"this is a \"string\""
'this is a "string"'
'this is a \'string\''
A string cannot directly contain a newline. To include a newline in a
string, use the escape "\A" (hexadecimal A is the line feed character
in Unicode, but represents the generic notion of "newline" in CSS).
See the 'content' property for an example.
It is possible to break strings over several lines, for esthetic or
other reasons, but in such a case the newline itself has to be escaped
with a backslash (\). For instance, the following two selectors are
exactly the same:
Example(s):
a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}
[INS: :INS]
4.3.8 Unsupported Values
If a UA does not support a particular value, it should ignore that
value when parsing style sheets, as if that value was an illegal
value. For example:
Example(s):
h3 {
display: inline;
display: run-in;
}
A UA that supports the 'run-in' value for the 'display' property will
accept the first display declaration and then "write over" that value
with the second display declaration. A UA that does not support the
'run-in' value will process the first display declaration and ignore
the second display declaration.
4.4 CSS document representation
A CSS style sheet is a sequence of characters from the Universal
Character Set (see [ISO10646]). For transmission and storage, these
characters must be encoded by a character encoding that supports the
set of characters available in US-ASCII (e.g., ISO 8859-x, SHIFT JIS,
etc.). For a good introduction to character sets and character
encodings, please consult the HTML 4.0 specification ([HTML40],
chapter 5), See also the XML 1.0 specification ([XML10], sections 2.2
and 4.3.3, and Appendix F.
When a style sheet is embedded in another document, such as in the
STYLE element or "style" attribute of HTML, the style sheet shares the
character encoding of the whole document.
When a style sheet resides in a separate file, user agents must
observe the following priorities when determining a document's
character encoding (from highest priority to lowest):
1. An HTTP "charset" parameter in a "Content-Type" field.
2. The @charset at-rule.
3. Mechanisms of the language of the referencing document (e.g., in
HTML, the "charset" attribute of the LINK element).
At most one @charset rule may appear in an external style sheet -- it
must not appear in an embedded style sheet -- and it must appear at
the very start of the document, not preceded by any characters. After
"@charset", authors specify the name of a character encoding. The name
must be a charset name as described in the IANA registry (See [IANA].
Also, see [CHARSETS] for a complete list of charsets). For example:
Example(s):
@charset "ISO-8859-1";
This specification does not mandate which character encodings a user
agent must support.
Note that reliance on the @charset construct theoretically poses a
problem since there is no a priori information on how it is encoded.
In practice, however, the encodings in wide use on the Internet are
either based on ASCII, UTF-16, UCS-4, or (rarely) on EBCDIC. This
means that in general, the initial byte values of a document enable a
user agent to detect the encoding family reliably, which provides
enough information to decode the @charset rule, which in turn
determines the exact character encoding.
4.4.1 Referring to characters not represented in a character encoding
A style sheet may have to refer to characters that cannot be
represented in the current character encoding. These characters must
be written as escaped references to ISO 10646 characters. These
escapes serve the same purpose as numeric character references in HTML
or XML documents (see [HTML40], chapters 5 and 25).
The character escape mechanism should be used when only a few
characters must be represented this way. If most of a document
requires escaping, authors should encode it with a more appropriate
encoding (e.g., if the document contains a lot of Greek characters,
authors might use "ISO-8859-7" or "UTF-8").
Intermediate processors using a different character encoding may
translate these escaped sequences into byte sequences of that
encoding. Intermediate processors must not, on the other hand, alter
escape sequences that cancel the special meaning of an ASCII
character.
Conforming user agents must correctly map to Unicode all characters in
any character encodings that they recognize (or they must behave as if
they did).
For example, a document transmitted as ISO-8859-1 (Latin-1) cannot
contain Greek letters directly: "kouro*s" (Greek: "kouros") has to be
written as "\3BA\3BF\3C5\3C1\3BF\3C2".
Note. In HTML 4.0, numeric character references are interpreted in
"style" attribute values but not in the content of the STYLE element.
Because of this asymmetry, we recommend that authors use the CSS
character escape mechanism rather than numeric character references
for both the "style" attribute and the STYLE element. For example, we
recommend:
<SPAN style="font-family: L\FC beck">...</SPAN>
rather than:
<SPAN style="font-family: Lübeck">...</SPAN>
_________________________________________________________________
_________________________________________________________________
5 Selectors
Contents
* 5.1 Pattern matching
* 5.2 Selector syntax
+ 5.2.1 Grouping
* 5.3 Universal selector
* 5.4 Type selectors
* 5.5 Descendant selectors
* 5.6 Child selectors
* 5.7 Adjacent sibling selectors
* 5.8 Attribute selectors
+ 5.8.1 Matching attributes and attribute values
+ 5.8.2 Default attribute values in DTDs
+ 5.8.3 Class selectors
* 5.9 ID selectors
* 5.10 Pseudo-elements and pseudo-classes
* 5.11 Pseudo-classes
+ 5.11.1 :first-child pseudo-class
+ 5.11.2 The link pseudo-classes: :link and :visited
+ 5.11.3 The dynamic pseudo-classes: :hover, :active, and
:focus
+ 5.11.4 The language pseudo-class: :lang
* 5.12 Pseudo-elements
+ 5.12.1 The :first-line pseudo-element
+ 5.12.2 The :first-letter pseudo-element
+ 5.12.3 The :before and :after pseudo-elements
5.1 Pattern matching
In CSS, pattern matching rules determine which style rules apply to
elements in the document tree. These patterns, called selectors, may
range from simple element names to rich contextual patterns. If all
conditions in the pattern are true for a certain element, the selector
matches the element.
The case-sensitivity of document language element names in selectors
depends on the document language. For example, in HTML, element names
are case-insensitive, but in XML they are case-sensitive.
The following table summarizes CSS 2.1 selector syntax:
Pattern Meaning Described in section
* Matches any element. Universal selector
E Matches any E element (i.e., an element of type E). Type selectors
E F Matches any F element that is a descendant of an E element.
Descendant selectors
E > F Matches any F element that is a child of an element E. Child
selectors
E:first-child Matches element E when E is the first child of its
parent. The :first-child pseudo-class
E:link
E:visited Matches element E if E is the source anchor of a hyperlink
of which the target is not yet visited (:link) or already visited
(:visited). The link pseudo-classes
E:active
E:hover
E:focus Matches E during certain user actions. The dynamic
pseudo-classes
E:lang(c) Matches element of type E if it is in (human) language c
(the document language specifies how language is determined). The
:lang() pseudo-class
E + F Matches any F element immediately preceded by an element E.
Adjacent selectors
E[foo] Matches any E element with the "foo" attribute set (whatever
the value). Attribute selectors
E[foo="warning"] Matches any E element whose "foo" attribute value is
exactly equal to "warning". Attribute selectors
E[foo~="warning"] Matches any E element whose "foo" attribute value is
a list of space-separated values, one of which is exactly equal to
"warning". Attribute selectors
E[lang|="en"] Matches any E element whose "lang" attribute has a
hyphen-separated list of values beginning (from the left) with "en".
Attribute selectors
DIV.warning Language specific. (In HTML, the same as
DIV[class~="warning"].) Class selectors
E#myid Matches any E element with ID equal to "myid". ID selectors
5.2 Selector syntax
A simple selector is either a type selector or universal selector
followed immediately by zero or more attribute selectors, ID
selectors, or pseudo-classes, in any order. The simple selector
matches if all of its components match.
Note: the terminology used here in CSS 2.1 is different from what is
used in CSS3. For example, a "simple selector" refers to a smaller
part of a selector in CSS3 than in CSS 2.1. See the CSS3 Selectors
module [CSS3SEL].
A selector is a chain of one or more simple selectors separated by
combinators. Combinators are: whitespace, ">", and "+". Whitespace may
appear between a combinator and the simple selectors around it.
The elements of the document tree that match a selector are called
subjects of the selector. A selector consisting of a single simple
selector matches any element satisfying its requirements. Prepending a
simple selector and combinator to a chain imposes additional matching
constraints, so the subjects of a selector are always a subset of the
elements matching the last simple selector.
One pseudo-element may be appended to the last simple selector in a
chain, in which case the style information applies to a subpart of
each subject.
5.2.1 Grouping
When several selectors share the same declarations, they may be
grouped into a comma-separated list.
Example(s):
In this example, we condense three rules with identical declarations
into one. Thus,
h1 { font-family: sans-serif }
h2 { font-family: sans-serif }
h3 { font-family: sans-serif }
is equivalent to:
h1, h2, h3 { font-family: sans-serif }
CSS offers other "shorthand" mechanisms as well, including multiple
declarations and shorthand properties.
5.3 Universal selector
The universal selector, written "*", matches the name of any element
type. It matches any single element in the document tree.
If the universal selector is not the only component of a simple
selector, the "*" may be omitted. For example:
* *[lang=fr] and [lang=fr] are equivalent.
* *.warning and .warning are equivalent.
* *#myid and #myid are equivalent.
5.4 Type selectors
A type selector matches the name of a document language element type.
A type selector matches every instance of the element type in the
document tree.
Example(s):
The following rule matches all H1 elements in the document tree:
h1 { font-family: sans-serif }
5.5 Descendant selectors
At times, authors may want selectors to match an element that is the
descendant of another element in the document tree (e.g., "Match those
EM elements that are contained by an H1 element"). Descendant
selectors express such a relationship in a pattern. A descendant
selector is made up of two or more selectors separated by whitespace.
A descendant selector of the form "A B" matches when an element B is
an arbitrary descendant of some ancestor element A.
Example(s):
For example, consider the following rules:
h1 { color: red }
em { color: red }
Although the intention of these rules is to add emphasis to text by
changing its color, the effect will be lost in a case such as:
<H1>This headline is <EM>very</EM> important</H1>
We address this case by supplementing the previous rules with a rule
that sets the text color to blue whenever an EM occurs anywhere within
an H1:
h1 { color: red }
em { color: red }
h1 em { color: blue }
The third rule will match the EM in the following fragment:
<H1>This <SPAN class="myclass">headline
is <EM>very</EM> important</SPAN></H1>
Example(s):
The following selector:
div * p
matches a P element that is a grandchild or later descendant of a DIV
element. Note the whitespace on either side of the "*" is not part of
the universal selector; the whitespace is the descendant selector
indicating that the DIV must be the ancestor of some element, and that
that element must be an ancestor of the P.
Example(s):
The selector in the following rule, which combines descendant and
attribute selectors, matches any element that (1) has the "href"
attribute set and (2) is inside a P that is itself inside a DIV:
div p *[href]
5.6 Child selectors
A child selector matches when an element is the child of some element.
A child selector is made up of two or more selectors separated by ">".
Example(s):
The following rule sets the style of all P elements that are children
of BODY:
body > P { line-height: 1.3 }
Example(s):
The following example combines descendant selectors and child
selectors:
div ol>li p
It matches a P element that is a descendant of an LI; the LI element
must be the child of an OL element; the OL element must be a
descendant of a DIV. Notice that the optional whitespace around the
">" combinator has been left out.
For information on selecting the first child of an element, please see
the section on the :first-child pseudo-class below.
5.7 Adjacent sibling selectors
Adjacent sibling selectors have the following syntax: E1 + E2, where
E2 is the subject of the selector. The selector matches if E1 and E2
share the same parent in the document tree and E1 immediately precedes
E2.
In some contexts, adjacent elements generate formatting objects whose
presentation is handled automatically (e.g., collapsing vertical
margins between adjacent boxes). The "+" selector allows authors to
specify additional style to adjacent elements.
Example(s):
Thus, the following rule states that when a P element immediately
follows a MATH element, it should not be indented:
math + p { text-indent: 0 }
The next example reduces the vertical space separating an H1 and an H2
that immediately follows it:
h1 + h2 { margin-top: -5mm }
Example(s):
The following rule is similar to the one in the previous example,
except that it adds an attribute selector. Thus, special formatting
only occurs when H1 has class="opener":
h1.opener + h2 { margin-top: -5mm }
5.8 Attribute selectors
CSS 2.1 allows authors to specify rules that match attributes defined
in the source document.
5.8.1 Matching attributes and attribute values
Attribute selectors may match in four ways:
[att]
Match when the element sets the "att" attribute, whatever the
value of the attribute.
[att=val]
Match when the element's "att" attribute value is exactly
"val".
[att~=val]
Match when the element's "att" attribute value is a
space-separated list of "words", one of which is exactly "val".
If this selector is used, the words in the value must not
contain spaces (since they are separated by spaces).
[att|=val]
Match when the element's "att" attribute value is a
hyphen-separated list of "words", beginning with "val". The
match always starts at the beginning of the attribute value.
This is primarily intended to allow language subcode matches
(e.g., the "lang" attribute in HTML) as described in RFC 1766
([RFC1766]).
Attribute values must be identifiers or strings. The case-sensitivity
of attribute names and values in selectors depends on the document
language.
Example(s):
For example, the following attribute selector matches all H1 elements
that specify the "title" attribute, whatever its value:
h1[title] { color: blue; }
Example(s):
In the following example, the selector matches all SPAN elements whose
"class" attribute has exactly the value "example":
span[class=example] { color: blue; }
Multiple attribute selectors can be used to refer to several
attributes of an element, or even several times to the same attribute.
Example(s):
Here, the selector matches all SPAN elements whose "hello" attribute
has exactly the value "Cleveland" and whose "goodbye" attribute has
exactly the value "Columbus":
span[hello="Cleveland"][goodbye="Columbus"] { color: blue; }
Example(s):
The following selectors illustrate the differences between "=" and
"~=". The first selector will match, for example, the value "copyright
copyleft copyeditor" for the "rel" attribute. The second selector will
only match when the "href" attribute has the value
"http://www.w3.org/".
a[rel~="copyright"]
a[href="http://www.w3.org/"]
Example(s):
The following rule hides all elements for which the value of the
"lang" attribute is "fr" (i.e., the language is French).
*[lang=fr] { display : none }
Example(s):
The following rule will match for values of the "lang" attribute that
begin with "en", including "en", "en-US", and "en-cockney":
*[lang|="en"] { color : red }
Example(s):
Similarly, the following aural style sheet rules allow a script to be
read aloud in different voices for each role:
DIALOGUE[character=romeo]
{ voice-family: "Lawrence Olivier", charles, male }
DIALOGUE[character=juliet]
{ voice-family: "Vivien Leigh", victoria, female }
5.8.2 Default attribute values in DTDs
Matching takes place on attribute values in the document tree. For
document languages other than HTML, default attribute values may be
defined in a DTD or elsewhere. Style sheets should be designed so that
they work even if the default values are not included in the document
tree.
Example(s):
For example, consider an element EXAMPLE with an attribute "notation"
that has a default value of "decimal". The DTD fragment might be
<!ATTLIST EXAMPLE notation (decimal,octal) "decimal">
If the style sheet contains the rules
EXAMPLE[notation=decimal] { /*... default property settings ...*/ }
EXAMPLE[notation=octal] { /*... other settings...*/ }
then to catch the cases where this attribute is set by default, and
not explicitly, the following rule might be added:
EXAMPLE { /*... default property settings ...*/ }
Because this selector is less specific than an attribute selector, it
will only be used for the default case. Care has to be taken that all
other attribute values that don't get the same style as the default
are explicitly covered.
5.8.3 Class selectors
Working with HTML, authors may use the period (.) notation as an
alternative to the ~= notation when representing the class attribute.
Thus, for HTML, div.value and div[class~=value] have the same meaning.
The attribute value must immediately follow the "period" (.). UAs may
apply selectors using the period (.) notation in XML documents if the
UA has namespace specific knowledge that allows it to determine which
attribute is the "class" attribute for the respective namespace. One
such example of namespace specific knowledge is the prose in the
specification for a particular namespace (e.g. SVG 1.0 [SVG10]
describes the SVG "class" attribute and how a UA should interpret it,
and similarly MathML 2.0 [MATH20] describes the MathML "class"
attribute.)
Example(s):
For example, we can assign style information to all elements with
class~="pastoral" as follows:
*.pastoral { color: green } /* all elements with class~=pastoral */
or just
.pastoral { color: green } /* all elements with class~=pastoral */
The following assigns style only to H1 elements with
class~="pastoral":
H1.pastoral { color: green } /* H1 elements with class~=pastoral */
Given these rules, the first H1 instance below would not have green
text, while the second would:
<H1>Not green</H1>
<H1 class="pastoral">Very green</H1>
To match a subset of "class" values, each value must be preceded by a
".", in any order.
Example(s):
For example, the following rule matches any P element whose "class"
attribute has been assigned a list of space-separated values that
includes "pastoral" and "marine":
p.pastoral.marine { color: green }
This rule matches when class="pastoral blue aqua marine" but does not
match for class="pastoral blue".
Note. CSS gives so much power to the "class" attribute, that authors
could conceivably design their own "document language" based on
elements with almost no associated presentation (such as DIV and SPAN
in HTML) and assigning style information through the "class"
attribute. Authors should avoid this practice since the structural
elements of a document language often have recognized and accepted
meanings and author-defined classes may not.
5.9 ID selectors
Document languages may contain attributes that are declared to be of
type ID. What makes attributes of type ID special is that no two such
attributes can have the same value; whatever the document language, an
ID attribute can be used to uniquely identify its element. In HTML all
ID attributes are named "id"; XML applications may name ID attributes
differently, but the same restriction applies.
The ID attribute of a document language allows authors to assign an
identifier to one element instance in the document tree. CSS ID
selectors match an element instance based on its identifier. A CSS ID
selector contains a "#" immediately followed by the ID value.
Example(s):
The following ID selector matches the H1 element whose ID attribute
has the value "chapter1":
h1#chapter1 { text-align: center }
In the following example, the style rule matches the element that has
the ID value "z98y". The rule will thus match for the P element:
<HEAD>
<TITLE>Match P</TITLE>
<STYLE type="text/css">
*#z98y { letter-spacing: 0.3em }
</STYLE>
</HEAD>
<BODY>
<P id=z98y>Wide text</P>
</BODY>
In the next example, however, the style rule will only match an H1
element that has an ID value of "z98y". The rule will not match the P
element in this example:
<HEAD>
<TITLE>Match H1 only</TITLE>
<STYLE type="text/css">
H1#z98y { letter-spacing: 0.5em }
</STYLE>
</HEAD>
<BODY>
<P id=z98y>Wide text</P>
</BODY>
ID selectors have a higher specificity than attribute selectors. For
example, in HTML, the selector #p123 is more specific than [id=p123]
in terms of the cascade.
Note. In XML 1.0 [XML10], the information about which attribute
contains an element's IDs is contained in a DTD. When parsing XML, UAs
do not always read the DTD, and thus may not know what the ID of an
element is. If a style sheet designer knows or suspects that this will
be the case, he should use normal attribute selectors instead:
[name=p371] instead of #p371. However, the cascading order of normal
attribute selectors is different from ID selectors. It may be
necessary to add an "!important" priority to the declarations:
[name=p371] {color: red ! important}. Of course, elements in XML 1.0
documents without a DTD do not have IDs at all.
5.10 Pseudo-elements and pseudo-classes
In CSS 2.1, style is normally attached to an element based on its
position in the document tree. This simple model is sufficient for
many cases, but some common publishing scenarios may not be possible
due to the structure of the document tree. For instance, in HTML 4.0
(see [HTML40]), no element refers to the first line of a paragraph,
and therefore no simple CSS selector may refer to it.
CSS introduces the concepts of pseudo-elements and pseudo-classes to
permit formatting based on information that lies outside the document
tree.
* Pseudo-elements create abstractions about the document tree beyond
those specified by the document language. For instance, document
languages do not offer mechanisms to access the first letter or
first line of an element's content. CSS pseudo-elements allow
style sheet designers to refer to this otherwise inaccessible
information. Pseudo-elements may also provide style sheet
designers a way to assign style to content that does not exist in
the source document (e.g., the :before and :after pseudo-elements
give access to generated content).
* Pseudo-classes classify elements on characteristics other than
their name, attributes or content; in principle characteristics
that cannot be deduced from the document tree. Pseudo-classes may
be dynamic, in the sense that an element may acquire or lose a
pseudo-class while a user interacts with the document. The
exceptions are ':first-child', which can be deduced from the
document tree, and ':lang()', which can be deduced from the
document tree in some cases.
Neither pseudo-elements nor pseudo-classes appear in the document
source or document tree.
Pseudo-classes are allowed anywhere in selectors while pseudo-elements
may only appear after the subject of the selector.
Pseudo-element and pseudo-class names are case-insensitive.
Some pseudo-classes are mutually exclusive, while others can be
applied simultaneously to the same element. In case of conflicting
rules, the normal cascading order determines the outcome.
Conforming HTML user agents may ignore all rules with :first-line or
:first-letter in the selector, or, alternatively, may only support a
subset of the properties on these pseudo-elements.
5.11 Pseudo-classes
5.11.1 :first-child pseudo-class
The :first-child pseudo-class matches an element that is the first
child of some other element.
Example(s):
In the following example, the selector matches any P element that is
the first child of a DIV element. The rule suppresses indentation for
the first paragraph of a DIV:
div > p:first-child { text-indent: 0 }
This selector would match the P inside the DIV of the following
fragment:
<P> The last P before the note.
<DIV class="note">
<P> The first P inside the note.
</DIV>
but would not match the second P in the following fragment:
<P> The last P before the note.
<DIV class="note">
<H2>Note</H2>
<P> The first P inside the note.
</DIV>
Example(s):
The following rule sets the font weight to 'bold' for any EM element
that is some descendant of a P element that is a first child:
p:first-child em { font-weight : bold }
Note that since anonymous boxes are not part of the document tree,
they are not counted when calculating the first child.
For example, the EM in:
<P>abc <EM>default</EM>
is the first child of the P.
The following two selectors are equivalent:
* > a:first-child /* A is first child of any element */
a:first-child /* Same */
5.11.2 The link pseudo-classes: :link and :visited
User agents commonly display unvisited links differently from
previously visited ones. CSS provides the pseudo-classes ':link' and
':visited' to distinguish them:
* The :link pseudo-class applies for links that have not yet been
visited.
* The :visited pseudo-class applies once the link has been visited
by the user.
Note. After a certain amount of time, user agents may choose to return
a visited link to the (unvisited) ':link' state.
The two states are mutually exclusive.
The document language determines which elements are hyperlink source
anchors. For example, in HTML 4.0, the link pseudo-classes apply to A
elements with an "href" attribute. Thus, the following two CSS 2.1
declarations have similar effect:
a:link { color: red }
:link { color: red }
Example(s):
If the following link:
<A class="external" href="http://out.side/">external link</A>
has been visited, this rule:
a.external:visited { color: blue }
will cause it to be blue.
Note. It is possible for stylesheet authors to abuse the :link and
:visited pseudo-classes to determine which sites a user has visited
without the user's consent. UAs may therefore treat all links as
unvisited links, or implement other measures to preserve the user's
privacy while rendering visited and unvisited links differently. See
[P3P] for more information about handling privacy.
5.11.3 The dynamic pseudo-classes: :hover, :active, and :focus
Interactive user agents sometimes change the rendering in response to
user actions. CSS provides three pseudo-classes for common cases:
* The :hover pseudo-class applies while the user designates an
element (with some pointing device), but does not activate it. For
example, a visual user agent could apply this pseudo-class when
the cursor (mouse pointer) hovers over a box generated by the
element. User agents not supporting interactive media do not have
to support this pseudo-class. Some conforming user agents
supporting interactive media may not be able to support this
pseudo-class (e.g., a pen device).
* The :active pseudo-class applies while an element is being
activated by the user. For example, between the times the user
presses the mouse button and releases it.
* The :focus pseudo-class applies while an element has the focus
(accepts keyboard events or other forms of text input).
An element may match several pseudo-classes at the same time.
CSS doesn't define which elements may be in the above states, or how
the states are entered and left. Scripting may change whether elements
react to user events or not, and different devices and UAs may have
different ways of pointing to, or activating elements.
User agents are not required to reflow a currently displayed document
due to pseudo-class transitions. For instance, a style sheet may
specify that the 'font-size' of an :active link should be larger than
that of an inactive link, but since this may cause letters to change
position when the reader selects the link, a UA may ignore the
corresponding style rule.
Example(s):
a:link { color: red } /* unvisited links */
a:visited { color: blue } /* visited links */
a:hover { color: yellow } /* user hovers */
a:active { color: lime } /* active links */
Note that the A:hover must be placed after the A:link and A:visited
rules, since otherwise the cascading rules will hide the 'color'
property of the A:hover rule. Similarly, because A:active is placed
after A:hover, the active color (lime) will apply when the user both
activates and hovers over the A element.
Example(s):
An example of combining dynamic pseudo-classes:
a:focus { background: yellow }
a:focus:hover { background: white }
The last selector matches A elements that are in pseudo-class :focus
and in pseudo-class :hover.
For information about the presentation of focus outlines, please
consult the section on dynamic focus outlines.
Note. In CSS1, the ':active' pseudo-class was mutually exclusive with
':link' and ':visited'. That is no longer the case. An element can be
both ':visited' and ':active' (or ':link' and ':active') and the
normal cascading rules determine which properties apply.
5.11.4 The language pseudo-class: :lang
If the document language specifies how the human language of an
element is determined, it is possible to write selectors in CSS that
match an element based on its language. For example, in HTML [HTML40],
the language is determined by a combination of the "lang" attribute,
the META element, and possibly by information from the protocol (such
as HTTP headers). XML uses an attribute called xml:lang, and there may
be other document language-specific methods for determining the
language.
The pseudo-class ':lang(C)' matches if the element is in language C.
Here C is a language code as specified in HTML 4.0 [HTML40] and
RFC 1766 [RFC1766]. It is matched the same way as for the '|='
operator.
Example(s):
The following rules set the quotation marks for an HTML document that
is either in French or German:
html:lang(fr) { quotes: '« ' ' »' }
html:lang(de) { quotes: '»' '«' '\2039' '\203A' }
:lang(fr) > Q { quotes: '« ' ' »' }
:lang(de) > Q { quotes: '»' '«' '\2039' '\203A' }
The second pair of rules actually set the 'quotes' property on Q
elements according to the language of its parent. This is done because
the choice of quote marks is typically based on the language of the
element around the quote, not the quote itself: like this piece of
French "à l'improviste" in the middle of an English text uses the
English quotation marks.
5.12 Pseudo-elements
5.12.1 The :first-line pseudo-element
The :first-line pseudo-element applies special styles to the first
formatted line of a paragraph. For instance:
p:first-line { text-transform: uppercase }
The above rule means "change the letters of the first line of every
paragraph to uppercase". However, the selector "P:first-line" does not
match any real HTML element. It does match a pseudo-element that
conforming user agents will insert at the beginning of every
paragraph.
Note that the length of the first line depends on a number of factors,
including the width of the page, the font size, etc. Thus, an ordinary
HTML paragraph such as:
<P>This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>
the lines of which happen to be broken as follows:
THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be identified by a fictional tag
sequence. The other lines will be treated as
ordinary lines in the paragraph.
might be "rewritten" by user agents to include the fictional tag
sequence for :first-line. This fictional tag sequence helps to show
how properties are inherited.
<P><P:first-line> This is a somewhat long HTML
paragraph that </P:first-line> will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>
If a pseudo-element breaks up a real element, the desired effect can
often be described by a fictional tag sequence that closes and then
re-opens the element. Thus, if we mark up the previous paragraph with
a SPAN element:
<P><SPAN class="test"> This is a somewhat long HTML
paragraph that will be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>
the user agent could generate the appropriate start and end tags for
SPAN when inserting the fictional tag sequence for :first-line.
<P><P:first-line><SPAN class="test"> This is a
somewhat long HTML
paragraph that will </SPAN></P:first-line><SPAN class="test"> be
broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>
The :first-line pseudo-element can only be attached to a block-level
element.
The :first-line pseudo-element is similar to an inline-level element,
but with certain restrictions. Only the following properties apply to
a :first-line pseudo-element: font properties, color properties,
background properties, 'word-spacing', 'letter-spacing',
'text-decoration', 'vertical-align', 'text-transform', 'line-height',
and 'clear'.
5.12.2 The :first-letter pseudo-element
The :first-letter pseudo-element may be used for "initial caps" and
"drop caps", which are common typographical effects. This type of
initial letter is similar to an inline-level element if its 'float'
property is 'none', otherwise it is similar to a floated element.
These are the properties that apply to :first-letter pseudo-elements:
font properties, color properties, background properties,
'text-decoration', 'vertical-align' (only if 'float' is 'none'),
'text-transform', 'line-height', margin properties, padding
properties, border properties, 'float', and 'clear'.
The following CSS 2.1 will make a drop cap initial letter span about
two lines:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Drop cap initial letter</TITLE>
<STYLE type="text/css">
P { font-size: 12pt; line-height: 1.2 }
P:first-letter { font-size: 200%; font-style: italic;
font-weight: bold; float: left }
SPAN { text-transform: uppercase }
</STYLE>
</HEAD>
<BODY>
<P><SPAN>The first</SPAN> few words of an article
in The Economist.</P>
</BODY>
</HTML>
This example might be formatted as follows:
Image illustrating the combined effect of the :first-letter and
:first-line pseudo-elements
The fictional tag sequence is:
<P>
<SPAN>
<P:first-letter>
T
</P:first-letter>he first
</SPAN>
few words of an article in the Economist.
</P>
Note that the :first-letter pseudo-element tags abut the content
(i.e., the initial character), while the :first-line pseudo-element
start tag is inserted right after the start tag of the element to
which it is attached.
In order to achieve traditional drop caps formatting, user agents may
approximate font sizes, for example to align baselines. Also, the
glyph outline may be taken into account when formatting.
Punctuation (i.e, characters defined in Unicode [UNICODE] in the
"open" (Ps), "close" (Pe), and "other" (Po) punctuation classes), that
precedes the first letter should be included, as in:
Quotes that precede the first letter should be included.
The :first-letter pseudo-element matches parts of block-level elements
only.
Some languages may have specific rules about how to treat certain
letter combinations. In Dutch, for example, if the letter combination
"ij" appears at the beginning of a word, both letters should be
considered within the :first-letter pseudo-element.
Example(s):
The following example illustrates how overlapping pseudo-elements may
interact. The first letter of each P element will be green with a font
size of '24pt'. The rest of the first formatted line will be 'blue'
while the rest of the paragraph will be 'red'.
p { color: red; font-size: 12pt }
p:first-letter { color: green; font-size: 200% }
p:first-line { color: blue }
<P>Some text that ends up on two lines</P>
Assuming that a line break will occur before the word "ends", the
fictional tag sequence for this fragment might be:
<P>
<P:first-line>
<P:first-letter>
S
</P:first-letter>ome text that
</P:first-line>
ends up on two lines
</P>
Note that the :first-letter element is inside the :first-line element.
Properties set on :first-line are inherited by :first-letter, but are
overridden if the same property is set on :first-letter.
5.12.3 The :before and :after pseudo-elements
The ':before' and ':after' pseudo-elements can be used to insert
generated content before or after an element's content. They are
explained in the section on generated text.
Example(s):
h1:before {content: counter(chapno, upper-roman) ". "}
When the :first-letter and :first-line pseudo-elements are combined
with :before and :after, they apply to the first letter or line of the
element including the inserted text.
Example(s):
p.special:before {content: "Special! "}
p.special:first-letter {color: #ffd800}
This will render the "S" of "Special!" in gold.
_________________________________________________________________
_________________________________________________________________
6 Assigning property values, Cascading, and Inheritance
Contents
* 6.1 Specified, computed, and actual values
+ 6.1.1 Specified values
+ 6.1.2 Computed values
+ 6.1.3 Actual values
* 6.2 Inheritance
+ 6.2.1 The 'inherit' value
* 6.3 The @import rule
* 6.4 The cascade
+ 6.4.1 Cascading order
+ 6.4.2 !important rules
+ 6.4.3 Calculating a selector's specificity
+ 6.4.4 Precedence of non-CSS presentational hints
6.1 Specified, computed, and actual values
Once a user agent has parsed a document and constructed a document
tree, it must assign, for every element in the tree, a value to every
property that applies to the target media type.
The final value of a property is the result of a three-step
calculation: the value is determined through specification (the
"specified value"), then resolved into an absolute value if necessary
(the "computed value"), and finally transformed according to the
limitations of the local environment (the "actual value").
6.1.1 Specified values
User agents must first assign a specified value to a property based on
the following mechanisms (in order of precedence):
1. If the cascade results in a value, use it.
2. Otherwise, if the property is inherited and the element is not the
root of the document tree, use the computed value of the parent
element.
3. Otherwise use the property's initial value. The initial value of
each property is indicated in the property's definition.
Since it has no parent, the root of the document tree cannot use
values from the parent element; in this case, the initial value is
used if necessary.
6.1.2 Computed values
Specified values may be absolute (i.e., they are not specified
relative to another value, as in 'red' or '2mm') or relative (i.e.,
they are specified relative to another value, as in 'auto', '2em', and
'12%'). For absolute values, no computation is needed to find the
computed value.
Relative values, on the other hand, must be transformed into computed
values: percentages must be multiplied by a reference value (each
property defines which value that is), values with relative units (em,
ex, px) must be made absolute by multiplying with the appropriate font
or pixel size, 'auto' values must be computed by the formulas given
with each property, certain keywords ('smaller', 'bolder', 'inherit')
must be replaced according to their definitions.
When the specified value is not 'inherit', the computed value of a
property is determined as specified by the Computed Value line in the
definition of the property. See the section on inheritance for the
definition of computed values when the specified value is 'inherit'.
The computed value exists even when the property doesn't apply, as
defined by the 'Applies To' [add reference] line. However, some
properties may define the computed value of a property for an element
to depend on whether the property applies to that element.
6.1.3 Actual values
A computed value is in principle ready to be used, but a user agent
may not be able to make use of the value in a given environment. For
example, a user agent may only be able to render borders with integer
pixel widths and may therefore have to approximate the computed width.
The actual value is the computed value after any approximations have
been applied.
6.2 Inheritance
Some values are inherited by the children of an element in the
document tree,as described above. Each property defines whether it is
inherited or not.
Suppose there is an H1 element with an emphasizing element (EM)
inside:
<H1>The headline <EM>is</EM> important!</H1>
If no color has been assigned to the EM element, the emphasized "is"
will inherit the color of the parent element, so if H1 has the color
blue, the EM element will likewise be in blue.
When inheritance occurs, elements inherit computed values. The
computed value from the parent element becomes both the specified
value and the computed value on the child.
Example(s):
For example, given the following style sheet:
body { font-size: 10pt }
h1 { font-size: 120% }
and this document fragment:
<BODY>
<H1>A <EM>large</EM> heading</H1>
</BODY>
the 'font-size' property for the H1 element will have the computed
value '12pt' (120% times 10pt, the parent's value). Since the computed
value of 'font-size' is inherited, the EM element will have the
computed value '12pt' as well. If the user agent does not have the
12pt font available, the actual value of 'font-size' for both H1 and
EM might be, for example, '11pt'.
6.2.1 The 'inherit' value
Each property may also have a specified value of 'inherit', which
means that, for a given element, the property takes the same computed
value as the property for the element's parent. The 'inherit' value
can be used to strengthen inherited values, and it can also be used on
properties that are not normally inherited.
Example(s):
In the example below, the 'color' and 'background' properties are set
on the BODY element. On all other elements, the 'color' value will be
inherited and the background will be transparent. If these rules are
part of the user's style sheet, black text on a white background will
be enforced throughout the document.
body {
color: black !important;
background: white !important;
}
* {
color: inherit !important;
background: transparent !important;
}
6.3 The @import rule
The '@import' rule allows users to import style rules from other style
sheets. Any @import rules must precede all rule sets in a style sheet.
The '@import' keyword must be followed by the URI of the style sheet
to include. A string is also allowed; it will be interpreted as if it
had url(...) around it.
Example(s):
The following lines are equivalent in meaning and illustrate both
'@import' syntaxes (one with "url()" and one with a bare string):
@import "mystyle.css";
@import url("mystyle.css");
So that user agents can avoid retrieving resources for unsupported
media types, authors may specify media-dependent @import rules. These
conditional imports specify comma-separated media types after the URI.
Example(s):
The following rules illustrate how @import rules can be made
media-dependent:
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
In the absence of any media types, the import is unconditional.
Specifying 'all' for the medium has the same effect.
6.4 The cascade
Style sheets may have three different origins: author, user, and user
agent.
* Author. The author specifies style sheets for a source document
according to the conventions of the document language. For
instance, in HTML, style sheets may be included in the document or
linked externally.
* User: The user may be able to specify style information for a
particular document. For example, the user may specify a file that
contains a style sheet or the user agent may provide an interface
that generates a user style sheet (or behaves as if it did).
* User agent: Conforming user agents must apply a default style
sheet (or behave as if they did) prior to all other style sheets
for a document. A user agent's default style sheet should present
the elements of the document language in ways that satisfy general
presentation expectations for the document language (e.g., for
visual browsers, the EM element in HTML is presented using an
italic font). See A sample style sheet for HTML for a recommended
default style sheet for HTML documents.
Note that the default style sheet may change if system settings
are modified by the user (e.g., system colors). However, due to
limitations in a user agent's internal implementation, it may be
impossible to change the values in the default style sheet.
Style sheets from these three origins will overlap in scope, and they
interact according to the cascade.
The CSS cascade assigns a weight to each style rule. When several
rules apply, the one with the greatest weight takes precedence.
By default, rules in author style sheets have more weight than rules
in user style sheets. Precedence is reversed, however, for
"!important" rules. All user and author rules have more weight than
rules in the UA's default style sheet.
Rules specified in a given style sheet override rules of the same
weight imported from other style sheets. Imported style sheets can
themselves import and override other style sheets, recursively, and
the same precedence rules apply.
6.4.1 Cascading order
To find the value for an element/property combination, user agents
must apply the following sorting order:
1. Find all declarations that apply to the element and property in
question, for the target media type. Declarations apply if the
associated selector matches the element in question.
2. Sort by weight (normal or important) and origin (author, user, or
user agent). In ascending order:
1. user agent style sheets
2. user normal style sheets
3. author normal style sheets
4. author important style sheets
5. user important style sheets
3. Sort by specificity of selector: more specific selectors will
override more general ones. Pseudo-elements and pseudo-classes are
counted as normal elements and classes, respectively.
4. Finally, sort by order specified: if two rules have the same
weight, origin and specificity, the latter specified wins. Rules
in imported style sheets are considered to be before any rules in
the style sheet itself.
Apart from the "!important" setting on individual declarations, this
strategy gives author's style sheets higher weight than those of the
reader. It is therefore important that the user agent give the user
the ability to turn off the influence of a certain style sheet, e.g.,
through a pull-down menu.
6.4.2 !important rules
CSS attempts to create a balance of power between author and user
style sheets. By default, rules in an author's style sheet override
those in a user's style sheet (see cascade rule 3).
However, for balance, an "!important" declaration (the keywords "!"
and "important" follow the declaration) takes precedence over a normal
declaration. Both author and user style sheets may contain
"!important" declarations, and user "!important" rules override author
"!important" rules. This CSS feature improves accessibility of
documents by giving users with special requirements (large fonts,
color combinations, etc.) control over presentation.
Declaring a shorthand property (e.g., 'background') to be "!important"
is equivalent to declaring all of its sub-properties to be
"!important".
Example(s):
The first rule in the user's style sheet in the following example
contains an "!important" declaration, which overrides the
corresponding declaration in the author's style sheet. The second
declaration will also win due to being marked "!important". However,
the third rule in the user's style sheet is not "!important" and will
therefore lose to the second rule in the author's style sheet (which
happens to set style on a shorthand property). Also, the third author
rule will lose to the second author rule since the second rule is
"!important". This shows that "!important" declarations have a
function also within author style sheets.
/* From the user's style sheet */
p { text-indent: 1em ! important }
p { font-style: italic ! important }
p { font-size: 18pt }
/* From the author's style sheet */
p { text-indent: 1.5em !important }
p { font: 12pt sans-serif !important }
p { font-size: 24pt }
6.4.3 Calculating a selector's specificity
A selector's specificity is calculated as follows:
* count 1 if the selector is a 'style' attribute rather than a
selector, 0 otherwise (= a) (In HTML, values of an element's
"style" attribute are style sheet rules. These rules have no
selectors, so a=1, b=0, c=0, and d=0.)
* count the number of ID attributes in the selector (= b)
* count the number of other attributes and pseudo-classes in the
selector (= c)
* count the number of element names and pseudo-elements in the
selector (= d)
Concatenating the four numbers a-b-c-d (in a number system with a
large base) gives the specificity.
Example(s):
Some examples:
* {} /* a=0 b=0 c=0 d=0 -> specificity = 0,0,0,0 */
li {} /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,1 */
li:first-line {} /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,2 */
ul li {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */
ul ol+li {} /* a=0 b=0 c=0 d=3 -> specificity = 0,0,0,3 */
h1 + *[rel=up]{} /* a=0 b=0 c=1 d=1 -> specificity = 0,0,1,1 */
ul ol li.red {} /* a=0 b=0 c=1 d=3 -> specificity = 0,0,1,3 */
li.red.level {} /* a=0 b=0 c=2 d=1 -> specificity = 0,0,2,1 */
#x34y {} /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */
style="" /* a=1 b=0 c=0 d=0 -> specificity = 1,0,0,0 */
<HEAD>
<STYLE type="text/css">
#x97z { color: red }
</STYLE>
</HEAD>
<BODY>
<P ID=x97z style="color: green">
</BODY>
In the above example, the color of the P element would be green. The
declaration in the "style" attribute will override the one in the
STYLE element because of cascading rule 3, since it has a higher
specificity.
Note: The specificity is based only on the form of the selector. In
particular, a selector of the form "[id=p33]" is counted as an
attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is
defined as an "ID" in the source document's DTD.
6.4.4 Precedence of non-CSS presentational hints
The UA may choose to honor presentational attributes in the source
document. If so, these attributes are translated to the corresponding
CSS rules with specificity equal to 0, and are treated as if they were
inserted at the start of the author style sheet. They may therefore be
overridden by subsequent style sheet rules. In a transition phase,
this policy will make it easier for stylistic attributes to coexist
with style sheets.
For HTML, any attribute that is not in the following list should be
considered presentational: abbr, accept-charset, accept, accesskey,
action, alt, archive, axis, charset, checked, cite, class, classid,
code, codebase, codetype, colspan, coords, data, datetime, declare,
defer, dir, disabled, enctype, for, headers, href, hreflang,
http-equiv, id, ismap, label, lang, language, longdesc, maxlength,
media, method, multiple, name, nohref, object, onblur, onchange,
onclick, ondblclick, onfocus, onkeydown, onkeypress, onkeyup, onload,
onload, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup,
onreset, onselect, onsubmit, onunload, onunload, profile, prompt,
readonly, rel, rev, rowspan, scheme, scope, selected, shape, span,
src, standby, start, style, summary, title, type, usemap, value,
valuetype, version.
For XHTML and other languages written in XML, no attribute should be
considered presentational. The styling of elements and
non-presentational attributes should be handled in the user agent
stylesheet.
Example(s):
The following user stylesheet would override the font weight of 'b'
elements in all documents, and the color of 'font' elements with color
attributes in XML documents. It would not affect the color of any
'font' elements with color attributes in HTML documents:
b { font-weight: normal; }
font[color] { color: orange; }
The following, however, would override the color of font elements in
all documents:
font[color] { color: orange ! important; }
_________________________________________________________________
_________________________________________________________________
7 Media types
Contents
* 7.1 Introduction to media types
* 7.2 Specifying media-dependent style sheets
+ 7.2.1 The @media rule
* 7.3 Recognized media types
+ 7.3.1 Media groups
7.1 Introduction to media types
One of the most important features of style sheets is that they
specify how a document is to be presented on different media: on the
screen, on paper, with a speech synthesizer, with a braille device,
etc.
Certain CSS properties are only designed for certain media (e.g., the
'page-break-before' property only applies to paged media). On
occasion, however, style sheets for different media types may share a
property, but require different values for that property. For example,
the 'font-size' property is useful both for screen and print media.
The two media types are different enough to require different values
for the common property; a document will typically need a larger font
on a computer screen than on paper. Therefore, it is necessary to
express that a style sheet, or a section of a style sheet, applies to
certain media types.
7.2 Specifying media-dependent style sheets
There are currently two ways to specify media dependencies for style
sheets:
* Specify the target medium from a style sheet with the @media or
@import at-rules.
Example(s):
@import url("fancyfonts.css") screen;
@media print {
/* style sheet for print goes here */
}
* Specify the target medium within the document language. For
example, in HTML 4.0 ([HTML40]), the "media" attribute on the LINK
element specifies the target media of an external style sheet:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Link to a target medium</TITLE>
<LINK REL="stylesheet" TYPE="text/css"
MEDIA="print, handheld" HREF="foo.css">
</HEAD>
<BODY>
<P>The body...
</BODY>
</HTML>
The @import rule is defined in the chapter on the cascade.
7.2.1 The @media rule
An @media rule specifies the target media types (separated by commas)
of a set of rules (delimited by curly braces). The @media construct
allows style sheet rules for various media in the same style sheet:
@media print {
body { font-size: 10pt }
}
@media screen {
body { font-size: 13px }
}
@media screen, print {
body { line-height: 1.2 }
}
7.3 Recognized media types
The names chosen for CSS media types reflect target devices for which
the relevant properties make sense. The names of media types are
normative. In the following list of CSS media types, the parenthetical
descriptions are not normative. Likewise, the "Media" field in the
description of each property is informative.
all
Suitable for all devices.
braille
Intended for braille tactile feedback devices.
embossed
Intended for paged braille printers.
handheld
Intended for handheld devices (typically small screen, limited
bandwidth).
print
Intended for paged material and for documents viewed on screen
in print preview mode. Please consult the section on paged
media for information about formatting issues that are specific
to paged media.
projection
Intended for projected presentations, for example projectors.
Please consult the section on paged media for information about
formatting issues that are specific to paged media.
screen
Intended primarily for color computer screens.
speech
Intended for speech synthesizers. Note: CSS2 had a similar
media type called 'aural' for this purpose. See the appendix on
aural style sheets for details.
tty
Intended for media using a fixed-pitch character grid (such as
teletypes, terminals, or portable devices with limited display
capabilities). Authors should not use pixel units with the
"tty" media type.
tv
Intended for television-type devices (low resolution, color,
limited-scrollability screens, sound available).
Media type names are case-insensitive.
Media types are mutually exclusive in the sense that a user agent can
only support one media type when rendering a document. However, user
agents may have different modes which support different media types.
Note. Future versions of CSS may extend this list. Authors should not
rely on media type names that are not yet defined by a CSS
specification.
7.3.1 Media groups
This section is informative, not normative.
Each CSS property definition specifies the media types for which the
property must be implemented by a conforming user agent. Since
properties generally apply to several media, the "Applies to media"
section of each property definition lists media groups rather than
individual media types. Each property applies to all media types in
the media groups listed in its definition.
CSS 2.1 defines the following media groups:
* continuous or paged.
* visual, audio, speech, or tactile.
* grid (for character grid devices), or bitmap.
* interactive (for devices that allow user interaction), or static
(for those that don't).
* all (includes all media types)
The following table shows the relationships between media groups and
media types:
CAPTION: Relationship between media groups and media types
Media Types Media Groups
continuous/paged visual/audio/speech/tactile grid/bitmap
interactive/static
braille continuous tactile grid both
emboss paged tactile grid static
handheld both visual, audio, speech both both
print paged visual bitmap static
projection paged visual bitmap interactive
screen continuous visual, audio bitmap both
speech continuous speech N/A both
tty continuous visual grid both
tv both visual, audio bitmap both
_________________________________________________________________
_________________________________________________________________
8 Box model
Contents
* 8.1 Box dimensions
* 8.2 Example of margins, padding, and borders
* 8.3 Margin properties: 'margin-top', 'margin-right',
'margin-bottom', 'margin-left', and 'margin'
+ 8.3.1 Collapsing margins
* 8.4 Padding properties: 'padding-top', 'padding-right',
'padding-bottom', 'padding-left', and 'padding'
* 8.5 Border properties
+ 8.5.1 Border width: 'border-top-width', 'border-right-width',
'border-bottom-width', 'border-left-width', and
'border-width'
+ 8.5.2 Border color: 'border-top-color', 'border-right-color',
'border-bottom-color', 'border-left-color', and
'border-color'
+ 8.5.3 Border style: 'border-top-style', 'border-right-style',
'border-bottom-style', 'border-left-style', and
'border-style'
+ 8.5.4 Border shorthand properties: 'border-top',
'border-bottom', 'border-right', 'border-left', and 'border'
+ 8.5.5 The border model for inline elements
The CSS box model describes the rectangular boxes that are generated
for elements in the document tree and laid out according to the visual
formatting model.
8.1 Box dimensions
Each box has a content area (e.g., text, an image, etc.) and optional
surrounding padding, border, and margin areas; the size of each area
is specified by properties defined below. The following diagram shows
how these areas relate and the terminology used to refer to pieces of
margin, border, and padding:
Image illustrating the relationship between content, padding, borders,
and margins.
The margin, border, and padding can be broken down into top, right,
bottom, and left segments (e.g., in the diagram, "LM" for left margin,
"RP" for right padding, "TB" for top border, etc.).
The perimeter of each of the four areas (content, padding, border, and
margin) is called an "edge", so each box has four edges:
content edge or inner edge
The content edge surrounds the element's rendered content.
padding edge
The padding edge surrounds the box padding. If the padding has
0 width, the padding edge is the same as the content edge.
border edge
The border edge surrounds the box's border. If the border has 0
width, the border edge is the same as the padding edge.
margin edge or outer edge
The margin edge surrounds the box margin. If the margin has 0
width, the margin edge is the same as the border edge.
Each edge may be broken down into a top, right, bottom, and left edge.
The dimensions of the content area of a box -- the content width and
content height -- depend on several factors: whether the element
generating the box has the 'width' or 'height' property set, whether
the box contains text or other boxes, whether the box is a table, etc.
Box widths and heights are discussed in the chapter on visual
formatting model details.
The box width is given by the sum of the left and right margins,
border, and padding, and the content width. The box height is given by
the sum of the top and bottom margins, border, and padding, and the
content height.
The background style of the content, padding, and border areas of a
box is specified by the 'background' property of the generating
element. Margin backgrounds are always transparent.
8.2 Example of margins, padding, and borders
This example illustrates how margins, padding, and borders interact.
The example HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Examples of margins, padding, and borders</TITLE>
<STYLE type="text/css">
UL {
background: yellow;
margin: 12px 12px 12px 12px;
padding: 3px 3px 3px 3px;
/* No borders set */
}
LI {
color: white; /* text color is white */
background: blue; /* Content, padding will be blue */
margin: 12px 12px 12px 12px;
padding: 12px 0px 12px 12px; /* Note 0px padding right */
list-style: none /* no glyphs before a list item */
/* No borders set */
}
LI.withborder {
border-style: dashed;
border-width: medium; /* sets border width on all sides */
border-color: lime;
}
</STYLE>
</HEAD>
<BODY>
<UL>
<LI>First element of list
<LI class="withborder">Second element of list is longer
to illustrate wrapping.
</UL>
</BODY>
</HTML>
results in a document tree with (among other relationships) a UL
element that has two LI children.
The first of the following diagrams illustrates what this example
would produce. The second illustrates the relationship between the
margins, padding, and borders of the UL elements and those of its
children LI elements.
Image illustrating how parent and child margins, borders, and padding
relate.
Note that:
* The content width for each LI box is calculated top-down; the
containing block for each LI box is established by the UL element.
* The height of each LI box is given by its content height, plus top
and bottom padding, borders, and margins. Note that vertical
margins between the LI boxes collapse.
* The right padding of the LI boxes has been set to zero width (the
'padding' property). The effect is apparent in the second
illustration.
* The margins of the LI boxes are transparent -- margins are always
transparent -- so the background color (green) of the UL padding
and content areas shines through them.
* The second LI element specifies a dashed border (the
'border-style' property).
8.3 Margin properties: 'margin-top', 'margin-right', 'margin-bottom',
'margin-left', and 'margin'
Margin properties specify the width of the margin area of a box. The
'margin' shorthand property sets the margin for all four sides while
the other margin properties only set their respective side. These
properties apply to all elements, but vertical margins will not have
any effect on non-replaced inline elements. Conforming HTML user
agents may ignore the margin properties on the HTML element.
The properties defined in this section refer to the <margin-width>
value type, which may take one of the following values:
<length>
Specifies a fixed width.
<percentage>
The percentage is calculated with respect to the width of the
generated box's containing block. Note that this is true for
'margin-top' and 'margin-bottom' as well.
auto
See the section on computing widths and margins for behavior.
Negative values for margin properties are allowed, but there may be
implementation-specific limits.
'margin-top', 'margin-bottom'
Value: <margin-width> | inherit
Initial: 0
Applies to: all elements but inline, non-replaced elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length
'margin-right', 'margin-left'
Value: <margin-width> | inherit
Initial: 0
Applies to: all elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length
These properties set the top, right, bottom, and left margin of a box.
Example(s):
h1 { margin-top: 2em }
'margin'
Value: <margin-width>{1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: see individual properties
The 'margin' property is a shorthand property for setting
'margin-top', 'margin-right', 'margin-bottom', and 'margin-left' at
the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom margins are set to the first value and the
right and left margins are set to the second. If there are three
values, the top is set to the first value, the left and right are set
to the second, and the bottom is set to the third. If there are four
values, they apply to the top, right, bottom, and left, respectively.
Example(s):
body { margin: 2em } /* all margins set to 2em */
body { margin: 1em 2em } /* top & bottom = 1em, right & left = 2em */
body { margin: 1em 2em 3em } /* top=1em, right=2em, bottom=3em, left=2em */
The last rule of the example above is equivalent to the example below:
body {
margin-top: 1em;
margin-right: 2em;
margin-bottom: 3em;
margin-left: 2em; /* copied from opposite side (right) */
}
8.3.1 Collapsing margins
In this specification, the expression collapsing margins means that
adjoining margins (no non-empty content, padding or border areas
separate them) of two or more boxes (which may be next to one another
or nested) combine to form a single margin.
In CSS 2.1, horizontal margins never collapse.
Vertical margins may collapse between certain boxes:
* Two or more adjoining vertical margins of block boxes in the
normal flow collapse. The resulting margin width is the maximum of
the adjoining margin widths. In the case of negative margins, the
absolute maximum of the negative adjoining margins is deducted
from the maximum of the positive adjoining margins. If there are
no positive margins, the absolute maximum of the negative
adjoining margins is deducted from zero. Note. Adjoining boxes may
be generated by elements that are not related as siblings or
ancestors.
* Vertical margins between a floated box and any other box do not
collapse.
* Margins of absolutely and relatively positioned boxes do not
collapse (not even with their in-flow children).
Collapsing is based on the computed value of 'padding', 'margin',
'border' and 'height': content is empty if its height is zero, padding
and border are absent if their width is zero, and the collapsed margin
is calculated over the computed value of the various margins.
Please consult the examples of margin, padding, and borders for an
illustration of collapsed margins.
8.4 Padding properties: 'padding-top', 'padding-right', 'padding-bottom',
'padding-left', and 'padding'
The padding properties specify the width of the padding area of a box.
The 'padding' shorthand property sets the padding for all four sides
while the other padding properties only set their respective side.
The properties defined in this section refer to the <padding-width>
value type, which may take one of the following values:
<length>
Specifies a fixed width.
<percentage>
The percentage is calculated with respect to the width of the
generated box's containing block, even for 'padding-top' and
'padding-bottom'.
Unlike margin properties, values for padding values cannot be
negative. Like margin properties, percentage values for padding
properties refer to the width of the generated box's containing block.
'padding-top', 'padding-right', 'padding-bottom', 'padding-left'
Value: <padding-width> | inherit
Initial: 0
Applies to: all elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length
These properties set the top, right, bottom, and left padding of a
box.
Example(s):
blockquote { padding-top: 0.3em }
'padding'
Value: <padding-width>{1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: see individual properties
The 'padding' property is a shorthand property for setting
'padding-top', 'padding-right', 'padding-bottom', and 'padding-left'
at the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom paddings are set to the first value and the
right and left paddings are set to the second. If there are three
values, the top is set to the first value, the left and right are set
to the second, and the bottom is set to the third. If there are four
values, they apply to the top, right, bottom, and left, respectively.
The surface color or image of the padding area is specified via the
'background' property:
Example(s):
h1 {
background: white;
padding: 1em 2em;
}
The example above specifies a '1em' vertical padding ('padding-top'
and 'padding-bottom') and a '2em' horizontal padding ('padding-right'
and 'padding-left'). The 'em' unit is relative to the element's font
size: '1em' is equal to the size of the font in use.
8.5 Border properties
The border properties specify the width, color, and style of the
border area of a box. These properties apply to all elements.
Conforming HTML user agents may ignore the border properties on the
HTML element.
Note. Notably for HTML, user agents may render borders for certain
elements (e.g., buttons, menus, etc.) differently than for "ordinary"
elements.
8.5.1 Border width: 'border-top-width', 'border-right-width',
'border-bottom-width', 'border-left-width', and 'border-width'
The border width properties specify the width of the border area. The
properties defined in this section refer to the <border-width> value
type, which may take one of the following values:
thin
A thin border.
medium
A medium border.
thick
A thick border.
<length>
The border's thickness has an explicit value. Explicit border
widths cannot be negative.
The interpretation of the first three values depends on the user
agent. The following relationships must hold, however:
'thin' <='medium' <= 'thick'.
Furthermore, these widths must be constant throughout a document.
'border-top-width', 'border-right-width', 'border-bottom-width',
'border-left-width'
Value: <border-width> | inherit
Initial: medium
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: absolute length; '0' if the border style is 'none'
or 'hidden'
These properties set the width of the top, right, bottom, and left
border of a box.
'border-width'
Value: <border-width>{1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
This property is a shorthand property for setting 'border-top-width',
'border-right-width', 'border-bottom-width', and 'border-left-width'
at the same place in the style sheet.
If there is only one value, it applies to all sides. If there are two
values, the top and bottom borders are set to the first value and the
right and left are set to the second. If there are three values, the
top is set to the first value, the left and right are set to the
second, and the bottom is set to the third. If there are four values,
they apply to the top, right, bottom, and left, respectively.
Example(s):
In the examples below, the comments indicate the resulting widths of
the top, right, bottom, and left borders:
h1 { border-width: thin } /* thin thin thin thin */
h1 { border-width: thin thick } /* thin thick thin thick */
h1 { border-width: thin thick medium } /* thin thick medium thick */
8.5.2 Border color: 'border-top-color', 'border-right-color',
'border-bottom-color', 'border-left-color', and 'border-color'
The border color properties specify the color of a box's border.
'border-top-color', 'border-right-color', 'border-bottom-color',
'border-left-color'
Value: <color> | transparent | inherit
Initial: the value of the 'color' property
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: when taken from the 'color' property, the computed
value of 'color'; otherwise, as specified
'border-color'
Value: [ <color> | transparent ]{1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border-color' property sets the color of the four borders. Values
have the following meanings:
<color>
Specifies a color value.
transparent
The border is transparent (though it may have width).
The 'border-color' property can have from one to four values, and the
values are set on the different sides as for 'border-width'.
If an element's border color is not specified with a border property,
user agents must use the value of the element's 'color' property as
the computed value for the border color.
Example(s):
In this example, the border will be a solid black line.
p {
color: black;
background: white;
border: solid;
}
8.5.3 Border style: 'border-top-style', 'border-right-style',
'border-bottom-style', 'border-left-style', and 'border-style'
The border style properties specify the line style of a box's border
(solid, double, dashed, etc.). The properties defined in this section
refer to the <border-style> value type, which make take one of the
following values:
none
No border.
hidden
Same as 'none', except in terms of border conflict resolution
for table elements.
dotted
The border is a series of dots.
dashed
The border is a series of short line segments.
solid
The border is a single line segment.
double
The border is two solid lines. The sum of the two lines and the
space between them equals the value of 'border-width'.
groove
The border looks as though it were carved into the canvas.
ridge
The opposite of 'groove': the border looks as though it were
coming out of the canvas.
inset
The border makes the box look as though it were embedded in the
canvas.
outset
The opposite of 'inset': the border makes the box look as
though it were coming out of the canvas.
All borders are drawn on top of the box's background. The color of
borders drawn for values of 'groove', 'ridge', 'inset', and 'outset'
depends on the element's border color properties, but UAs may choose
their own algorithm to calculate the actual colors used. For instance,
if the 'border-color' has the value 'silver', then a UA could use a
gradient of colors from white to dark gray to indicate a sloping
border.
Conforming HTML user agents may interpret 'dotted', 'dashed',
'double', 'groove', 'ridge', 'inset', and 'outset' to be 'solid'.
'border-top-style', 'border-right-style', 'border-bottom-style',
'border-left-style'
Value: <border-style> | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
'border-style'
Value: <border-style>{1,4} | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border-style' property sets the style of the four borders. It can
have from one to four values, and the values are set on the different
sides as for 'border-width' above.
Example(s):
#xy34 { border-style: solid dotted }
In the above example, the horizontal borders will be 'solid' and the
vertical borders will be 'dotted'.
Since the initial value of the border styles is 'none', no borders
will be visible unless the border style is set.
8.5.4 Border shorthand properties: 'border-top', 'border-bottom',
'border-right', 'border-left', and 'border'
[Use <'border-top-color'> as a type, in order to include 'transparent'
as a legal value. BB]
'border-top', 'border-right', 'border-bottom', 'border-left'
Value: [ <border-width> || <border-style> || 'border-top-color' ] |
inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
This is a shorthand property for setting the width, style, and color
of the top, right, bottom, and left border of a box.
Example(s):
h1 { border-bottom: thick solid red }
The above rule will set the width, style, and color of the border
below the H1 element. Omitted values are set to their initial values.
Since the following rule does not specify a border color, the border
will have the color specified by the 'color' property:
H1 { border-bottom: thick solid }
'border'
Value: [ <border-width> || <border-style> || 'border-top-color' ] |
inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'border' property is a shorthand property for setting the same
width, color, and style for all four borders of a box. Unlike the
shorthand 'margin' and 'padding' properties, the 'border' property
cannot set different values on the four borders. To do so, one or more
of the other border properties must be used.
Example(s):
For example, the first rule below is equivalent to the set of four
rules shown after it:
p { border: solid red }
p {
border-top: solid red;
border-right: solid red;
border-bottom: solid red;
border-left: solid red
}
Since, to some extent, the properties have overlapping functionality,
the order in which the rules are specified is important.
Example(s):
Consider this example:
blockquote {
border-color: red;
border-left: double;
color: black
}
In the above example, the color of the left border is black, while the
other borders are red. This is due to 'border-left' setting the width,
style, and color. Since the color value is not given by the
'border-left' property, it will be taken from the 'color' property.
The fact that the 'color' property is set after the 'border-left'
property is not relevant.
8.5.5 The border model for inline elements
For each line box, UAs must take the inline boxes generated for each
element and render the borders and backgrounds in visual order (not
logical order). The results (what part of the background is behind
which element) may differ based on the line wrapping.
When the element's 'direction' property is 'ltr', the left-most
generated box of the first line box in which the element appears has a
left border, and the right-most generated box of the last line box in
which the element appears has a right border.
When the element's 'direction' property is 'rtl', the right-most
generated box of the first line box in which the element appears has a
right border, and the left-most generated box of the left line box in
which the element appears has a left border.
_________________________________________________________________
_________________________________________________________________
9 Visual formatting model
Contents
* 9.1 Introduction to the visual formatting model
+ 9.1.1 The viewport
+ 9.1.2 Containing blocks
* 9.2 Controlling box generation
+ 9.2.1 Block-level elements and block boxes
o Anonymous block boxes
+ 9.2.2 Inline-level elements and inline boxes
o Anonymous inline boxes
+ 9.2.3 Run-in boxes
+ 9.2.4 The 'display' property
* 9.3 Positioning schemes
+ 9.3.1 Choosing a positioning scheme: 'position' property
+ 9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'
* 9.4 Normal flow
+ 9.4.1 Block formatting context
+ 9.4.2 Inline formatting context
+ 9.4.3 Relative positioning
* 9.5 Floats
+ 9.5.1 Positioning the float: the 'float' property
+ 9.5.2 Controlling flow next to floats: the 'clear' property
* 9.6 Absolute positioning
+ 9.6.1 Fixed positioning
* 9.7 Relationships between 'display', 'position', and 'float'
* 9.8 Comparison of normal flow, floats, and absolute positioning
+ 9.8.1 Normal flow
+ 9.8.2 Relative positioning
+ 9.8.3 Floating a box
+ 9.8.4 Absolute positioning
* 9.9 Layered presentation
+ 9.9.1 Specifying the stack level: the 'z-index' property
* 9.10 Text direction: the 'direction' and 'unicode-bidi' properties
9.1 Introduction to the visual formatting model
This chapter and the next describe the visual formatting model: how
user agents process the document tree for visual media.
In the visual formatting model, each element in the document tree
generates zero or more boxes according to the box model. The layout of
these boxes is governed by:
* box dimensions and type.
* positioning scheme (normal flow, float, and absolute).
* relationships between elements in the document tree.
* external information (e.g., viewport size, intrinsic dimensions of
images, etc.).
The properties defined in this chapter and the next apply to both
continuous media and paged media. However, the meanings of the margin
properties vary when applied to paged media (see the page model for
details).
The visual formatting model does not specify all aspects of formatting
(e.g., it does not specify a letter-spacing algorithm). Conforming
user agents may behave differently for those formatting issues not
covered by this specification.
9.1.1 The viewport
User agents for continuous media generally offer users a viewport (a
window or other viewing area on the screen) through which users
consult a document. User agents may change the document's layout when
the viewport is resized (see the initial containing block).
When the viewport is smaller than the area of the canvas on which the
document is rendered, the user agent should offer a scrolling
mechanism. There is at most one viewport per canvas, but user agents
may render to more than one canvas (i.e., provide different views of
the same document).
9.1.2 Containing blocks
In CSS 2.1, many box positions and sizes are calculated with respect
to the edges of a rectangular box called a containing block. In
general, generated boxes act as containing blocks for descendant
boxes; we say that a box "establishes" the containing block for its
descendants. The phrase "a box's containing block" means "the
containing block in which the box lives," not the one it generates.
Each box is given a position with respect to its containing block, but
it is not confined by this containing block; it may overflow.
User agents may treat float as 'none' and/or position as 'static' on
the root element.
The details of how a containing block's dimensions are calculated are
described in the next chapter.
9.2 Controlling box generation
The following sections describe the types of boxes that may be
generated in CSS 2.1. A box's type affects, in part, its behavior in
the visual formatting model. The 'display' property, described below,
specifies a box's type.
9.2.1 Block-level elements and block boxes
Block-level elements are those elements of the source document that
are formatted visually as blocks (e.g., paragraphs). Several values of
the 'display' property make an element block-level: 'block',
'list-item', and 'run-in' (part of the time; see run-in boxes), and
'table'.
Block-level elements generate a principal block box that only contains
block boxes. The principal block box establishes the containing block
for descendant boxes and generated content and is also the box
involved in any positioning scheme. Principal block boxes participate
in a block formatting context.
Some block-level elements generate additional boxes outside of the
principal box: 'list-item' elements. These additional boxes are placed
with respect to the principal box.
Anonymous block boxes
In a document like this:
<DIV>
Some text
<P>More text
</DIV>
(and assuming the DIV and the P both have 'display: block'), the DIV
appears to have both inline content and block content. To make it
easier to define the formatting, we assume that there is an anonymous
block box around "Some text".
diagram showing the three boxes for the example above
Diagram showing the three boxes, of which one is anonymous, for the
example above.
In other words: if a block box (such as that generated for the DIV
above) has another block box inside it (such as the P above), then we
force it to have only block boxes inside it, by wrapping any inline
boxes in an anonymous block box.
Example(s):
This model would apply in the following example if the following
rules:
body { display: inline }
p { display: block }
were used with this HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HEAD>
<TITLE>Anonymous text interrupted by a block</TITLE>
</HEAD>
<BODY>
This is anonymous text before the P.
<P>This is the content of P.</P>
This is anonymous text after the P.
</BODY>
The BODY element contains a chunk (C1) of anonymous text followed by a
block-level element followed by another chunk (C2) of anonymous text.
The resulting boxes would be an anonymous block box for BODY,
containing an anonymous block box around C1, the P block box, and
another anonymous block box around C2.
The properties of anonymous boxes are inherited from the enclosing
non-anonymous box (in the example: the one for DIV). Non-inherited
properties have their initial value. For example, the font of the
anonymous box is inherited from the DIV, but the margins will be 0.
Properties set on elements that are turned into anonymous block boxes
still apply to the content of the element. For example, if a border
had been set on the BODY element in the above example, the border
would be drawn around C1 and C2.
9.2.2 Inline-level elements and inline boxes
Inline-level elements are those elements of the source document that
do not form new blocks of content; the content is distributed in lines
(e.g., emphasized pieces of text within a paragraph, inline images,
etc.). Several values of the 'display' property make an element
inline: 'inline', 'inline-table', and 'run-in' (part of the time; see
run-in boxes). Inline-level elements generate inline boxes.
Anonymous inline boxes
In a document like this:
<P>Some <EM>emphasized</EM> text</P>
The P generates a block box, with several inline boxes inside it. The
box for "emphasized" is an inline box generated by an inline element
(EM), but the other boxes ("Some" and "text") are inline boxes
generated by a block-level element (P). The latter are called
anonymous inline boxes, because they don't have an associated
inline-level element.
Such anonymous inline boxes inherit inheritable properties from their
block parent box. Non-inherited properties have their initial value.
In the example, the color of the anonymous inline boxes is inherited
from the P, but the background is transparent.
If it is clear from the context which type of anonymous box is meant,
both anonymous inline boxes and anonymous block boxes are simply
called anonymous boxes in this specification.
There are more types of anonymous boxes that arise when formatting
tables.
9.2.3 Run-in boxes
A run-in box behaves as follows:
* If a block box (that does not float and is not absolutely
positioned) follows the run-in box, the run-in box becomes the
first inline box of the block box.
* Otherwise, the run-in box becomes a block box.
A 'run-in' box is useful for run-in headers, as in this example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>A run-in box example</TITLE>
<STYLE type="text/css">
H3 { display: run-in }
</STYLE>
</HEAD>
<BODY>
<H3>A run-in heading.</H3>
<P>And a paragraph of text that
follows it.
</BODY>
</HTML>
This example might be formatted as:
A run-in heading. And a
paragraph of text that
follows it.
Despite appearing visually part of the following block box, a run-in
element still inherits properties from its parent in the source tree.
Please consult the section on generated content for information about
how run-in boxes interact with generated content.
9.2.4 The 'display' property
'display'
Value: inline | block | list-item | run-in | inline-block | table |
inline-table | table-row-group | table-header-group |
table-footer-group | table-row | table-column-group | table-column |
table-cell | table-caption | none | inherit
Initial: inline
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: as specified
The values of this property have the following meanings:
block
This value causes an element to generate a block box.
inline-block
This value causes an element to generate a block box, which
itself is flowed as a single inline box, similar to a replaced
element. The inside of an inline-block is formatted as a block
box, and the element itself is formatted as a replaced element
on the line.
inline
This value causes an element to generate one or more inline
boxes.
list-item
This value causes an element (e.g., LI in HTML) to generate a
principal block box and a list-item inline box. For information
about lists and examples of list formatting, please consult the
section on lists.
none
This value causes an element to generate no boxes in the
formatting structure (i.e., the element has no effect on
layout). Descendant elements do not generate any boxes either;
this behavior cannot be overridden by setting the 'display'
property on the descendants.
Please note that a display of 'none' does not create an
invisible box; it creates no box at all. CSS includes
mechanisms that enable an element to generate boxes in the
formatting structure that affect formatting but are not visible
themselves. Please consult the section on visibility for
details.
run-in
This value creates either block or inline boxes, depending on
context. Properties apply to run-in boxes based on their final
status (inline-level or block-level).
table, inline-table, table-row-group, table-column,
table-column-group, table-header-group, table-footer-group,
table-row, table-cell, and table-caption
These values cause an element to behave like a table element
(subject to restrictions described in the chapter on tables).
Note that although the initial value of 'display' is 'inline', rules
in the user agent's default style sheet may override this value. See
the sample style sheet for HTML 4.0 in the appendix.
Example(s):
Here are some examples of the 'display' property:
p { display: block }
em { display: inline }
li { display: list-item }
img { display: none } /* Don't display images */
9.3 Positioning schemes
In CSS 2.1, a box may be laid out according to three positioning
schemes:
1. Normal flow. In CSS 2.1, normal flow includes block formatting of
block boxes, inline formatting of inline boxes, relative
positioning of block or inline boxes, and positioning of run-in
boxes.
2. Floats. In the float model, a box is first laid out according to
the normal flow, then taken out of the flow and shifted to the
left or right as far as possible. Content may flow along the side
of a float.
3. Absolute positioning. In the absolute positioning model, a box is
removed from the normal flow entirely (it has no impact on later
siblings) and assigned a position with respect to a containing
block.
Note. CSS 2.1's positioning schemes help authors make their documents
more accessible by allowing them to avoid mark-up tricks (e.g.,
invisible images) used for layout effects.
9.3.1 Choosing a positioning scheme: 'position' property
The 'position' and 'float' properties determine which of the CSS 2.1
positioning algorithms is used to calculate the position of a box.
'position'
Value: static | relative | absolute | fixed | inherit
Initial: static
Applies to: all elements, but not to generated content
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
The values of this property have the following meanings:
static
The box is a normal box, laid out according to the normal flow.
The 'top', 'right', 'bottom', and 'left' properties do not
apply.
relative
The box's position is calculated according to the normal flow
(this is called the position in normal flow). Then the box is
offset relative to its normal position. When a box B is
relatively positioned, the position of the following box is
calculated as though B were not offset.
absolute
The box's position (and possibly size) is specified with the
'top', 'right', 'bottom', and 'left' properties. These
properties specify offsets with respect to the box's containing
block. Absolutely positioned boxes are taken out of the normal
flow. This means they have no impact on the layout of later
siblings. Also, though absolutely positioned boxes have
margins, they do not collapse with any other margins.
fixed
The box's position is calculated according to the 'absolute'
model, but in addition, the box is fixed with respect to some
reference. In the case of continuous media, the box is fixed
with respect to the viewport (and doesn't move when scrolled).
In the case of paged media, the box is fixed with respect to
the page, even if that page is seen through a viewport (in the
case of a print-preview, for example). Authors may wish to
specify 'fixed' in a media-dependent way. For instance, an
author may want a box to remain at the top of the viewport on
the screen, but not at the top of each printed page. The two
specifications may be separated by using an @media rule, as in:
Example(s):
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'
An element is said to be positioned if its 'position' property has a
value other than 'static'. Positioned elements generate positioned
boxes, laid out according to four properties:
'top'
Value: <length> | <percentage> | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: absolute length; 'auto' if the property does not
apply
This property specifies how far an absolutely positioned box's top
margin edge is offset below the top edge of the box's containing
block. For relatively positioned boxes, the offset is with respect to
the top edges of the box itself (i.e., the box is given a position in
the normal flow, then offset from that position according to these
properties).
'right'
Value: <length> | <percentage> | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length; 'auto' if the property does not
apply
Like 'top', but specifies how far a box's right margin edge is offset
to the left of the right edge of the box's containing block. For
relatively positioned boxes, the offset is with respect to the right
edge of the box itself.
'bottom'
Value: <length> | <percentage> | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: absolute length; 'auto' if the property does not
apply
Like 'top', but specifies how far a box's bottom margin edge is offset
above the bottom of the box's containing block. For relatively
positioned boxes, the offset is with respect to the bottom edge of the
box itself.
'left'
Value: <length> | <percentage> | auto | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length; 'auto' if the property does not
apply
Like 'top', but specifies how far a box's left margin edge is offset
to the right of the left edge of the box's containing block. For
relatively positioned boxes, the offset is with respect to the left
edge of the box itself.
The values for the four properties have the following meanings:
<length>
The offset is a fixed distance from the reference edge.
<percentage>
The offset is a percentage of the containing block's width (for
'left' or 'right') or height (for 'top' and 'bottom'). For
'top' and 'bottom', if the height of the containing block is
not specified explicitly (i.e., it depends on content height),
the percentage value is interpreted like 'auto'.
auto
The effect of this value depends on which of related properties
have the value 'auto' as well. See the sections on the width
and height of absolutely positioned, non-replaced elements for
details.
9.4 Normal flow
Boxes in the normal flow belong to a formatting context, which may be
block or inline, but not both simultaneously. Block boxes participate
in a block formatting context. Inline boxes participate in an inline
formatting context.
9.4.1 Block formatting context
In a block formatting context, boxes are laid out one after the other,
vertically, beginning at the top of a containing block. The vertical
distance between two sibling boxes is determined by the 'margin'
properties. Vertical margins between adjacent block boxes in a block
formatting context collapse.
In a block formatting context, each box's left outer edge touches the
left edge of the containing block (for right-to-left formatting, right
edges touch). This is true even in the presence of floats (although a
box's line boxes may shrink due to the floats).
For information about page breaks in paged media, please consult the
section on allowed page breaks.
9.4.2 Inline formatting context
In an inline formatting context, boxes are laid out horizontally, one
after the other, beginning at the top of a containing block.
Horizontal margins, borders, and padding are respected between these
boxes. The boxes may be aligned vertically in different ways: their
bottoms or tops may be aligned, or the baselines of text within them
may be aligned. The rectangular area that contains the boxes that form
a line is called a line box.
The width of a line box is determined by a containing block and the
presence of floats. The height of a line box is determined by the
rules given in the section on line height calculations.
A line box is always tall enough for all of the boxes it contains.
However, it may be taller than the tallest box it contains (if, for
example, boxes are aligned so that baselines line up). When the height
of a box B is less than the height of the line box containing it, the
vertical alignment of B within the line box is determined by the
'vertical-align' property. When several inline boxes cannot fit
horizontally within a single line box, they are distributed among two
or more vertically-stacked line boxes. Thus, a paragraph is a vertical
stack of line boxes. Line boxes are stacked with no vertical
separation and they never overlap.
In general, the left edge of a line box touches the left edge of its
containing block and the right edge touches the right edge of its
containing block. However, floating boxes may come between the
containing block edge and the line box edge. Thus, although line boxes
in the same inline formatting context generally have the same width
(that of the containing block), they may vary in width if available
horizontal space is reduced due to floats. Line boxes in the same
inline formatting context generally vary in height (e.g., one line
might contain a tall image while the others contain only text).
When the total width of the inline boxes on a line is less than the
width of the line box containing them, their horizontal distribution
within the line box is determined by the 'text-align' property. If
that property has the value 'justify', the user agent may stretch the
inline boxes as well.
When an inline box exceeds the width of a line box, it is split into
several boxes and these boxes are distributed across several line
boxes. If an inline box cannot be split (e.g. if the inline box
contains a single character, or language specific word breaking rules
disallow a break within the inline box, or if the inline box is
affected by a white-space value of nowrap or pre), then the inline box
overflows the line box.
When an inline box is split, margins, borders, and padding have no
visual effect where the split occurs (or at any split, when there are
several). Formatting of margins, borders, and padding may not be fully
defined if the split occurs within a bidirectional embedding.
Inline boxes may also be split into several boxes within the same line
box due to bidirectional text processing.
Here is an example of inline box construction. The following paragraph
(created by the HTML block-level element P) contains anonymous text
interspersed with the elements EM and STRONG:
<P>Several <EM>emphasized words</EM> appear
<STRONG>in this</STRONG> sentence, dear.</P>
The P element generates a block box that contains five inline boxes,
three of which are anonymous:
* Anonymous: "Several"
* EM: "emphasized words"
* Anonymous: "appear"
* STRONG: "in this"
* Anonymous: "sentence, dear."
To format the paragraph, the user agent flows the five boxes into line
boxes. In this example, the box generated for the P element
establishes the containing block for the line boxes. If the containing
block is sufficiently wide, all the inline boxes will fit into a
single line box:
Several emphasized words appear in this sentence, dear.
If not, the inline boxes will be split up and distributed across
several line boxes. The previous paragraph might be split as follows:
Several emphasized words appear
in this sentence, dear.
or like this:
Several emphasized
words appear in this
sentence, dear.
In the previous example, the EM box was split into two EM boxes (call
them "split1" and "split2"). Margins, borders, padding, or text
decorations have no visible effect after split1 or before split2.
Consider the following example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Example of inline flow on several lines</TITLE>
<STYLE type="text/css">
EM {
padding: 2px;
margin: 1em;
border-width: medium;
border-style: dashed;
line-height: 2.4em;
}
</STYLE>
</HEAD>
<BODY>
<P>Several <EM>emphasized words</EM> appear here.</P>
</BODY>
</HTML>
Depending on the width of the P, the boxes may be distributed as
follows:
Image illustrating the effect of line breaking on the display of
margins, borders, and padding.
* The margin is inserted before "emphasized" and after "words".
* The padding is inserted before, above, and below "emphasized" and
after, above, and below "words". A dashed border is rendered on
three sides in each case.
9.4.3 Relative positioning
Once a box has been laid out according to the normal flow or floated,
it may be shifted relative to this position. This is called relative
positioning. Offsetting a box (B1) in this way has no effect on the
box (B2) that follows: B2 is given a position as if B1 were not offset
and B2 is not re-positioned after B1's offset is applied. This implies
that relative positioning may cause boxes to overlap.
A relatively positioned box keeps its normal flow size, including line
breaks and the space originally reserved for it. The section on
containing blocks explains when a relatively positioned box
establishes a new containing block.
For relatively positioned elements, 'left' and 'right' move the
box(es) horizontally, without changing their size. 'left' moves the
boxes to the right, and 'right' moves them to the left. Since boxes
are not split or stretched as a result of 'left' or 'right', the
computed values are always: left = -right.
If both 'left' and 'right' are 'auto' (their initial values), the
computed values are '0' (i.e., the boxes stay in their original
position).
If 'left' is 'auto', its computed value is minus the value of 'right'
(i.e., the boxes move to the left by the value of 'right').
If 'right' is specified as 'auto', its computed value is minus the
value of 'left'.
If neither 'left' nor 'right' is 'auto', the position is
over-constrained, and one of them has to be ignored. If the
'direction' property is 'ltr', the value of 'left' wins and 'right'
becomes -'left'. If 'direction' is 'rtl', 'right' wins and 'left' is
ignored.
Example(s):
Example. The following three rules are equivalent:
div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
The 'top' and 'bottom' properties move relatively positioned
element(s) up or down without changing their size. 'top' moves the
boxes down, and 'bottom' moves them up. Since boxes are not split or
stretched as a result of 'top' or 'bottom', the computed values are
always: top = -bottom. If both are 'auto', their computed values are
both '0'. If one of them is 'auto', it becomes the negative of the
other. If neither is 'auto', 'bottom' is ignored (i.e., the computed
value of 'bottom' will be minus the value of 'top').
Dynamic movement of relatively positioned boxes can produce animation
effects in scripting environments (see also the 'visibility'
property). Relative positioning may also be used as a general form of
superscripting and subscripting except that line height is not
automatically adjusted to take the positioning into consideration. See
the description of line height calculations for more information.
Examples of relative positioning are provided in the section comparing
normal flow, floats, and absolute positioning.
9.5 Floats
A float is a box that is shifted to the left or right on the current
line. The most interesting characteristic of a float (or "floated" or
"floating" box) is that content may flow along its side (or be
prohibited from doing so by the 'clear' property). Content flows down
the right side of a left-floated box and down the left side of a
right-floated box. The following is an introduction to float
positioning and content flow; the exact rules governing float behavior
are given in the description of the 'float' property.
A floated box must have an explicit width (assigned via the 'width'
property, or its intrinsic width in the case of replaced elements).
Any floated box becomes a block box that is shifted to the left or
right until its outer edge touches the containing block edge or the
outer edge of another float. The top of the floated box is aligned
with the top of the current line box (or bottom of the preceding block
box if no line box exists).
If there isn't enough horizontal room for the float, it is shifted
downward until either it fits or there are no more floats present.
Since a float is not in the flow, non-positioned block boxes created
before and after the float box flow vertically as if the float didn't
exist. However, line boxes created next to the float are shortened to
make room for the floated box. If a shortened line box is too small to
contain any further content, then it is shifted downward until either
it fits or there are no more floats present. Any content in the
current line before a floated box is reflowed in the first available
line on the other side of the float.
Example(s):
Example. In the following document fragment, the containing block is
too short to contain the content, so the content gets moved to below
the floats where it is aligned in the line box according to the
text-align property.
p { width: 10em; border: solid aqua; }
span { float: left; width: 5em; height: 5em; border: solid blue; }
...
<p>
<span> </span>
Supercalifragilisticexpialidocious
</p>
This fragment might look like this:
Image illustrating the effect of an unbreakable piece of content being
reflowed to just after a float which left insufficient room next to it
for the content to fit.
Several floats may be adjacent, and this model also applies to
adjacent floats in the same line.
Example(s):
The following rule floats all IMG boxes with class="icon" to the left
(and sets the left margin to '0'):
img.icon {
float: left;
margin-left: 0;
}
Consider the following HTML source and style sheet:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Float example</TITLE>
<STYLE type="text/css">
IMG { float: left }
BODY, P, IMG { margin: 2em }
</STYLE>
</HEAD>
<BODY>
<P><IMG src=img.png alt="This image will illustrate floats">
Some sample text that has no other...
</BODY>
</HTML>
The IMG box is floated to the left. The content that follows is
formatted to the right of the float, starting on the same line as the
float. The line boxes to the right of the float are shortened due to
the float's presence, but resume their "normal" width (that of the
containing block established by the P element) after the float. This
document might be formatted as:
Image illustrating how floating boxes interact with margins.
Formatting would have been exactly the same if the document had been:
<BODY>
<P>Some sample text
<IMG src=img.png alt="This image will illustrate floats">
that has no other...
</BODY>
because the content to the left of the float is displaced by the float
and reflowed down its right side.
The margins of floating boxes never collapse with margins of adjacent
boxes. Thus, in the previous example, vertical margins do not collapse
between the P box and the floated IMG box.
A float can overlap other boxes in the normal flow (e.g., when a
normal flow box next to a float has negative margins). When an inline
box overlaps with a float, the content, background, and borders of the
inline box are rendered in front of the float. When a block box
overlaps, the background and borders of the block box are rendered
behind the float and are only visible where the float is transparent.
The content of the block box is rendered in front of the float.
Example(s):
Here is another illustration, showing what happens when a float
overlaps borders of elements in the normal flow.
Image showing a floating image that overlaps the borders of two
paragraphs: the borders are interrupted by the image.
A floating image obscures borders of block boxes it overlaps.
The following example illustrates the use of the 'clear' property to
prevent content from flowing next to a float.
Example(s):
Assuming a rule such as this:
p { clear: left }
formatting might look like this:
Image showing a floating image and the effect of 'clear: left' on the
two paragraphs.
Both paragraphs have set 'clear: left', which causes the second
paragraph to be "pushed down" to a position below the float -- its top
margin expands to accomplish this (see the 'clear' property).
9.5.1 Positioning the float: the 'float' property
'float'
Value: left | right | none | inherit
Initial: none
Applies to: all but generated content, but see 9.7
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies whether a box should float to the left, right,
or not at all. It may be set for elements that generate boxes that are
not absolutely positioned. The values of this property have the
following meanings:
left
The element generates a block box that is floated to the left.
Content flows on the right side of the box, starting at the top
(subject to the 'clear' property). A floated element becomes
block-level, regardless of the 'display' property, unless it
has the value 'none'
right
Same as 'left', but content flows on the left side of the box,
starting at the top.
none
The box is not floated.
Here are the precise rules that govern the behavior of floats:
1. The left outer edge of a left-floating box may not be to the left
of the left edge of its containing block. An analogous rule holds
for right-floating elements.
2. If the current box is left-floating, and there are any
left-floating boxes generated by elements earlier in the source
document, then for each such earlier box, either the left outer
edge of the current box must be to the right of the right outer
edge of the earlier box, or its top must be lower than the bottom
of the earlier box. Analogous rules hold for right-floating boxes.
3. The right outer edge of a left-floating box may not be to the
right of the left outer edge of any right-floating box that is to
the right of it. Analogous rules hold for right-floating elements.
4. A floating box's outer top may not be higher than the top of its
containing block.
5. The outer top of a floating box may not be higher than the outer
top of any block or floated box generated by an element earlier in
the source document.
6. The outer top of an element's floating box may not be higher than
the top of any line-box containing a box generated by an element
earlier in the source document.
7. A left-floating box that has another left-floating box to its left
may not have its right outer edge to the right of its containing
block's right edge. (Loosely: a left float may not stick out at
the right edge, unless it is already as far to the left as
possible.) An analogous rule holds for right-floating elements.
8. A floating box must be placed as high as possible.
9. A left-floating box must be put as far to the left as possible, a
right-floating box as far to the right as possible. A higher
position is preferred over one that is further to the left/right.
9.5.2 Controlling flow next to floats: the 'clear' property
'clear'
Value: none | left | right | both | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property indicates which sides of an element's box(es) may not be
adjacent to an earlier floating box. (It may be that the element
itself has floating descendants; the 'clear' property has no effect on
those.)
For run-in boxes, this property applies to the final block box to
which the run-in box belongs.
Values have the following meanings when applied to non-floating block
boxes:
left
The top margin of the generated box is increased enough that
the top border edge is below the bottom outer edge of any
left-floating boxes that resulted from elements earlier in the
source document.
right
The top margin of the generated box is increased enough that
the top border edge is below the bottom outer edge of any
right-floating boxes that resulted from elements earlier in the
source document.
both
The top margin of the generated box is increased enough that
the top border edge is below the bottom outer edge of any
right-floating and left-floating boxes that resulted from
elements earlier in the source document.
none
No constraint on the box's position with respect to floats.
When applied to inline elements, rather than increasing the top margin
as explained above, one or more empty line boxes are inserted to move
the top of the cleared inline's line box to below the respective
floating box(es).
Example:
span { clear: left }
When the property is set on floating elements, it results in a
modification of the rules for positioning the float. An extra
constraint (#10) is added:
* The top outer edge of the float must be below the bottom outer
edge of all earlier left-floating boxes (in the case of 'clear:
left'), or all earlier right-floating boxes (in the case of
'clear: right'), or both ('clear: both').
9.6 Absolute positioning
In the absolute positioning model, a box is explicitly offset with
respect to its containing block. It is removed from the normal flow
entirely (it has no impact on later siblings). An absolutely
positioned box establishes a new containing block for normal flow
children and positioned descendants. However, the contents of an
absolutely positioned element do not flow around any other boxes. They
may obscure the contents of another box (or be obscured themselves),
depending on the stack levels of the overlapping boxes.
References in this specification to an absolutely positioned element
(or its box) imply that the element's 'position' property has the
value 'absolute' or 'fixed'.
9.6.1 Fixed positioning
Fixed positioning is a subcategory of absolute positioning. The only
difference is that for a fixed positioned box, the containing block is
established by the viewport. For continuous media, fixed boxes do not
move when the document is scrolled. In this respect, they are similar
to fixed background images. For paged media, boxes with fixed
positions are repeated on every page. This is useful for placing, for
instance, a signature at the bottom of each page.
Authors may use fixed positioning to create frame-like presentations.
Consider the following frame layout:
Image illustrating a frame-like layout with position='fixed'.
This might be achieved with the following HTML document and style
rules:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>A frame document with CSS 2.1</TITLE>
<STYLE type="text/css">
BODY { height: 8.5in } /* Required for percentage heights below */
#header {
position: fixed;
width: 100%;
height: 15%;
top: 0;
right: 0;
bottom: auto;
left: 0;
}
#sidebar {
position: fixed;
width: 10em;
height: auto;
top: 15%;
right: auto;
bottom: 100px;
left: 0;
}
#main {
position: fixed;
width: auto;
height: auto;
top: 15%;
right: 0;
bottom: 100px;
left: 10em;
}
#footer {
position: fixed;
width: 100%;
height: 100px;
top: auto;
right: 0;
bottom: 0;
left: 0;
}
</STYLE>
</HEAD>
<BODY>
<DIV id="header"> ... </DIV>
<DIV id="sidebar"> ... </DIV>
<DIV id="main"> ... </DIV>
<DIV id="footer"> ... </DIV>
</BODY>
</HTML>
9.7 Relationships between 'display', 'position', and 'float'
The three properties that affect box generation and layout --
'display', 'position', and 'float' -- interact as follows:
1. If 'display' has the value 'none', then 'position' and 'float' do
not apply. In this case, the element generates no box.
2. Otherwise, if 'position' has the value 'absolute' or 'fixed', the
box is absolutely positioned, the computed value of 'float' is
'none', and display is set according to this table:
Specified value Computed value
inline-table table
inline, run-in, table-row-group, table-column, table-column-group,
table-header-group, table-footer-group, table-row, table-cell,
table-caption, inline-block block
others same as specified
The position of the box will be determined by the 'top', 'right',
'bottom' and 'left' properties and the box's containing block.
3. Otherwise, if 'float' has a value other than 'none', the box is
floated and 'display' is set according to this table:
Specified value Computed value
inline-table table
inline, run-in, table-row-group, table-column, table-column-group,
table-header-group, table-footer-group, table-row, table-cell,
table-caption, inline-block block
others same as specified
4. Otherwise, the remaining 'display' property values apply as
specified.
9.8 Comparison of normal flow, floats, and absolute positioning
To illustrate the differences between normal flow, relative
positioning, floats, and absolute positioning, we provide a series of
examples based on the following HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Comparison of positioning schemes</TITLE>
</HEAD>
<BODY>
<P>Beginning of body contents.
<SPAN id="outer"> Start of outer contents.
<SPAN id="inner"> Inner contents.</SPAN>
End of outer contents.</SPAN>
End of body contents.
</P>
</BODY>
</HTML>
In this document, we assume the following rules:
body { display: block; font-size:12px; line-height: 200%;
width: 400px; height: 400px }
p { display: block }
span { display: inline }
The final positions of boxes generated by the outer and inner elements
vary in each example. In each illustration, the numbers to the left of
the illustration indicate the normal flow position of the
double-spaced (for clarity) lines.
Note. The diagrams in this section are illustrative and not to scale.
They are meant to highlight the differences between the various
positioning schemes in CSS 2.1, and are not intended to be reference
renderings of the examples given.
9.8.1 Normal flow
Consider the following CSS declarations for outer and inner that don't
alter the normal flow of boxes:
#outer { color: red }
#inner { color: blue }
The P element contains all inline content: anonymous inline text and
two SPAN elements. Therefore, all of the content will be laid out in
an inline formatting context, within a containing block established by
the P element, producing something like:
Image illustrating the normal flow of text between parent and sibling
boxes.
9.8.2 Relative positioning
To see the effect of relative positioning, we specify:
#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }
Text flows normally up to the outer element. The outer text is then
flowed into its normal flow position and dimensions at the end of line
1. Then, the inline boxes containing the text (distributed over three
lines) are shifted as a unit by '-12px' (upwards).
The contents of inner, as a child of outer, would normally flow
immediately after the words "of outer contents" (on line 1.5).
However, the inner contents are themselves offset relative to the
outer contents by '12px' (downwards), back to their original position
on line 2.
Note that the content following outer is not affected by the relative
positioning of outer.
Image illustrating the effects of relative positioning on a box's
content.
Note also that had the offset of outer been '-24px', the text of outer
and the body text would have overlapped.
9.8.3 Floating a box
Now consider the effect of floating the inner element's text to the
right by means of the following rules:
#outer { color: red }
#inner { float: right; width: 130px; color: blue }
Text flows normally up to the inner box, which is pulled out of the
flow and floated to the right margin (its 'width' has been assigned
explicitly). Line boxes to the left of the float are shortened, and
the document's remaining text flows into them.
Image illustrating the effects of floating a box.
To show the effect of the 'clear' property, we add a sibling element
to the example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Comparison of positioning schemes II</TITLE>
</HEAD>
<BODY>
<P>Beginning of body contents.
<SPAN id=outer> Start of outer contents.
<SPAN id=inner> Inner contents.</SPAN>
<SPAN id=sibling> Sibling contents.</SPAN>
End of outer contents.</SPAN>
End of body contents.
</P>
</BODY>
</HTML>
The following rules:
#inner { float: right; width: 130px; color: blue }
#sibling { color: red }
cause the inner box to float to the right as before and the document's
remaining text to flow into the vacated space:
Image illustrating the effects of floating a box without setting the
clear property to control the flow of text around the box.
However, if the 'clear' property on the sibling element is set to
'right' (i.e., the generated sibling box will not accept a position
next to floating boxes to its right), the sibling content begins to
flow below the float:
#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }
Image illustrating the effects of floating an element with setting the
clear property to control the flow of text around the element.
9.8.4 Absolute positioning
Finally, we consider the effect of absolute positioning. Consider the
following CSS declarations for outer and inner:
#outer {
position: absolute;
top: 200px; left: 200px;
width: 200px;
color: red;
}
#inner { color: blue }
which cause the top of the outer box to be positioned with respect to
its containing block. The containing block for a positioned box is
established by the nearest positioned ancestor (or, if none exists,
the initial containing block, as in our example). The top side of the
outer box is '200px' below the top of the containing block and the
left side is '200px' from the left side. The child box of outer is
flowed normally with respect to its parent.
Image illustrating the effects of absolutely positioning a box.
The following example shows an absolutely positioned box that is a
child of a relatively positioned box. Although the parent outer box is
not actually offset, setting its 'position' property to 'relative'
means that its box may serve as the containing block for positioned
descendants. Since the outer box is an inline box that is split across
several lines, the first inline box's top and left edges (depicted by
thick dashed lines in the illustration below) serve as references for
'top' and 'left' offsets.
#outer {
position: relative;
color: red
}
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
This results in something like the following:
Image illustrating the effects of absolutely positioning a box with
respect to a containing block.
If we do not position the outer box:
#outer { color: red }
#inner {
position: absolute;
top: 200px; left: -100px;
height: 130px; width: 130px;
color: blue;
}
the containing block for inner becomes the initial containing block
(in our example). The following illustration shows where the inner box
would end up in this case.
Image illustrating the effects of absolutely positioning a box with
respect to a containing block established by a normally positioned
parent.
Relative and absolute positioning may be used to implement change
bars, as shown in the following example. The following fragment:
<P style="position: relative; margin-right: 10px; left: 10px;">
I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
<SPAN style="position: absolute; top: auto; left: -1em; color: red;">--</SPAN>
word.</P>
might result in something like:
Image illustrating the use of floats to create a changebar effect.
First, the paragraph (whose containing block sides are shown in the
illustration) is flowed normally. Then it is offset '10px' from the
left edge of the containing block (thus, a right margin of '10px' has
been reserved in anticipation of the offset). The two hyphens acting
as change bars are taken out of the flow and positioned at the current
line (due to 'top: auto'), '-1em' from the left edge of its containing
block (established by the P in its final position). The result is that
the change bars seem to "float" to the left of the current line.
9.9 Layered presentation
9.9.1 Specifying the stack level: the 'z-index' property
'z-index'
Value: auto | <integer> | inherit
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
For a positioned box, the 'z-index' property specifies:
1. The stack level of the box in the current stacking context.
2. Whether the box establishes a local stacking context.
Values have the following meanings:
<integer>
This integer is the stack level of the generated box in the
current stacking context. The box also establishes a local
stacking context in which its stack level is '0'.
auto
The stack level of the generated box in the current stacking
context is the same as its parent's box. The box does not
establish a new local stacking context.
In this section, the expression "in front of" means closer to the user
as the user faces the screen.
In CSS 2.1, each box has a position in three dimensions. In addition
to their horizontal and vertical positions, boxes lie along a "z-axis"
and are formatted one on top of the other. Z-axis positions are
particularly relevant when boxes overlap visually. This section
discusses how boxes may be positioned along the z-axis.
Each box belongs to one stacking context. Each box in a given stacking
context has an integer stack level, which is its position on the
z-axis relative to other boxes in the same stacking context. Boxes
with greater stack levels are always formatted in front of boxes with
lower stack levels. Boxes may have negative stack levels. Boxes with
the same stack level in a stacking context are stacked bottom-to-top
according to document tree order.
The root element creates a root stacking context, but other elements
may establish local stacking contexts. Stacking contexts are
inherited. A local stacking context is atomic; boxes in other stacking
contexts may not come between any of its boxes.
An element that establishes a local stacking context generates a box
that has two stack levels: one for the stacking context it creates
(always '0') and one for the stacking context to which it belongs
(given by the 'z-index' property).
An element's box has the same stack level as its parent's box unless
given a different stack level with the 'z-index' property.
In the following example, the stack levels of the boxes (named with
their "id" attributes) are: "text2"=0, "image"=1, "text3"=2, and
"text1"=3. The "text2" stack level is inherited from the root box. The
others are specified with the 'z-index' property.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Z-order positioning</TITLE>
<STYLE type="text/css">
.pile {
position: absolute;
left: 2in;
top: 2in;
width: 3in;
height: 3in;
}
</STYLE>
</HEAD>
<BODY>
<P>
<IMG id="image" class="pile"
src="butterfly.png" alt="A butterfly image"
style="z-index: 1">
<DIV id="text1" class="pile"
style="z-index: 3">
This text will overlay the butterfly image.
</DIV>
<DIV id="text2">
This text will be beneath everything.
</DIV>
<DIV id="text3" class="pile"
style="z-index: 2">
This text will underlay text1, but overlay the butterfly image
</DIV>
</BODY>
</HTML>
This example demonstrates the notion of transparency. The default
behavior of a box is to allow boxes behind it to be visible through
transparent areas in its content. In the example, each box
transparently overlays the boxes below it. This behavior can be
overridden by using one of the existing background properties.
9.10 Text direction: the 'direction' and 'unicode-bidi' properties
Conforming user agents that do not support bidirectional text may
ignore the 'direction' and 'unicode-bidi' properties described in this
section.
The characters in certain scripts are written from right to left. In
some documents, in particular those written with the Arabic or Hebrew
script, and in some mixed-language contexts, text in a single
(visually displayed) block may appear with mixed directionality. This
phenomenon is called bidirectionality, or "bidi" for short.
The Unicode standard ([UNICODE], section 3.11) defines a complex
algorithm for determining the proper directionality of text. The
algorithm consists of an implicit part based on character properties,
as well as explicit controls for embeddings and overrides. CSS 2.1
relies on this algorithm to achieve proper bidirectional rendering.
The 'direction' and 'unicode-bidi' properties allow authors to specify
how the elements and attributes of a document language map to this
algorithm.
If a document contains right-to-left characters, and if the user agent
displays these characters in right-to-left order, the user agent must
apply the bidirectional algorithm. (UAs that render right-to-left
characters simply because a font on the system contains them but do
not support the concept of right-to-left text direction are exempt
from this requirement.) This seemingly one-sided requirement reflects
the fact that, although not every Hebrew or Arabic document contains
mixed-directionality text, such documents are much more likely to
contain left-to-right text (e.g., numbers, text from other languages)
than are documents written in left-to-right languages.
Because the directionality of a text depends on the structure and
semantics of the document language, these properties should in most
cases be used only by designers of document type descriptions (DTDs),
or authors of special documents. If a default style sheet specifies
these properties, authors and users should not specify rules to
override them.
The HTML 4.0 specification ([HTML40], section 8.2) defines
bidirectionality behavior for HTML elements. The style sheet rules
that would achieve the bidi behavior specified in [HTML40] are given
in the sample style sheet. The HTML 4.0 specification also contains
more information on bidirectionality issues.
'direction'
Value: ltr | rtl | inherit
Initial: ltr
Applies to: all elements, but see prose
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the base writing direction of blocks and the
direction of embeddings and overrides (see 'unicode-bidi') for the
Unicode bidirectional algorithm. In addition, it specifies the
direction of table column layout, the direction of horizontal
overflow, and the position of an incomplete last line in a block in
case of 'text-align: justify'.
Values for this property have the following meanings:
ltr
Left-to-right direction.
rtl
Right-to-left direction.
For the 'direction' property to have any effect on inline-level
elements, the 'unicode-bidi' property's value must be 'embed' or
'override'.
Note. The 'direction' property, when specified for table column
elements, is not inherited by cells in the column since columns are
not the ancestors of the cells in the document tree. Thus, CSS cannot
easily capture the "dir" attribute inheritance rules described in
[HTML40], section 11.3.2.1.
'unicode-bidi'
Value: normal | embed | bidi-override | inherit
Initial: normal
Applies to: all elements, but see prose
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Values for this property have the following meanings:
normal
The element does not open an additional level of embedding with
respect to the bidirectional algorithm. For inline-level
elements, implicit reordering works across element boundaries.
embed
If the element is inline-level, this value opens an additional
level of embedding with respect to the bidirectional algorithm.
The direction of this embedding level is given by the
'direction' property. Inside the element, reordering is done
implicitly. This corresponds to adding a LRE (U+202A; for
'direction: ltr') or RLE (U+202B; for 'direction: rtl') at the
start of the element and a PDF (U+202C) at the end of the
element.
bidi-override
If the element is inline-level or a block-level element that
contains only inline-level elements, this creates an override.
This means that inside the element, reordering is strictly in
sequence according to the 'direction' property; the implicit
part of the bidirectional algorithm is ignored. This
corresponds to adding a LRO (U+202D; for 'direction: ltr') or
RLO (U+202E; for 'direction: rtl') at the start of the element
and a PDF (U+202C) at the end of the element.
The final order of characters in each block-level element is the same
as if the bidi control codes had been added as described above, markup
had been stripped, and the resulting character sequence had been
passed to an implementation of the Unicode bidirectional algorithm for
plain text that produced the same line-breaks as the styled text. In
this process, non-textual entities such as images are treated as
neutral characters, unless their 'unicode-bidi' property has a value
other than 'normal', in which case they are treated as strong
characters in the 'direction' specified for the element.
Please note that in order to be able to flow inline boxes in a uniform
direction (either entirely left-to-right or entirely right-to-left),
more inline boxes (including anonymous inline boxes) may have to be
created, and some inline boxes may have to be split up and reordered
before flowing.
Because the Unicode algorithm has a limit of 61 levels of embedding,
care should be taken not to use 'unicode-bidi' with a value other than
'normal' unless appropriate. In particular, a value of 'inherit'
should be used with extreme caution. However, for elements that are,
in general, intended to be displayed as blocks, a setting of
'unicode-bidi: embed' is preferred to keep the element together in
case display is changed to inline (see example below).
The following example shows an XML document with bidirectional text.
It illustrates an important design principle: DTD designers should
take bidi into account both in the language proper (elements and
attributes) and in any accompanying style sheets. The style sheets
should be designed so that bidi rules are separate from other style
rules. The bidi rules should not be overridden by other style sheets
so that the document language's or DTD's bidi behavior is preserved.
Example(s):
In this example, lowercase letters stand for inherently left-to-right
characters and uppercase letters represent inherently right-to-left
characters:
<HEBREW>
<PAR>HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5</PAR>
<PAR>HEBREW6 <EMPH>HEBREW7</EMPH> HEBREW8</PAR>
</HEBREW>
<ENGLISH>
<PAR>english9 english10 english11 HEBREW12 HEBREW13</PAR>
<PAR>english14 english15 english16</PAR>
<PAR>english17 <HE-QUO>HEBREW18 english19 HEBREW20</HE-QUO></PAR>
</ENGLISH>
Since this is XML, the style sheet is responsible for setting the
writing direction. This is the style sheet:
/* Rules for bidi */
HEBREW, HE-QUO {direction: rtl; unicode-bidi: embed}
ENGLISH {direction: ltr; unicode-bidi: embed}
/* Rules for presentation */
HEBREW, ENGLISH, PAR {display: block}
EMPH {font-weight: bold}
The HEBREW element is a block with a right-to-left base direction, the
ENGLISH element is a block with a left-to-right base direction. The
PARs are blocks that inherit the base direction from their parents.
Thus, the first two PARs are read starting at the top right, the final
three are read starting at the top left. Please note that HEBREW and
ENGLISH are chosen as element names for explicitness only; in general,
element names should convey structure without reference to language.
The EMPH element is inline-level, and since its value for
'unicode-bidi' is 'normal' (the initial value), it has no effect on
the ordering of the text. The HE-QUO element, on the other hand,
creates an embedding.
The formatting of this text might look like this if the line length is
long:
5WERBEH 4WERBEH english3 2WERBEH 1WERBEH
8WERBEH 7WERBEH 6WERBEH
english9 english10 english11 13WERBEH 12WERBEH
english14 english15 english16
english17 20WERBEH english19 18WERBEH
Note that the HE-QUO embedding causes HEBREW18 to be to the right of
english19.
If lines have to be broken, it might be more like this:
2WERBEH 1WERBEH
-EH 4WERBEH english3
5WERB
-EH 7WERBEH 6WERBEH
8WERB
english9 english10 en-
glish11 12WERBEH
13WERBEH
english14 english15
english16
english17 18WERBEH
20WERBEH english19
Because HEBREW18 must be read before english19, it is on the line
above english19. Just breaking the long line from the earlier
formatting would not have worked. Note also that the first syllable
from english19 might have fit on the previous line, but hyphenation of
left-to-right words in a right-to-left context, and vice versa, is
usually suppressed to avoid having to display a hyphen in the middle
of a line.
_________________________________________________________________
_________________________________________________________________
10 Visual formatting model details
Contents
* 10.1 Definition of "containing block"
* 10.2 Content width: the 'width' property
* 10.3 Computing widths and margins
+ 10.3.1 Inline, non-replaced elements
+ 10.3.2 Inline, replaced elements
+ 10.3.3 Block-level, non-replaced elements in normal flow
+ 10.3.4 Block-level, replaced elements in normal flow
+ 10.3.5 Floating, non-replaced elements
+ 10.3.6 Floating, replaced elements
+ 10.3.7 Absolutely positioned, non-replaced elements
+ 10.3.8 Absolutely positioned, replaced elements
+ 10.3.9 'Inline-block', non-replaced elements in normal flow
+ 10.3.10 'Inline-block', replaced elements in normal flow
* 10.4 Minimum and maximum widths: 'min-width' and 'max-width'
* 10.5 Content height: the 'height' property
* 10.6 Computing heights and margins
+ 10.6.1 Inline, non-replaced elements
+ 10.6.2 Inline replaced elements, block-level replaced
elements in normal flow, 'inline-block' replaced elements in
normal flow and floating replaced elements
+ 10.6.3 Block-level and 'inline-block', non-replaced elements
in normal flow and floating, non-replaced elements
+ 10.6.4 Absolutely positioned, non-replaced elements
+ 10.6.5 Absolutely positioned, replaced elements
* 10.7 Minimum and maximum heights: 'min-height' and 'max-height'
* 10.8 Line height calculations: the 'line-height' and
'vertical-align' properties
+ 10.8.1 Leading and half-leading
10.1 Definition of "containing block"
The position and size of an element's box(es) are sometimes computed
relative to a certain rectangle, called the containing block of the
element. The containing block of an element is defined as follows:
1. The containing block in which the root element lives is chosen by
the user agent. (It could be related to the viewport.) This
containing block is called the initial containing block.
2. For other elements, if the element's position is 'relative' or
'static', the containing block is formed by the content edge of
the nearest block-level, table cell or inline-block ancestor box.
3. If the element has 'position: fixed', the containing block is
established by the viewport.
4. If the element has 'position: absolute', the containing block is
established by the nearest ancestor with a 'position' of
'absolute', 'relative' or 'fixed', in the following way:
1. In the case that the ancestor is block-level, the containing
block is formed by the padding edge of the ancestor.
2. In the case that the ancestor is inline-level, the containing
block depends on the 'direction' property of the ancestor:
1. If the 'direction' is 'ltr', the top and left of the
containing block are the top and left content edges of
the first box generated by the ancestor, and the bottom
and right are the bottom and right content edges of the
last box of the ancestor.
2. If the 'direction' is 'rtl', the top and right are the
top and right edges of the first box generated by the
ancestor, and the bottom and left are the bottom and
left content edges of the last box of the ancestor.
If there is no such ancestor, the containing block is the initial
containing block.
Example(s):
With no positioning, the containing blocks (C.B.) in the following
document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Illustration of containing blocks</TITLE>
</HEAD>
<BODY id="body">
<DIV id="div1">
<P id="p1">This is text in the first paragraph...</P>
<P id="p2">This is text <EM id="em1"> in the
<STRONG id="strong1">second</STRONG> paragraph.</EM></P>
</DIV>
</BODY>
</HTML>
are established as follows:
For box generated by C.B. is established by
html initial C.B. (UA-dependent)
body html
div1 body
p1 div1
p2 div1
em1 p2
strong1 p2
If we position "div1":
#div1 { position: absolute; left: 50px; top: 50px }
its containing block is no longer "body"; it becomes the initial
containing block (since there are no other positioned ancestor boxes).
If we position "em1" as well:
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
the table of containing blocks becomes:
For box generated by C.B. is established by
html initial C.B. (UA-dependent)
body html
div1 initial C.B.
p1 div1
p2 div1
em1 div1
strong1 em1
By positioning "em1", its containing block becomes the nearest
positioned ancestor box (i.e., that generated by "div1").
10.2 Content width: the 'width' property
'width'
Value: <length> | <percentage> | auto | inherit
Initial: auto
Applies to: all elements but non-replaced inline elements, table
rows, and row groups
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length; 'auto' if the property does not
apply
This property specifies the content width of boxes generated by
block-level and replaced elements.
This property does not apply to non-replaced inline-level elements.
The content width of a non-replaced inline element's boxes is that of
the rendered content within them (before any relative offset of
children). Recall that inline boxes flow into line boxes. The width of
line boxes is given by the their containing block, but may be shorted
by the presence of floats.
The width of a replaced element's box is intrinsic and may be scaled
by the user agent if the value of this property is different than
'auto'.
Values have the following meanings:
<length>
Specifies the width of the content area using a length unit.
<percentage>
Specifies a percentage width. The percentage is calculated with
respect to the width of the generated box's containing block.
auto
The width depends on the values of other properties. See the
sections below.
Negative values for 'width' are illegal.
Example(s):
For example, the following rule fixes the content width of paragraphs
at 100 pixels:
p { width: 100px }
10.3 Computing widths and margins
The computed values of an element's 'width', 'margin-left',
'margin-right', 'left' and 'right' properties depend on the type of
box generated and on each other. In principle, the computed values are
the same as the specified values, with 'auto' replaced by some
suitable value, but there are exceptions. The following situations
need to be distinguished:
1. inline, non-replaced elements
2. inline, replaced elements
3. block-level, non-replaced elements in normal flow
4. block-level, replaced elements in normal flow
5. floating, non-replaced elements
6. floating, replaced elements
7. absolutely positioned, non-replaced elements
8. absolutely positioned, replaced elements
9. 'inline-block', non-replaced elements in normal flow
10. 'inline-block', replaced elements in normal flow
Points 1-6 and 9-10 include relative positioning.
10.3.1 Inline, non-replaced elements
The 'width' property does not apply. A specified value of 'auto' for
'left', 'right', 'margin-left' or 'margin-right' becomes a computed
value of '0'.
10.3.2 Inline, replaced elements
A specified value of 'auto' for 'left', 'right', 'margin-left' or
'margin-right' becomes a computed value of '0'. If 'width' has a
specified value of 'auto' and 'height' also has a specified value of
'auto', the element's intrinsic width is the computed value of
'width'. If 'width' has a specified value of 'auto' and 'height' has
some other specified value, then the computed value of 'width' is:
(intrinsic width) * ( (computed height) / (intrinsic height) )
10.3.3 Block-level, non-replaced elements in normal flow
If 'left' or 'right' are given as 'auto', their computed value is 0.
The following constraints must hold between the other properties:
'margin-left' + 'border-left-width' + 'padding-left' + 'width' +
'padding-right' + 'border-right-width' + 'margin-right' = width of
containing block
(If the border style is 'none', use '0' as the border width.) If all
of the above have a specified value other than 'auto', the values are
said to be "over-constrained" and one of the computed values will have
to be different from its specified value. If the 'direction' property
has the value 'ltr', the specified value of 'margin-right' is ignored
and the value is computed so as to make the equality true. If the
value of 'direction' is 'rtl', this happens to 'margin-left' instead.
If there is exactly one value specified as 'auto', its computed value
follows from the equality.
If 'width' is set to 'auto', any other 'auto' values become '0' and
'width' follows from the resulting equality.
If both 'margin-left' and 'margin-right' are 'auto', their computed
values are equal. This horizontally centers the element with respect
to the edges of the containing block.
10.3.4 Block-level, replaced elements in normal flow
If 'left' or 'right' are 'auto', their computed value is 0. The
computed value of 'width' is determined as for inline replaced
elements. If one of the margins is 'auto', its computed value is given
by the constraints above. Furthermore, if both margins are 'auto',
their computed values are equal.
10.3.5 Floating, non-replaced elements
If 'left', 'right', 'margin-left', or 'margin-right' are specified as
'auto', their computed value is '0'.
If 'width' is specified as 'auto', the computed value is the
"shrink-to-fit" width.
Calculation of the shrink-to-fit width is similar to computing the
width of a table cell using the automatic table layout algorithm.
Roughly: calculate the preferred width by formatting the content
without breaking lines other than where explicit line breaks occur,
and also calculate the preferred minimum width, e.g., by trying all
possible line breaks. CSS 2.1 does not define the exact algorithm.
Thirdly, compute the available width: in this case, this is the width
of the containing block minus 'left', 'right', 'margin-left' and
'margin-right'. (Omit 'left' and 'right' if they do not apply to this
element.)
Then the shrink-to-fit width is: min(max(preferred minimum width,
available width), preferred width).
10.3.6 Floating, replaced elements
If 'left', 'right', 'margin-left' or 'margin-right' are specified as
'auto', their computed value is '0'. The computed value of 'width' is
determined as for inline replaced elements.
10.3.7 Absolutely positioned, non-replaced elements
For the purposes of this section and the next, the term "static
position" (of an element) refers, roughly, to the position an element
would have had in the normal flow. More precisely:
* The static position for 'left' is the distance from the left edge
of the containing block to the left margin edge of a hypothetical
box that would have been the first box of the element if its
'position' property had been 'static'. The value is negative if
the hypothetical box is to the left of the containing block.
* The static position for 'right' is the distance from the right
edge of the containing block to the right margin edge of the same
hypothetical box as above. The value is positive if the
hypothetical box is to the left of the containing block's edge.
But rather than actually computing that hypothetical box, user agents
are free to make a guess at its probable position.
The constraint that determines the computed values for these elements
is:
'left' + 'margin-left' + 'border-left-width' + 'padding-left' +
'width' + 'padding-right' + 'border-right-width' + 'margin-right' +
'right' = width of containing block
If all three of 'left', 'width', and 'right' are 'auto': if
'direction' is 'ltr' set 'left' to the static position and apply rule
number three below; otherwise, set 'right' to the static position and
apply rule number one below.
If none of the three is 'auto': If both 'margin-left' and
'margin-right' are 'auto', solve the equation under the extra
constraint that the two margins get equal values. If one of
'margin-left' or 'margin-right' is 'auto', solve the equation for that
value. If the values are over-constrained, ignore the value for 'left'
(in case 'direction' is 'rtl') or 'right' (in case 'direction' is
'ltr') and solve for that value.
Otherwise, set 'auto' values for 'margin-left' and 'margin-right' to
0, and pick the one of the following six rules that applies.
1. 'left' and 'width' are 'auto' and 'right' is not 'auto', then the
width is shrink-to-fit. Then solve for 'left'
2. 'left' and 'right' are 'auto' and 'width' is not 'auto', then if
'direction' is 'ltr' set 'left' to the static position, otherwise
set 'right' to the static position. Then solve for 'left' (if
'direction is 'rtl') or 'right' (if 'direction' is 'ltr').
3. 'width' and 'right' are 'auto' and 'left' is not 'auto', then the
width is shrink-to-fit . Then solve for 'right'
4. 'left' is 'auto', 'width' and 'right' are not 'auto', then solve
for 'left'
5. 'width' is 'auto', 'left' and 'right' are not 'auto', then solve
for 'width'
6. 'right' is 'auto', 'left' and 'width' are not 'auto', then solve
for 'right'
Calculation of the shrink-to-fit width is similar to computing the
width of a table cell using the automatic table layout algorithm.
Roughly: calculate the preferred width by formatting the content
without breaking lines other than where explicit line breaks occur,
and also calculate the preferred minimum width, e.g., by trying all
possible line breaks. CSS 2.1 does not define the exact algorithm.
Thirdly, compute the available width: this is computed by solving for
'width' after setting 'left' (in case 1) or 'right (in case 3) to 0.
Then the shrink-to-fit width is: min(max(preferred minimum width,
available width), preferred width).
10.3.8 Absolutely positioned, replaced elements
This situation is similar to the previous one, except that the element
has an intrinsic width. The sequence of substitutions is now:
1. The computed value of 'width' is determined as for inline replaced
elements.
2. If 'left' has the value 'auto' while 'direction' is 'ltr', replace
'auto' with the static position.
3. If 'right' has the value 'auto' while 'direction' is 'rtl',
replace 'auto' with the static position.
4. If 'left' or 'right' are 'auto', replace any 'auto' on
'margin-left' or 'margin-right' with '0'.
5. If at this point both 'margin-left' and 'margin-right' are still
'auto', solve the equation under the extra constraint that the two
margins must get equal values.
6. If at this point there is only one 'auto' left, solve the equation
for that value.
7. If at this point the values are over-constrained, ignore the value
for either 'left' (in case 'direction' is 'rtl') or 'right' (in
case 'direction' is 'ltr') and solve for that value.
10.3.9 'Inline-block', non-replaced elements in normal flow
If 'width' is 'auto', the computed value is the shrink-to-fit width as
for floating elements.
A specified value of 'auto' for 'left', 'right', 'margin-left' or
'margin-right' becomes a computed value of '0'.
10.3.10 'Inline-block', replaced elements in normal flow
Exactly as inline replaced elements.
10.4 Minimum and maximum widths: 'min-width' and 'max-width'
'min-width'
Value: <length> | <percentage> | inherit
Initial: 0
Applies to: all elements except non-replaced inline elements and
table elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length
'max-width'
Value: <length> | <percentage> | none | inherit
Initial: none
Applies to: all elements except non-replaced inline elements and
table elements
Inherited: no
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length or 'none'
These two properties allow authors to constrain box widths to a
certain range. Values have the following meanings:
<length>
Specifies a fixed minimum or maximum computed width.
<percentage>
Specifies a percentage for determining the computed value. The
percentage is calculated with respect to the width of the
generated box's containing block.
none
(Only on 'max-width') No limit on the width of the box.
The following algorithm describes how the two properties influence the
computed value of the 'width' property:
1. The width is computed (without 'min-width' and 'max-width')
following the rules under "Computing widths and margins" above.
2. If the computed value of 'min-width' is greater than the value of
'max-width', 'max-width' is set to the value of 'min-width'.
3. If the computed width is greater than 'max-width', the rules above
are applied again, but this time using the value of 'max-width' as
the specified value for 'width'.
4. If the computed width is smaller than 'min-width', the rules above
are applied again, but this time using the value of 'min-width' as
the specified value for 'width'.
The user agent may define a non-negative minimum value for the
'min-width' property, which may vary from element to element and even
depend on other properties. If 'min-width' goes below this limit,
either because it was set explicitly, or because it was 'auto' and the
rules below would make it too small, the user agent may use the
minimum value as the computed value.
10.5 Content height: the 'height' property
'height'
Value: <length> | <percentage> | auto | inherit
Initial: auto
Applies to: all elements but non-replaced inline elements, table
columns, and column groups
Inherited: no
Percentages: see prose
Media: visual
Computed value: absolute length; 'auto' if the property does not
apply
This property specifies the content height of boxes generated by
block-level, inline-block and replaced elements.
This property does not apply to non-replaced inline-level elements.
The height of a non-replaced inline element's boxes is given by the
element's (possibly inherited) 'line-height' value.
Values have the following meanings:
<length>
Specifies the height of the content area using a length value.
<percentage>
Specifies a percentage height. The percentage is calculated
with respect to the height of the generated box's containing
block. If the height of the containing block is not specified
explicitly (i.e., it depends on content height), and this
element is not positioned, the value is interpreted like
'auto'.
auto
The height depends on the values of other properties. See the
prose below.
A UA may compute a percentage height on the root element relative to
the viewport.
Negative values for 'height' are illegal.
Example(s):
For example, the following rule sets the content height of paragraphs
to 100 pixels:
p { height: 100px }
Paragraphs of which the height of the contents exceeds 100 pixels will
overflow according to the 'overflow' property.
10.6 Computing heights and margins
For computing the values of 'top', 'margin-top', 'height',
'margin-bottom', and 'bottom' a distinction must be made between
various kinds of boxes:
1. inline, non-replaced elements
2. inline, replaced elements
3. block-level, non-replaced elements in normal flow
4. block-level, replaced elements in normal flow
5. floating, non-replaced elements
6. floating, replaced elements
7. absolutely positioned, non-replaced elements
8. absolutely positioned, replaced elements
9. 'inline-block', non-replaced elements in normal flow
10. 'inline-block', replaced elements in normal flow
Points 1-6 and 9-10 include relative positioning.
10.6.1 Inline, non-replaced elements
If 'top' or 'bottom' are 'auto', their computed value is 0.
The 'height' property doesn't apply. The height of the content area
should be based on the font, but this specification does not specify
how. A UA may, e.g., use the em-box or the maximum ascender and
descender of the font. (The latter would ensure that glyphs with parts
above or below the em-box still fall within the content area, but
leads to differently sized boxes for different fonts.)
Note: level 3 of CSS will probably include a property to select which
measure of the font is used for the content height.
The vertical padding, border and margin of an inline, non-replaced box
start at the top and bottom of the content area, not the
'line-height'. But only the 'line-height' is used to compute the
height of the line box.
If more than one font is used (this could happen when glyphs are found
in different fonts), the height of the content area is not defined by
this specification. However, we suggest that the height is chosen such
that the content area is just high enough for either (1) the em-boxes
or (2) the maximum ascenders and descenders of all the fonts in the
element. Note that this may be larger than any of the font sizes
involved, depending on the baseline alignment of the fonts.
10.6.2 Inline replaced elements, block-level replaced elements in normal
flow, 'inline-block' replaced elements in normal flow and floating replaced
elements
If 'top', 'bottom', 'margin-top', or 'margin-bottom' are 'auto', their
computed value is 0. If 'height' has a specified value of 'auto' and
'width' also has a specified value of 'auto', the element's intrinsic
height is the computed value of 'height'. If 'height' has a specified
value of 'auto' and 'width' has some other specified value, then the
computed value of 'height' is:
(intrinsic height) * ( (computed width) / (intrinsic width) )
10.6.3 Block-level and 'inline-block', non-replaced elements in normal flow
and floating, non-replaced elements
If 'top', 'bottom', 'margin-top', or 'margin-bottom' are 'auto', their
computed value is 0. If 'height' is 'auto', the height depends on
whether the element has any block-level children and whether it has
padding or borders.
If it only has inline-level children, the height is the distance
between the top of the topmost line box and the bottom of the
bottommost line box.
If it has block-level children, the height is the distance between the
top border-edge of the topmost block-level child box and the bottom
border-edge of the bottommost block-level child box. However, if the
element has a non-zero top padding and/or top border, then the content
starts at the top margin edge of the topmost child. (The first case
expresses the fact that the top and bottom margins of the element
collapse with those of the topmost and bottommost children, while in
the second case the presence of the padding/border prevents the top
margins from collapsing.) Similarly, if the element has a non-zero
bottom padding and/or bottom border, then the content ends at the
bottom margin edge of the bottommost child.
Only children in the normal flow are taken into account (i.e.,
floating boxes and absolutely positioned boxes are ignored, and
relatively positioned boxes are considered without their offset). Note
that the child box may be an anonymous block box.
10.6.4 Absolutely positioned, non-replaced elements
For the purposes of this section and the next, the term "static
position" (of an element) refers, roughly, to the position an element
would have had in the normal flow. More precisely, the static position
for 'top' is the distance from the top edge of the containing block to
the top margin edge of a hypothetical box that would have been the
first box of the element if its 'position' property had been 'static'.
The value is negative if the hypothetical box is above the containing
block.
But rather than actually computing that hypothetical box, user agents
are free to make a guess at its probable position.
For absolutely positioned elements, the vertical dimensions must
satisfy this constraint:
'top' + 'margin-top' + 'border-top-width' + 'padding-top' +
'height' + 'padding-bottom' + 'border-bottom-width' +
'margin-bottom' + 'bottom' = height of containing block
If all three of 'top', 'height', and 'bottom' are auto, set 'top' to
the static position and apply rule number three below.
If none of the three are 'auto': If both 'margin-top' and
'margin-bottom' are 'auto', solve the equation under the extra
constraint that the two margins get equal values. If one of
'margin-top' or 'margin-bottom' is 'auto', solve the equation for that
value. If the values are over-constrained, ignore the value for
'bottom' and solve for that value.
Otherwise, pick the one of the following six rules that applies.
1. 'top' and 'height' are 'auto' and 'bottom' is not 'auto', then the
height is based on the content, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'top'
2. 'top' and 'bottom' are 'auto' and 'height' is not 'auto', then set
'top' to the static position, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'bottom'
3. 'height' and 'bottom' are 'auto' and 'top' is not 'auto', then the
height is based on the content, set 'auto' values for 'margin-top'
and 'margin-bottom' to 0, and solve for 'bottom'
4. 'top' is 'auto', 'height' and 'bottom' are not 'auto', then set
'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve
for 'top'
5. 'height' is 'auto', 'top' and 'bottom' are not 'auto', then 'auto'
values for 'margin-top' and 'margin-bottom' are set to 0 and solve
for 'height'
6. 'bottom' is 'auto', 'top' and 'height' are not 'auto', then set
'auto' values for 'margin-top' and 'margin-bottom' to 0 and solve
for 'bottom'
10.6.5 Absolutely positioned, replaced elements
This situation is similar to the previous one, except that the element
has an intrinsic height. The sequence of substitutions is now:
1. The computed value of 'height' is determined as for inline
replaced elements.
2. If 'top' has the value 'auto', replace it with the element's
static position.
3. If 'bottom' is 'auto', replace any 'auto' on 'margin-top' or
'margin-bottom' with '0'.
4. If at this point both 'margin-top' and 'margin-bottom' are still
'auto', solve the equation under the extra constraint that the two
margins must get equal values.
5. If at this point there is only one 'auto' left, solve the equation
for that value.
6. If at this point the values are over-constrained, ignore the value
for 'bottom' and solve for that value.
10.7 Minimum and maximum heights: 'min-height' and 'max-height'
It is sometimes useful to constrain the height of elements to a
certain range. Two properties offer this functionality:
'min-height'
Value: <length> | <percentage> | inherit
Initial: 0
Applies to: all elements except non-replaced inline elements and
table elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: absolute length
'max-height'
Value: <length> | <percentage> | none | inherit
Initial: none
Applies to: all elements except non-replaced inline elements and
table elements
Inherited: no
Percentages: refer to height of containing block
Media: visual
Computed value: absolute length or 'none'
These two properties allow authors to constrain box heights to a
certain range. Values have the following meanings:
<length>
Specifies a fixed minimum or maximum computed height.
<percentage>
Specifies a percentage for determining the computed value. The
percentage is calculated with respect to the height of the
generated box's containing block. If the height of the
containing block is not specified explicitly (i.e., it depends
on content height), the percentage value computes to '0' (for
'min-height') or 'none' (for 'max-height').
none
(Only on 'max-height') No limit on the height of the box.
The following algorithm describes how the two properties influence the
computed value of the 'height' property:
1. The height is computed (without 'min-height' and 'max-height')
following the rules under "Computing heights and margins" above.
2. If the computed value of 'min-height' is greater than the value of
'max-height', 'max-height' is set to the value of 'min-height'.
3. If the computed height is greater than 'max-height', the rules
above are applied again, but this time using the value of
'max-height' as the specified value for 'height'.
4. If the computed height is smaller than 'min-height', the rules
above are applied again, but this time using the value of
'min-height' as the specified value for 'height'.
10.8 Line height calculations: the 'line-height' and 'vertical-align'
properties
As described in the section on inline formatting contexts, user agents
flow inline boxes into a vertical stack of line boxes. The height of a
line box is determined as follows:
1. The height of each inline box in the line box is calculated (see
"Computing heights and margins" and the 'line-height' property).
2. The inline boxes are aligned vertically according to their
'vertical-align' property.
3. The line box height is the distance between the uppermost box top
and the lowermost box bottom.
Empty inline elements generate empty inline boxes, but these boxes
still have margins, padding, borders and a line height, and thus
influence these calculations just like elements with content.
10.8.1 Leading and half-leading
Since the value of 'line-height' may be different from the height of
the content area there may be space above and below rendered glyphs.
The difference between the content height and the computed value of
'line-height' is called the leading. Half the leading is called the
half-leading.
User agents center glyphs vertically in an inline box, adding
half-leading on the top and bottom. For example, if a piece of text is
'12pt' high and the 'line-height' value is '14pt', 2pts of extra space
should be added: 1pt above and 1pt below the letters. (This applies to
empty boxes as well, as if the empty box contained an infinitely
narrow letter.)
When the 'line-height' value is less than the content height, the
final inline box height will be less than the font size and the
rendered glyphs will "bleed" outside the box. If such a box touches
the edge of a line box, the rendered glyphs will also "bleed" into the
adjacent line box.
Although margins, borders, and padding of non-replaced elements do not
enter into the line box calculation, they are still rendered around
inline boxes. This means that if the height specified by 'line-height'
is less than the content height of contained boxes, backgrounds and
colors of padding and borders may "bleed" into adjacent line boxes.
[DEL: However, in this case, some user agents may use the line box to
"clip" the border and padding areas (i.e., not render them). :DEL]
[INS: User agents should render the boxes in document order. This will
cause the borders on subsequent lines to paint over the borders and
text of previous lines. :INS]
'line-height'
Value: normal | <number> | <length> | <percentage> | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: refer to the font size of the element itself
Media: visual
Computed value: for <length> and <percentage> the absolute value;
otherwise as specified
If the property is set on a block-level element whose content is
composed of inline-level elements, it specifies the minimal height of
line boxes within the element. The minimum height consist of a minimum
height above the block's baseline and a minimum depth below it,
exactly as if each line box starts with a zero-width inline box with
the block's font and line height properties (what T[E]X calls a
"strut").
If the property is set on an inline-level element, it specifies the
height that is used in the calculation of the line box height (except
for inline replaced elements, where the height of the box is given by
the 'height' property).
Values for this property have the following meanings:
normal
Tells user agents to set the computed value to a "reasonable"
value based on the font of the element. The value has the same
meaning as <number>. We recommend a computed value for 'normal'
between 1.0 to 1.2.
<length>
The specified length is used in the calculation of the line box
height. Negative values are illegal.
<number>
The computed value of the property is this number multiplied by
the element's font size. Negative values are illegal. However,
the number, not the computed value, is inherited.
<percentage>
The computed value of the property is this percentage
multiplied by the element's computed font size. Negative values
are illegal.
Example(s):
The three rules in the example below have the same resultant line
height:
div { line-height: 1.2; font-size: 10pt } /* number */
div { line-height: 1.2em; font-size: 10pt } /* length */
div { line-height: 120%; font-size: 10pt } /* percentage */
[DEL: When an element contains text that is rendered in more than one
font, user agents should determine the 'line-height' value according
to the largest font size. :DEL]
Generally, when there is only one value of 'line-height' for all
inline boxes in a paragraph (and no tall images), the above will
ensure that baselines of successive lines are exactly 'line-height'
apart. This is important when columns of text in different fonts have
to be aligned, for example in a table.
'vertical-align'
Value: baseline | sub | super | top | text-top | middle | bottom |
text-bottom | <percentage> | <length> | inherit
Initial: baseline
Applies to: inline-level and 'table-cell' elements
Inherited: no
Percentages: refer to the 'line-height' of the element itself
Media: visual
Computed value: for <percentage> and <length> the absolute length,
otherwise as specified
This property affects the vertical positioning inside a line box of
the boxes generated by an inline-level element. The following values
only have meaning with respect to a parent inline-level element, or to
a parent block-level element.
Note. Values of this property have slightly different meanings in the
context of tables. Please consult the section on table height
algorithms for details.
baseline
Align the baseline of the box with the baseline of the parent
box. If the box doesn't have a baseline, align the bottom
margin edge with the parent's baseline.
middle
Align the vertical midpoint of the box with the baseline of the
parent box plus half the x-height of the parent.
sub
Lower the baseline of the box to the proper position for
subscripts of the parent's box. (This value has no effect on
the font size of the element's text.)
super
Raise the baseline of the box to the proper position for
superscripts of the parent's box. (This value has no effect on
the font size of the element's text.)
text-top
Align the top of the box with the top of the parent element's
font.
text-bottom
Align the bottom of the box with the bottom of the parent
element's font.
<percentage>
Raise (positive value) or lower (negative value) the box by
this distance (a percentage of the 'line-height' value). The
value '0%' means the same as 'baseline'.
<length>
Raise (positive value) or lower (negative value) the box by
this distance. The value '0cm' means the same as 'baseline'.
top
Align the top of the box with the top of the line box.
bottom
Align the bottom of the box with the bottom of the line box.
_________________________________________________________________
_________________________________________________________________
11 Visual effects
Contents
* 11.1 Overflow and clipping
+ 11.1.1 Overflow: the 'overflow' property
+ 11.1.2 Clipping: the 'clip' property
* 11.2 Visibility: the 'visibility' property
11.1 Overflow and clipping
Generally, the content of a block box is confined to the content edges
of the box. In certain cases, a box may overflow, meaning its content
lies partly or entirely outside of the box, e.g.:
* A line cannot be broken, causing the line box to be wider than the
block box.
* A block-level box is too wide for the containing block. This may
happen when an element's 'width' property has a value that causes
the generated block box to spill over sides of the containing
block.
* An element's height exceeds an explicit height assigned to the
containing block (i.e., the containing block's height is
determined by the 'height' property, not by content height).
* A descendent box is positioned absolutely, partly outside the box.
* A descendent box has negative margins, causing it to be positioned
partly outside the box.
Whenever overflow occurs, the 'overflow' property specifies whether a
box is clipped to its content box, and if so, whether a scrolling
mechanism is provided to access any clipped out content.
11.1.1 Overflow: the 'overflow' property
'overflow'
Value: visible | hidden | scroll | auto | inherit
Initial: visible
Applies to: block-level and replaced elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies whether the content of a block-level element
is clipped when it overflows the element's box (which is acting as a
containing block for the content). Values have the following meanings:
visible
This value indicates that content is not clipped, i.e., it may
be rendered outside the block box.
hidden
This value indicates that the content is clipped and that no
scrolling mechanism should be provided to view the content
outside the clipping region; users will not have access to
clipped content.
scroll
This value indicates that the content is clipped and that if
the user agent uses a scrolling mechanism that is visible on
the screen (such as a scroll bar or a panner), that mechanism
should be displayed for a box whether or not any of its content
is clipped. This avoids any problem with scrollbars appearing
and disappearing in a dynamic environment. When this value is
specified and the target medium is 'print', overflowing content
should may be printed.
auto
The behavior of the 'auto' value is user agent-dependent, but
should cause a scrolling mechanism to be provided for
overflowing boxes.
Even if 'overflow' is set to 'visible', content may be clipped to a
UA's document window by the native operating environment.
[INS: :INS]
HTML UAs may apply the overflow property from the BODY or HTML
elements to the viewport.
Example(s):
Consider the following example of a block quotation (BLOCKQUOTE) that
is too big for its containing block (established by a DIV). Here is
the source document:
<DIV>
<BLOCKQUOTE>
<P>I didn't like the play, but then I saw
it under adverse conditions - the curtain was up.
<CITE>- Groucho Marx</CITE>
</BLOCKQUOTE>
</DIV>
Here is the style sheet controlling the sizes and style of the
generated boxes:
div { width : 100px; height: 100px;
border: thin solid red;
}
blockquote { width : 125px; height : 100px;
margin-top: 50px; margin-left: 50px;
border: thin dashed black
}
cite { display: block;
text-align : right;
border: none
}
The initial value of 'overflow' is 'visible', so the BLOCKQUOTE would
be formatted without clipping, something like this:
Rendered overflow
Setting 'overflow' to 'hidden' for the DIV element, on the other hand,
causes the BLOCKQUOTE to be clipped by the containing block:
Clipped overflow
A value of 'scroll' would tell UAs that support a visible scrolling
mechanism to display one so that users could access the clipped
content.
11.1.2 Clipping: the 'clip' property
A clipping region defines what portion of an element's border box is
visible. By default, the clipping region has the same size and shape
as the element's border box. However, the clipping region may be
modified by the 'clip' property.
'clip'
Value: <shape> | auto | inherit
Initial: auto
Applies to: absolutely positioned elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: For rectangle values, a rectangle consisting of four
computed lengths; otherwise, as specified
The 'clip' property applies only to absolutely positioned elements.
Values have the following meanings:
auto
The clipping region has the same size and location as the
element's box(es).
<shape>
In CSS 2.1, the only valid <shape> value is: rect (<top>,
<right>, <bottom>, <left>) where <top> and <bottom> specify
offsets from the top border edge of the box, and <right>, and
<left> specify offsets from the left border edge of the box in
left-to-right text and from the right border edge of the box in
right-to-left text. Authors should separate offset values with
commas. User agents must support separation with commas, but
may also support separation without commas, because a previous
version of this specification was ambiguous in this respect.
<top>, <right>, <bottom>, and <left> may either have a <length>
value or 'auto'. Negative lengths are permitted. The value
'auto' means that a given edge of the clipping region will be
the same as the edge of the element's generated box (i.e.,
'auto' means the same as '0' for <top> and <left> (in
left-to-right text, <right> in right-to-left text), the same as
the computed value of the height plus the sum of vertical
padding and border widths for <bottom>, and the same as the
computed value of the width plus the sum of the horizontal
padding and border widths for <right> (in left-to-right text,
<left> in right-to-left text), such that four 'auto' values
result in the clipping region being fit with the border edge).
When coordinates are rounded to pixel coordinates, care should
be taken that no pixels remain visible when <left> and <right>
have the same value (or <top> and <bottom> have the same
value), and conversely that no pixels within the element's box
remain hidden when these values are 'auto'.
The element's ancestors may also have clipping regions (e.g. if their
'overflow' property is not 'visible'); what is rendered is the
intersection of the various clipping regions.
If the clipping region exceeds the bounds of the UA's document window,
content may be clipped to that window by the native operating
environment.
Example(s):
The following two rules:
p { clip: rect(5px, 40px, 45px, 5px); }
p { clip: rect(5px, 55px, 45px, 5px); }
will create the rectangular clipping regions delimited by the dashed
lines in the following illustrations:
Two clipping regions
Note. In CSS 2.1, all clipping regions are rectangular. We anticipate
future extensions to permit non-rectangular clipping. Future versions
may also reintroduce a syntax for offsetting shapes from each edge
instead of offsetting from a point.
11.2 Visibility: the 'visibility' property
'visibility'
Value: visible | hidden | collapse | inherit
Initial: visble
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The 'visibility' property specifies whether the boxes generated by an
element are rendered. Invisible boxes still affect layout (set the
'display' property to 'none' to suppress box generation altogether).
Values have the following meanings:
visible
The generated box is visible.
hidden
The generated box is invisible (fully transparent), but still
affects layout. Furthermore, descendents of the element will be
visible if they have 'visibility: visible'.
collapse
Please consult the section on dynamic row and column effects in
tables. If used on elements other than rows or columns,
'collapse' has the same meaning as 'hidden'.
This property may be used in conjunction with scripts to create
dynamic effects.
In the following example, pressing either form button invokes a
user-defined script function that causes the corresponding box to
become visible and the other to be hidden. Since these boxes have the
same size and position, the effect is that one replaces the other.
(The script code is in a hypothetical script language. It may or may
not have any effect in a CSS-capable UA.)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<STYLE type="text/css">
<!--
#container1 { position: absolute;
top: 2in; left: 2in; width: 2in }
#container2 { position: absolute;
top: 2in; left: 2in; width: 2in;
visibility: hidden; }
-->
</STYLE>
</HEAD>
<BODY>
<P>Choose a suspect:</P>
<DIV id="container1">
<IMG alt="Al Capone"
width="100" height="100"
src="suspect1.jpg">
<P>Name: Al Capone</P>
<P>Residence: Chicago</P>
</DIV>
<DIV id="container2">
<IMG alt="Lucky Luciano"
width="100" height="100"
src="suspect2.jpg">
<P>Name: Lucky Luciano</P>
<P>Residence: New York</P>
</DIV>
<FORM method="post"
action="http://www.suspect.org/process-bums">
<P>
<INPUT name="Capone" type="button"
value="Capone"
onclick='show("container1");hide("container2")'>
<INPUT name="Luciano" type="button"
value="Luciano"
onclick='show("container2");hide("container1")'>
</FORM>
</BODY>
</HTML>
_________________________________________________________________
_________________________________________________________________
12 Generated content, automatic numbering, and lists
Contents
* 12.1 The :before and :after pseudo-elements
* 12.2 The 'content' property
* 12.3 Interaction of :before and :after with 'run-in' elements
* 12.4 Quotation marks
+ 12.4.1 Specifying quotes with the 'quotes' property
+ 12.4.2 Inserting quotes with the 'content' property
* 12.5 Automatic counters and numbering
+ 12.5.1 Nested counters and scope
+ 12.5.2 Counter styles
+ 12.5.3 Counters in elements with 'display: none'
* 12.6 Lists
+ 12.6.1 Lists: the 'list-style-type', 'list-style-image',
'list-style-position', and 'list-style' properties
In some cases, authors may want user agents to render content that
does not come from the document tree. One familiar example of this is
a numbered list; the author does not want to list the numbers
explicitly, he or she wants the user agent to generate them
automatically. Similarly, authors may want the user agent to insert
the word "Figure" before the caption of a figure, or "Chapter 7"
before the seventh chapter title. For audio or braille in particular,
user agents should be able to insert these strings.
In CSS 2.1, content may be generated by two mechanisms:
* The 'content' property, in conjunction with the :before and :after
pseudo-elements.
* Elements with a value of 'list-item' for the 'display' property.
12.1 The :before and :after pseudo-elements
Authors specify the style and location of generated content with the
:before and :after pseudo-elements. As their names indicate, the
:before and :after pseudo-elements specify the location of content
before and after an element's document tree content. The 'content'
property, in conjunction with these pseudo-elements, specifies what is
inserted.
Example(s):
For example, the following rule inserts the string "Note: " before the
content of every P element whose "class" attribute has the value
"note":
p.note:before { content: "Note: " }
The formatting objects (e.g., boxes) generated by an element include
generated content. So, for example, changing the above style sheet to:
p.note:before { content: "Note: " }
p.note { border: solid green }
would cause a solid green border to be rendered around the entire
paragraph, including the initial string.
The :before and :after pseudo-elements inherit any inheritable
properties from the element in the document tree to which they are
attached.
Example(s):
For example, the following rules insert an open quote mark before
every Q element. The color of the quote mark will be red, but the font
will be the same as the font of the rest of the Q element:
q:before {
content: open-quote;
color: red
}
In a :before or :after pseudo-element declaration, non-inherited
properties take their initial values.
Example(s):
So, for example, because the initial value of the 'display' property
is 'inline', the quote in the previous example is inserted as an
inline box (i.e., on the same line as the element's initial text
content). The next example explicitly sets the 'display' property to
'block', so that the inserted text becomes a block:
body:after {
content: "The End";
display: block;
margin-top: 2em;
text-align: center;
}
User agents must ignore the following properties with :before and
:after pseudo-elements: 'position', 'float', list properties, and
table properties.
The :before and :after pseudo-elements elements allow values of the
'display' property as follows:
* If the subject of the selector is a block-level element, allowed
values are 'none', 'inline', 'block', and 'marker'. If the value
of the pseudo-element's 'display' property has any other value,
the pseudo-element will behave as if its value were 'block'.
* If the subject of the selector is an inline-level element, allowed
values are 'none' and 'inline'. If the value of the
pseudo-element's 'display' property has any other value, the
pseudo-element will behave as if its value were 'inline'.
12.2 The 'content' property
'content'
Value: [ <string> | <counter> | attr(<identifier>) | open-quote |
close-quote | no-open-quote | no-close-quote ]+ | inherit
Initial: empty string
Applies to: :before and :after pseudo-elements
Inherited: no
Percentages: N/A
Media: all
Computed value: for URI values, the absolute URI; for attr() values,
the resulting string; otherwise as specified
This property is used with the :before and :after pseudo-elements to
generate content in a document. Values have the following meanings:
normal
On the :before and :after pseudo-elements, this value is the
same as 'none'.
none
No content is generated.
<string>
Text content (see the section on strings).
<uri>
The value is a URI that designates an external resource. If a
user agent cannot support the resource because of the media
types it supports, it must ignore the resource.
<counter>
Counters may be specified with two different functions:
'counter()' or 'counters()'. The former has two forms:
'counter(name)' or 'counter(name, style)'. The generated text
is the value of the named counter at this point in the
formatting structure; it is formatted in the indicated style
('decimal' by default). The latter function also has two forms:
'counters(name, string)' or 'counters(name, string, style)'.
The generated text is the value of all counters with the given
name at this point in the formatting structure, separated by
the specified string. The counters are rendered in the
indicated style ('decimal' by default). See the section on
automatic counters and numbering for more information.
open-quote and close-quote
These values are replaced by the appropriate string from the
'quotes' property.
no-open-quote and no-close-quote
Same as 'none', but increments (decrements) the level of
nesting for quotes.
attr(X)
This function returns as a string the value of attribute X for
the subject of the selector. The string is not parsed by the
CSS processor. If the subject of the selector doesn't have an
attribute X, an empty string is returned. The case-sensitivity
of attribute names depends on the document language. Note. In
CSS 2.1, it is not possible to refer to attribute values for
other elements than the subject of the selector.
The 'display' property controls whether the content is placed in a
block, inline, or marker box.
Example(s):
The following rule causes the string "Chapter: " to be generated
before each H1 element:
H1:before {
content: "Chapter: ";
display: inline;
}
Authors may include newlines in the generated content by writing the
"\A" escape sequence in one of the strings after the 'content'
property. This inserts a forced line break, similar to the BR element
in HTML. See "Strings" and "Characters and case" for more information
on the "\A" escape sequence.
Example(s):
h1:before {
display: block;
text-align: center;
content: "chapter\A hoofdstuk\A chapitre"
}
Generated content does not alter the document tree. In particular, it
is not fed back to the document language processor (e.g., for
reparsing).
12.3 Interaction of :before and :after with 'run-in' elements
The following cases can occur:
1. A 'run-in' element has a :before pseudo-element of type 'inline':
the pseudo-element is rendered inside the same block box as the
element.
2. A 'run-in' element has an :after pseudo-element of type 'inline':
The rules of the previous point apply.
3. A 'run-in' element has a :before pseudo-element of type 'block':
the pseudo-element is formatted as a block above the element.
4. A 'run-in' element has an :after pseudo-element of type 'block':
both the element and its :after pseudo-element are formatted as
block boxes. The element is not formatted as an inline box in its
own :after pseudo-element.
5. The element following a 'run-in' element has a :before of type
'block': the decision how to format the 'run-in' element is made
with respect to the block box resulting from the :before
pseudo-element.
6. The element following a 'run-in' element has an :before of type
'inline': the decision how to format the 'run-in' element depends
on the 'display' value of the element to which the :before is
attached.
Example(s):
Here is an example of a 'run-in' header with an :after pseudo-element,
followed by a paragraph with a :before pseudo-element. All
pseudo-elements are inline (the default) in this example. When the
style sheet:
h3 { display: run-in }
h3:after { content: ": " }
p:before { content: "... " }
is applied to this source document:
<h3>Centaurs</h3>
<p>have hoofs
<p>have a tail
The visual formatting will resemble:
Centaurs: ... have hoofs
... have a tail
12.4 Quotation marks
In CSS 2.1, authors may specify, in a style-sensitive and
context-dependent manner, how user agents should render quotation
marks. The 'quotes' property specifies pairs of quotation marks for
each level of embedded quotation. The 'content' property gives access
to those quotation marks and causes them to be inserted before and
after a quotation.
12.4.1 Specifying quotes with the 'quotes' property
'quotes'
Value: [<string> <string>]+ | none | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies quotation marks for any number of embedded
quotations. Values have the following meanings:
none
The 'open-quote' and 'close-quote' values of the 'content'
property produce no quotation marks.
[<string> <string>]+
Values for the 'open-quote' and 'close-quote' values of the
'content' property are taken from this list of pairs of
quotation marks (opening and closing). The first (leftmost)
pair represents the outermost level of quotation, the second
pair the first level of embedding, etc. The user agent must
apply the appropriate pair of quotation marks according to the
level of embedding.
Example(s):
For example, applying the following style sheet:
/* Specify pairs of quotes for two levels in two languages */
q:lang(en) { quotes: '"' '"' "'" "'" }
q:lang(no) { quotes: "«" "»" '"' '"' }
/* Insert quotes before and after Q element content */
q:before { content: open-quote }
q:after { content: close-quote }
to the following HTML fragment:
<HTML lang="en">
<HEAD>
<TITLE>Quotes</TITLE>
</HEAD>
<BODY>
<P><Q>Quote me!</Q>
</BODY>
</HTML>
would allow a user agent to produce:
"Quote me!"
while this HTML fragment:
<HTML lang="no">
<HEAD>
<TITLE>Quotes</TITLE>
</HEAD>
<BODY>
<P><Q>Trøndere gråter når <Q>Vinsjan på kaia</Q> blir deklamert.</Q>
</BODY>
</HTML>
would produce:
«Trøndere gråter når "Vinsjan på kaia" blir deklamert.»
Note. While the quotation marks specified by 'quotes' in the previous
examples are conveniently located on computer keyboards, high quality
typesetting would require different ISO 10646 characters. The
following informative table lists some of the ISO 10646 quotation mark
characters:
Character Approximate rendering ISO 10646 code (hex) Description
" " 0022 QUOTATION MARK [the ASCII double quotation mark]
' ' 0027 APOSTROPHE [the ASCII single quotation mark]
< < 2039 SINGLE LEFT-POINTING ANGLE QUOTATION MARK
> > 203A SINGLE RIGHT-POINTING ANGLE QUOTATION MARK
« « 00AB LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
» » 00BB RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
` ` 2018 LEFT SINGLE QUOTATION MARK [single high-6]
' ' 2019 RIGHT SINGLE QUOTATION MARK [single high-9]
" `` 201C LEFT DOUBLE QUOTATION MARK [double high-6]
" '' 201D RIGHT DOUBLE QUOTATION MARK [double high-9]
" ,, 201E DOUBLE LOW-9 QUOTATION MARK [double low-9]
12.4.2 Inserting quotes with the 'content' property
Quotation marks are inserted in appropriate places in a document with
the 'open-quote' and 'close-quote' values of the 'content' property.
Each occurrence of 'open-quote' or 'close-quote' is replaced by one of
the strings from the value of 'quotes', based on the depth of nesting.
'Open-quote' refers to the first of a pair of quotes, 'close-quote'
refers to the second. Which pair of quotes is used depends on the
nesting level of quotes: the number of occurrences of 'open-quote' in
all generated text before the current occurrence, minus the number of
occurrences of 'close-quote'. If the depth is 0, the first pair is
used, if the depth is 1, the second pair is used, etc. If the depth is
greater than the number of pairs, the last pair is repeated. A
'close-quote' that would make the depth negative is in error and is
ignored (at rendering time): the depth stays at 0 and no quote mark is
rendered (although the rest of the 'content' property's value is still
inserted).
Note. The quoting depth is independent of the nesting of the source
document or the formatting structure.
Some typographic styles require open quotation marks to be repeated
before every paragraph of a quote spanning several paragraphs, but
only the last paragraph ends with a closing quotation mark. In CSS,
this can be achieved by inserting "phantom" closing quotes. The
keyword 'no-close-quote' decrements the quoting level, but does not
insert a quotation mark.
Example(s):
The following style sheet puts opening quotation marks on every
paragraph in a BLOCKQUOTE, and inserts a single closing quote at the
end:
blockquote p:before { content: open-quote }
blockquote p:after { content: no-close-quote }
blockquote p.last:after { content: close-quote }
This relies on the last paragraph being marked with a class "last".
For symmetry, there is also a 'no-open-quote' keyword, which inserts
nothing, but increments the quotation depth by one.
12.5 Automatic counters and numbering
Automatic numbering in CSS2 is controlled with two properties,
'counter-increment' and 'counter-reset'. The counters defined by these
properties are used with the counter() and counters() functions of the
the 'content' property.
'counter-reset'
Value: [ <identifier> <integer>? ]+ | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: as specified
'counter-increment'
Value: [ <identifier> <integer>? ]+ | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: all
Computed value: as specified
The 'counter-increment' property accepts one or more names of counters
(identifiers), each one optionally followed by an integer. The integer
indicates by how much the counter is incremented for every occurrence
of the element. The default increment is 1. Zero and negative integers
are allowed.
The 'counter-reset' property also contains a list of one or more names
of counters, each one optionally followed by an integer. The integer
gives the value that the counter is set to on each occurrence of the
element. The default is 0.
If 'counter-increment' refers to a counter that is not in the scope
(see below) of any 'counter-reset', the counter is assumed to have
been reset to 0 by the root element.
Example(s):
This example shows a way to number chapters and sections with "Chapter
1", "1.1", "1.2", etc.
H1:before {
content: "Chapter " counter(chapter) ". ";
counter-increment: chapter; /* Add 1 to chapter */
counter-reset: section; /* Set section to 0 */
}
H2:before {
content: counter(chapter) "." counter(section) " ";
counter-increment: section;
}
If an element increments/resets a counter and also uses it (in the
'content' property of its :before or :after pseudo-element), the
counter is used after being incremented/reset.
If an element both resets and increments a counter, the counter is
reset first and then incremented.
The 'counter-reset' property follows the cascading rules. Thus, due to
cascading, the following style sheet:
H1 { counter-reset: section -1 }
H1 { counter-reset: imagenum 99 }
will only reset 'imagenum'. To reset both counters, they have to be
specified together:
H1 { counter-reset: section -1 imagenum 99 }
12.5.1 Nested counters and scope
Counters are "self-nesting", in the sense that re-using a counter in a
child element automatically creates a new instance of the counter.
This is important for situations like lists in HTML, where elements
can be nested inside themselves to arbitrary depth. It would be
impossible to define uniquely named counters for each level.
Example(s):
Thus, the following suffices to number nested list items. The result
is very similar to that of setting 'display:list-item' and
'list-style: inside' on the LI element:
OL { counter-reset: item }
LI { display: block }
LI:before { content: counter(item) ". "; counter-increment: item }
The self-nesting is based on the principle that every element that has
a 'counter-reset' for a counter X, creates a fresh counter X, the
scope of which is the element, its preceding siblings, and all the
descendants of the element and its preceding siblings.
In the example above, an OL will create a counter, and all children of
the OL will refer to that counter.
If we denote by item[n] the nth instance of the "item" counter, and by
"(" and ")" the beginning and end of a scope, then the following HTML
fragment will use the indicated counters. (We assume the style sheet
as given in the example above).
<OL> <!-- (set item[0] to 0 -->
<LI>item <!-- increment item[0] (= 1) -->
<LI>item <!-- increment item[0] (= 2) -->
<OL> <!-- (set item[1] to 0 -->
<LI>item <!-- increment item[1] (= 1) -->
<LI>item <!-- increment item[1] (= 2) -->
<LI>item <!-- increment item[1] (= 3) -->
<OL> <!-- (set item[2] to 0 -->
<LI>item <!-- increment item[2] (= 1) -->
</OL> <!-- ) -->
<OL> <!-- (set item[3] to 0 -->
<LI> <!-- increment item[3] (= 1) -->
</OL> <!-- ) -->
<LI>item <!-- increment item[1] (= 4) -->
</OL> <!-- ) -->
<LI>item <!-- increment item[0] (= 3) -->
<LI>item <!-- increment item[0] (= 4) -->
</OL> <!-- ) -->
<OL> <!-- (reset item[4] to 0 -->
<LI>item <!-- increment item[4] (= 1) -->
<LI>item <!-- increment item[4] (= 2) -->
</OL> <!-- ) -->
The 'counters()' function generates a string composed of the values of
all counters with the same name, separated by a given string.
Example(s):
The following style sheet numbers nested list items as "1", "1.1",
"1.1.1", etc.
OL { counter-reset: item }
LI { display: block }
LI:before { content: counters(item, "."); counter-increment: item }
12.5.2 Counter styles
By default, counters are formatted with decimal numbers, but all the
styles available for the 'list-style-type' property are also available
for counters. The notation is:
counter(name)
for the default style, or:
counter(name, 'list-style-type')
All the styles are allowed, including 'disc', 'circle', 'square', and
'none'.
Example(s):
H1:before { content: counter(chno, upper-latin) ". " }
H2:before { content: counter(section, upper-roman) " - " }
BLOCKQUOTE:after { content: " [" counter(bq, hebrew) "]" }
DIV.note:before { content: counter(notecntr, disc) " " }
P:before { content: counter(p, none) }
12.5.3 Counters in elements with 'display: none'
An element that is not displayed ('display' set to 'none') cannot
increment or reset a counter.
Example(s):
For example, with the following style sheet, H2s with class "secret"
do not increment 'count2'.
H2.secret {counter-increment: count2; display: none}
Elements with 'visibility' set to 'hidden', on the other hand, do
increment counters.
12.6 Lists
CSS 2.1 offers basic visual formatting of lists. An element with
'display: list-item' generates a principal box for the element's
content and an optional marker box as a visual indication that the
element is a list item.
The list properties describe basic visual formatting of lists: they
allow style sheets to specify the marker type (image, glyph, or
number), and the marker position with respect to the principal box
(outside it or within it before content). They do not allow authors to
specify distinct style (colors, fonts, alignment, etc.) for the list
marker or adjust its position with respect to the principal box.
The background properties apply to the principal box only; an
'outside' marker box is transparent.
12.6.1 Lists: the 'list-style-type', 'list-style-image',
'list-style-position', and 'list-style' properties
'list-style-type'
Value: disc | circle | square | decimal | decimal-leading-zero |
lower-roman | upper-roman | lower-latin | upper-latin | none | inherit
Initial: disc
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies appearance of the list item marker if
'list-style-image' has the value 'none' or if the image pointed to by
the URI cannot be displayed. The value 'none' specifies no marker,
otherwise there are three types of marker: glyphs, numbering systems,
and alphabetic systems.
Glyphs are specified with disc, circle, and square. Their exact
rendering depends on the user agent.
Numbering systems are specified with:
decimal
Decimal numbers, beginning with 1.
decimal-leading-zero
Decimal numbers padded by initial zeros (e.g., 01, 02, 03, ...,
98, 99).
lower-roman
Lowercase roman numerals (i, ii, iii, iv, v, etc.).
upper-roman
Uppercase roman numerals (I, II, III, IV, V, etc.).
A user agent that does not recognize a numbering system should use
'decimal'.
Alphabetic systems are specified with:
lower-latin or lower-alpha
Lowercase ascii letters (a, b, c, ... z).
upper-latin or upper-alpha
Uppercase ascii letters (A, B, C, ... Z).
This specification does not define how alphabetic systems wrap at the
end of the alphabet. For instance, after 26 list items, 'lower-latin'
rendering is undefined. Therefore, for long lists, we recommend that
authors specify true numbers.
For example, the following HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Lowercase latin numbering</TITLE>
<STYLE type="text/css">
ol { list-style-type: lower-roman }
</STYLE>
</HEAD>
<BODY>
<OL>
<LI> This is the first item.
<LI> This is the second item.
<LI> This is the third item.
</OL>
</BODY>
</HTML>
might produce something like this:
i This is the first item.
ii This is the second item.
iii This is the third item.
The list marker alignment (here, right justified) depends on the user
agent.
'list-style-image'
Value: <uri> | none | inherit
Initial: none
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: absolute URI or 'none'
This property sets the image that will be used as the list item
marker. When the image is available, it will replace the marker set
with the 'list-style-type' marker.
Example(s):
The following example sets the marker at the beginning of each list
item to be the image "ellipse.png".
ul { list-style-image: url("http://png.com/ellipse.png") }
'list-style-position'
Value: inside | outside | inherit
Initial: outside
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the position of the marker box in the
principal block box. Values have the following meanings:
outside
The marker box is outside the principal block box. CSS 2.1 does
not specify the precise location of the marker box.
inside
The marker box is the first inline box in the principal block
box, after which the element's content flows.
For example:
<HTML>
<HEAD>
<TITLE>Comparison of inside/outside position</TITLE>
<STYLE type="text/css">
ul { list-style: outside }
ul.compact { list-style: inside }
</STYLE>
</HEAD>
<BODY>
<UL>
<LI>first list item comes first
<LI>second list item comes second
</UL>
<UL class="compact">
<LI>first list item comes first
<LI>second list item comes second
</UL>
</BODY>
</HTML>
The above example may be formatted as:
Difference between inside and outside list style position
In right-to-left text, the markers would have been on the right side
of the box.
'list-style'
Value: [ 'list-style-type' || 'list-style-position' ||
'list-style-image' ] | inherit
Initial: see individual properties
Applies to: elements with 'display: list-item'
Inherited: yes
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'list-style' property is a shorthand notation for setting the
three properties 'list-style-type', 'list-style-image', and
'list-style-position' at the same place in the style sheet.
Example(s):
ul { list-style: upper-roman inside } /* Any "ul" element */
ul > li > ul { list-style: circle outside } /* Any "ul" child
of an "li" child
of a "ul" element */
Although authors may specify 'list-style' information directly on list
item elements (e.g., "li" in HTML), they should do so with care. The
following rules look similar, but the first declares a descendant
selector and the second a (more specific) child selector.
ol.alpha li { list-style: lower-alpha } /* Any "li" descendant of an "ol" */
ol.alpha > li { list-style: lower-alpha } /* Any "li" child of an "ol" */
Authors who use only the descendant selector may not achieve the
results they expect. Consider the following rules:
<HTML>
<HEAD>
<TITLE>WARNING: Unexpected results due to cascade</TITLE>
<STYLE type="text/css">
ol.alpha li { list-style: lower-alpha }
ul li { list-style: disc }
</STYLE>
</HEAD>
<BODY>
<OL class="alpha">
<LI>level 1
<UL>
<LI>level 2
</UL>
</OL>
</BODY>
</HTML>
The desired rendering would have level 1 list items with 'lower-alpha'
labels and level 2 items with 'disc' labels. However, the cascading
order will cause the first style rule (which includes specific class
information) to mask the second. The following rules solve the problem
by employing a child selector instead:
ol.alpha > li { list-style: lower-alpha }
ul li { list-style: disc }
Another solution would be to specify 'list-style' information only on
the list type elements:
ol.alpha { list-style: lower-alpha }
ul { list-style: disc }
Inheritance will transfer the 'list-style' values from OL and UL
elements to LI elements. This is the recommended way to specify list
style information.
Example(s):
A URI value may be combined with any other value, as in:
ul { list-style: url("http://png.com/ellipse.png") disc }
In the example above, the 'disc' will be used when the image is
unavailable.
A value of 'none' for the 'list-style' property sets both
'list-style-type' and 'list-style-image' to 'none':
ul { list-style: none }
The result is that no list-item marker is displayed.
_________________________________________________________________
_________________________________________________________________
13 Paged media
Contents
* 13.1 Introduction to paged media
* 13.2 Page boxes: the @page rule
+ 13.2.1 Page margins
o Rendering page boxes that do not fit a target sheet
o Positioning the page box on the sheet
+ 13.2.2 Page selectors: selecting left, right, and first pages
+ 13.2.3 Content outside the page box
* 13.3 Page breaks
+ 13.3.1 Page break properties: 'page-break-before',
'page-break-after', 'page-break-inside'
+ 13.3.2 Breaks inside elements: 'orphans', 'widows'
+ 13.3.3 Allowed page breaks
+ 13.3.4 Forced page breaks
+ 13.3.5 "Best" page breaks
* 13.4 Cascading in the page context
13.1 Introduction to paged media
Paged media (e.g., paper, transparencies, pages that are displayed on
computer screens, etc.) differ from continuous media in that the
content of the document is split into one or more discrete pages. To
handle page breaks, CSS2 extends the visual formatting model as
follows:
1. The page box extends the box model to allow authors to specify
page margins.
2. The page model extends the visual formatting model to account for
page breaks.
The CSS 2.1 page model specifies how a document is formatted within
the page box. The page box does not necessarily correspond to the real
sheet where the document will ultimately be rendered (paper,
transparency, screen, etc.). The user agent is responsible for
transferring the page box to the sheet. Transfer possibilities
include:
* Transferring one page box to one sheet (e.g., single-sided
printing).
* Transferring two page boxes to both sides of the same sheet (e.g.,
double-sided printing).
* Transferring N (small) page boxes to one sheet (called "n-up").
* Transferring one (large) page box to N x M sheets (called
"tiling").
* Creating signatures. A signature is a group of pages printed on a
sheet, which, when folded and trimmed like a book, appear in their
proper sequence.
* Printing one document to several output trays.
* Outputting to a file.
13.2 Page boxes: the @page rule
The page box is a rectangular region that contains two areas:
* The page area. The page area includes the boxes laid out on that
page. The edges of the page area act as the initial containing
block for layout that occurs between page breaks.
* The margin area, which surrounds the page area.
Authors can specify the margins of a page box inside an @page rule. An
@page rule consists of the keyword "@page", followed by an optional
page selector, followed by a block of declarations. The declarations
are said to be in the page context.
The page selector specifies for which pages the declarations apply. In
CSS 2.1, page selectors may designate the first page, all left pages,
or all right pages
13.2.1 Page margins
The margin properties ('margin-top', 'margin-right', 'margin-bottom',
'margin-left', and 'margin') apply within the page context. The
following diagram shows the relationships between the sheet, page box,
and page margins:
Illustration of sheet, page box, margin, and page area. [D]
Example(s):
Here is a simple example which sets all page margins on all pages:
@page {
margin: 3cm;
}
The page context has no notion of fonts, so 'em' and 'ex' units are
not allowed. Percentage values on the margin properties are relative
to the dimensions of the page box; for left and right margins, they
refer to page box width while for top and bottom margins, they refer
to page box height. All other units associated with the respective
CSS 2.1 properties are allowed.
Due to negative margin values (either on the page box or on elements)
or absolute positioning content may end up outside the page box, but
this content may be "cut" -- by the user agent, the printer, or
ultimately, the paper cutter.
The computed value of box margins at the top or bottom of the page
area is zero.
Rendering page boxes that do not fit a target sheet
If a page box does not fit the target sheet dimensions, the user agent
may choose to:
* Rotate the page box 90° if this will make the page box fit.
* Scale the page to fit the target.
The user agent should consult the user before performing these
operations.
Positioning the page box on the sheet
When the page box is smaller than the target size, the user agent is
free to place the page box anywhere on the sheet. However, it is
recommended that the page box be centered on the sheet since this will
align double-sided pages and avoid accidental loss of information that
is printed near the edge of the sheet.
13.2.2 Page selectors: selecting left, right, and first pages
When printing double-sided documents, the page boxes on left and right
pages may be different. This can be expressed through two CSS
pseudo-classes that may be used in page selectors.
All pages are automatically classified by user agents into either the
:left or :right pseudo-class.
Example(s):
@page :left {
margin-left: 4cm;
margin-right: 3cm;
}
@page :right {
margin-left: 3cm;
margin-right: 4cm;
}
Authors may also specify style for the first page of a document with
the :first pseudo-class:
Example(s):
@page { margin: 2cm } /* All margins set to 2cm */
@page :first {
margin-top: 10cm /* Top margin on first page 10cm */
}
Properties specified in a :left or :right @page rule override those
specified in an @page rule that has no pseudo-class specified.
Properties specified in a :first @page rule override those specified
in :left or :right @page rules.
Margin declarations on left, right, and first pages may result in
different page area widths. To simplify implementations, user agents
may use a single page area width on left, right, and first pages. In
this case, the page area width of the first page should be used.
13.2.3 Content outside the page box
When formatting content in the page model, some content may end up
outside the page box. For example, an element whose 'white-space'
property has the value 'pre' may generate a box that is wider than the
page box. Also, when boxes are positioned absolutely, they may end up
in "inconvenient" locations. For example, images may be placed on the
edge of the page box or 100,000 meters below the page box.
The exact formatting of such elements lies outside the scope of this
specification. However, we recommend that authors and user agents
observe the following general principles concerning content outside
the page box:
* Content should be allowed slightly beyond the page box to allow
pages to "bleed".
* User agents should avoid generating a large number of empty page
boxes to honor the positioning of elements (e.g., you don't want
to print 100 blank pages).
* Authors should not position elements in inconvenient locations
just to avoid rendering them.
* User agents may handle boxes positioned outside the page box in
several ways, including discarding them or creating page boxes for
them at the end of the document.
13.3 Page breaks
This section describes page breaks in CSS 2.1. Five properties
indicate where the user agent may or should break pages, and on what
page (left or right) the subsequent content should resume. Each page
break ends layout in the current page box and causes remaining pieces
of the document tree to be laid out in a new page box.
13.3.1 Page break properties: 'page-break-before', 'page-break-after',
'page-break-inside'
'page-break-before'
Value: auto | always | avoid | left | right | inherit
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual, paged
Computed value: as specified
'page-break-after'
Value: auto | always | avoid | left | right | inherit
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual, paged
Computed value: as specified
'page-break-inside'
Value: avoid | auto | inherit
Initial: auto
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
Values for these properties have the following meanings:
auto
Neither force nor forbid a page break before (after, inside)
the generated box.
always
Always force a page break before (after) the generated box.
avoid
Avoid a page break before (after, inside) the generated box.
left
Force one or two page breaks before (after) the generated box
so that the next page is formatted as a left page.
right
Force one or two page breaks before (after) the generated box
so that the next page is formatted as a right page.
Whether the first page of a document is :left or :right depends on the
major writing direction of the document. A conforming user agent may
interpret the values 'left' and 'right' as 'always'.
A potential page break location is typically under the influence of
the parent element's 'page-break-inside' property, the
'page-break-after' property of the preceding element, and the
'page-break-before' property of the following element. When these
properties have values other than 'auto', the values 'always', 'left',
and 'right' take precedence over 'avoid'.
These properties only apply to non-floating block-level elements.
Also, page breaks cannot be forced to occur inside table cells,
absolutely positioned boxes, and fixed positioned boxes. Page breaks
set before, inside, or after such elements must be ignored.
13.3.2 Breaks inside elements: 'orphans', 'widows'
'orphans'
Value: <integer> | inherit
Initial: 2
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
'widows'
Value: <integer> | inherit
Initial: 2
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual, paged
Computed value: as specified
The 'orphans' property specifies the minimum number of lines of a
paragraph that must be left at the bottom of a page. The 'widows'
property specifies the minimum number of lines of a paragraph that
must be left at the top of a page. Examples of how they are used to
control page breaks are given below.
For information about paragraph formatting, please consult the section
on line boxes.
13.3.3 Allowed page breaks
In the normal flow, page breaks can occur at the following places:
1. In the vertical margin between block boxes. When a page break
occurs here, the computed values of the relevant 'margin-top' and
'margin-bottom' properties are set to '0'.
2. Between line boxes inside a block box.
These breaks are subject to the following rules:
* Rule A: Breaking at (1) is allowed only if the 'page-break-after'
and 'page-break-before' properties of all the elements generating
boxes that meet at this margin allow it, which is when at least
one of them has the value 'always', 'left', or 'right', or when
all of them are 'auto'.
* Rule B: However, if all of them are 'auto' and the nearest common
ancestor of all the elements has a 'page-break-inside' value of
'avoid', then breaking here is not allowed.
* Rule C: Breaking at (2) is allowed only if the number of line
boxes between the break and the start of the enclosing block box
is the value of 'orphans' or more, and the number of line boxes
between the break and the end of the box is the value of 'widows'
or more.
* Rule D: In addition, breaking at (2) is allowed only if the
'page-break-inside' property is 'auto'.
If the above doesn't provide enough break points to keep content from
overflowing the page boxes, then rules B and D are dropped in order to
find additional breakpoints.
If that still does not lead to sufficient break points, rules A and C
are dropped as well, to find still more break points.
13.3.4 Forced page breaks
A page break must occur at (1) if, among the 'page-break-after' and
'page-break-before' properties of all the elements generating boxes
that meet at this margin, there is at least one with the value
'always', 'left', or 'right'.
13.3.5 "Best" page breaks
CSS2 does not define which of a set of allowed page breaks must be
used; CSS2 does not forbid a user agent from breaking at every
possible break point, or not to break at all. But CSS2 does recommend
that user agents observe the following heuristics (while recognizing
that they are sometimes contradictory):
* Break as few times as possible.
* Make all pages that don't end with a forced break appear to have
about the same height.
* Avoid breaking inside a block that has a border.
* Avoid breaking inside a table.
* Avoid breaking inside a floated element
Example(s):
Suppose, for example, that the style sheet contains 'orphans: 4',
'widows: 2', and there are 20 lines (line boxes) available at the
bottom of the current page:
* If a paragraph at the end of the current page contains 20 lines or
fewer, it should be placed on the current page.
* If the paragraph contains 21 or 22 lines, the second part of the
paragraph must not violate the 'widows' constraint, and so the
second part must contain exactly two lines
* If the paragraph contains 23 lines or more, the first part should
contain 20 lines and the second part the remaining lines.
Now suppose that 'orphans' is '10', 'widows' is '20', and there are 8
lines available at the bottom of the current page:
* If a paragraph at the end of the current page contains 8 lines or
fewer, it should be placed on the current page.
* If the paragraph contains 9 lines or more, it cannot be split
(that would violate the orphan constraint), so it should move as a
block to the next page.
13.4 Cascading in the page context
Declarations in the page context obey the cascade just like normal
CSS2 declarations.
Example(s):
Consider the following example:
@page {
margin-left: 3cm;
}
@page :left {
margin-left: 4cm;
}
Due to the higher specificity of the pseudo-class selector, the left
margin on left pages will be '4cm' and all other pages (i.e., the
right pages) will have a left margin of '3cm'.
_________________________________________________________________
_________________________________________________________________
14 Colors and Backgrounds
Contents
* 14.1 Foreground color: the 'color' property
* 14.2 The background
+ 14.2.1 Background properties: 'background-color',
'background-image', 'background-repeat',
'background-attachment', 'background-position', and
'background'
* 14.3 Gamma correction
CSS properties allow authors to specify the foreground color and
background of an element. Backgrounds may be colors or images.
Background properties allow authors to position a background image,
repeat it, and declare whether it should be fixed with respect to the
viewport or scrolled along with the document.
See the section on color units for the syntax of valid color values.
14.1 Foreground color: the 'color' property
'color'
Value: <color> | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property describes the foreground color of an element's text
content. There are different ways to specify red:
Example(s):
em { color: red } /* predefined color name */
em { color: rgb(255,0,0) } /* RGB range 0-255 */
14.2 The background
Authors may specify the background of an element (i.e., its rendering
surface) as either a color or an image. In terms of the box model,
"background" refers to the background of the content, padding and
border areas. Border colors and styles are set with the border
properties. Margins are always transparent.
Background properties are not inherited, but the parent box's
background will shine through by default because of the initial
'transparent' value on 'background-color'.
The background of the box generated by the root element covers the
entire canvas.
For HTML documents, however, we recommend that authors specify the
background for the BODY element rather than the HTML element. User
agents should observe the following precedence rules to fill in the
background of the canvas: if the value of the 'background' property
for the HTML element is different from 'transparent' then use it, else
use the value of the 'background' property for the BODY element. If
the resulting value is 'transparent', the rendering is undefined.
(This does not apply to XHTML documents.)
According to these rules, the canvas underlying the following HTML
document will have a "marble" background:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Setting the canvas background</TITLE>
<STYLE type="text/css">
BODY { background: url("http://example.com/marble.png") }
</STYLE>
</HEAD>
<BODY>
<P>My background is marble.
</BODY>
</HTML>
14.2.1 Background properties: 'background-color', 'background-image',
'background-repeat', 'background-attachment', 'background-position', and
'background'
'background-color'
Value: <color> | transparent | inherit
Initial: transparent
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
This property sets the background color of an element, either a
<color> value or the keyword 'transparent', to make the underlying
colors shine through.
Example(s):
h1 { background-color: #F00 }
'background-image'
Value: <uri> | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: absolute URI
This property sets the background image of an element. When setting a
background image, authors should also specify a background color that
will be used when the image is unavailable. When the image is
available, it is rendered on top of the background color. (Thus, the
color is visible in the transparent parts of the image).
Values for this property are either <uri>, to specify the image, or
'none', when no image is used.
Example(s):
body { background-image: url("marble.png") }
p { background-image: none }
'background-repeat'
Value: repeat | repeat-x | repeat-y | no-repeat | inherit
Initial: repeat
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
If a background image is specified, this property specifies whether
the image is repeated (tiled), and how. All tiling covers the content,
padding and border areas of a box.
The tiling and positioning of the background-image on inline elements
is undefined in this specification. A future level of CSS may define
the tiling and positioning of the background-image on inline elements.
Values have the following meanings:
repeat
The image is repeated both horizontally and vertically.
repeat-x
The image is repeated horizontally only.
repeat-y
The image is repeated vertically only.
no-repeat
The image is not repeated: only one copy of the image is drawn.
Example(s):
body {
background: white url("pendant.png");
background-repeat: repeat-y;
background-position: center;
}
A centered background image, with copies repeated up and down the
padding and content areas.
One copy of the background image is centered, and other copies are put
above and below it to make a vertical band behind the element.
'background-attachment'
Value: scroll | fixed | inherit
Initial: scroll
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
If a background image is specified, this property specifies whether it
is fixed with regard to the viewport ('fixed') or scrolls along with
the containing block ('scroll').
Note that there is only one viewport per view. If an element has a
scrolling mechanism (see 'overflow'), a 'fixed' background doesn't
move with the element, and a 'scroll' background doesn't move with the
scrolling mechanism.
Even if the image is fixed, it is still only visible when it is in the
background, padding or border area of the element. Thus, unless the
image is tiled ('background-repeat: repeat'), it may be invisible.
Example(s):
This example creates an infinite vertical band that remains "glued" to
the viewport when the element is scrolled.
body {
background: red url("pendant.png");
background-repeat: repeat-y;
background-attachment: fixed;
}
User agents that do not support 'fixed' backgrounds (for example due
to limitations of the hardware platform) should ignore declarations
with the keyword 'fixed'. For example:
body {
background: white url(paper.png) scroll; /* for all UAs */
background: white url(ledger.png) fixed; /* for UAs that do fixed backgrounds
*/
}
See the section on conformance for details.
'background-position'
Value: [ [<percentage> | <length> | top | center | bottom] ||
[<percentage> | <length> | left | center | right] ] | inherit
Initial: 0% 0%
Applies to: block-level and replaced elements
Inherited: no
Percentages: refer to the size of the box itself
Media: visual
Computed value: for <length> the absolute value, otherwise a
percentage
If a background image has been specified, this property specifies its
initial position. Values have the following meanings:
<percentage> <percentage>
With a value pair of '0% 0%', the upper left corner of the
image is aligned with the upper left corner of the box's
padding edge. A value pair of '100% 100%' places the lower
right corner of the image in the lower right corner of padding
area. With a value pair of '14% 84%', the point 14% across and
84% down the image is to be placed at the point 14% across and
84% down the padding area.
<length> <length>
With a value pair of '2cm 1cm', the upper left corner of the
image is placed 2cm to the right and 1cm below the upper left
corner of the padding area.
top left and left top
Same as '0% 0%'.
top, top center, and center top
Same as '50% 0%'.
right top and top right
Same as '100% 0%'.
left, left center, and center left
Same as '0% 50%'.
center and center center
Same as '50% 50%'.
right, right center, and center right
Same as '100% 50%'.
bottom left and left bottom
Same as '0% 100%'.
bottom, bottom center, and center bottom
Same as '50% 100%'.
bottom right and right bottom
Same as '100% 100%'.
If only one percentage or length value is given, it sets the
horizontal position only, and the vertical position will be 50%. If
two values are given, the horizontal position comes first.
Combinations of length and percentage values are allowed, (e.g., '50%
2cm'). Negative positions are allowed. Keywords cannot be combined
with percentage values or length values (all possible combinations are
given above).
Example(s):
body { background: url("banner.jpeg") right top } /* 100% 0% */
body { background: url("banner.jpeg") top center } /* 50% 0% */
body { background: url("banner.jpeg") center } /* 50% 50% */
body { background: url("banner.jpeg") bottom } /* 50% 100% */
If the background image is fixed within the viewport (see the
'background-attachment' property), the image is placed relative to the
viewport instead of the element's padding area. For example,
Example(s):
body {
background-image: url("logo.png");
background-attachment: fixed;
background-position: 100% 100%;
background-repeat: no-repeat;
}
In the example above, the (single) image is placed in the lower-right
corner of the viewport.
'background'
Value: ['background-color' || 'background-image' ||
'background-repeat' || 'background-attachment' ||
'background-position'] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: allowed on 'background-position'
Media: visual
Computed value: see individual properties
The 'background' property is a shorthand property for setting the
individual background properties (i.e., 'background-color',
'background-image', 'background-repeat', 'background-attachment' and
'background-position') at the same place in the style sheet.
Given a valid declaration, the 'background' property first sets all
the individual background properties to their initial values, then
assigns explicit values given in the declaration.
Example(s):
In the first rule of the following example, only a value for
'background-color' has been given and the other individual properties
are set to their initial value. In the second rule, all individual
properties have been specified.
BODY { background: red }
P { background: url("chess.png") gray 50% repeat fixed }
14.3 Gamma correction
For information about gamma issues, please consult the Gamma Tutorial
in the PNG specification ([PNG10]).
In the computation of gamma correction, UAs displaying on a CRT may
assume an ideal CRT and ignore any effects on apparent gamma caused by
dithering. That means the minimal handling they need to do on current
platforms is:
PC using MS-Windows
none
Unix using X11
none
Mac using QuickDraw
apply gamma 1.45 [ICC32] (ColorSync-savvy applications may
simply pass the sRGB ICC profile to ColorSync to perform
correct color correction)
SGI using X
apply the gamma value from /etc/config/system.glGammaVal (the
default value being 1.70; applications running on Irix 6.2 or
above may simply pass the sRGB ICC profile to the color
management system)
NeXT using NeXTStep
apply gamma 2.22
"Applying gamma" means that each of the three R, G and B must be
converted to R'=Rgamma, G'=Ggamma, B'=B^gamma, before being handed to
the OS.
This may be done rapidly by building a 256-element lookup table once
per browser invocation thus:
for i := 0 to 255 do
raw := i / 255.0;
corr := pow (raw, gamma);
table[i] := trunc (0.5 + corr * 255.0)
end
which then avoids any need to do transcendental math per color
attribute, far less per pixel.
_________________________________________________________________
_________________________________________________________________
15 Fonts
Contents
* 15.1 Introduction
* 15.2 Font matching algorithm
* 15.3 Font family: the 'font-family' property
* 15.4 Font styling: the 'font-style' property
* 15.5 Small-caps: the 'font-variant' property
* 15.6 Font boldness: the 'font-weight' property
* 15.7 Font size: the 'font-size' property
* 15.8 Shorthand font property: the 'font' property
15.1 Introduction
Setting font properties will be among the most common uses of style
sheets. Unfortunately, there exists no well-defined and universally
accepted taxonomy for classifying fonts, and terms that apply to one
font family may not be appropriate for others. E.g. 'italic' is
commonly used to label slanted text, but slanted text may also be
labeled as being Oblique, Slanted, Incline, Cursive or Kursiv.
Therefore it is not a simple problem to map typical font selection
properties to a specific font.
15.2 Font matching algorithm
Because there is no accepted, universal taxonomy of font properties,
matching of properties to font faces must be done carefully. The
properties are matched in a well-defined order to insure that the
results of this matching process are as consistent as possible across
UAs (assuming that the same library of font faces is presented to each
of them).
1. The User Agent makes (or accesses) a database of relevant CSS 2.1
properties of all the fonts of which the UA is aware. If there are
two fonts with exactly the same properties, the user agent selects
one of them.
2. At a given element and for each character in that element, the UA
assembles the font properties applicable to that element. Using
the complete set of properties, the UA uses the 'font-family'
property to choose a tentative font family. The remaining
properties are tested against the family according to the matching
criteria described with each property. If there are matches for
all the remaining properties, then that is the matching font face
for the given element.
3. If there is no matching font face within the 'font-family' being
processed by step 2, and if there is a next alternative
'font-family' in the font set, then repeat step 2 with the next
alternative 'font-family'.
4. If there is a matching font face, but it doesn't contain a glyph
for the current character, and if there is a next alternative
'font-family' in the font sets, then repeat step 2 with the next
alternative 'font-family'.
5. If there is no font within the family selected in 2, then use a
UA-dependent default 'font-family' and repeat step 2, using the
best match that can be obtained within the default font. If a
particular character cannot be displayed using this font, then the
UA has no suitable font for that character. The UA should map each
character for which it has no suitable font to a visible symbol
chosen by the UA, preferably a "missing character" glyph from one
of the font faces available to the UA.
(The above algorithm can be optimized to avoid having to revisit the
CSS 2.1 properties for each character.)
The per-property matching rules from (2) above are as follows:
1. 'font-style' is tried first. 'italic' will be satisfied if there
is either a face in the UA's font database labeled with the CSS
keyword 'italic' (preferred) or 'oblique'. Otherwise the values
must be matched exactly or font-style will fail.
2. 'font-variant' is tried next. 'normal' matches a font not labeled
as 'small-caps'; 'small-caps' matches (1) a font labeled as
'small-caps', (2) a font in which the small caps are synthesized,
or (3) a font where all lowercase letters are replaced by upper
case letters. A small-caps font may be synthesized by
electronically scaling uppercase letters from a normal font.
3. 'font-weight' is matched next, it will never fail. (See
'font-weight' below.)
4. 'font-size' must be matched within a UA-dependent margin of
tolerance. (Typically, sizes for scalable fonts are rounded to the
nearest whole pixel, while the tolerance for bitmapped fonts could
be as large as 20%.) Further computations, e.g. by 'em' values in
other properties, are based on the computed value of 'font-size'.
15.3 Font family: the 'font-family' property
'font-family'
Value: [[ <family-name> | <generic-family> ] [, <family-name>|
<generic-family>]* ] | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The value is a prioritized list of font family names and/or generic
family names. Unlike most other CSS properties, values are separated
by a comma to indicate that they are alternatives:
body { font-family: Gill, Helvetica, sans-serif }
Although many fonts provide the "missing character" glyph, typically
an open box, as its name implies this should not be considered a match
for characters that cannot be found in the font. (It should, however,
be considered a match for U+FFFD, the "missing character" character's
code point).
There are two types of font family names:
<family-name>
The name of a font family of choice. In the last example,
"Gill" and "Helvetica" are font families.
<generic-family>
In the example above, the last value is a generic family name.
The following generic families are defined:
+ 'serif' (e.g. Times)
+ 'sans-serif' (e.g. Helvetica)
+ 'cursive' (e.g. Zapf-Chancery)
+ 'fantasy' (e.g. Western)
+ 'monospace' (e.g. Courier)
Style sheet designers are encouraged to offer a generic font
family as a last alternative. Generic font family names are
keywords and must NOT be quoted.
[INS: :INS]
If an unquoted font family name contains parentheses, brackets, and/or
braces, they must still be either balanced or escaped per CSS grammar
rules. Similarly, quote marks, semicolons, exclamation marks and
commas within unquoted font family names must be escaped. Font names
containing any such characters or whitespace should be quoted:
[DEL: :DEL]
Font names containing whitespace should be quoted:
body { font-family: "New Century Schoolbook", serif }
<BODY STYLE="font-family: 'My own font', fantasy">
If quoting is omitted, any whitespace characters before and after the
font name are ignored and any sequence of whitespace characters inside
the font name is converted to a single space.
15.4 Font styling: the 'font-style' property
'font-style'
Value: normal | italic | oblique | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
The 'font-style' property selects between normal (sometimes referred
to as "roman" or "upright"), italic and oblique faces within a font
family.
A value of 'normal' selects a font that is classified as 'normal' in
the UA's font database, while 'oblique' selects a font that is labeled
'oblique'. A value of 'italic' selects a font that is labeled
'italic', or, if that is not available, one labeled 'oblique'.
The font that is labeled 'oblique' in the UA's font database may
actually have been generated by electronically slanting a normal font.
Fonts with Oblique, Slanted or Incline in their names will typically
be labeled 'oblique' in the UA's font database. Fonts with Italic,
Cursive or Kursiv in their names will typically be labeled 'italic'.
h1, h2, h3 { font-style: italic }
h1 em { font-style: normal }
In the example above, emphasized text within 'H1' will appear in a
normal face.
15.5 Small-caps: the 'font-variant' property
'font-variant'
Value: normal | small-caps | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
Another type of variation within a font family is the small-caps. In a
small-caps font the lower case letters look similar to the uppercase
ones, but in a smaller size and with slightly different proportions.
The 'font-variant' property selects that font.
A value of 'normal' selects a font that is not a small-caps font,
'small-caps' selects a small-caps font. It is acceptable (but not
required) in CSS 2.1 if the small-caps font is a created by taking a
normal font and replacing the lower case letters by scaled uppercase
characters. As a last resort, uppercase letters will be used as
replacement for a small-caps font.
The following example results in an 'H3' element in small-caps, with
any emphasized words in oblique, and any emphasized words within an
'H3' oblique small-caps:
h3 { font-variant: small-caps }
em { font-style: oblique }
There may be other variants in the font family as well, such as fonts
with old-style numerals, small-caps numerals, condensed or expanded
letters, etc. CSS 2.1 has no properties that select those.
Note: insofar as this property causes text to be transformed to
uppercase, the same considerations as for 'text-transform' apply.
15.6 Font boldness: the 'font-weight' property
'font-weight'
Value: normal | bold | bolder | lighter | 100 | 200 | 300 | 400 |
500 | 600 | 700 | 800 | 900 | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: see text
The 'font-weight' property selects the weight of the font. The values
'100' to '900' form an ordered sequence, where each number indicates a
weight that is at least as dark as its predecessor. The keyword
'normal' is synonymous with '400', and 'bold' is synonymous with
'700'. Keywords other than 'normal' and 'bold' have been shown to be
often confused with font names and a numerical scale was therefore
chosen for the 9-value list.
p { font-weight: normal } /* 400 */
h1 { font-weight: 700 } /* bold */
The 'bolder' and 'lighter' values select font weights that are
relative to the weight inherited from the parent:
strong { font-weight: bolder }
Child elements inherit the resultant weight, not the keyword value.
Fonts (the font data) typically have one or more properties whose
values are names that are descriptive of the "weight" of a font. There
is no accepted, universal meaning to these weight names. Their primary
role is to distinguish faces of differing darkness within a single
font family. Usage across font families is quite variant; for example,
a font that one might think of as being bold might be described as
being Regular, Roman, Book, Medium, Semi- or DemiBold, Bold, or Black,
depending on how black the "normal" face of the font is within the
design. Because there is no standard usage of names, the weight
property values in CSS 2.1 are given on a numerical scale in which the
value '400' (or 'normal') corresponds to the "normal" text face for
that family. The weight name associated with that face will typically
be Book, Regular, Roman, Normal or sometimes Medium.
The association of other weights within a family to the numerical
weight values is intended only to preserve the ordering of darkness
within that family. However, the following heuristics tell how the
assignment is done in typical cases:
* If the font family already uses a numerical scale with nine values
(like e.g. OpenType does), the font weights should be mapped
directly.
* If there is both a face labeled Medium and one labeled Book,
Regular, Roman or Normal, then the Medium is normally assigned to
the '500'.
* The font labeled "Bold" will often correspond to the weight value
'700'.
* If there are fewer then 9 weights in the family, the default
algorithm for filling the "holes" is as follows. If '500' is
unassigned, it will be assigned the same font as '400'. If any of
the values '600', '700', '800' or '900' remains unassigned, they
are assigned to the same face as the next darker assigned keyword,
if any, or the next lighter one otherwise. If any of '300', '200'
or '100' remains unassigned, it is assigned to the next lighter
assigned keyword, if any, or the next darker otherwise.
The following two examples show typical mappings.
Assume four weights in the "Rattlesnake" family, from lightest to
darkest: Regular, Medium, Bold, Heavy.
CAPTION: First example of font-weight mapping
Available faces Assignments Filling the holes
"Rattlesnake Regular" 400 100, 200, 300
"Rattlesnake Medium" 500
"Rattlesnake Bold" 700 600
"Rattlesnake Heavy" 800 900
Assume six weights in the "Ice Prawn" family: Book, Medium, Bold,
Heavy, Black, ExtraBlack. Note that in this instance the user agent
has decided not to assign a numeric value to "Ice Prawn ExtraBlack".
CAPTION: Second example of font-weight mapping
Available faces Assignments Filling the holes
"Ice Prawn Book" 400 100, 200, 300
"Ice Prawn Medium" 500
"Ice Prawn Bold" 700 600
"Ice Prawn Heavy" 800
"Ice Prawn Black" 900
"Ice Prawn ExtraBlack" (none)
Since the intent of the relative keywords 'bolder' and 'lighter' is to
darken or lighten the face within the family and because a family may
not have faces aligned with all the symbolic weight values, the
matching of 'bolder' is to the next darker face available on the
client within the family and the matching of 'lighter' is to the next
lighter face within the family. To be precise, the meaning of the
relative keywords 'bolder' and 'lighter' is as follows:
* 'bolder' selects the next weight that is assigned to a font that
is darker than the inherited one. If there is no such weight, it
simply results in the next darker numerical value (and the font
remains unchanged), unless the inherited value was '900' in which
case the resulting weight is also '900'.
* 'lighter' is similar, but works in the opposite direction: it
selects the next lighter keyword with a different font from the
inherited one, unless there is no such font, in which case it
selects the next lighter numerical value (and keeps the font
unchanged).
There is no guarantee that there will be a darker face for each of the
'font-weight' values; for example, some fonts may have only a normal
and a bold face, while others may have eight face weights. There is no
guarantee on how a UA will map font faces within a family to weight
values. The only guarantee is that a face of a given value will be no
less dark than the faces of lighter values.
15.7 Font size: the 'font-size' property
'font-size'
Value: <absolute-size> | <relative-size> | <length> | <percentage> |
inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: refer to parent element's font size
Media: visual
Computed value: absolute length
The font size corresponds to the em square, a concept used in
typography. Note that certain glyphs may bleed outside their em
squares. Values have the following meanings:
<absolute-size>
An <absolute-size> keyword is an index to a table of font sizes
computed and kept by the UA. Possible values are:
[ xx-small | x-small | small | medium | large | x-large |
xx-large ]
The following table provides user agent guidelines for the
absolute-size scaling factor and their mapping to HTML heading
and absolute font-sizes. The 'medium' value is used as the
reference middle value. The user agent may fine tune these
values for different fonts or different types of display
devices.
CSS absolute-size values xx-small x-small small medium large x-large
xx-large
scaling factor 3/5 3/4 8/9 1 6/5 3/2 2/1 3/1
HTML headings h6 h5 h4 h3 h2 h1
HTML font sizes 1 2 3 4 5 6 7
Different media may need different scaling factors. Also, the
UA should take the quality and availability of fonts into
account when computing the table. The table may be different
from one font family to another.
Note 1. To preserve readability, a UA applying these guidelines
should nevertheless avoid creating font-size resulting in less
than 9 pixels per EM unit on a computer display.
Note 2. In CSS1, the suggested scaling factor between adjacent
indexes was 1.5 which user experience proved to be too large.
In CSS2, the suggested scaling factor for computer screen
between adjacent indexes was 1.2 which still created issues for
the small sizes. The new scaling factor varies between each
index to provide better readability.
<relative-size>
A <relative-size> keyword is interpreted relative to the table
of font sizes and the font size of the parent element. Possible
values are: [ larger | smaller ]. For example, if the parent
element has a font size of 'medium', a value of 'larger' will
make the font size of the current element be 'large'. If the
parent element's size is not close to a table entry, the UA is
free to interpolate between table entries or round off to the
closest one. The UA may have to extrapolate table values if the
numerical value goes beyond the keywords.
Length and percentage values should not take the font size table into
account when calculating the font size of the element.
Negative values are not allowed.
On all other properties, 'em' and 'ex' length values refer to the
computed font size of the current element. On the 'font-size'
property, these length units refer to the computed font size of the
parent element.
Note that an application may reinterpret an explicit size, depending
on the context. E.g., inside a VR scene a font may get a different
size because of perspective distortion.
Examples:
p { font-size: 16px; }
@media print {
p { font-size: 12pt; }
}
blockquote { font-size: larger }
em { font-size: 150% }
em { font-size: 1.5em }
15.8 Shorthand font property: the 'font' property
'font'
Value: [ [ 'font-style' || 'font-variant' || 'font-weight' ]?
'font-size' [ / 'line-height' ]? 'font-family' ] | caption | icon |
menu | message-box | small-caption | status-bar | inherit
Initial: see individual properties
Applies to: all elements
Inherited: yes
Percentages: allowed on 'font-size' and 'line-height'
Media: visual
Computed value: see individual properties
The 'font' property is, except as described below, a shorthand
property for setting 'font-style', 'font-variant', 'font-weight',
'font-size', 'line-height' and 'font-family' at the same place in the
style sheet. The syntax of this property is based on a traditional
typographical shorthand notation to set multiple properties related to
fonts.
All font-related properties are first reset to their initial values,
including those listed in the preceding paragraph. Then, those
properties that are given explicit values in the 'font' shorthand are
set to those values. For a definition of allowed and initial values,
see the previously defined properties.
p { font: 12px/14px sans-serif }
p { font: 80% sans-serif }
p { font: x-large/110% "New Century Schoolbook", serif }
p { font: bold italic large Palatino, serif }
p { font: normal small-caps 120%/120% fantasy }
In the second rule, the font size percentage value ('80%') refers to
the font size of the parent element. In the third rule, the line
height percentage refers to the font size of the element itself.
In the first three rules above, the 'font-style', 'font-variant' and
'font-weight' are not explicitly mentioned, which means they are all
three set to their initial value ('normal'). The fourth rule sets the
'font-weight' to 'bold', the 'font-style' to 'italic' and implicitly
sets 'font-variant' to 'normal'.
The fifth rule sets the 'font-variant' ('small-caps'), the 'font-size'
(120% of the parent's font), the 'line-height' (120% times the font
size) and the 'font-family' ('fantasy'). It follows that the keyword
'normal' applies to the two remaining properties: 'font-style' and
'font-weight'.
The following values refer to system fonts:
caption
The font used for captioned controls (e.g., buttons,
drop-downs, etc.).
icon
The font used to label icons.
menu
The font used in menus (e.g., dropdown menus and menu lists).
message-box
The font used in dialog boxes.
small-caption
The font used for labeling small controls.
status-bar
The font used in window status bars.
System fonts may only be set as a whole; that is, the font family,
size, weight, style, etc. are all set at the same time. These values
may then be altered individually if desired. If no font with the
indicated characteristics exists on a given platform, the user agent
should either intelligently substitute (e.g., a smaller version of the
'caption' font might be used for the 'small-caption' font), or
substitute a user agent default font. As for regular fonts, if, for a
system font, any of the individual properties are not part of the
operating system's available user preferences, those properties should
be set to their initial values.
That is why this property is "almost" a shorthand property: system
fonts can only be specified with this property, not with 'font-family'
itself, so 'font' allows authors to do more than the sum of its
subproperties. However, the individual properties such as
'font-weight' are still given values taken from the system font, which
can be independently varied.
Example(s):
button { font: 300 italic 1.3em/1.7em "FB Armada", sans-serif }
button p { font: menu }
button p em { font-weight: bolder }
If the font used for dropdown menus on a particular system happened to
be, for example, 9-point Charcoal, with a weight of 600, then P
elements that were descendants of BUTTON would be displayed as if this
rule were in effect:
button p { font: 600 9px Charcoal }
Because the 'font' shorthand property resets any property not
explicitly given a value to its initial value, this has the same
effect as this declaration:
button p {
font-family: Charcoal;
font-style: normal;
font-variant: normal;
font-weight: 600;
font-size: 9px;
line-height: normal;
}
_________________________________________________________________
_________________________________________________________________
16 Text
Contents
* 16.1 Indentation: the 'text-indent' property
* 16.2 Alignment: the 'text-align' property
* 16.3 Decoration
+ 16.3.1 Underlining, overlining, striking, and blinking: the
'text-decoration' property
* 16.4 Letter and word spacing: the 'letter-spacing' and
'word-spacing' properties
* 16.5 Capitalization: the 'text-transform' property
* 16.6 Whitespace: the 'white-space' property
+ 16.6.1 The 'white-space' processing model
The properties defined in the following sections affect the visual
presentation of characters, spaces, words, and paragraphs.
16.1 Indentation: the 'text-indent' property
'text-indent'
Value: <length> | <percentage> | inherit
Initial: 0
Applies to: block-level elements
Inherited: yes
Percentages: refer to width of containing block
Media: visual
Computed value: absolute length
This property specifies the indentation of the first line of text in a
block. More precisely, it specifies the indentation of the first box
that flows into the block's first line box. The box is indented with
respect to the left (or right, for right-to-left layout) edge of the
line box. User agents should render this indentation as blank space.
Values have the following meanings:
<length>
The indentation is a fixed length.
<percentage>
The indentation is a percentage of the containing block width.
The value of 'text-indent' may be negative, but there may be
implementation-specific limits. If the value of 'text-indent' is
negative, the value of 'overflow' will affect whether the text is
visible.
Example(s):
The following example causes a '3em' text indent.
p { text-indent: 3em }
Note: Since the 'text-indent' property inherits, when specified on a
block element, it will affect descendent inline-block elements. For
this reason, it is often wise to specify 'text-indent: 0' on elements
that are specified 'display:inline-block'.
16.2 Alignment: the 'text-align' property
'text-align'
Value: left | right | center | justify | inherit
Initial: depends on user agent and writing direction
Applies to: block-level elements and table cells
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property describes how inline content of a block is aligned.
Values have the following meanings:
left, right, center, justify
Left, right, center, and justify text, respectively.
A block of text is a stack of line boxes. In the case of 'left',
'right' and 'center', this property specifies how the inline boxes
within each line box align with respect to the line box's left and
right sides; alignment is not with respect to the viewport. In the
case of 'justify', the UA may stretch the inline boxes in addition to
adjusting their positions. (See also 'letter-spacing' and
'word-spacing'.)
Example(s):
In this example, note that since 'text-align' is inherited, all
block-level elements inside the DIV element with 'class=important'
will have their inline content centered.
div.important { text-align: center }
Note. The actual justification algorithm used depends on the
user-agent and the language/script of the text.
Conforming user agents may interpret the value 'justify' as 'left' or
'right', depending on whether the element's default writing direction
is left-to-right or right-to-left, respectively.
16.3 Decoration
16.3.1 Underlining, overlining, striking, and blinking: the 'text-decoration'
property
'text-decoration'
Value: none | [ underline || overline || line-through || blink ] |
inherit
Initial: none
Applies to: all elements
Inherited: no (see prose)
Percentages: N/A
Media: visual
Computed value: as specified
This property describes decorations that are added to the text of an
element. If the property is specified for a block-level element, it
affects all inline-level descendants of the element. If it is
specified for (or affects) an inline-level element, it affects all
boxes generated by the element. If the element has no content or no
text content (e.g., the IMG element in HTML), user agents must ignore
this property.
Values have the following meanings:
none
Produces no text decoration.
underline
Each line of text is underlined.
overline
Each line of text has a line above it.
line-through
Each line of text has a line through the middle.
blink
Text blinks (alternates between visible and invisible).
Conforming user agents may simply not blink the text. Note that
not blinking the text is one technique to satisfy checkpoint
3.3 of WAI-UAAG.
The color(s) required for the text decoration must be derived from the
'color' property value.
This property is not inherited, but descendant boxes of a block box
should be formatted with the same decoration (e.g., they should all be
underlined). The color of decorations should remain the same even if
descendant elements have different 'color' values.
Example(s):
In the following example for HTML, the text content of all A elements
acting as hyperlinks (whether visited or not) will be underlined:
a:visited,a:link { text-decoration: underline }
16.4 Letter and word spacing: the 'letter-spacing' and 'word-spacing'
properties
'letter-spacing'
Value: normal | <length> | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: 'normal' or absolute length
This property specifies spacing behavior between text characters.
Values have the following meanings:
normal
The spacing is the normal spacing for the current font. This
value allows the user agent to alter the space between
characters in order to justify text.
<length>
This value indicates inter-character space in addition to the
default space between characters. Values may be negative, but
there may be implementation-specific limits. User agents may
not further increase or decrease the inter-character space in
order to justify text.
Character spacing algorithms are user agent-dependent. Character
spacing may also be influenced by justification (see the 'text-align'
property).
Example(s):
In this example, the space between characters in BLOCKQUOTE elements
is increased by '0.1em'.
blockquote { letter-spacing: 0.1em }
In the following example, the user agent is not permitted to alter
inter-character space:
blockquote { letter-spacing: 0cm } /* Same as '0' */
When the resultant space between two characters is not the same as the
default space, user agents should not use ligatures.
'word-spacing'
Value: normal | <length> | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: for 'normal' the value '0'; otherwise the absolute
length
This property specifies spacing behavior between words. Values have
the following meanings:
normal
The normal inter-word space, as defined by the current font
and/or the UA.
<length>
This value indicates inter-word space in addition to the
default space between words. Values may be negative, but there
may be implementation-specific limits.
Word spacing algorithms are user agent-dependent. Word spacing is also
influenced by justification (see the 'text-align' property).
Example(s):
In this example, the word-spacing between each word in H1 elements is
increased by '1em'.
h1 { word-spacing: 1em }
16.5 Capitalization: the 'text-transform' property
'text-transform'
Value: capitalize | uppercase | lowercase | none | inherit
Initial: none
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property controls capitalization effects of an element's text.
Values have the following meanings:
capitalize
Puts the first character of each word in uppercase.
uppercase
Puts all characters of each word in uppercase.
lowercase
Puts all characters of each word in lowercase.
none
No capitalization effects.
The actual transformation in each case is written language dependent.
See RFC 2070 ([RFC2070]) for ways to find the language of an element.
Conforming user agents may consider the value of 'text-transform' to
be 'none' for characters that are not from the Latin-1 repertoire and
for elements in languages for which the transformation is different
from that specified by the case-conversion tables of ISO 10646
([ISO10646]).
Example(s):
In this example, all text in an H1 element is transformed to uppercase
text.
h1 { text-transform: uppercase }
16.6 Whitespace: the 'white-space' property
'white-space'
Value: normal | pre | nowrap | pre-wrap | pre-line | inherit
Initial: normal
Applies to: block-level elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property declares how whitespace inside the element is handled.
Values have the following meanings:
normal
This value directs user agents to collapse sequences of
whitespace, and break lines as necessary to fill line boxes.
pre
This value prevents user agents from collapsing sequences of
whitespace. Lines are only broken at newlines in the source, or
at occurrences of "\A" in generated content.
nowrap
This value collapses whitespace as for 'normal', but suppresses
line breaks within text.
pre-wrap
This value prevents user agents from collapsing sequences of
whitespace. Lines are broken at newlines in the source, at
occurrences of "\A" in generated content, and as necessary to
fill line boxes.
pre-line
This value collapses whitespace as for 'normal', except
occurrences of newlines in the source or "\A" in generated
content do cause line breaks.
Example(s):
The following examples show what whitespace behavior is expected from
the PRE and P elements, the "nowrap" attribute in HTML, and in
generated content.
pre { white-space: pre }
p { white-space: normal }
td[nowrap] { white-space: nowrap }
:before,:after { white-space: pre-line }
In addition, the effect of an HTML PRE element with the non-standard
"wrap" attribute is demonstrated by the following example:
pre[wrap] { white-space: pre-wrap }
[INS: :INS]
16.6.1 The 'white-space' processing model
Any text that is directly contained inside a block (not inside an
inline) should be treated as an anonymous inline element.
For each inline (including anonymous inlines), the following steps are
performed, ignoring bidi formatting characters as if they were not
there:
1. Each non-linefeed whitespace character surrounding a linefeed
character is removed if 'white-space' is set to 'normal',
'no-wrap', or 'pre-line'.
2. If 'white-space' is set to 'pre' or 'pre-wrap', any sequence of
spaces (U+0020) unbroken by an element boundary is treated as a
sequence of non-breaking spaces. However, a line breaking
opportunity exists at the end of the sequence.
3. If 'white-space' is set to 'normal' or 'nowrap', linefeed
characters are transformed for rendering purpose into one of the
following characters: a space character, a zero width space
character (U+200B), or no character (i.e. not rendered), according
to UA-specific algorithms based on the content script.
4. If 'white-space' is set to 'normal', 'nowrap', or 'pre-line',
1. every tab (U+0009) is converted to a space (U+0020)
2. any space (U+0020) following another space (U+0020) -- even a
space before the inline, if that space also has 'white-space
set to 'normal', 'nowrap' or 'pre-line' -- is removed.
Then, the entire block is rendered. Inlines are laid out, taking bidi
reordering into account, and wrapping as specified by the
'white-space' property.
As each line is laid out,
1. If a space (U+0020) at the beginning of a line has 'white-space'
set to 'normal', 'nowrap', or 'pre-line', it is removed.
2. All tabs (U+0009) are rendered as a horizontal shift that lines up
the start edge of the next glyph with the next tab stop. Tab stops
occur at points that are multiples of 8 times the width of a space
(U+0020) rendered in the block's font from the block's starting
content edge.
3. If a space (U+0020) at the end of a line has 'white-space' set to
'normal', 'nowrap', or 'pre-line', it is also removed.
_________________________________________________________________
_________________________________________________________________
17 Tables
Contents
* 17.1 Introduction to tables
* 17.2 The CSS table model
+ 17.2.1 Anonymous table objects
* 17.3 Column selectors
* 17.4 Tables in the visual formatting model
+ 17.4.1 Caption position and alignment
* 17.5 Visual layout of table contents
+ 17.5.1 Table layers and transparency
+ 17.5.2 Table width algorithms: the 'table-layout' property
o Fixed table layout
o Automatic table layout
+ 17.5.3 Table height algorithms
+ 17.5.4 Horizontal alignment in a column
+ 17.5.5 Dynamic row and column effects
* 17.6 Borders
+ 17.6.1 The separated borders model
o Borders and Backgrounds around empty cells: the
'empty-cells' property
+ 17.6.2 The collapsing border model
o Border conflict resolution
+ 17.6.3 Border styles
17.1 Introduction to tables
Tables represent relationships between data. Authors specify these
relationships in the document language and specify their presentation
in CSS, in two ways: visually and aurally.
Authors may specify the visual formatting of a table as a rectangular
grid of cells. Rows and columns of cells may be organized into row
groups and column groups. Rows, columns, row groups, column groups,
and cells may have borders drawn around them (there are two border
models in CSS 2.1). Authors may align data vertically or horizontally
within a cell and align data in all cells of a row or column.
Authors may also specify the aural rendering of a table; how headers
and data will be spoken. In the document language, authors may label
cells and groups of cells so that when rendered aurally, cell headers
are spoken before cell data. In effect, this "serializes" the table:
users browsing the table aurally hear a sequence of headers followed
by data.
Example(s):
Here is a simple three-row, three-column table described in HTML 4.0:
<TABLE>
<CAPTION>This is a simple 3x3 table</CAPTION>
<TR id="row1">
<TH>Header 1 <TD>Cell 1 <TD>Cell 2
<TR id="row2">
<TH>Header 2 <TD>Cell 3 <TD>Cell 4
<TR id="row3">
<TH>Header 3 <TD>Cell 5 <TD>Cell 6
</TABLE>
This code creates one table (the TABLE element), three rows (the TR
elements), three header cells (the TH elements), and six data cells
(the TD elements). Note that the three columns of this example are
specified implicitly: there are as many columns in the table as
required by header and data cells.
The following CSS rule centers the text horizontally in the header
cells and present the data with a bold font weight:
th { text-align: center; font-weight: bold }
The next rules align the text of the header cells on their baseline
and vertically centers the text in each data cell:
th { vertical-align: baseline }
td { vertical-align: middle }
The next rules specify that the top row will be surrounded by a 3px
solid blue border and each of the other rows will be surrounded by a
1px solid black border:
table { border-collapse: collapse }
tr#row1 { border-top: 3px solid blue }
tr#row2 { border-top: 1px solid black }
tr#row3 { border-top: 1px solid black }
Note, however, that the borders around the rows overlap where the rows
meet. What color (black or blue) and thickness (1px or 3px) will the
border between row1 and row2 be? We discuss this in the section on
border conflict resolution.
The following rule puts the table caption above the table:
caption { caption-side: top }
[DEL: Finally, the following rule specifies that, when rendered
aurally, each row of data is to be spoken as a "Header, Data, Data":
:DEL]
[DEL: th { speak-header: once } :DEL]
[DEL: For instance, the first row would be spoken "Header1 Cell1
Cell2". On the other hand, with the following rule: :DEL]
[DEL: th { speak-header: always } :DEL]
[DEL: it would be spoken "Header1 Cell1 Header1 Cell2". :DEL]
The preceding example shows how CSS works with HTML 4.0 elements; in
HTML 4.0, the semantics of the various table elements (TABLE, CAPTION,
THEAD, TBODY, TFOOT, COL, COLGROUP, TH, and TD) are well-defined. In
other document languages (such as XML applications), there may not be
pre-defined table elements. Therefore, CSS 2.1 allows authors to "map"
document language elements to table elements via the 'display'
property. For example, the following rule makes the FOO element act
like an HTML TABLE element and the BAR element act like a CAPTION
element:
FOO { display : table }
BAR { display : table-caption }
We discuss the various table elements in the following section. In
this specification, the term table element refers to any element
involved in the creation of a table. An "internal" table element is
one that produces a row, row group, column, column group, or cell.
17.2 The CSS table model
The CSS table model is based on the HTML 4.0 table model, in which the
structure of a table closely parallels the visual layout of the table.
In this model, a table consists of an optional caption and any number
of rows of cells. The table model is said to be "row primary" since
authors specify rows, not columns, explicitly in the document
language. Columns are derived once all the rows have been specified --
the first cell of each row belongs to the first column, the second to
the second column, etc.). Rows and columns may be grouped structurally
and this grouping reflected in presentation (e.g., a border may be
drawn around a group of rows).
Thus, the table model consists of tables, captions, rows, row groups,
columns, column groups, and cells.
The CSS model does not require that the document language include
elements that correspond to each of these components. For document
languages (such as XML applications) that do not have pre-defined
table elements, authors must map document language elements to table
elements; this is done with the 'display' property. The following
'display' values assign table formatting rules to an arbitrary
element:
table (In HTML: TABLE)
Specifies that an element defines a block-level table: it is a
rectangular block that participates in a block formatting
context.
inline-table (In HTML: TABLE)
Specifies that an element defines an inline-level table: it is
a rectangular block that participates in an inline formatting
context).
table-row (In HTML: TR)
Specifies that an element is a row of cells.
table-row-group (In HTML: TBODY)
Specifies that an element groups one or more rows.
table-header-group (In HTML: THEAD)
Like 'table-row-group', but for visual formatting, the row
group is always displayed before all other rows and rowgroups
and after any top captions. Print user agents may repeat header
rows on each page spanned by a table.
table-footer-group (In HTML: TFOOT)
Like 'table-row-group', but for visual formatting, the row
group is always displayed after all other rows and rowgroups
and before any bottom captions. Print user agents may repeat
footer rows on each page spanned by a table.
table-column (In HTML: COL)
Specifies that an element describes a column of cells.
table-column-group (In HTML: COLGROUP)
Specifies that an element groups one or more columns.
table-cell (In HTML: TD, TH)
Specifies that an element represents a table cell.
table-caption (In HTML: CAPTION)
Specifies a caption for the table.
Elements with 'display' set to 'table-column' or 'table-column-group'
are not rendered (exactly as if they had 'display: none'), but they
are useful, because they may have attributes which induce a certain
style for the columns they represent.
The default style sheet for HTML 4.0 in the appendix illustrates the
use of these values for HTML 4.0:
table { display: table }
tr { display: table-row }
thead { display: table-header-group }
tbody { display: table-row-group }
tfoot { display: table-footer-group }
col { display: table-column }
colgroup { display: table-column-group }
td, th { display: table-cell }
caption { display: table-caption }
User agents may ignore these 'display' property values for HTML table
elements, since HTML tables may be rendered using other algorithms
intended for backwards compatible rendering. However, this is not
meant to discourage the use of 'display: table' on other, non-table
elements in HTML.
17.2.1 Anonymous table objects
Document languages other than HTML may not contain all the elements in
the CSS 2.1 table model. In these cases, the "missing" elements must
be assumed in order for the table model to work. Any table element
will automatically generate necessary anonymous table objects around
itself, consisting of at least three nested objects corresponding to a
'table'/'inline-table' element, a 'table-row' element, and a
'table-cell' element. Missing elements generate anonymous objects
(e.g., anonymous boxes in visual table layout) according to the
following rules:
1. If the parent P of a 'table-cell' element T is not a 'table-row',
an object corresponding to a 'table-row' will be generated between
P and T. This object will span all consecutive 'table-cell'
siblings (in the document tree) of T.
2. If the parent P of a 'table-row' element T is not a 'table',
'inline-table', or 'table-row-group' element, an object
corresponding to a 'table' element will be generated between P and
T. This object will span all consecutive siblings (in the document
tree) of T that require a 'table' parent: 'table-row',
'table-row-group', 'table-header-group', 'table-footer-group',
'table-column', 'table-column-group', and 'table-caption'.
3. If the parent P of a 'table-column' element T is not a 'table',
'inline-table', or 'table-column-group' element, an object
corresponding to a 'table' element will be generated between P and
T. This object will span all consecutive siblings (in the document
tree) of T that require a 'table' parent: 'table-row',
'table-row-group', 'table-header-group', 'table-footer-group',
'table-column', 'table-column-group', and 'table-caption'.
4. If the parent P of a 'table-row-group' (or 'table-header-group',
'table-footer-group', or 'table-column-group') element T is not a
'table' or 'inline-table', an object corresponding to a 'table'
element will be generated between P and T. This object will span
all consecutive siblings (in the document tree) of T that require
a 'table' parent: 'table-row', 'table-row-group',
'table-header-group', 'table-footer-group', 'table-column',
'table-column-group', and 'table-caption'.
5. If a child T of a 'table' element (or 'inline-table') P is not a
'table-row-group', 'table-header-group', 'table-footer-group', or
'table-row' element, an object corresponding to a 'table-row'
element will be generated between P and T. This object spans all
consecutive siblings of T that are not 'table-row-group',
'table-header-group', 'table-footer-group', or 'table-row'
elements.
6. If a child T of a 'table-row-group' element (or
'table-header-group' or 'table-footer-group') P is not a
'table-row' element, an object corresponding to a 'table-row'
element will be generated between P and T. This object spans all
consecutive siblings of T that are not 'table-row' elements.
7. If a child T of a 'table-row' element P is not a 'table-cell'
element, an object corresponding to a 'table-cell' element will be
generated between P and T. This object spans all consecutive
siblings of T that are not 'table-cell' elements.
Example(s):
In this XML example, a 'table' element is assumed to contain the HBOX
element:
<HBOX>
<VBOX>George</VBOX>
<VBOX>4287</VBOX>
<VBOX>1998</VBOX>
</HBOX>
because the associated style sheet is:
HBOX { display: table-row }
VBOX { display: table-cell }
Example(s):
In this example, three 'table-cell' elements are assumed to contain
the text in the ROWs. Note that the text is further encapsulated in
anonymous inline boxes, as explained in visual formatting model:
<STACK>
<ROW>This is the <D>top</D> row.</ROW>
<ROW>This is the <D>middle</D> row.</ROW>
<ROW>This is the <D>bottom</D> row.</ROW>
</STACK>
The style sheet is:
STACK { display: inline-table }
ROW { display: table-row }
D { display: inline; font-weight: bolder }
HTML user agents are not required to create anonymous objects
according to the above rules.
17.3 Column selectors
Table cells may belong to two contexts: rows and columns. However, in
the source document cells are descendants of rows, never of columns.
Nevertheless, some aspects of cells can be influenced by setting
properties on columns.
The following properties apply to column and column-group elements:
'border'
The various border properties apply to columns only if
'border-collapse' is set to 'collapse' on the table element. In
that case, borders set on columns and column groups are input
to the conflict resolution algorithm that selects the border
styles at every cell edge.
'background'
The background properties set the background for cells in the
column, but only if both the cell and row have transparent
backgrounds. See "Table layers and transparency."
'width'
The 'width' property gives the minimum width for the column.
'visibility'
If the 'visibility' of a column is set to 'collapse', none of
the cells in the column are rendered, and cells that span into
other columns are clipped. In addition, the width of the table
is diminished by the width the column would have taken up. See
"Dynamic effects" below. Other values for 'visibility' have no
effect.
Example(s):
Here are some examples of style rules that set properties on columns.
The first two rules together implement the "rules" attribute of HTML
4.0 with a value of "cols". The third rule makes the "totals" column
blue, the final two rules shows how to make a column a fixed size, by
using the fixed layout algorithm.
col { border-style: none solid }
table { border-style: hidden }
col.totals { background: blue }
table { table-layout: fixed }
col.totals { width: 5em }
17.4 Tables in the visual formatting model
In terms of the visual formatting model, a table may behave like a
block-level or replaced inline-level element. Tables have content,
padding, borders, and margins.
In both cases, the table element generates an anonymous box that
contains the table box itself and the caption's box (if present). The
table and caption boxes retain their own content, padding, margin, and
border areas, and the dimensions of the rectangular anonymous box are
the smallest required to contain both. Vertical margins collapse where
the table box and caption box touch. Any repositioning of the table
must move the entire anonymous box, not just the table box, so that
the caption follows the table.
A table with a caption above it; both have margins and the margins
between them are collapsed, as is normal for vertical margins.
Diagram of a table with a caption above it; the bottom margin of the
caption is collapsed with the top margin of the table.
17.4.1 Caption position and alignment
'caption-side'
Value: top | bottom | left | right | inherit
Initial: top
Applies to: 'table-caption' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property specifies the position of the caption box with respect
to the table box. Values have the following meanings:
top
Positions the caption box above the table box.
bottom
Positions the caption box below the table box.
Captions above or below a 'table' element are formatted very much as
if they were a block element before or after the table, except that
(1) they inherit inheritable properties from the table, and (2) they
are not considered to be a block box for the purposes of any 'compact'
or 'run-in' element that may precede the table.
A caption that is above or below a table box also behaves like a block
box for width calculations; the width is computed with respect to the
width of the table box's containing block.
To align caption content horizontally within the caption box, use the
'text-align' property.
Example(s):
In this example, the 'caption-side' property places captions below
tables. The caption will be as wide as the parent of the table, and
caption text will be left-justified.
caption { caption-side: bottom;
width: auto;
text-align: left }
17.5 Visual layout of table contents
Like other elements of the document language, internal table elements
generate rectangular boxes with content and borders. Cells have
padding as well. Internal table elements do not have margins.
The visual layout of these boxes is governed by a rectangular,
irregular grid of rows and columns. Each box occupies a whole number
of grid cells, determined according to the following rules. These
rules do not apply to HTML 4.0 or earlier HTML versions; HTML imposes
its own limitations on row and column spans.
1. Each row box occupies one row of grid cells. Together, the row
boxes fill the table from top to bottom in the order they occur in
the source document (i.e., the table occupies exactly as many grid
rows as there are row elements).
2. A row group occupies the same grid cells as the rows it contains.
3. A column box occupies one or more columns of grid cells. Column
boxes are placed next to each other in the order they occur. The
first column box may be either on the left or on the right,
depending on the value of the 'direction' property of the table.
4. A column group box occupies the same grid cells as the columns it
contains.
5. Cells may span several rows or columns. (Although CSS 2.1 doesn't
define how the number of spanned rows or columns is determined, a
user agent may have special knowledge about the source document; a
future version of CSS may provide a way to express this knowledge
in CSS syntax.) Each cell is thus a rectangular box, one or more
grid cells wide and high. The top row of this rectangle is in the
row specified by the cell's parent. The rectangle must be as far
to the left as possible, but it may not overlap with any other
cell box, and must be to the right of all cells in the same row
that are earlier in the source document. (This constraint holds if
the 'direction' property of the table is 'ltr'; if the 'direction'
is 'rtl', interchange "left" and "right" in the previous
sentence.)
6. A cell box cannot extend beyond the last row box of a table or
row-group; the user agents must shorten it until it fits.
Note. Table cells may be positioned, but this is not recommended:
absolute and fixed positioning, as well as floating, remove a box from
the flow, affecting table size.
Here are two examples. The first is assumed to occur in an HTML
document:
<TABLE>
<TR><TD>1 <TD rowspan="2">2 <TD>3 <TD>4
<TR><TD colspan="2">5
</TABLE>
<TABLE>
<ROW><CELL>1 <CELL rowspan="2">2 <CELL>3 <CELL>4
<ROW><CELL colspan="2">5
</TABLE>
The second table is formatted as in the figure on the right. However,
the HTML table's rendering is explicitly undefined by HTML, and CSS
doesn't try to define it. User agents are free to render it, e.g., as
in the figure on the left.
One table with overlapping cells and one without
On the left, one possible rendering of an erroneous HTML 4.0 table; on
the right, the only possible formatting of a similar, non-HTML table.
17.5.1 Table layers and transparency
For the purposes of finding the background of each table cell, the
different table elements may be thought of as being on six
superimposed layers. The background set on an element in one of the
layers will only be visible if the layers above it have a transparent
background.
schema of table layers
Schema of table layers.
1. The lowest layer is a single plane, representing the table box
itself. Like all boxes, it may be transparent.
2. The next layer contains the column groups. Each column group
extends from the top of the cells in the top row to the bottom of
the cells on the bottom row and from the left edge of its leftmost
column to the right edge of its rightmost column. The background
extends to cover the full area of all cells that originate in the
column group, but this extension does not affect background image
positioning.
3. On top of the column groups are the areas representing the column
boxes. Each column is as tall as the column groups and as wide as
a normal (single-column-spanning) cell in the column. The
background extends to cover the full area of all cells that
originate in the column, even if they span outside the column, but
this this extension does not affect background image positioning.
4. Next is the layer containing the row groups. Each row group
extends from the top left corner of its topmost cell in the first
column to the bottom right corner of its bottommost cell in the
last column.
5. The next to last layer contains the rows. Each row is as wide as
the row groups and as tall as a normal (single-row-spanning) cell
in the row. As with columns, the background extends to cover the
full area of all cells that originate in the row, even if they
span outside the row, but this this extension does not affect
background image positioning.
6. The topmost layer contains the cells themselves. As the figure
shows, although all rows contain the same number of cells, not
every cell may have specified content. If the value of their
'empty-cells' property is 'hide' these "empty" cells are
transparent through the cell, row, row group, column and column
group backgrounds, letting the table background show through.
The edges of the rows, columns, row groups and column groups in the
collapsing borders model coincide with the hypothetical grid lines on
which the borders of the cells are centered. (And thus, in this model,
the rows together exactly cover the table, leaving no gaps; ditto for
the columns.) In the separated borders model, the edges coincide with
the border edges of cells. (And thus, in this model, there may be gaps
between the rows and columns, corresponding to the 'border-spacing'
property.)
In the following example, the first row contains four cells, but the
second row contains no cells, and thus the table background shines
through, except where a cell from the first row spans into this row.
The following HTML code and style rules
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<STYLE type="text/css">
TABLE { background: #ff0; border-collapse: collapse }
TD { background: red; border: double black }
</STYLE>
</HEAD>
<BODY>
<P>
<TABLE>
<TR>
<TD> 1
<TD rowspan="2"> 2
<TD> 3
<TD> 4
</TR>
<TR><TD></TD></TR>
</TABLE>
</BODY>
</HTML>
might be formatted as follows:
Table with three empty cells in bottom row
Table with three empty cells in the bottom row.
Note that if the table has 'border-collapse: separate', the background
of the area given by the 'border-spacing' property is always the
background of the table element. See the separated borders model.
17.5.2 Table width algorithms: the 'table-layout' property
CSS does not define an "optimal" layout for tables since, in many
cases, what is optimal is a matter of taste. CSS does define
constraints that user agents must respect when laying out a table.
User agents may use any algorithm they wish to do so, and are free to
prefer rendering speed over precision, except when the "fixed layout
algorithm" is selected.
Note that this section overrides the rules that apply to calculating
widths as described in section 10.3. In particular, if the margins of
a table are set to '0' and the width to 'auto', the table will not
automatically size to fill its containing block. However, once the
calculated value of 'width' for the table is found (using the
algorithms given below or, when appropriate, some other UA dependant
algorithm) then the other parts of section 10.3 do apply. Therefore a
table can be centered using left and right 'auto' margins, for
instance.
Future versions of CSS may introduce ways of making tables
automatically fit their containing blocks.
'table-layout'
Value: auto | fixed | inherit
Initial: auto
Applies to: 'table' and 'inline-table' elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
The 'table-layout' property controls the algorithm used to lay out the
table cells, rows, and columns. Values have the following meaning:
fixed
Use the fixed table layout algorithm
auto
Use any automatic table layout algorithm
The two algorithms are described below.
Fixed table layout
With this (fast) algorithm, the horizontal layout of the table does
not depend on the contents of the cells; it only depends on the
table's width, the width of the columns, and borders or cell spacing.
The table's width may be specified explicitly with the 'width'
property. A value of 'auto' (for both 'display: table' and 'display:
inline-table') means use the automatic table layout algorithm.
However, if the table is a block-level table ('display: table') in
normal flow, a UA may (but does not have to) use the algorithm of
10.3.3 to compute a width and apply fixed table layout even if the
specified width is 'auto'.
Example(s):
If a UA supports fixed table layout when 'width' is 'auto', the
following will create a table that is 4em narrower than its containing
block:
table { table-layout: fixed;
margin-left: 2em;
margin-right: 2em }
In the fixed table layout algorithm, the width of each column is
determined as follows:
1. A column element with a value other than 'auto' for the 'width'
property sets the width for that column.
2. Otherwise, a cell in the first row with a value other than 'auto'
for the 'width' property sets the width for that column. If the
cell spans more than one column, the width is divided over the
columns.
3. Any remaining columns equally divide the remaining horizontal
table space (minus borders or cell spacing).
The width of the table is then the greater of the value of the 'width'
property for the table element and the sum of the column widths (plus
cell spacing or borders). If the table is wider than the columns, the
extra space should be distributed over the columns.
In this manner, the user agent can begin to lay out the table once the
entire first row has been received. Cells in subsequent rows do not
affect column widths. Any cell that has content that overflows uses
the 'overflow' property to determine whether to clip the overflow
content.
Automatic table layout
In this algorithm (which generally requires no more than two passes),
the table's width is given by the width of its columns (and
intervening borders). This algorithm reflects the behavior of several
popular HTML user agents at the writing of this specification. UAs are
not required to implement this algorithm to determine the table layout
in the case that 'table-layout' is 'auto'; they can use any other
algorithm.
This algorithm may be inefficient since it requires the user agent to
have access to all the content in the table before determining the
final layout and may demand more than one pass.
Column widths are determined as follows:
1. Calculate the minimum content width (MCW) of each cell: the
formatted content may span any number of lines but may not
overflow the cell box. If the specified 'width' (W) of the cell is
greater than MCW, W is the minimum cell width. A value of 'auto'
means that MCW is the minimum cell width.
Also, calculate the "maximum" cell width of each cell: formatting
then content without breaking lines other than where explicit line
breaks occur.
2. For each column, determine a maximum and minimum column width from
the cells that span only that column. The minimum is that required
by the cell with the largest minimum cell width (or the column
'width', whichever is larger). The maximum is that required by the
cell with the largest maximum cell width (or the column 'width',
whichever is larger).
3. For each cell that spans more than one column, increase the
minimum widths of the columns it spans so that together, they are
at least as wide as the cell. Do the same for the maximum widths.
If possible, widen all spanned columns by approximately the same
amount.
This gives a maximum and minimum width for each column. Column widths
influence the final table width as follows:
1. If the 'table' or 'inline-table' element's 'width' property has a
specified value (W) other than 'auto', the property's computed
value is the greater of W and the minimum width required by all
the columns plus cell spacing or borders (MIN). If W is greater
than MIN, the extra width should be distributed over the columns.
2. If the 'table' or 'inline-table' element has 'width: auto', the
computed table width is the greater of the table's containing
block width and MIN. However, if the maximum width required by the
columns plus cell spacing or borders (MAX) is less than that of
the containing block, use MAX.
A percentage value for a column width is relative to the table width.
If the table has 'width: auto', a percentage represents a constraint
on the column's width, which a UA should try to satisfy. (Obviously,
this is not always possible: if the column's width is '110%', the
constraint cannot be satisfied.)
Note. In this algorithm, rows (and row groups) and columns (and column
groups) both constrain and are constrained by the dimensions of the
cells they contain. Setting the width of a column may indirectly
influence the height of a row, and vice versa.
17.5.3 Table height algorithms
The height of a table is given by the 'height' property for the
'table' or 'inline-table' element. A value of 'auto' means that the
height is the sum of the row heights plus any cell spacing or borders.
Any other value specifies the height explicitly; the table may thus be
taller or shorter than the height of its rows. CSS 2.1 does not
specify rendering when the specified table height differs from the
content height, in particular whether content height should override
specified height; if it doesn't, how extra space should be distributed
among rows that add up to less than the specified table height; or, if
the content height exceeds the specified table height, whether the UA
should provide a scrolling mechanism. Note. Future versions of CSS may
specify this further.
The height of a 'table-row' element's box is calculated once the user
agent has all the cells in the row available: it is the maximum of the
row's specified 'height' and the minimum height (MIN) required by the
cells. A 'height' value of 'auto' for a 'table-row' means the computed
row height is MIN. MIN depends on cell box heights and cell box
alignment (much like the calculation of a line box height). CSS 2.1
does not define what percentage values of 'height' refer to when
specified for table rows and row groups.
In CSS 2.1, the height of a cell box is the maximum of the table
cell's 'height' property and the minimum height required by the
content (MIN). A value of 'auto' for 'height' implies a computed value
of MIN. CSS 2.1 does not define what percentage values of 'height'
refer to when specified for table cells.
CSS 2.1 does not specify how cells that span more than row affect row
height calculations except that the sum of the row heights involved
must be great enough to encompass the cell spanning the rows.
The 'vertical-align' property of each table cell determines its
alignment within the row. Each cell's content has a baseline, a top, a
middle, and a bottom, as does the row itself. In the context of
tables, values for 'vertical-align' have the following meanings:
baseline
The baseline of the cell is put at the same height as the
baseline of the first of the rows it spans (see below for the
definition of baselines of cells and rows).
top
The top of the cell box is aligned with the top of the first
row it spans.
bottom
The bottom of the cell box is aligned with the bottom of the
last row it spans.
middle
The center of the cell is aligned with the center of the rows
it spans.
sub, super, text-top, text-bottom
These values do not apply to cells; the cell is aligned at the
baseline instead.
The baseline of a cell is the baseline of the first line box in the
cell. If there is no text, the baseline is the baseline of whatever
object is displayed in the cell, or, if it has none, the bottom of the
cell box. The maximum distance between the top of the cell box and the
baseline over all cells that have 'vertical-align: baseline' is used
to set the baseline of the row. Here is an example:
Example of vertically aligning the cells
Diagram showing the effect of various values of 'vertical-align' on
table cells.
Cell boxes 1 and 2 are aligned at their baselines. Cell box 2 has the
largest height above the baseline, so that determines the baseline of
the row. Note that if there is no cell box aligned at its baseline,
the row will not have (nor need) a baseline.
To avoid ambiguous situations, the alignment of cells proceeds in the
following order:
1. First the cells that are aligned on their baseline are positioned.
This will establish the baseline of the row. Next the cells with
'vertical-align: top' are positioned.
2. The row now has a top, possibly a baseline, and a provisional
height, which is the distance from the top to the lowest bottom of
the cells positioned so far. (See conditions on the cell padding
below.)
3. If any of the remaining cells, those aligned at the bottom or the
middle, have a height that is larger than the current height of
the row, the height of the row will be increased to the maximum of
those cells, by lowering the bottom.
4. Finally the remaining cells are positioned.
Cell boxes that are smaller than the height of the row receive extra
top or bottom padding.
17.5.4 Horizontal alignment in a column
The horizontal alignment of a cell's content within a cell box is
specified with the 'text-align' property.
17.5.5 Dynamic row and column effects
The 'visibility' property takes the value 'collapse' for row, row
group, column, and column group elements. This value causes the entire
row or column to be removed from the display, and the space normally
taken up by the row or column to be made available for other content.
The suppression of the row or column, however, does not otherwise
affect the layout of the table. This allows dynamic effects to remove
table rows or columns without forcing a re-layout of the table in
order to account for the potential change in column constraints.
17.6 Borders
There are two distinct models for setting borders on table cells in
CSS. One is most suitable for so-called separated borders around
individual cells, the other is suitable for borders that are
continuous from one end of the table to the other. Many border styles
can be achieved with either model, so it is often a matter of taste
which one is used.
'border-collapse'
Value: collapse | separate | inherit
Initial: separate
Applies to: 'table' and 'inline-table' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
This property selects a table's border model. The value 'separate'
selects the separated borders border model. The value 'collapse'
selects the collapsing borders model. The models are described below.
17.6.1 The separated borders model
'border-spacing'
Value: <length> <length>? | inherit
Initial: 0
Applies to: 'table' and 'inline-table' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: two absolute lengths
The lengths specify the distance that separates adjacent cell borders.
If one length is specified, it gives both the horizontal and vertical
spacing. If two are specified, the first gives the horizontal spacing
and the second the vertical spacing. Lengths may not be negative.
In this model, each cell has an individual border. The
'border-spacing' property specifies the distance between the borders
of adjacent cells. In this space, the row, column, row group, and
column group backgrounds are invisible, allowing the table background
to show through. Rows, columns, row groups, and column groups cannot
have borders (i.e., user agents must ignore the border properties for
those elements).
Example(s):
The table in the figure below could be the result of a style sheet
like this:
table { border: outset 10pt;
border-collapse: separate;
border-spacing: 15pt }
td { border: inset 5pt }
td.special { border: inset 10pt } /* The top-left cell */
A table with border-spacing
A table with 'border-spacing' set to a length value. Note that each
cell has its own border, and the table has a separate border as well.
Borders and Backgrounds around empty cells: the 'empty-cells' property
'empty-cells'
Value: show | hide | inherit
Initial: show
Applies to: 'table-cell' elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
In the separated borders model, this property controls the rendering
of borders and backgrounds around cells that have no visible content.
Empty cells and cells with the 'visibility' property set to 'hidden'
are considered to have no visible content. Visible content includes
" " and other whitespace except ASCII CR ("\0D"), LF ("\0A"), tab
("\09"), and space ("\20").
When this property has the value 'show', borders and backgrounds are
drawn around/behind empty cells (like normal cells).
A value of 'hide' means that no borders or backgrounds are drawn
around/behind empty cells (see point 6 in 17.5.1). Furthermore, if all
the cells in a row have a value of 'hide' and have no visible content,
the entire row behaves as if it had 'display: none'.
Example(s):
The following rule causes borders and backgrounds to be drawn around
all cells:
table { empty-cells: show }
17.6.2 The collapsing border model
In the collapsing border model, it is possible to specify borders that
surround all or part of a cell, row, row group, column, and column
group. Borders for HTML's "rule" attribute can be specified this way.
Borders are centered on the grid lines between the cells. User agents
must find a consistent rule for rounding off in the case of an odd
number of discrete units (screen pixels, printer dots).
The diagram below shows how the width of the table, the widths of the
borders, the padding, and the cell width interact. Their relation is
given by the following equation, which holds for every row of the
table:
row-width = (0.5 * border-width[0]) + padding-left[1] + width[1] +
padding-right[1] + border-width[1] + padding-left[2] +...+
padding-right[n] + (0.5 * border-width[n])
Here n is the number of cells in the row, and border-width[i] refers
to the border between cells i and i + 1. Note only half of the two
exterior borders are counted in the table width; the other half of
these two borders lies in the margin area. Padding-left[i] and
padding-right[i] refer to the left (resp., right) padding of cell i.
Schema showing the widths of cells and borders and the padding of
cells
Schema showing the widths of cells and borders and the padding of
cells.
Note that in this model, the width of the table includes half the
table border. Also, in this model, a table doesn't have padding (but
does have margins).
Border conflict resolution
In the collapsing border model, borders at every edge of every cell
may be specified by border properties on a variety of elements that
meet at that edge (cells, rows, row groups, columns, column groups,
and the table itself), and these borders may vary in width, style, and
color. The rule of thumb is that at each edge the most "eye catching"
border style is chosen, except that any occurrence of the style
'hidden' unconditionally turns the border off.
The following rules determine which border style "wins" in case of a
conflict:
1. Borders with the 'border-style' of 'hidden' take precedence over
all other conflicting borders. Any border with this value
suppresses all borders at this location.
2. Borders with a style of 'none' have the lowest priority. Only if
the border properties of all the elements meeting at this edge are
'none' will the border be omitted (but note that 'none' is the
default value for the border style.)
3. If none of the styles is 'hidden' and at least one of them is not
'none', then narrow borders are discarded in favor of wider ones.
If several have the same 'border-width' then styles are preferred
in this order: 'double', 'solid', 'dashed', 'dotted', 'ridge',
'outset', 'groove', and the lowest: 'inset'.
4. If border styles differ only in color, then a style set on a cell
wins over one on a row, which wins over a row group, column,
column group and, lastly, table. It is undefined which color is
used when two elements of the same type disagree.
Example(s):
The following example illustrates the application of these precedence
rules. This style sheet:
table { border-collapse: collapse;
border: 5px solid yellow; }
*#col1 { border: 3px solid black; }
td { border: 1px solid red; padding: 1em; }
td.solid-blue { border: 5px dashed blue; }
td.solid-green { border: 5px solid green; }
with this HTML source:
<P>
<TABLE>
<COL id="col1"><COL id="col2"><COL id="col3">
<TR id="row1">
<TD> 1
<TD> 2
<TD> 3
</TR>
<TR id="row2">
<TD> 4
<TD class="solid-blue"> 5
<TD class="solid-green"> 6
</TR>
<TR id="row3">
<TD> 7
<TD> 8
<TD> 9
</TR>
<TR id="row4">
<TD> 10
<TD> 11
<TD> 12
</TR>
<TR id="row5">
<TD> 13
<TD> 14
<TD> 15
</TR>
</TABLE>
would produce something like this:
An example of a table with collapsed borders
An example of a table with collapsed borders.
Example(s):
The next example shows a table with horizontal rules between the rows.
The top border of the table is set to 'hidden' to suppress the top
border of the first row. This implements the "rules" attribute of HTML
4.0 (rules="rows").
table[rules=rows] tr { border-top: solid }
table[rules=rows] { border-collapse: collapse;
border-top: hidden }
Table with horizontal rules
Table with horizontal rules between the rows.
In this case the same effect can also be achieved without setting a
'hidden' border on TABLE, by addressing the first row separately.
Which method is preferred is a matter of taste.
tr:first-child { border-top: none }
tr { border-top: solid }
Example(s):
Here is another example of hidden collapsing borders:
Table with two omitted borders
Table with two omitted internal borders.
HTML source:
<TABLE style="border-collapse: collapse; border: solid;">
<TR><TD style="border-right: hidden; border-bottom: hidden">foo</TD>
<TD style="border: solid">bar</TD></TR>
<TR><TD style="border: none">foo</TD>
<TD style="border: solid">bar</TD></TR>
</TABLE>
17.6.3 Border styles
Some of the values of the 'border-style' have different meanings in
tables than for other elements. In the list below they are marked with
an asterisk.
none
No border.
*hidden
Same as 'none', but in the collapsing border model, also
inhibits any other border (see the section on border
conflicts).
dotted
The border is a series of dots.
dashed
The border is a series of short line segments.
solid
The border is a single line segment.
double
The border is two solid lines. The sum of the two lines and the
space between them equals the value of 'border-width'.
groove
The border looks as though it were carved into the canvas.
ridge
The opposite of 'grove': the border looks as though it were
coming out of the canvas.
*inset
In the separated borders model, the border makes the entire box
look as though it were embedded in the canvas. In the
collapsing border model, same as 'groove'.
*outset
In the separated borders model, the border makes the entire box
look as though it were coming out of the canvas. In the
collapsing border model, same as 'ridge'.
_________________________________________________________________
_________________________________________________________________
18 User interface
Contents
* 18.1 Cursors: the 'cursor' property
* 18.2 User preferences for colors
* 18.3 User preferences for fonts
* 18.4 Dynamic outlines: the 'outline' property
+ 18.4.1 Outlines and the focus
* 18.5 Magnification
18.1 Cursors: the 'cursor' property
'cursor'
Value: [ [<uri> ,]* [ auto | crosshair | default | pointer | move |
e-resize | ne-resize | nw-resize | n-resize | se-resize | sw-resize |
s-resize | w-resize| text | wait | help | progress ] ] | inherit
Initial: auto
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual, interactive
Computed value: absolute URI; otherwise as specified
This property specifies the type of cursor to be displayed for the
pointing device. Values have the following meanings:
auto
The UA determines the cursor to display based on the current
context.
crosshair
A simple crosshair (e.g., short line segments resembling a "+"
sign).
default
The platform-dependent default cursor. Often rendered as an
arrow.
pointer
The cursor is a pointer that indicates a link.
move
Indicates something is to be moved.
e-resize, ne-resize, nw-resize, n-resize, se-resize, sw-resize,
s-resize, w-resize
Indicate that some edge is to be moved. For example, the
'se-resize' cursor is used when the movement starts from the
south-east corner of the box.
text
Indicates text that may be selected. Often rendered as an
I-bar.
wait,
Indicates that the program is busy and the user should wait.
Often rendered as a watch or hourglass.
progress
A progress indicator. The program is performing some
processing, but is different from 'wait' in that the user may
still interact with the program. Often rendered as a spinning
beach ball or an arrow with a small hourglass next to it.
help
Help is available for the object under the cursor. Often
rendered as a question mark or a balloon.
<uri>
The user agent retrieves the cursor from the resource
designated by the URI. If the user agent cannot handle the
first cursor of a list of cursors, it should attempt to handle
the second, etc. If the user agent cannot handle any
user-defined cursor, it must use the generic cursor at the end
of the list.
Example(s):
:link,:visited { cursor: url(example.svg#linkcursor), url(hyper.cur), pointer }
This example sets the cursor on all hyperlinks (whether visited or
not) to an external SVG cursor. User agents that don't support SVG
cursors would simply skip to the next value and attempt to use the
"hyper.cur" cursor. If that cursor format was also not supported, the
UA would skip to the next value and simply render the 'pointer'
cursor.
18.2 User preferences for colors
In addition to being able to assign pre-defined color values to text,
backgrounds, etc., CSS 2.1 allows authors to specify colors in a
manner that integrates them into the user's graphic environment. Style
rules that take into account user preferences thus offer the following
advantages:
1. They produce pages that fit the user's defined look and feel.
2. They produce pages that may be more accessible as the current user
settings may be related to a disability.
For systems that do not have a corresponding value, the specified
value should be mapped to the nearest system attribute, or to a
default color.
The following lists additional values for color-related CSS attributes
and their general meaning. Any color property (e.g., 'color' or
'background-color') can take one of the following names. Although
these are case-insensitive, it is recommended that the mixed
capitalization shown below be used, to make the names more legible.
ActiveBorder
Active window border.
ActiveCaption
Active window caption.
AppWorkspace
Background color of multiple document interface.
Background
Desktop background.
ButtonFace
Face color for three-dimensional display elements.
ButtonHighlight
Highlight color for three-dimensional display elements (for
edges facing away from the light source).
ButtonShadow
Shadow color for three-dimensional display elements.
ButtonText
Text on push buttons.
CaptionText
Text in caption, size box, and scrollbar arrow box.
GrayText
Grayed (disabled) text. This color is set to #000 if the
current display driver does not support a solid gray color.
Highlight
Item(s) selected in a control.
HighlightText
Text of item(s) selected in a control.
InactiveBorder
Inactive window border.
InactiveCaption
Inactive window caption.
InactiveCaptionText
Color of text in an inactive caption.
InfoBackground
Background color for tooltip controls.
InfoText
Text color for tooltip controls.
Menu
Menu background.
MenuText
Text in menus.
Scrollbar
Scroll bar gray area.
ThreeDDarkShadow
Dark shadow for three-dimensional display elements.
ThreeDFace
Face color for three-dimensional display elements.
ThreeDHighlight
Highlight color for three-dimensional display elements.
ThreeDLightShadow
Light color for three-dimensional display elements (for edges
facing the light source).
ThreeDShadow
Dark shadow for three-dimensional display elements.
Window
Window background.
WindowFrame
Window frame.
WindowText
Text in windows.
Example(s):
For example, to set the foreground and background colors of a
paragraph to the same foreground and background colors of the user's
window, write the following:
p { color: WindowText; background-color: Window }
18.3 User preferences for fonts
As for colors, authors may specify fonts in a way that makes use of a
user's system resources. Please consult the 'font' property for
details.
18.4 Dynamic outlines: the 'outline' property
At times, style sheet authors may want to create outlines around
visual objects such as buttons, active form fields, image maps, etc.,
to make them stand out. CSS 2.1 outlines differ from borders in the
following ways:
1. Outlines do not take up space.
2. Outlines may be non-rectangular.
The outline properties control the style of these dynamic outlines.
'outline'
Value: [ 'outline-color' || 'outline-style' || 'outline-width' ] |
inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: see individual properties
'outline-width'
Value: <border-width> | inherit
Initial: medium
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: absolute length; '0' if the outline style is 'none'
or 'hidden'
'outline-style'
Value: <border-style> | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: as specified
'outline-color'
Value: <color> | invert | inherit
Initial: invert
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual, interactive
Computed value: as specified
The outline created with the outline properties is drawn "over" a box,
i.e., the outline is always on top, and doesn't influence the position
or size of the box, or of any other boxes. Therefore, displaying or
suppressing outlines does not cause reflow.
The outline may be drawn starting just outside the border edge.
Outlines may be non-rectangular. For example, if the element is broken
across several lines, the outline is the minimum outline that encloses
all the element's boxes. In contrast to borders, the outline is not
open at the line box's end or start, but is always fully connected if
possible.
The 'outline-width' property accepts the same values as
'border-width'.
The 'outline-style' property accepts the same values as
'border-style', except that 'hidden' is not a legal outline style.
The 'outline-color' accepts all colors, as well as the keyword
'invert'. 'Invert' is expected to perform a color inversion on the
pixels on the screen. This is a common trick to ensure the focus
border is visible, regardless of color background.
[INS: :INS]
Conformant UAs may ignore the 'invert' value. If UA does not support
the 'invert' value then the initial value of the 'outline-color'
property is defined to be UA-specific.
The 'outline' property is a shorthand property, and sets all three of
'outline-style', 'outline-width', and 'outline-color'.
Note that the outline is the same on all sides. In contrast to
borders, there is no 'outline-top' or 'outline-left' property.
This specification does not define how multiple overlapping outlines
are drawn, or how outlines are drawn for boxes that are partially
obscured behind other elements.
Note. Since the focus outline does not affect formatting (i.e., no
space is left for it in the box model), it may well overlap other
elements on the page.
Example(s):
Here's an example of drawing a thick outline around a BUTTON element:
button { outline-width : thick }
Scripts may be used to dynamically change the width of the outline,
without provoking a reflow.
18.4.1 Outlines and the focus
Graphical user interfaces may use outlines around elements to tell the
user which element on the page has the focus. These outlines are in
addition to any borders, and switching outlines on and off should not
cause the document to reflow. The focus is the subject of user
interaction in a document (e.g., for entering text, selecting a
button, etc.). User agents supporting the interactive media group must
keep track of where the focus lies and must also represent the focus.
This may be done by using dynamic outlines in conjunction with the
:focus pseudo-class.
Example(s):
For example, to draw a thick black line around an element when it has
the focus, and a thick red line when it is active, the following rules
can be used:
:focus { outline: thick solid black }
:active { outline: thick solid red }
18.5 Magnification
The CSS working group considers that the magnification of a document
or portions of a document should not be specified through style
sheets. User agents may support such magnification in different ways
(e.g., larger images, louder sounds, etc.)
When magnifying a page, UAs should preserve the relationships between
positioned elements. For example, a comic strip may be composed of
images with overlaid text elements. When magnifying this page, a user
agent should keep the text within the comic strip balloon.
_________________________________________________________________
_________________________________________________________________
Appendix A. Aural style sheets
Contents
* A.1 The media type 'aural'
* A.2 Introduction to aural style sheets
+ A.2.1 Angles
+ A.2.2 Times
+ A.2.3 Frequencies
* A.3 Volume properties: 'volume'
* A.4 Speaking properties: 'speak'
* A.5 Pause properties: 'pause-before', 'pause-after', and 'pause'
* A.6 Cue properties: 'cue-before', 'cue-after', and 'cue'
* A.7 Mixing properties: 'play-during'
* A.8 Spatial properties: 'azimuth' and 'elevation'
* A.9 Voice characteristic properties: 'speech-rate',
'voice-family', 'pitch', 'pitch-range', 'stress', and 'richness'
* A.10 Speech properties: 'speak-punctuation' and 'speak-numeral'
* A.11 Audio rendering of tables
+ A.11.1 Speaking headers: the 'speak-header' property
* A.12 Sample style sheet for HTML
* A.13 Emacspeak
This chapter is informative. UAs are not required to implement the
properties of this chapter in order to conform to CSS 2.1.
A.1 The media type 'aural'
We expect that in a future level of CSS there will be new properties
and values defined for speech output. Therefore CSS 2.1 reserves the
'speech' media type (see chapter 7, "Media types"), but does not yet
define which properties do or do not apply to it.
The properties in this appendix apply to a media type 'aural', that
was introduced in CSS2. The type 'aural' is now deprecated.
This means that a style sheet such as
@media speech {
body { voice-family: Paul }
}
is valid, but that its meaning is not defined by CSS 2.1, while
@media aural {
body { voice-family: Paul }
}
is deprecated, but defined by this appendix.
A.2 Introduction to aural style sheets
The aural rendering of a document, already commonly used by the blind
and print-impaired communities, combines speech synthesis and
"auditory icons." Often such aural presentation occurs by converting
the document to plain text and feeding this to a screen reader --
software or hardware that simply reads all the characters on the
screen. This results in less effective presentation than would be the
case if the document structure were retained. Style sheet properties
for aural presentation may be used together with visual properties
(mixed media) or as an aural alternative to visual presentation.
Besides the obvious accessibility advantages, there are other large
markets for listening to information, including in-car use, industrial
and medical documentation systems (intranets), home entertainment, and
to help users learning to read or who have difficulty reading.
When using aural properties, the canvas consists of a
three-dimensional physical space (sound surrounds) and a temporal
space (one may specify sounds before, during, and after other sounds).
The CSS properties also allow authors to vary the quality of
synthesized speech (voice type, frequency, inflection, etc.).
Example(s):
h1, h2, h3, h4, h5, h6 {
voice-family: paul;
stress: 20;
richness: 90;
cue-before: url("ping.au")
}
p.heidi { azimuth: center-left }
p.peter { azimuth: right }
p.goat { volume: x-soft }
This will direct the speech synthesizer to speak headers in a voice (a
kind of "audio font") called "paul", on a flat tone, but in a very
rich voice. Before speaking the headers, a sound sample will be played
from the given URL. Paragraphs with class "heidi" will appear to come
from front left (if the sound system is capable of spatial audio), and
paragraphs of class "peter" from the right. Paragraphs with class
"goat" will be very soft.
A.2.1 Angles
Angle values are denoted by <angle> in the text. Their format is a
<number> immediately followed by an angle unit identifier.
Angle unit identifiers are:
* deg: degrees
* grad: grads
* rad: radians
Angle values may be negative. They should be normalized to the range
0-360deg by the user agent. For example, -10deg and 350deg are
equivalent.
For example, a right angle is '90deg' or '100grad' or
'1.570796326794897rad'.
Like for <length>, the unit may be omitted, if the value is zero:
'0deg' may be written as '0'.
A.2.2 Times
Time values are denoted by <time> in the text. Their format is a
<number> immediately followed by a time unit identifier.
Time unit identifiers are:
* ms: milliseconds
* s: seconds
Time values may not be negative.
Like for <length>, the unit may be omitted, if the value is zero: '0s'
may be written as '0'.
A.2.3 Frequencies
Frequency values are denoted by <frequency> in the text. Their format
is a <number> immediately followed by a frequency unit identifier.
Frequency unit identifiers are:
* Hz: Hertz
* kHz: kilohertz
Frequency values may not be negative.
For example, 200Hz (or 200hz) is a bass sound, and 6kHz is a treble
sound.
Like for <length>, the unit may be omitted, if the value is zero:
'0Hz' may be written as '0'.
A.3 Volume properties: 'volume'
'volume'
Value: <number> | <percentage> | silent | x-soft | soft | medium |
loud | x-loud | inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: refer to inherited value
Media: aural
Computed value: number
Volume refers to the median volume of the waveform. In other words, a
highly inflected voice at a volume of 50 might peak well above that.
The overall values are likely to be human adjustable for comfort, for
example with a physical volume control (which would increase both the
0 and 100 values proportionately); what this property does is adjust
the dynamic range.
Values have the following meanings:
<number>
Any number between '0' and '100'. '0' represents the minimum
audible volume level and 100 corresponds to the maximum
comfortable level.
<percentage>
Percentage values are calculated relative to the inherited
value, and are then clipped to the range '0' to '100'.
silent
No sound at all. The value '0' does not mean the same as
'silent'.
x-soft
Same as '0'.
soft
Same as '25'.
medium
Same as '50'.
loud
Same as '75'.
x-loud
Same as '100'.
User agents should allow the values corresponding to '0' and '100' to
be set by the listener. No one setting is universally applicable;
suitable values depend on the equipment in use (speakers, headphones),
the environment (in car, home theater, library) and personal
preferences. Some examples:
* A browser for in-car use has a setting for when there is lots of
background noise. '0' would map to a fairly high level and '100'
to a quite high level. The speech is easily audible over the road
noise but the overall dynamic range is compressed. Cars with
better insulation might allow a wider dynamic range.
* Another speech browser is being used in an apartment, late at
night, or in a shared study room. '0' is set to a very quiet level
and '100' to a fairly quiet level, too. As with the first example,
there is a low slope; the dynamic range is reduced. The actual
volumes are low here, whereas they were high in the first example.
* In a quiet and isolated house, an expensive hi-fi home theater
setup. '0' is set fairly low and '100' to quite high; there is
wide dynamic range.
The same author style sheet could be used in all cases, simply by
mapping the '0' and '100' points suitably at the client side.
A.4 Speaking properties: 'speak'
'speak'
Value: normal | none | spell-out | inherit
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property specifies whether text will be rendered aurally and if
so, in what manner. The possible values are:
none
Suppresses aural rendering so that the element requires no time
to render. Note, however, that descendants may override this
value and will be spoken. (To be sure to suppress rendering of
an element and its descendants, use the 'display' property).
normal
Uses language-dependent pronunciation rules for rendering an
element and its children.
spell-out
Spells the text one letter at a time (useful for acronyms and
abbreviations).
Note the difference between an element whose 'volume' property has a
value of 'silent' and an element whose 'speak' property has the value
'none'. The former takes up the same time as if it had been spoken,
including any pause before and after the element, but no sound is
generated. The latter requires no time and is not rendered (though its
descendants may be).
A.5 Pause properties: 'pause-before', 'pause-after', and 'pause'
'pause-before'
Value: <time> | <percentage> | inherit
Initial: 0
Applies to: all elements
Inherited: no
Percentages: see prose
Media: aural
Computed value: time
'pause-after'
Value: <time> | <percentage> | inherit
Initial: 0
Applies to: all elements
Inherited: no
Percentages: see prose
Media: aural
Computed value: time;;
These properties specify a pause to be observed before (or after)
speaking an element's content. Values have the following meanings:
<time>
Expresses the pause in absolute time units (seconds and
milliseconds).
<percentage>
Refers to the inverse of the value of the 'speech-rate'
property. For example, if the speech-rate is 120 words per
minute (i.e., a word takes half a second, or 500ms) then a
'pause-before' of 100% means a pause of 500 ms and a
'pause-before' of 20% means 100ms.
The pause is inserted between the element's content and any
'cue-before' or 'cue-after' content.
Authors should use relative units to create more robust style sheets
in the face of large changes in speech-rate.
'pause'
Value: [ [<time> | <percentage>]{1,2} ] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: see descriptions of 'pause-before' and 'pause-after'
Media: aural
Computed value: see individual properties
The 'pause' property is a shorthand for setting 'pause-before' and
'pause-after'. If two values are given, the first value is
'pause-before' and the second is 'pause-after'. If only one value is
given, it applies to both properties.
Example(s):
h1 { pause: 20ms } /* pause-before: 20ms; pause-after: 20ms */
h2 { pause: 30ms 40ms } /* pause-before: 30ms; pause-after: 40ms */
h3 { pause-after: 10ms } /* pause-before unspecified; pause-after: 10ms */
A.6 Cue properties: 'cue-before', 'cue-after', and 'cue'
'cue-before'
Value: <uri> | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: absolute URI or 'none'
'cue-after'
Value: <uri> | none | inherit
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: absolute URI or 'none'
Auditory icons are another way to distinguish semantic elements.
Sounds may be played before and/or after the element to delimit it.
Values have the following meanings:
<uri>
The URI must designate an auditory icon resource. If the URI
resolves to something other than an audio file, such as an
image, the resource should be ignored and the property treated
as if it had the value 'none'.
none
No auditory icon is specified.
Example(s):
a {cue-before: url("bell.aiff"); cue-after: url("dong.wav") }
h1 {cue-before: url("pop.au"); cue-after: url("pop.au") }
'cue'
Value: [ 'cue-before' || 'cue-after' ] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: see individual properties
The 'cue' property is a shorthand for setting 'cue-before' and
'cue-after'. If two values are given, the first value is 'cue-before'
and the second is 'cue-after'. If only one value is given, it applies
to both properties.
Example(s):
The following two rules are equivalent:
h1 {cue-before: url("pop.au"); cue-after: url("pop.au") }
h1 {cue: url("pop.au") }
If a user agent cannot render an auditory icon (e.g., the user's
environment does not permit it), we recommend that it produce an
alternative cue.
Please see the sections on the :before and :after pseudo-elements for
information on other content generation techniques. 'cue-before'
sounds and 'pause-before' gaps are inserted before content from the
':before' pseudo-element. Similarly, 'pause-after' gaps and
'cue-after' sounds are inserted after content from the ':after'
pseudo-element.
A.7 Mixing properties: 'play-during'
'play-during'
Value: <uri> [ mix || repeat ]? | auto | none | inherit
Initial: auto
Applies to: all elements
Inherited: no
Percentages: N/A
Media: aural
Computed value: absolute URI, rest as specified
Similar to the 'cue-before' and 'cue-after' properties, this property
specifies a sound to be played as a background while an element's
content is spoken. Values have the following meanings:
<uri>
The sound designated by this <uri> is played as a background
while the element's content is spoken.
mix
When present, this keyword means that the sound inherited from
the parent element's 'play-during' property continues to play
and the sound designated by the <uri> is mixed with it. If
'mix' is not specified, the element's background sound replaces
the parent's.
repeat
When present, this keyword means that the sound will repeat if
it is too short to fill the entire duration of the element.
Otherwise, the sound plays once and then stops. This is similar
to the 'background-repeat' property. If the sound is too long
for the element, it is clipped once the element has been
spoken.
auto
The sound of the parent element continues to play (it is not
restarted, which would have been the case if this property had
been inherited).
none
This keyword means that there is silence. The sound of the
parent element (if any) is silent during the current element
and continues after the current element.
Example(s):
blockquote.sad { play-during: url("violins.aiff") }
blockquote Q { play-during: url("harp.wav") mix }
span.quiet { play-during: none }
A.8 Spatial properties: 'azimuth' and 'elevation'
Spatial audio is an important stylistic property for aural
presentation. It provides a natural way to tell several voices apart,
as in real life (people rarely all stand in the same spot in a room).
Stereo speakers produce a lateral sound stage. Binaural headphones or
the increasingly popular 5-speaker home theater setups can generate
full surround sound, and multi-speaker setups can create a true
three-dimensional sound stage. VRML 2.0 also includes spatial audio,
which implies that in time consumer-priced spatial audio hardware will
become more widely available.
'azimuth'
Value: <angle> | [[ left-side | far-left | left | center-left |
center | center-right | right | far-right | right-side ] || behind ] |
leftwards | rightwards | inherit
Initial: center
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: normalized angle
Values have the following meanings:
<angle>
Position is described in terms of an angle within the range
'-360deg' to '360deg'. The value '0deg' means directly ahead in
the center of the sound stage. '90deg' is to the right,
'180deg' behind, and '270deg' (or, equivalently and more
conveniently, '-90deg') to the left.
left-side
Same as '270deg'. With 'behind', '270deg'.
far-left
Same as '300deg'. With 'behind', '240deg'.
left
Same as '320deg'. With 'behind', '220deg'.
center-left
Same as '340deg'. With 'behind', '200deg'.
center
Same as '0deg'. With 'behind', '180deg'.
center-right
Same as '20deg'. With 'behind', '160deg'.
right
Same as '40deg'. With 'behind', '140deg'.
far-right
Same as '60deg'. With 'behind', '120deg'.
right-side
Same as '90deg'. With 'behind', '90deg'.
leftwards
Moves the sound to the left, relative to the current angle.
More precisely, subtracts 20 degrees. Arithmetic is carried out
modulo 360 degrees. Note that 'leftwards' is more accurately
described as "turned counter-clockwise," since it always
subtracts 20 degrees, even if the inherited azimuth is already
behind the listener (in which case the sound actually appears
to move to the right).
rightwards
Moves the sound to the right, relative to the current angle.
More precisely, adds 20 degrees. See 'leftwards' for
arithmetic.
This property is most likely to be implemented by mixing the same
signal into different channels at differing volumes. It might also use
phase shifting, digital delay, and other such techniques to provide
the illusion of a sound stage. The precise means used to achieve this
effect and the number of speakers used to do so are user
agent-dependent; this property merely identifies the desired end
result.
Example(s):
h1 { azimuth: 30deg }
td.a { azimuth: far-right } /* 60deg */
#12 { azimuth: behind far-right } /* 120deg */
p.comment { azimuth: behind } /* 180deg */
If spatial-azimuth is specified and the output device cannot produce
sounds behind the listening position, user agents should convert
values in the rearwards hemisphere to forwards hemisphere values. One
method is as follows:
* if 90deg < x <= 180deg then x := 180deg - x
* if 180deg < x <= 270deg then x := 540deg - x
'elevation'
Value: <angle> | below | level | above | higher | lower | inherit
Initial: level
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: normalized angle
Values of this property have the following meanings:
<angle>
Specifies the elevation as an angle, between '-90deg' and
'90deg'. '0deg' means on the forward horizon, which loosely
means level with the listener. '90deg' means directly overhead
and '-90deg' means directly below.
below
Same as '-90deg'.
level
Same as '0deg'.
above
Same as '90deg'.
higher
Adds 10 degrees to the current elevation.
lower
Subtracts 10 degrees from the current elevation.
The precise means used to achieve this effect and the number of
speakers used to do so are undefined. This property merely identifies
the desired end result.
Example(s):
h1 { elevation: above }
tr.a { elevation: 60deg }
tr.b { elevation: 30deg }
tr.c { elevation: level }
A.9 Voice characteristic properties: 'speech-rate', 'voice-family', 'pitch',
'pitch-range', 'stress', and 'richness'
'speech-rate'
Value: <number> | x-slow | slow | medium | fast | x-fast | faster |
slower | inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: number
This property specifies the speaking rate. Note that both absolute and
relative keyword values are allowed (compare with 'font-size'). Values
have the following meanings:
<number>
Specifies the speaking rate in words per minute, a quantity
that varies somewhat by language but is nevertheless widely
supported by speech synthesizers.
x-slow
Same as 80 words per minute.
slow
Same as 120 words per minute
medium
Same as 180 - 200 words per minute.
fast
Same as 300 words per minute.
x-fast
Same as 500 words per minute.
faster
Adds 40 words per minute to the current speech rate.
slower
Subtracts 40 words per minutes from the current speech rate.
'voice-family'
Value: [[<specific-voice> | <generic-voice> ],]* [<specific-voice> |
<generic-voice> ] | inherit
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
The value is a comma-separated, prioritized list of voice family names
(compare with 'font-family'). Values have the following meanings:
<generic-voice>
Values are voice families. Possible values are 'male',
'female', and 'child'.
<specific-voice>
Values are specific instances (e.g., comedian, trinoids,
carlos, lani).
Example(s):
h1 { voice-family: announcer, male }
p.part.romeo { voice-family: romeo, male }
p.part.juliet { voice-family: juliet, female }
Names of specific voices may be quoted, and indeed must be quoted if
any of the words that make up the name does not conform to the syntax
rules for identifiers. It is also recommended to quote specific voices
with a name consisting of more than one word. If quoting is omitted,
any whitespace characters before and after the voice family name are
ignored and any sequence of whitespace characters inside the voice
family name is converted to a single space.
'pitch'
Value: <frequency> | x-low | low | medium | high | x-high | inherit
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: frequency
Specifies the average pitch (a frequency) of the speaking voice. The
average pitch of a voice depends on the voice family. For example, the
average pitch for a standard male voice is around 120Hz, but for a
female voice, it's around 210Hz.
Values have the following meanings:
<frequency>
Specifies the average pitch of the speaking voice in hertz
(Hz).
x-low, low, medium, high, x-high
These values do not map to absolute frequencies since these
values depend on the voice family. User agents should map these
values to appropriate frequencies based on the voice family and
user environment. However, user agents must map these values in
order (i.e., 'x-low' is a lower frequency than 'low', etc.).
'pitch-range'
Value: <number> | inherit
Initial: 50
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
Specifies variation in average pitch. The perceived pitch of a human
voice is determined by the fundamental frequency and typically has a
value of 120Hz for a male voice and 210Hz for a female voice. Human
languages are spoken with varying inflection and pitch; these
variations convey additional meaning and emphasis. Thus, a highly
animated voice, i.e., one that is heavily inflected, displays a high
pitch range. This property specifies the range over which these
variations occur, i.e., how much the fundamental frequency may deviate
from the average pitch.
Values have the following meanings:
<number>
A value between '0' and '100'. A pitch range of '0' produces a
flat, monotonic voice. A pitch range of 50 produces normal
inflection. Pitch ranges greater than 50 produce animated
voices.
'stress'
Value: <number> | inherit
Initial: 50
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
Specifies the height of "local peaks" in the intonation contour of a
voice. For example, English is a stressed language, and different
parts of a sentence are assigned primary, secondary, or tertiary
stress. The value of 'stress' controls the amount of inflection that
results from these stress markers. This property is a companion to the
'pitch-range' property and is provided to allow developers to exploit
higher-end auditory displays.
Values have the following meanings:
<number>
A value, between '0' and '100'. The meaning of values depends
on the language being spoken. For example, a level of '50' for
a standard, English-speaking male voice (average pitch =
122Hz), speaking with normal intonation and emphasis would have
a different meaning than '50' for an Italian voice.
'richness'
Value: <number> | inherit
Initial: 50
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
Specifies the richness, or brightness, of the speaking voice. A rich
voice will "carry" in a large room, a smooth voice will not. (The term
"smooth" refers to how the wave form looks when drawn.)
Values have the following meanings:
<number>
A value between '0' and '100'. The higher the value, the more
the voice will carry. A lower value will produce a soft,
mellifluous voice.
A.10 Speech properties: 'speak-punctuation' and 'speak-numeral'
An additional speech property, speak-header, is described below.
'speak-punctuation'
Value: code | none | inherit
Initial: none
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property specifies how punctuation is spoken. Values have the
following meanings:
code
Punctuation such as semicolons, braces, and so on are to be
spoken literally.
none
Punctuation is not to be spoken, but instead rendered naturally
as various pauses.
'speak-numeral'
Value: digits | continuous | inherit
Initial: continuous
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property controls how numerals are spoken. Values have the
following meanings:
digits
Speak the numeral as individual digits. Thus, "237" is spoken
"Two Three Seven".
continuous
Speak the numeral as a full number. Thus, "237" is spoken "Two
hundred thirty seven". Word representations are
language-dependent.
A.11 Audio rendering of tables
When a table is spoken by a speech generator, the relation between the
data cells and the header cells must be expressed in a different way
than by horizontal and vertical alignment. Some speech browsers may
allow a user to move around in the 2-dimensional space, thus giving
them the opportunity to map out the spatially represented relations.
When that is not possible, the style sheet must specify at which
points the headers are spoken.
A.11.1 Speaking headers: the 'speak-header' property
'speak-header'
Value: once | always | inherit
Initial: once
Applies to: elements that have table header information
Inherited: yes
Percentages: N/A
Media: aural
Computed value: as specified
This property specifies whether table headers are spoken before every
cell, or only before a cell when that cell is associated with a
different header than the previous cell. Values have the following
meanings:
once
The header is spoken one time, before a series of cells.
always
The header is spoken before every pertinent cell.
Each document language may have different mechanisms that allow
authors to specify headers. For example, in HTML 4.0 ([HTML40]), it is
possible to specify header information with three different attributes
("headers", "scope", and "axis"), and the specification gives an
algorithm for determining header information when these attributes
have not been specified.
Image of a table created in MS Word
Image of a table with header cells ("San Jose" and "Seattle") that are
not in the same column or row as the data they apply to.
This HTML example presents the money spent on meals, hotels and
transport in two locations (San Jose and Seattle) for successive days.
Conceptually, you can think of the table in terms of an n-dimensional
space. The headers of this space are: location, day, category and
subtotal. Some cells define marks along an axis while others give
money spent at points within this space. The markup for this table is:
<TABLE>
<CAPTION>Travel Expense Report</CAPTION>
<TR>
<TH></TH>
<TH>Meals</TH>
<TH>Hotels</TH>
<TH>Transport</TH>
<TH>subtotal</TH>
</TR>
<TR>
<TH id="san-jose" axis="san-jose">San Jose</TH>
</TR>
<TR>
<TH headers="san-jose">25-Aug-97</TH>
<TD>37.74</TD>
<TD>112.00</TD>
<TD>45.00</TD>
<TD></TD>
</TR>
<TR>
<TH headers="san-jose">26-Aug-97</TH>
<TD>27.28</TD>
<TD>112.00</TD>
<TD>45.00</TD>
<TD></TD>
</TR>
<TR>
<TH headers="san-jose">subtotal</TH>
<TD>65.02</TD>
<TD>224.00</TD>
<TD>90.00</TD>
<TD>379.02</TD>
</TR>
<TR>
<TH id="seattle" axis="seattle">Seattle</TH>
</TR>
<TR>
<TH headers="seattle">27-Aug-97</TH>
<TD>96.25</TD>
<TD>109.00</TD>
<TD>36.00</TD>
<TD></TD>
</TR>
<TR>
<TH headers="seattle">28-Aug-97</TH>
<TD>35.00</TD>
<TD>109.00</TD>
<TD>36.00</TD>
<TD></TD>
</TR>
<TR>
<TH headers="seattle">subtotal</TH>
<TD>131.25</TD>
<TD>218.00</TD>
<TD>72.00</TD>
<TD>421.25</TD>
</TR>
<TR>
<TH>Totals</TH>
<TD>196.27</TD>
<TD>442.00</TD>
<TD>162.00</TD>
<TD>800.27</TD>
</TR>
</TABLE>
By providing the data model in this way, authors make it possible for
speech enabled-browsers to explore the table in rich ways, e.g., each
cell could be spoken as a list, repeating the applicable headers
before each data cell:
San Jose, 25-Aug-97, Meals: 37.74
San Jose, 25-Aug-97, Hotels: 112.00
San Jose, 25-Aug-97, Transport: 45.00
...
The browser could also speak the headers only when they change:
San Jose, 25-Aug-97, Meals: 37.74
Hotels: 112.00
Transport: 45.00
26-Aug-97, Meals: 27.28
Hotels: 112.00
...
A.12 Sample style sheet for HTML
This style sheet describes a possible rendering of HTML 4.0:
@media aural {
h1, h2, h3,
h4, h5, h6 { voice-family: paul, male; stress: 20; richness: 90 }
h1 { pitch: x-low; pitch-range: 90 }
h2 { pitch: x-low; pitch-range: 80 }
h3 { pitch: low; pitch-range: 70 }
h4 { pitch: medium; pitch-range: 60 }
h5 { pitch: medium; pitch-range: 50 }
h6 { pitch: medium; pitch-range: 40 }
li, dt, dd { pitch: medium; richness: 60 }
dt { stress: 80 }
pre, code, tt { pitch: medium; pitch-range: 0; stress: 0; richness: 80 }
em { pitch: medium; pitch-range: 60; stress: 60; richness: 50 }
strong { pitch: medium; pitch-range: 60; stress: 90; richness: 90 }
dfn { pitch: high; pitch-range: 60; stress: 60 }
s, strike { richness: 0 }
i { pitch: medium; pitch-range: 60; stress: 60; richness: 50 }
b { pitch: medium; pitch-range: 60; stress: 90; richness: 90 }
u { richness: 0 }
a:link { voice-family: harry, male }
a:visited { voice-family: betty, female }
a:active { voice-family: betty, female; pitch-range: 80; pitch: x-high }
}
A.13 Emacspeak
For information, here is the list of properties implemented by
Emacspeak, a speech subsystem for the Emacs editor.
* voice-family
* stress (but with a different range of values)
* richness (but with a different range of values)
* pitch (but with differently named values)
* pitch-range (but with a different range of values)
(We thank T. V. Raman for the information about implementation status
of aural properties.)
_________________________________________________________________
_________________________________________________________________
Appendix D. A sample style sheet for HTML 4.0
This appendix is informative, not normative.
This style sheet describes the typical formatting of all HTML 4.0
([HTML40]) elements based on extensive research into current UA
practice. Developers are encouraged to use it as a default style sheet
in their implementations.
The full presentation of some HTML elements cannot be expressed in
CSS 2.1, including replaced elements ("img", "object"), scripting
elements ("script", "applet"), form control elements, and frame
elements.
For other elements, the legacy presentation can be described in CSS
but the solution removes the element. For example, the FONT element
can be replaced by attaching CSS declarations to other elements (e.g.,
DIV). Likewise, legacy presentation of presentational attributes
(e.g., the "border" attribute on TABLE) can be described in CSS, but
the markup in the source document must be changed.
address,
blockquote,
body, dd, div,
dl, dt, fieldset, form,
frame, frameset,
h1, h2, h3, h4,
h5, h6, noframes,
ol, p, ul, center,
dir, hr, menu, pre { display: block }
li { display: list-item }
head { display: none }
table { display: table }
tr { display: table-row }
thead { display: table-header-group }
tbody { display: table-row-group }
tfoot { display: table-footer-group }
col { display: table-column }
colgroup { display: table-column-group }
td, th { display: table-cell; }
caption { display: table-caption }
th { font-weight: bolder; text-align: center }
caption { text-align: center }
body { padding: 8px; line-height: 1.12em }
h1 { font-size: 2em; margin: .67em 0 }
h2 { font-size: 1.5em; margin: .75em 0 }
h3 { font-size: 1.17em; margin: .83em 0 }
h4, p,
blockquote, ul,
fieldset, form,
ol, dl, dir,
menu { margin: 1.12em 0 }
h5 { font-size: .83em; margin: 1.5em 0 }
h6 { font-size: .75em; margin: 1.67em 0 }
h1, h2, h3, h4,
h5, h6, b,
strong { font-weight: bolder }
blockquote { margin-left: 40px; margin-right: 40px }
i, cite, em,
var, address { font-style: italic }
pre, tt, code,
kbd, samp { font-family: monospace }
pre { white-space: pre }
button, textarea,
input, object,
select, img { display:inline-block; }
big { font-size: 1.17em }
small, sub, sup { font-size: .83em }
sub { vertical-align: sub }
sup { vertical-align: super }
s, strike, del { text-decoration: line-through }
hr { border: 1px inset }
ol, ul, dir,
menu, dd { margin-left: 40px }
ol { list-style-type: decimal }
ol ul, ul ol,
ul ul, ol ol { margin-top: 0; margin-bottom: 0 }
u, ins { text-decoration: underline }
br:before { content: "\A" }
center { text-align: center }
abbr, acronym { font-variant: small-caps; letter-spacing: 0.1em }
:link, :visited { text-decoration: underline }
:focus { outline: thin dotted invert }
/* Begin bidirectionality settings (do not change) */
BDO[DIR="ltr"] { direction: ltr; unicode-bidi: bidi-override }
BDO[DIR="rtl"] { direction: rtl; unicode-bidi: bidi-override }
*[DIR="ltr"] { direction: ltr; unicode-bidi: embed }
*[DIR="rtl"] { direction: rtl; unicode-bidi: embed }
@media print {
h1 { page-break-before: always }
h1, h2, h3,
h4, h5, h6 { page-break-after: avoid }
ul, ol, dl { page-break-before: avoid }
}
_________________________________________________________________
_________________________________________________________________
Appendix C. Changes
Contents
* C.1 Additional property values
+ C.1.1 Section 4.3.5 Colors
+ C.1.2 Section 9.2.4 The 'display' property
+ C.1.3 Section 18.1 Cursors: the 'cursor' property
+ C.1.4 Section 16.6 Whitespace: the 'white-space' property
* C.2 Changes
+ C.2.1 Section 3.2 Conformance
+ C.2.2 Section 6.4.3 Calculating a selector's specificity
+ C.2.3 Section 6.4.4 Precedence of non-CSS presentational
hints
+ C.2.4 Chapter 9 Visual formatting model
+ C.2.5 Section 10.3.7 Absolutely positioned, non-replaced
elements
+ C.2.6 Section 10.6.4 Absolutely positioned, non-replaced
elements
+ C.2.7 Section 11.1.2 Clipping: the 'clip' property
+ C.2.8 17.4.1 Caption position and alignment
+ C.2.9 17.5.4 Horizontal alignment in a column
+ C.2.10 Section 17.6 Borders
+ C.2.11 Chapter 12 Generated content, automatic numbering, and
lists
+ C.2.12 Section 12.2 The 'content' property
+ C.2.13 Chapter 13 Paged media
+ C.2.14 Chapter 15 Fonts
+ C.2.15 Chapter 16 Text
+ C.2.16 Appendix A. Aural style sheets
+ C.2.17 Other
* C.3 Errors
+ C.3.1 Shorthand properties
+ C.3.2 Section 4.1.1 (and G2)
+ C.3.3 4.1.3 Characters and case
+ C.3.4 Section 4.3 (Double sign problem)
+ C.3.5 Section 4.3.2 Lengths
+ C.3.6 Section 4.3.6
+ C.3.7 5.10 Pseudo-elements and pseudo-classes
+ C.3.8 8.2 Example of margins, padding, and borders
+ C.3.9 Section 8.5.2 Border color: 'border-top-color',
'border-right-color', 'border-bottom-color',
'border-left-color', and 'border-color'
+ C.3.10 Section 8.4 Padding properties
+ C.3.11 8.5.3 Border style
+ C.3.12 Section 8.5.4 Border shorthand properties:
'border-top', 'border-bottom', 'border-right', 'border-left',
and 'border'
+ C.3.13 8.5.4 Border shorthand properties: 'border-top',
'border-bottom', 'border-right', 'border-left', and 'border'
+ C.3.14 Section 9.3.1
+ C.3.15 Section 9.3.2
+ C.3.16 Section 9.4.3
+ C.3.17 Section 9.7 Relationships between 'display',
'position', and 'float'
+ C.3.18 Section 10.3.2 Inline, replaced elements (and 10.3.4,
10.3.6, and 10.3.8)
+ C.3.19 Section 10.3.3
+ C.3.20 Section 10.6.2 Inline, replaced elements ... (and
10.6.5)
+ C.3.21 Section 10.6.3
+ C.3.22 Section 11.1.1
+ C.3.23 11.2 Visibility: the 'visibility' property
+ C.3.24 12.6.2 Lists
+ C.3.25 Section 15.2.6
+ C.3.26 Section 15.5
+ C.3.27 Section 16.6 Whitespace: the 'white-space' property
+ C.3.28 Section 17.2 The CSS table model
+ C.3.29 17.2.1 Anonymous table objects
+ C.3.30 17.5 Visual layout of table contents
+ C.3.31 17.5 Visual layout of table contents
+ C.3.32 Section 17.5.1 Table layers and transparency
+ C.3.33 Section 17.6.1 The separated borders model
+ C.3.34 Appendix D.2 Lexical scanner
* C.4 Clarifications
+ C.4.1 2.2 A brief CSS2 tutorial for XML
+ C.4.2 Section 4.1.1
+ C.4.3 Section 5.5
+ C.4.4 Section 5.9 ID selectors
+ C.4.5 Section 5.12.1 The :first-line pseudo-element
+ C.4.6 Section 6.2.1
+ C.4.7 6.4 The Cascade
+ C.4.8 Section 6.4.3 Calculating a selector's specificity
+ C.4.9 Section 7.3 Recognized media types
+ C.4.10 Section 8.1
+ C.4.11 Section 8.3.1
+ C.4.12 Section 9.4.2
+ C.4.13 Section 9.4.3
+ C.4.14 Section 9.10
+ C.4.15 10.3.3 Block-level, non-replaced elements in normal
flow
+ C.4.16 Section 10.5 Content height: the 'height' property
+ C.4.17 Section 10.8.1
+ C.4.18 Section 11.1
+ C.4.19 Section 11.1.1
+ C.4.20 Section 11.1.2
+ C.4.21 12.1 The :before and :after pseudo-elements
+ C.4.22 Section 12.4.2 Inserting quotes with the 'content'
property
+ C.4.23 Lists 12.6.2
+ C.4.24 14.2 The background
+ C.4.25 14.2.1 Background properties
+ C.4.26 Section 16.1
+ C.4.27 16.2 Alignment: the 'text-align' property
+ C.4.28 Section 17.5.1 Table layers and transparency
+ C.4.29 Section 17.5.2 Table width algorithms
+ C.4.30 17.6.1 The separated borders model
+ C.4.31 Borders around empty cells: the 'empty-cells' property
+ C.4.32 Section 17.6.2 The collapsing borders model
+ C.4.33 Section 18.2
+ C.4.34 Section A.3
+ C.4.35 Appendix G.2 Lexical scanner
+ C.4.36 Appendix E. References
This appendix is informative, not normative.
CSS 2.1 is an updated version of CSS2. The changes between the CSS2
specification (see [CSS2]) and this specification fall into five
groups: known errors, typographical errors, clarifications, changes
and additions. Typographical errors are not listed here.
C.1 Additional property values
C.1.1 Section 4.3.5 Colors
New color value: 'orange'
C.1.2 Section 9.2.4 The 'display' property
New 'display' value: 'inline-block'
C.1.3 Section 18.1 Cursors: the 'cursor' property
New 'cursor' value: 'progress'
C.1.4 Section 16.6 Whitespace: the 'white-space' property
New 'white-space' values: 'pre-wrap' and 'pre-line'
C.2 Changes
C.2.1 Section 3.2 Conformance
Support for user style sheets is now required (in most cases), rather
than just recommended.
C.2.2 Section 6.4.3 Calculating a selector's specificity
The "style" attribute now has a higher specificity than any style
rule.
C.2.3 Section 6.4.4 Precedence of non-CSS presentational hints
"Non-CSS presentational hints" no longer exist, with the exception of
a small set of attributes in HTML.
C.2.4 Chapter 9 Visual formatting model
The value 'compact' for 'display' does not exist in CSS 2.1.
C.2.5 Section 10.3.7 Absolutely positioned, non-replaced elements
Absolutely positioned elements can now "shrink-wrap" their contents:
When both 'width' and 'right' (or 'width' and 'left') are 'auto', the
element's computed width is the width of the contents (using an
algorithm similar to that for table cells) and then 'right' (or
'left') is solved. CSS2 incorrectly said that 'right' (or 'left') was
set to 0 in that case, and then width was solved.
C.2.6 Section 10.6.4 Absolutely positioned, non-replaced elements
Like normal-flow block-level elements, absolutely positioned elements
by default take on the height of their contents ("shrink-wrap"). If
'height' and 'bottom' are both 'auto', the computed value of 'height'
is set to the height of the contents and then 'bottom' is solved. CSS2
incorrectly said the reverse: 'bottom' was set to 0 and then height
was solved.
C.2.7 Section 11.1.2 Clipping: the 'clip' property
While CSS2 specified that values of "rect()" give offsets from the
respective sides of the box, current implementations interpret values
with respect to the top and left edges for all four values (top,
right, bottom, and left). This is now the correct interpretation.
C.2.8 17.4.1 Caption position and alignment
The 'left' and 'right' values on 'caption-side' have been removed.
C.2.9 17.5.4 Horizontal alignment in a column
The <string> value for 'text-align' is not part of CSS 2.1.
C.2.10 Section 17.6 Borders
Several popular browsers assume an initial value for 'border-collapse'
of 'separate' rather than 'collapse' or exhibit behavior that is close
to that value, even if they do not actually implement the CSS table
model. 'Separate' is now the initial value.
C.2.11 Chapter 12 Generated content, automatic numbering, and lists
The 'marker' value for 'display' does not exist in CSS 2.1
C.2.12 Section 12.2 The 'content' property
The '<uri>' value is dropped.
C.2.13 Chapter 13 Paged media
The 'size', 'marks' and 'page' properties are not in CSS 2.1.
C.2.14 Chapter 15 Fonts
The 'font-stretch' and 'font-size-adjust' properties don't exist in
CSS 2.1.
Font descriptors and the '@font-face' declaration don't exist in
CSS 2.1.
C.2.15 Chapter 16 Text
The 'text-shadow' property is not in CSS 2.1.
C.2.16 Appendix A. Aural style sheets
Chapter 19 on aural style sheets has become appendix A and is not
normative in CSS 2.1. Related units (deg, grad, rad, ms, s, Hz, kHz)
are also moved to this appendix, as is the 'speak-header' property
from the "tables" chapter.
C.2.17 Other
The former informative appendix C, "Implementation and performance
notes for fonts," is left out of CSS 2.1.
C.3 Errors
C.3.1 Shorthand properties
Shorthand properties take a list of subproperty values or the value
'inherit'. One cannot mix 'inherit' with other subproperty values as
it would not be possible to specify the subproperty to which 'inherit'
applied. The definitions of a number of shorthand properties did not
enforce this rule: 'border-top', 'border-right', 'border-bottom',
'border-left', 'border', 'background', 'font', 'list-style', 'cue',
and 'outline'.
C.3.2 Section 4.1.1 (and G2)
* The "nmchar" token also allows the range "A-Z".
* In the rule for "any" (in the core syntax), changed "FUNCTION" to
"FUNCTION any* ')'".
The underscore character ("_") is allowed in identifiers. The
definitions of the lexical macros "nmstart" and "nmchar" now include
it.
C.3.3 4.1.3 Characters and case
In the third bullet, added to point 1:
1.with a space (or other whitespace character): "\26 B" ("&B")
the following text: "In this case, user agents should treat a "CR/LF"
pair (13/10) as a single whitespace character."
The underscore is allowed in identifiers. Changed "In CSS2,
identifiers [...] can contain only the characters [A-Za-z0-9] and ISO
10646 characters 161 and higher, plus the hyphen (-)" to:
In CSS2, identifiers [...] contain only the characters [A-Za-z0-9]
and ISO 10646 characters 161 and higher, plus the hyphen (-) and
the underscore (_)
C.3.4 Section 4.3 (Double sign problem)
Several values described in subsections of this section incorrectly
allowed two "+" or "-" signs at their beginnings.
C.3.5 Section 4.3.2 Lengths
The suggested reference pixel is based on a [INS: 96 dpi :INS] device,
not 90 dpi. The visual angle is thus about [INS: 0.0213 degrees :INS]
instead of 0.0227, and a pixel at arm's length is about [INS: 0.26 mm
:INS] instead of 0.28
C.3.6 Section 4.3.6
Deleted the comments about range restriction after the following
examples:
em { color: rgb(255,0,0) }
em { color: rgb(100%, 0%, 0%) }
C.3.7 5.10 Pseudo-elements and pseudo-classes
In the second bullet, the following sentence was incomplete: "The
exception is ':first-child', which can be deduced from the document
tree." The ':lang()' pseudo-class can be deduced from the document in
some cases.
C.3.8 8.2 Example of margins, padding, and borders
The colors in the example HTML did not match the colors in the image.
C.3.9 Section 8.5.2 Border color: 'border-top-color', 'border-right-color',
'border-bottom-color', 'border-left-color', and 'border-color'
The value 'transparent' is also allowed on 'border-top-color',
'border-right-color', etc. Changed the line "Value: <color> | inherit"
to
Value: <color> | transparent | inherit
C.3.10 Section 8.4 Padding properties
The five properties related to padding ('padding', 'padding-top',
'padding-right', 'padding-bottom', and 'padding-left') now say that
they don't apply to table rows, row groups, header groups, footer
groups, columns, and column groups.
C.3.11 8.5.3 Border style
Changed the sentence "The color of borders drawn for values of
'groove', 'ridge', 'inset', and 'outset' depends on the element's
'color' property" to
The color of borders drawn for values of 'groove', 'ridge',
'inset', and 'outset' should be based on the element's
'border-color' property, but UAs may choose their own algorithm to
calculate the actual colors used. For instance, if the
'border-color' has the value 'silver', then a UA could use a
gradient of colors from white to dark gray to indicate a sloping
border.
C.3.12 Section 8.5.4 Border shorthand properties: 'border-top',
'border-bottom', 'border-right', 'border-left', and 'border'
Changed <'border-top-width'> to <border-width> as the first value
option for 'border-top', 'border-right', 'border-bottom', and
'border-left', and changed <'border-style'> to <border-style>. For
'border', changed <'border-width'> to <border-width> and
<'border-style'> to <border-style>.
The value 'transparent' is also allowed on 'border-top',
'border-bottom', 'border-right', 'border-left', and 'border'.
Changed the two lines "Value: [ <'border-top-width'> ||
<'border-style'> || <color> | inherit" to
Value: [ <border-top-width> || <border-style> || [<color> |
transparent] | inherit
C.3.13 8.5.4 Border shorthand properties: 'border-top', 'border-bottom',
'border-right', 'border-left', and 'border'
Changed <'border-top-width'> to <border-width> as the first value
option for 'border-top', 'border-right', 'border-bottom', and
'border-left', and changed <'border-style'> to <border-style>. For
'border', changed <'border-width'> to <border-width> and
<'border-style'> to <border-style>.
C.3.14 Section 9.3.1
The definition of the value 'static' now says that the properties
'top', 'right', 'bottom', and 'left' do not apply.
C.3.15 Section 9.3.2
The properties 'top', 'right', 'bottom', and 'left', incorrectly
referred to offsets with respect to a box's content edge. The proper
edge is the margin edge. Thus, for 'top', the description now reads:
"This property specifies how far a box's top margin edge is offset
below the top edge of the box's containing block."
C.3.16 Section 9.4.3
In the first sentence, added to the end of "Once a box has been laid
out according to the normal flow" the words "or floated,".
C.3.17 Section 9.7 Relationships between 'display', 'position', and 'float'
If an element floats, the 'display' property is set to a block-level
value, but not necessarily 'block'. In bullet 3, changed "Otherwise,
if 'float' has a value other than 'none', 'display' is set to 'block'
and the box is floated" to a table with the proper computed values.
C.3.18 Section 10.3.2 Inline, replaced elements (and 10.3.4, 10.3.6, and
10.3.8)
Changed:
A specified value of 'auto' for 'width' gives the element's
intrinsic width as the computed value.
to:
If 'width' has a specified value of 'auto' and 'height' also has a
specified value of 'auto', the element's intrinsic width is the
computed value of 'width'. If 'width' has a specified value of
'auto' and 'height' has some other specified value, then the
computed value of 'width' is (intrinsic width) * ( (computed
height) / (intrinsic height) ).
C.3.19 Section 10.3.3
In the last sentence of the paragraph following the equation ("If the
value of 'direction' is 'ltr', this happens to 'margin-left' instead")
substituted 'rtl' for 'ltr'.
C.3.20 Section 10.6.2 Inline, replaced elements ... (and 10.6.5)
Changed:
If 'height' is 'auto', the computed value is the intrinsic height.
to:
If 'height' has a specified value of 'auto' and 'width' also has a
specified value of 'auto', the element's intrinsic height is the
computed value of 'height'. If 'height' has a specified value of
'auto' and 'width' has some other specified value, then the
computed value of 'height' is (intrinsic height) * ( (computed
width) / (intrinsic width) ).
C.3.21 Section 10.6.3
The height calculation for block-level, non-replaced elements in
normal flow, and floating, non-replaced elements was not quite
correct. It now takes into account the case when margins do not
collapse, due to the presence of a padding or border.
C.3.22 Section 11.1.1
The example of a DIV element containing a BLOCKQUOTE containing
another DIV was not rendered correctly. The first style rule applied
to both DIVs, so the second DIV box should have been rendered with a
red border as well. The second DIV has now been changed to a CITE,
which doesn't have a red border.
C.3.23 11.2 Visibility: the 'visibility' property
Changed "initial" and "inherited" to:
Initial: visible
Inherited: yes
This has the same effect as the original definition, but removes the
undefined state of the root element (which was a problem for DOM
implementations).
C.3.24 12.6.2 Lists
Under the 'list-style' property, the example:
ul > ul { list-style: circle outside } /* Any UL child of a UL */
could never match valid HTML markup (since a UL element cannot be a
child of another UL element). An LI has been inserted in between.
C.3.25 Section 15.2.6
'Totum' and 'Kodic' is not a 'serif' but 'sans-serif'. 'pathang' is
not a 'sans-serif' but 'serif'.
C.3.26 Section 15.5
In bullet 2, changed "the UA uses the 'font-family' descriptor" to
"the UA uses the 'font-family' property".
In bullet 6, changed "steps 3, 4 and 5" to "steps 2, 3, 4 and 5".
C.3.27 Section 16.6 Whitespace: the 'white-space' property
The 'white-space' property applies to all elements, not just
block-level elements.
C.3.28 Section 17.2 The CSS table model
In the definition of table-header-group, changed "footer" to "header"
in "Print user agents may repeat footer rows on each page spanned by a
table."
C.3.29 17.2.1 Anonymous table objects
Moved the first bullet text to the prose before the list of generation
rules and added missing rules.
C.3.30 17.5 Visual layout of table contents
The following note:
Note. Table cells may be relatively and absolutely positioned, but
this is not recommended: positioning and floating remove a box from
the flow, affecting table alignment.
has been amended as follows:
Note. Table cells may be positioned, but this is not recommended:
absolute and fixed positioning, as well as floating, remove a box
from the flow, affecting table size.
C.3.31 17.5 Visual layout of table contents
Changed:
Like other elements of the document language, internal table
elements generate rectangular boxes with content, padding, and
borders. They do not have margins, however.
to:
Like other elements of the document language, internal table
elements generate rectangular boxes with content and borders. Cells
have padding as well. Internal table elements do not have margins.
C.3.32 Section 17.5.1 Table layers and transparency
The rows and columns only cover the whole table in the collapsed
borders model, not in the separated borders model. The points 2, 3, 4
and 5 have been corrected to define the area covered by rows, columns,
row groups and column groups in terms of the cells they cover.
C.3.33 Section 17.6.1 The separated borders model
In the image, changed "cell-spacing" to "border-spacing".
C.3.34 Appendix D.2 Lexical scanner
The underscore character ("_") is be allowed in identifiers. The
definitions of the lexical macros "nmstart" and "nmchar" have been
fixed.
Note that the tokenizer is case-insensitive, so uppercase A-Z is
matched as well.
(Same change in section 4.1.1, see above.)
C.4 Clarifications
C.4.1 2.2 A brief CSS2 tutorial for XML
The specification for the XML style sheet PI was written after CSS2
was finalized. The first line of the full XML example should not have
been be <?XML:stylesheet type="text/css" href="bach.css"?>, but
<?xml-stylesheet type="text/css" href="bach.css"?>
C.4.2 Section 4.1.1
DELIM should not have included single or double quote. Refer also to
section 4.1.6 on strings, which must have matching single or double
quotes around them.
C.4.3 Section 5.5
Near the end of the section, the text 'Note the whitespace on either
side of the "*"' was misleading. The note was not meant to imply that
whitespace is required on both sides of the "*" (since the grammar
does not require it in this case) but that one may use whitespace in
this case.
C.4.4 Section 5.9 ID selectors
The word "precedence" in the last but one paragraph should have been
"specificity."
C.4.5 Section 5.12.1 The :first-line pseudo-element
Added some clarifications at the end of the section about the
fictional tag sequence in the case of nested block-level elements
C.4.6 Section 6.2.1
The 'inherit' value causes the properties value to be inherited. This
applies even to properties for which values do not otherwise inherit.
C.4.7 6.4 The Cascade
Changed "Rules specified in a given style sheet override rules
imported from other style sheets." to "Rules specified in a given
style sheet override rules of the same weight imported from other
style sheets."
C.4.8 Section 6.4.3 Calculating a selector's specificity
Added a note:
The specificity is based only on the form of the selector. In
particular, a selector of the form "[id=p33]" is counted as an
attribute selector (a=0, b=1, c=0), even if the id attribute is
defined as an "ID" in the source document's DTD.
C.4.9 Section 7.3 Recognized media types
Text has been added to clarify that media types are mutually
exclusive.
C.4.10 Section 8.1
* From the definition of "padding edge", deleted the sentence "The
padding edge of a box defines the edges of the containing block
established by the box." For information about containing blocks,
consult Section 10.1.
* Border backgrounds are not specified by border properties. Changed
the last paragraph of 8.1 to:
The background style of the content, padding, and border areas of a
box is specified by the 'background' property of the generating
element. Margin backgrounds are always transparent.
C.4.11 Section 8.3.1
Added this clarifying note to the first bullet of the explanation of
vertical collapsing of margins:
Note. Adjoining boxes may be generated by elements that are not
related as siblings or ancestors.
C.4.12 Section 9.4.2
The statement "When an inline box is split, margins, borders, and
padding have no visual effect where the split occurs." has been
generalized. Margins, borders, and padding have no visual effect where
one or more splits occur.
C.4.13 Section 9.4.3
Relatively positioned boxes do not always establish new containing
blocks. Changed the second paragraph accordingly.
Added clarifying text and an example about the 'left', 'right', 'top'
and 'bottom' properties for relative positioning.
C.4.14 Section 9.10
In this sentence of the last paragraph:
Conforming HTML user agents may therefore ignore the 'direction'
and 'unicode-bidi' properties in author and user style sheets.
the word "ignore" meant that if a 'unicode-bidi' or 'direction' value
conflicts with the HTML 4.0 "dir" attribute value, then user agents
may choose to use the "dir" value rather than the CSS properties.
User agents are not required to support the 'direction' and
'unicode-bidi' properties to conform to CSS2 unless they support
bi-directional text rendering (except for the case of HTML 4.0 as
noted above).
The sentence has been rewritten to be clearer.
C.4.15 10.3.3 Block-level, non-replaced elements in normal flow
Added the following note at the end of the section:
Note that 'width' may not be greater than 'max-width' and not less
than 'min-width'. In particular, it may not be negative. See the
rules in section 10.4 below.
C.4.16 Section 10.5 Content height: the 'height' property
The UA is free to chose the containing block for the root element (see
10.1), therefore this sentence has been added as a suggestion:
A UA may compute a percentage height on the root element relative
to the viewport.
C.4.17 Section 10.8.1
Clarified this paragraph:
Note that replaced elements have a 'font-size' and a 'line-height'
property, even if they are not used directly to determine the
height of the box. The 'font-size' is, however, used to define the
'em' and 'ex' units, and the 'line-height' has a role in the
'vertical-align' property.
as follows:
Note that replaced elements have a 'font-size' and a 'line-height'
property, even if they are not used directly to determine the
height of the box: 'em' and 'ex' values are relative to values of
'font-size' and percentage values for 'vertical-align' are relative
to values of 'line-height'.
Under 'line-height', after the sentence "If the property is set on a
block-level element whose content is composed of inline-level
elements, it specifies the minimal height of each generated inline
box," added the following clarification:
The minimum height consist of a minimum height above the block's
baseline and a minimum depth below it, exactly as if each line box
starts with a zero-width inline box with the block's font and line
height properties (what T[E]X calls a "strut").
C.4.18 Section 11.1
Clarifications to the last two bullets on when overflow may occur:
* A descendent box is positioned absolutely partly outside of the
box.
* A descendent box has negative margins, causing it to be positioned
partly outside the box.
C.4.19 Section 11.1.1
Removed 'projection' from this sentence under the value 'scroll'
When this value is specified and the target medium is 'print' or
'projection', overflowing content should be printed.
C.4.20 Section 11.1.2
Values of "rect()" should be separated by commas. Thus, the definition
of <shape> now starts:
In CSS2, the only valid <shape> value is: rect (<top>, <right>,
<bottom>, <left>) ...
Due to this ambiguity, user agents may support separation of offsets
in "rect()" with or without commas.
C.4.21 12.1 The :before and :after pseudo-elements
Clarification to the following lines:
The :before and :after pseudo-elements elements allow values of the
'display' property as follows:
* If the subject of the selector is a block-level element, allowed
values are 'none', 'inline' and 'block'. If the value of the
pseudo-element's 'display' property has any other value, the
pseudo-element will behave as if its value were 'block'.
* If the subject of the selector is an inline-level element, allowed
values are 'none' and 'inline'. If the value of the
pseudo-element's 'display' property has any other value, the
pseudo-element will behave as if its value were 'inline'.
C.4.22 Section 12.4.2 Inserting quotes with the 'content' property
Added the following sentence at the end of the 2nd paragraph:
A 'close-quote' that would make the depth negative is in error and
is ignored: the depth stays at 0 and no quote mark is rendered
(although the rest of the 'content' property's value is still
inserted).
C.4.23 Lists 12.6.2
To clarify Hebrew numbering, added "(Alef, Bet, ... Tet Vav, Tet
Zayin, ... Yod Tet, Kaf ...)".
C.4.24 14.2 The background
Second sentence: "In terms of the box model, 'background' refers to
the background of the content and the padding areas" now also mentions
the border area. (See also errata to section 8.1 above.) Thus:
In terms of the box model, "background" refers to the background of
the content, padding and border areas.
In the fourth paragraph, added to the end of "User agents should
observe the following precedence rules to fill in the background" the
following words: "of the canvas".
C.4.25 14.2.1 Background properties
Added this note after the first paragraph after
'background-attachment':
Note that there is only one viewport per document. I.e., even if an
element has a scrolling mechanism (see 'overflow'), a 'fixed'
background doesn't move with it.
Under 'background-repeat', the sentence "All tiling covers the content
and padding areas [...]" has been corrected to
"All tiling covers the content, padding [INS: and border :INS]
areas [...]".
Under 'background-attachment', the sentence "Even if the image is
fixed [...] background or padding area of the element" has been
corrected to
Even if the image is fixed, it is still only visible when it is in
the background, padding [INS: or border :INS] area of the element.
C.4.26 Section 16.1
Added to:
The value of 'text-indent' may be negative, but there may be
implementation-specific limits.
the following clarification: "If the value of 'text-indent' is
negative, the value of 'overflow' will affect whether the text is
visible."
C.4.27 16.2 Alignment: the 'text-align' property
Changed "double justify" to "justify" under "left, right, center, and
justify".
C.4.28 Section 17.5.1 Table layers and transparency
In point 6, changed 'These "empty" cells are transparent' to:
If the value of their 'empty-cells' property is 'hide' these
"empty" cells are transparent through the cell, row, row group,
column, and column group backgrounds, letting the table background
show through.
To remove ambiguity about the position of backgrounds on rows and
column, the following paragraph was added after point 6:
the edges of the rows, columns, row groups and column groups in the
collapsing borders model coincide with the hypothetical grid lines
on which the borders of the cells are centered. (And thus, in this
model, the rows together exactly cover the table, leaving no gaps;
ditto for the columns.) In the separated borders model, the edges
coincide with the border edges of cells. (And thus, in this model,
there may be gaps between the rows and columns, corresponding to
the 'border-spacing' property.)
At the end of the section added the following paragraph:
Note that if the table has 'border-collapse: separate', the
background of the area given by the 'border-spacing' property is
always the background of the table element. See 17.6.1
C.4.29 Section 17.5.2 Table width algorithms
Added the following paragraph after the initial paragraph of this
section:
Note that this section overrides the rules that apply to
calculating widths as described in section 10.3. In particular, if
the margins of a table are set to '0' and the width to 'auto', the
table will not automatically size to fill its containing block.
However, once the calculated value of 'width' for the table is
found (using the algorithms given below or, when appropriate, some
other UA dependant algorithm) then the other parts of section 10.3
do apply. Therefore a table can be centered using left and right
'auto' margins, for instance.
The WG may introduce ways of automatically making tables fit their
containing blocks in CSS3.
C.4.30 17.6.1 The separated borders model
Added clarification about alignment of row/column backgrounds. The
sentence "This space is filled with the background of the table
element" was replaced by:
In this space, the row, column, row group, and column group
backgrounds are invisible, allowing the table background to show
through.
C.4.31 Borders around empty cells: the 'empty-cells' property
The 'empty-cells' property not only controls the borders, but also the
background.
C.4.32 Section 17.6.2 The collapsing borders model
In the sentence after the question, added "and padding-left[i] and
padding-right[i] refer to the left (resp., right) padding of cell i."
C.4.33 Section 18.2
For the 'ButtonHighlight' value, changed the description from "Dark
shadow" to "Highlight color".
C.4.34 Section A.3
The parenthetical phrase "somewhat analogous to the 'display'
property" was misleading. The 'speak' property resembles 'visibility'
in some ways and 'display' in others.
C.4.35 Appendix G.2 Lexical scanner
Removed the following line from the scanner as it does not appear in
the grammar:
"@"{ident} {return ATKEYWORD;}
The DIMEN token is in the scanner to ensure that a number followed by
an identifier is read as one token rather than two. This case is
considered an error in CSS2.
C.4.36 Appendix E. References
The entry for "[URI]" referred to a draft that has become an RFC. The
entry has been changed.
_________________________________________________________________
_________________________________________________________________
Appendix G. Grammar of CSS 2.1
Contents
* G.1 Grammar
* G.2 Lexical scanner
* G.3 Comparison of tokenization in CSS 2.1 and CSS1
This appendix is normative.
The grammar below defines the syntax of CSS 2.1. It is in some sense,
however, a superset of CSS 2.1 as this specification imposes
additional semantic constraints not expressed in this grammar. A
conforming UA must also adhere to the forward-compatible parsing
rules, the property and value notation, and the unit notation. In
addition, the document language may impose restrictions, e.g. HTML
imposes restrictions on the possible values of the "class" attribute.
G.1 Grammar
The grammar below is LL(1) (but note that most UA's should not use it
directly, since it doesn't express the parsing conventions, only the
CSS 2.1 syntax). The format of the productions is optimized for human
consumption and some shorthand notation beyond Yacc (see [YACC]) is
used:
* *: 0 or more
* +: 1 or more
* ?: 0 or 1
* |: separates alternatives
* [ ]: grouping
The productions are:
stylesheet
: [ CHARSET_SYM S* STRING S* ';' ]?
[S|CDO|CDC]* [ import [S|CDO|CDC]* ]*
[ [ ruleset | media | page ] [S|CDO|CDC]* ]*
;
import
: IMPORT_SYM S*
[STRING|URI] S* [ medium [ ',' S* medium]* ]? ';' S*
;
media
: MEDIA_SYM S* medium [ ',' S* medium ]* '{' S* ruleset* '}' S*
;
medium
: IDENT S*
;
page
: PAGE_SYM S* pseudo_page? S*
'{' S* declaration [ ';' S* declaration ]* '}' S*
;
pseudo_page
: ':' IDENT
;
operator
: '/' S* | ',' S* | /* empty */
;
combinator
: '+' S* | '>' S* | /* empty */
;
unary_operator
: '-' | '+'
;
property
: IDENT S*
;
ruleset
: selector [ ',' S* selector ]*
'{' S* declaration [ ';' S* declaration ]* '}' S*
;
selector
: simple_selector [ combinator simple_selector ]*
;
simple_selector
: element_name? [ HASH | class | attrib | pseudo ]* S*
;
class
: '.' IDENT
;
element_name
: IDENT | '*'
;
attrib
: '[' S* IDENT S* [ [ '=' | INCLUDES | DASHMATCH ] S*
[ IDENT | STRING ] S* ]? ']'
;
pseudo
: ':' [ IDENT | FUNCTION S* IDENT S* ')' ]
;
declaration
: property ':' S* expr prio?
| /* empty */
;
prio
: IMPORTANT_SYM S*
;
expr
: term [ operator term ]*
;
term
: unary_operator?
[ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
TIME S* | FREQ S* | function ]
| STRING S* | IDENT S* | URI S* | UNICODERANGE S* | hexcolor
;
function
: FUNCTION S* expr ')' S*
;
/*
* There is a constraint on the color that it must
* have either 3 or 6 hex-digits (i.e., [0-9a-fA-F])
* after the "#"; e.g., "#000" is OK, but "#abcd" is not.
*/
hexcolor
: HASH S*
;
G.2 Lexical scanner
The following is the tokenizer, written in Flex (see [FLEX]) notation.
The tokenizer is case-insensitive.
The two occurrences of "\377" represent the highest character number
that current versions of Flex can deal with (decimal 255). They should
be read as "\4177777" (decimal 1114111), which is the highest possible
code point in Unicode/ISO-10646.
%option case-insensitive
h [0-9a-f]
nonascii [\200-\377]
unicode \\{h}{1,6}[ \t\r\n\f]?
escape {unicode}|\\[ -~\200-\377]
nmstart [_a-z]|{nonascii}|{escape}
nmchar [_a-zA-Z0-9-]|{nonascii}|{escape}
string1 \"([\t !#$%&(-~]|\\{nl}|\'|{nonascii}|{escape})*\"
string2 \'([\t !#$%&(-~]|\\{nl}|\"|{nonascii}|{escape})*\'
ident {nmstart}{nmchar}*
name {nmchar}+
num [0-9]+|[0-9]*"."[0-9]+
string {string1}|{string2}
url ([!#$%&*-~]|{nonascii}|{escape})*
w [ \t\r\n\f]*
nl \n|\r\n|\r|\f
range \?{1,6}|{h}(\?{0,5}|{h}(\?{0,4}|{h}(\?{0,3}|{h}(\?{0,2}|{h}(\??
|{h})))))
%%
[ \t\r\n\f]+ {return S;}
\/\*[^*]*\*+([^/*][^*]*\*+)*\/ /* ignore comments */
"<!--" {return CDO;}
"-->" {return CDC;}
"~=" {return INCLUDES;}
"|=" {return DASHMATCH;}
{string} {return STRING;}
{ident} {return IDENT;}
"#"{name} {return HASH;}
"@import" {return IMPORT_SYM;}
"@page" {return PAGE_SYM;}
"@media" {return MEDIA_SYM;}
"@charset" {return CHARSET_SYM;}
"!"{w}"important" {return IMPORTANT_SYM;}
{num}em {return EMS;}
{num}ex {return EXS;}
{num}px {return LENGTH;}
{num}cm {return LENGTH;}
{num}mm {return LENGTH;}
{num}in {return LENGTH;}
{num}pt {return LENGTH;}
{num}pc {return LENGTH;}
{num}deg {return ANGLE;}
{num}rad {return ANGLE;}
{num}grad {return ANGLE;}
{num}ms {return TIME;}
{num}s {return TIME;}
{num}Hz {return FREQ;}
{num}kHz {return FREQ;}
{num}{ident} {return DIMEN;}
{num}% {return PERCENTAGE;}
{num} {return NUMBER;}
"url("{w}{string}{w}")" {return URI;}
"url("{w}{url}{w}")" {return URI;}
{ident}"(" {return FUNCTION;}
U\+{range} {return UNICODERANGE;}
U\+{h}{1,6}-{h}{1,6} {return UNICODERANGE;}
. {return *yytext;}
G.3 Comparison of tokenization in CSS 2.1 and CSS1
There are some differences in the syntax specified in the CSS1
recommendation ([CSS1]), and the one above. Most of these are due to
new tokens in CSS2 that didn't exist in CSS1. Others are because the
grammar has been rewritten to be more readable. However, there are
some incompatible changes, that were felt to be errors in the CSS1
syntax. They are explained below.
* CSS1 style sheets could only be in 1-byte-per-character encodings,
such as ASCII and ISO-8859-1. CSS 2.1 has no such limitation. In
practice, there was little difficulty in extrapolating the CSS1
tokenizer, and some UAs have accepted 2-byte encodings.
* CSS1 only allowed four hex-digits after the backslash (\) to refer
to Unicode characters, CSS2 allows six. Furthermore, CSS2 allows a
whitespace character to delimit the escape sequence. E.g.,
according to CSS1, the string "\abcdef" has 3 letters (\abcd, e,
and f), according to CSS2 it has only one (\abcdef).
* The tab character (ASCII 9) was not allowed in strings. However,
since strings in CSS1 were only used for font names and for URLs,
the only way this can lead to incompatibility between CSS1 and
CSS2 is if a style sheet contains a font family that has a tab in
its name.
* Similarly, newlines (escaped with a backslash) were not allowed in
strings in CSS1.
* CSS2 parses a number immediately followed by an identifier as a
DIMEN token (i.e., an unknown unit), CSS1 parsed it as a number
and an identifier. That means that in CSS1, the declaration 'font:
10pt/1.2serif' was correct, as was 'font: 10pt/12pt serif'; in
CSS2, a space is required before "serif". (Some UAs accepted the
first example, but not the second.)
* In CSS1, a class name could start with a digit (".55ft"), unless
it was a dimension (".55in"). In CSS2, such classes are parsed as
unknown dimensions (to allow for future additions of new units).
To make ".55ft" a valid class, CSS2 requires the first digit to be
escaped (".\35 5ft")
_________________________________________________________________
_________________________________________________________________
Appendix B. Bibliography
Contents
* B.1 Normative references
* B.2 Informative references
B.1 Normative references
[COLORIMETRY]
"Colorimetry, Second Edition", CIE Publication 15.2-1986, ISBN
3-900-734-00-3.
Available at
http://www.hike.te.chiba-u.ac.jp/ikeda/CIE/publ/abst/15-2-86.ht
ml.
[CSS1]
"Cascading Style Sheets, level 1", H. W. Lie and B. Bos, 17
December 1996.
The latest version is available at
http://www.w3.org/TR/REC-CSS1
[CSS2]
"Cascading Style Sheets, level 2, CSS2 Specification", B. Bos,
H. W. Lie, C. Lilley and I. Jacobs, 12 May 1998,
The latest version is available at
http://www.w3.org/TR/REC-CSS2
[FLEX]
"Flex: The Lexical Scanner Generator", Version 2.3.7, ISBN
1882114213.
[HTML40]
"HTML 4.0 Specification", D. Raggett, A. Le Hors, I. Jacobs, 18
December 1997.
The latest version of the specification is available at
http://www.w3.org/TR/REC-html40/. The Recommendation defines
three document type definitions: Strict, Transitional, and
Frameset, all reachable from the Recommendation.
[IANA]
"Assigned Numbers", STD 2, RFC 1700, USC/ISI, J. Reynolds and
J. Postel, October 1994.
Available at http://www.ietf.org/rfc/rfc1700.txt.
[ICC32]
"ICC Profile Format Specification, version 3.2", 1995.
Available at ftp://sgigate.sgi.com/pub/icc/ICC32.pdf.
[ISO8879]
ISO 8879:1986 "Information Processing -- Text and Office
Systems -- Standard Generalized Markup Language (SGML)", ISO
8879:1986.
For the list of SGML entities, consult
ftp://ftp.ifi.uio.no/pub/SGML/ENTITIES/.
[ISO10646]
"Information Technology - Universal Multiple- Octet Coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane", ISO/IEC 10646-1:1993. The current
specification also takes into consideration the first five
amendments to ISO/IEC 10646-1:1993. Useful roadmap of the BMP
and roadmap of plane 1 documents show which scripts sit at
which numeric ranges.
[PNG10]
"PNG (Portable Network Graphics) Specification, Version 1.0
specification", T. Boutell ed., 1 October 1996.
Available at http://www.w3.org/pub/WWW/TR/REC-png-multi.html.
[RFC1808]
"Relative Uniform Resource Locators", R. Fielding, June 1995.
Available at http://www.ietf.org/rfc/rfc1808.txt.
[RFC2045]
"Multipurpose Internet Mail Extensions (MIME) Part One: Format
of Internet Message Bodies", N. Freed and N. Borenstein,
November 1996.
Available at http://www.ietf.org/rfc/rfc2045.txt. Note that
this RFC obsoletes RFC1521, RFC1522, and RFC1590.
[RFC2068]
"HTTP Version 1.1 ", R. Fielding, J. Gettys, J. Mogul, H.
Frystyk Nielsen, and T. Berners-Lee, January 1997.
Available at http://www.ietf.org/rfc/rfc2068.txt.
[RFC2070]
"Internationalization of the HyperText Markup Language", F.
Yergeau, G. Nicol, G. Adams, and M. Dürst, January 1997.
Available at http://www.ietf.org/rfc/rfc2070.txt.
[RFC2119]
"Key words for use in RFCs to Indicate Requirement Levels", S.
Bradner, March 1997.
Available at http://www.ietf.org/rfc/rfc2119.txt.
[RFC2318]
"The text/css Media Type", H. Lie, B. Bos, C. Lilley, March
1998.
Available at http://www.ietf.org/rfc/rfc2318.txt.
[RFC1738]
"Uniform Resource Locators", T. Berners-Lee, L. Masinter, and
M. McCahill, December 1994.
Available at http://www.ietf.org/rfc/rfc1738.txt.
[SRGB]
"Proposal for a Standard Color Space for the Internet - sRGB",
M. Anderson, R. Motta, S. Chandrasekar, M. Stokes.
Available at http://www.w3.org/Graphics/Color/sRGB.html.
[UNICODE]
"The Unicode Standard: Version 2.0", The Unicode Consortium,
Addison-Wesley Developers Press, 1996. For bidirectionality,
see also the corrigenda at
http://www.unicode.org/unicode/uni2errata/bidi.htm. For more
information, consult the Unicode Consortium's home page at
http://www.unicode.org/.
The latest version of Unicode. For more information, consult
the Unicode Consortium's home page at http://www.unicode.org/.
[URI]
"Uniform Resource Identifiers (URI): Generic Syntax", T.
Berners-Lee, R. Fielding, L. Masinter, August 1998.
Available at http://www.ietf.org/rfc/rfc2396.txt.
[XML10]
"Extensible Markup Language (XML) 1.0" T. Bray, J. Paoli, C.M.
Sperberg-McQueen, editors, 10 February 1998.
Available at http://www.w3.org/TR/REC-xml.
[YACC]
"YACC - Yet another compiler compiler", S. C. Johnson,
Technical Report, Murray Hill, 1975.
B.2 Informative references
[CHARSETS]
Registered charset values. Download a list of registered
charset values from
ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets.
[CSS3SEL]
"Selectors", D. Glazman, T. Çelik, I. Hickson, 13 November 2001
Available at
http://www.w3.org/TR/2001/CR-css3-selectors-20011113
[DOM]
"Document Object Model Specification", L. Wood, A. Le Hors, 9
October 1997.
Available at http://www.w3.org/TR/WD-DOM/
[MATH20]
"Mathematical Markup Language (MathML) Version 2.0", D.
Carlisle, P. Ion, R. Miner, N. Poppelier, 21 February 2001
Available at http://www.w3.org/TR/2001/REC-MathML2-20010221
[P3P]
"The Platform for Privacy Preferences 1.0 (P3P1.0)
Specification", L. Cranor, M. Langheinrich, M. Marchiori, M.
Presler-Marshall, J. Reagle, 16 April 2002
Available at http://www.w3.org/TR/2002/REC-P3P-20020416
[RFC1766]
"Tags for the Identification of Languages", H. Alvestrand,
March 1995.
Available at http://www.ietf.org/rfc/rfc1766.txt.
[SVG10]
"Scalable Vector Graphics (SVG) 1.0 Specification", J.
Ferraiolo, 4 September 2001
Available at http://www.w3.org/TR/2001/REC-SVG-20010904
[WAI-PAGEAUTH]
"Web Content Accessibility Guidelines", W. Chisholm, G.
Vanderheiden, I. Jacobs eds.
Available at: http://www.w3.org/TR/WD-WAI-PAGEAUTH/.
[XHTML]
"XHTML 1.0 The Extensible HyperText Markup Language", various
authors,
Available at: http://www.w3.org/TR/xhtml1/.
_________________________________________________________________
_________________________________________________________________
Appendix E. Property index
This appendix is informative, not normative.
Name Values Initial value Applies to
(Default: all) Inherited? Percentages
(Default: N/A) Media groups
'azimuth' <angle> | [[ left-side | far-left | left | center-left |
center | center-right | right | far-right | right-side ] || behind ] |
leftwards | rightwards | inherit center yes aural
'background-attachment' scroll | fixed | inherit scroll no visual
'background-color' <color> | transparent | inherit transparent no
visual
'background-image' <uri> | none | inherit none no visual
'background-position' [ [<percentage> | <length> | top | center |
bottom] || [<percentage> | <length> | left | center | right] ] |
inherit 0% 0% block-level and replaced elements no refer to the size
of the box itself visual
'background-repeat' repeat | repeat-x | repeat-y | no-repeat | inherit
repeat no visual
'background' ['background-color' || 'background-image' ||
'background-repeat' || 'background-attachment' ||
'background-position'] | inherit see individual properties no
allowed on 'background-position' visual
'border-collapse' collapse | separate | inherit separate 'table' and
'inline-table' elements yes visual
'border-color' [ <color> | transparent ]{1,4} | inherit see individual
properties no visual
'border-spacing' <length> <length>? | inherit 0 'table' and
'inline-table' elements yes visual
'border-style' <border-style>{1,4} | inherit see individual properties
no visual
'border-top' 'border-right' 'border-bottom' 'border-left' [
<border-width> || <border-style> || 'border-top-color' ] | inherit see
individual properties no visual
'border-top-color' 'border-right-color' 'border-bottom-color'
'border-left-color' <color> | transparent | inherit the value of the
'color' property no visual
'border-top-style' 'border-right-style' 'border-bottom-style'
'border-left-style' <border-style> | inherit none no visual
'border-top-width' 'border-right-width' 'border-bottom-width'
'border-left-width' <border-width> | inherit medium no visual
'border-width' <border-width>{1,4} | inherit see individual properties
no visual
'border' [ <border-width> || <border-style> || 'border-top-color' ] |
inherit see individual properties no visual
'bottom' <length> | <percentage> | auto | inherit auto positioned
elements no refer to height of containing block visual
'caption-side' top | bottom | left | right | inherit top
'table-caption' elements yes visual
'clear' none | left | right | both | inherit none no visual
'clip' <shape> | auto | inherit auto absolutely positioned elements no
visual
'color' <color> | inherit depends on user agent yes visual
'content' [ <string> | <counter> | attr(<identifier>) | open-quote |
close-quote | no-open-quote | no-close-quote ]+ | inherit empty string
:before and :after pseudo-elements no all
'counter-increment' [ <identifier> <integer>? ]+ | none | inherit none
no all
'counter-reset' [ <identifier> <integer>? ]+ | none | inherit none
no all
'cue-after' <uri> | none | inherit none no aural
'cue-before' <uri> | none | inherit none no aural
'cue' [ 'cue-before' || 'cue-after' ] | inherit see individual
properties no aural
'cursor' [ [<uri> ,]* [ auto | crosshair | default | pointer | move |
e-resize | ne-resize | nw-resize | n-resize | se-resize | sw-resize |
s-resize | w-resize| text | wait | help | progress ] ] | inherit auto
yes visual, interactive
'direction' ltr | rtl | inherit ltr all elements, but see prose yes
visual
'display' inline | block | list-item | run-in | inline-block | table |
inline-table | table-row-group | table-header-group |
table-footer-group | table-row | table-column-group | table-column |
table-cell | table-caption | none | inherit inline no all
'elevation' <angle> | below | level | above | higher | lower | inherit
level yes aural
'empty-cells' show | hide | inherit show 'table-cell' elements yes
visual
'float' left | right | none | inherit none all but generated content,
but see 9.7 no visual
'font-family' [[ <family-name> | <generic-family> ] [, <family-name>|
<generic-family>]* ] | inherit depends on user agent yes visual
'font-size' <absolute-size> | <relative-size> | <length> |
<percentage> | inherit medium yes refer to parent element's font
size visual
'font-style' normal | italic | oblique | inherit normal yes visual
'font-variant' normal | small-caps | inherit normal yes visual
'font-weight' normal | bold | bolder | lighter | 100 | 200 | 300 | 400
| 500 | 600 | 700 | 800 | 900 | inherit normal yes visual
'font' [ [ 'font-style' || 'font-variant' || 'font-weight' ]?
'font-size' [ / 'line-height' ]? 'font-family' ] | caption | icon |
menu | message-box | small-caption | status-bar | inherit see
individual properties yes allowed on 'font-size' and 'line-height'
visual
'height' <length> | <percentage> | auto | inherit auto all elements
but non-replaced inline elements, table columns, and column groups no
see prose visual
'left' <length> | <percentage> | auto | inherit auto positioned
elements no refer to width of containing block visual
'letter-spacing' normal | <length> | inherit normal yes visual
'line-height' normal | <number> | <length> | <percentage> | inherit
normal yes refer to the font size of the element itself visual
'list-style-image' <uri> | none | inherit none elements with 'display:
list-item' yes visual
'list-style-position' inside | outside | inherit outside elements with
'display: list-item' yes visual
'list-style-type' disc | circle | square | decimal |
decimal-leading-zero | lower-roman | upper-roman | lower-latin |
upper-latin | none | inherit disc elements with 'display: list-item'
yes visual
'list-style-type' disc | circle | square | decimal |
decimal-leading-zero | lower-roman | upper-roman | lower-greek |
lower-alpha | lower-latin | upper-alpha | upper-latin | hebrew |
armenian | georgian | cjk-ideographic | hiragana | katakana |
hiragana-iroha | katakana-iroha | none | inherit disc elements with
'display: list-item' yes visual
'list-style' [ 'list-style-type' || 'list-style-position' ||
'list-style-image' ] | inherit see individual properties elements with
'display: list-item' yes visual
'margin-right' 'margin-left' <margin-width> | inherit 0 no refer to
width of containing block visual
'margin-top' 'margin-bottom' <margin-width> | inherit 0 all elements
but inline, non-replaced elements no refer to width of containing
block visual
'margin' <margin-width>{1,4} | inherit see individual properties no
refer to width of containing block visual
'max-height' <length> | <percentage> | none | inherit none all
elements except non-replaced inline elements and table elements no
refer to height of containing block visual
'max-width' <length> | <percentage> | none | inherit none all elements
except non-replaced inline elements and table elements no refer to
width of containing block visual
'min-height' <length> | <percentage> | inherit 0 all elements except
non-replaced inline elements and table elements no refer to height of
containing block visual
'min-width' <length> | <percentage> | inherit 0 all elements except
non-replaced inline elements and table elements no refer to width of
containing block visual
'orphans' <integer> | inherit 2 block-level elements yes visual,
paged
'outline-color' <color> | invert | inherit invert no visual,
interactive
'outline-style' <border-style> | inherit none no visual,
interactive
'outline-width' <border-width> | inherit medium no visual,
interactive
'outline' [ 'outline-color' || 'outline-style' || 'outline-width' ] |
inherit see individual properties no visual, interactive
'overflow' visible | hidden | scroll | auto | inherit visible
block-level and replaced elements no visual
'padding-top' 'padding-right' 'padding-bottom' 'padding-left'
<padding-width> | inherit 0 no refer to width of containing block
visual
'padding' <padding-width>{1,4} | inherit see individual properties
no refer to width of containing block visual
'page-break-after' auto | always | avoid | left | right | inherit auto
block-level elements no visual, paged
'page-break-before' auto | always | avoid | left | right | inherit
auto block-level elements no visual, paged
'page-break-inside' avoid | auto | inherit auto block-level elements
yes visual, paged
'pause-after' <time> | <percentage> | inherit 0 no see prose aural
'pause-before' <time> | <percentage> | inherit 0 no see prose aural
'pause' [ [<time> | <percentage>]{1,2} ] | inherit see individual
properties no see descriptions of 'pause-before' and 'pause-after'
aural
'pitch-range' <number> | inherit 50 yes aural
'pitch' <frequency> | x-low | low | medium | high | x-high | inherit
medium yes aural
'play-during' <uri> [ mix || repeat ]? | auto | none | inherit auto
no aural
'position' static | relative | absolute | fixed | inherit static all
elements, but not to generated content no visual
'quotes' [<string> <string>]+ | none | inherit depends on user agent
yes visual
'richness' <number> | inherit 50 yes aural
'right' <length> | <percentage> | auto | inherit auto positioned
elements no refer to width of containing block visual
'speak-header' once | always | inherit once elements that have table
header information yes aural
'speak-numeral' digits | continuous | inherit continuous yes aural
'speak-punctuation' code | none | inherit none yes aural
'speak' normal | none | spell-out | inherit normal yes aural
'speech-rate' <number> | x-slow | slow | medium | fast | x-fast |
faster | slower | inherit medium yes aural
'stress' <number> | inherit 50 yes aural
'table-layout' auto | fixed | inherit auto 'table' and 'inline-table'
elements no visual
'text-align' <string> | left | right | center | justify | inherit
depends on user agent and writing direction block-level elements and
table cells yes visual
'text-align' left | right | center | justify | inherit depends on user
agent and writing direction block-level elements and table cells yes
visual
'text-decoration' none | [ underline || overline || line-through ||
blink ] | inherit none no (see prose) visual
'text-indent' <length> | <percentage> | inherit 0 block-level elements
yes refer to width of containing block visual
'text-transform' capitalize | uppercase | lowercase | none | inherit
none yes visual
'top' <length> | <percentage> | auto | inherit auto positioned
elements no refer to height of containing block visual
'unicode-bidi' normal | embed | bidi-override | inherit normal all
elements, but see prose no visual
'vertical-align' baseline | sub | super | top | text-top | middle |
bottom | text-bottom | <percentage> | <length> | inherit baseline
inline-level and 'table-cell' elements no refer to the 'line-height'
of the element itself visual
'visibility' visible | hidden | collapse | inherit visble yes
visual
'voice-family' [[<specific-voice> | <generic-voice> ],]*
[<specific-voice> | <generic-voice> ] | inherit depends on user agent
yes aural
'volume' <number> | <percentage> | silent | x-soft | soft | medium |
loud | x-loud | inherit medium yes refer to inherited value aural
'white-space' normal | pre | nowrap | pre-wrap | pre-line | inherit
normal block-level elements yes visual
'widows' <integer> | inherit 2 block-level elements yes visual,
paged
'width' <length> | <percentage> | auto | inherit auto all elements but
non-replaced inline elements, table rows, and row groups no refer to
width of containing block visual
'word-spacing' normal | <length> | inherit normal yes visual
'z-index' auto | <integer> | inherit auto positioned elements no
visual
The following properties were present in CSS2, but are not part of CSS
2.1: 'font-size-adjust', font-stretch, 'marker-offset', 'marks',
'orphans',, 'size',, 'text-shadow', 'widows'.
The CSS2 aural properties are now described in Appendix A and the
following properties therefore do not appear in the table above:
'azimuth', 'cue', 'cue-after', 'cue-before', 'elevation', 'pause',
'pause-after', 'pause-before', 'pitch', 'pitch-range', 'play-during',
'richness', 'speak', 'speak-header', 'speak-numeral',
'speak-punctuation', 'speech-rate', 'stress', 'voice-family',
'volume'.
_________________________________________________________________
_________________________________________________________________
Appendix F. Index
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
* :active, 2
* :after, 1, 2
* :before, 1, 2
* :first, 1
* :first-child, 1
* :first-letter, 1
* :first-line, 1
* :focus, 1
* :hover, 1
* :lang, 1
* :left, 1
* :link, 1
* :right, 1
* :visited, 1
* =, 1
* ~=, 1
* |=, 1
* @charset, 1
* @import, 1, 2, 3
* @media, 1, 2
* @page, 1
* absolute length, 1
* absolutely positioned element, 1
* active (pseudo-class), 1
* actual value, 1
* after, 1
* 'all' media group, 1
* ancestor, 1
* <angle>, 1, 2
+ definition of, 1
* anonymous, 1
* anonymous inline boxes, 1
* at-rule, 1
* at-rules, 1
* attr(), 1
* attribute, 1
* 'audio' media group, 1
* auditory icon, 1
* authoring tool, 1
* automatic numbering, 1
* 'azimuth', 1
* 'background', 1
* 'background-attachment', 1
* 'background-color', 1
* 'background-image', 1
* 'background-position', 1
* 'background-repeat', 1
* backslash escapes, 1
* before, 1
* bidirectionality (bidi), 1
* 'bitmap' media group, 1
* block, 1
* block box, 1
* 'block', definition of, 1
* block-level element, 1
* border edge, 1
* 'border', 1
* 'border-bottom', 1
* 'border-bottom-color', 1
* 'border-bottom-style', 1
* 'border-bottom-width', 1
* 'border-collapse', 1
* 'border-color', 1
* 'border-left', 1
* 'border-left-color', 1
* 'border-left-style', 1
* 'border-left-width', 1
* 'border-right', 1
* 'border-right-color', 1
* 'border-right-style', 1
* 'border-right-width', 1
* 'border-spacing', 1
* <border-style>, 1
* <border-style>, definition of, 1
* 'border-style', 1
* 'border-top', 1
* 'border-top-color', 1
* 'border-top-style', 1
* 'border-top-width', 1
* <border-width>
+ definition of, 1
* 'border-width', 1
* border
+ of a box, 1
* <bottom>
+ definition of, 1
* 'bottom', 1
* box
+ border, 1
+ content, 1
+ content height, 1
+ content width, 1
+ height, 1
+ margin, 1
+ overflow, 1
+ padding, 1
+ width, 1
* canvas, 1, 2
* 'caption-side', 1
* cascade, 1
* case sensitivity, 1
* character encoding, 1
+ default, 1
+ user agent's determination of, 1
* child, 1
* child selector, 1
* circle, 1
* 'clear', 1
* 'clip', 1
* clipping region, 1
* close-quote, 1, 2
* collapsing margin, 1
* color, 1
* <color>, 1, 2
+ definition of, 1
* 'color', 1
* combinator, 1
* comments, 1
* computed value, 1
* conditional import, 1
* conformance, 1, 2, 3, 4
* containing block, 1, 2, 3
+ initial, 1
* content, 1
* content edge, 1
* 'content', 1
* content
+ of a box, 1
+ rendered, 1
* 'continuous' media group, 1
* <counter>, 1
* <counter>, definition of, 1
* counter(), 1
* 'counter-increment', 1
* 'counter-reset', 1
* counters, 1
* 'cue', 1
* 'cue-after', 1
* 'cue-before', 1
* 'cursor', 1
* 'dashed', 1, 2
* decimal, 1
* decimal-leading-zero, 1
* declaration, 1
* declaration-block, 1
* default style sheet, 1
* default
+ character encoding, 1
* descendant, 1
* descendant-selectors, 1
* 'direction', 1
* disc, 1
* 'display', 1
* document language, 1
* document tree, 1
* 'dotted', 1, 2
* 'double', 1, 2
* drop caps, 1
* DTD, 1, 2
* element, 1
+ following, 1
+ preceding, 1
* 'elevation', 1
* em (unit), 1
* empty, 1
* 'empty-cells', 1
* ex (unit), 1
* exact matching, 1
* fictional tag sequence, 1, 2, 3
* first-child, 1
* first-letter, 1
* first-line, 1
* float rules, 1
* 'float', 1
* focus, 1
* focus (pseudo-class), 1
* following element, 1
* 'font', 1
* 'font-family', 1
* 'font-size', 1
* 'font-style', 1
* 'font-variant', 1
* 'font-weight', 1
* forced line break, 1
* formatting context, 1
* formatting structure, 1
* forward-compatible parsing, 1
* <frequency>, 1
+ definition of, 1
* generated content, 1
* <generic-voice>, definition of, 1
* 'grid' media group, 1
* 'groove', 1, 2
* half-leading, 1
* 'height', 1
* 'hidden, 1
* 'hidden', 1
* horizontal margin, 1
* hover (pseudo-class), 1
* hyphen-separated matching, 1
* identifier, 1
* identifier, definition of, 1
* ignore, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19
* illegal, 1
* inherit, definition of, 1
* initial caps, 1
* initial containing block, 1
* initial value, 1
* 'inline', definition of, 1
* 'inline-block', definition of, 1
* inline-level element, 1
* inline-table, 1
* 'inline-table', 1
* inner edge, 1
* 'inset', 1, 2
* <integer>, 1
+ definition of, 1
* 'interactive media group, 1
* internal table element, 1
* intrinsic dimensions, 1
* invert, 1
* iso-10646, 1
* lang (pseudo-class), 1
* language (human), 1
* language code, 1, 2
* leading, 1
* <left>
+ definition of, 1
* 'left', 1
* <length>, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
+ definition of, 1
* 'letter-spacing', 1
* ligatures, 1
* line box, 1
* line-box, 1
* 'line-height', 1
* link (pseudo-class), 1
* list properties, 1
* 'list-item', definition of, 1
* 'list-style', 1
* 'list-style-image', 1
* 'list-style-position', 1
* 'list-style-type', 1
* LL(1), 1
* local stacking context, 1
* lower-latin, 1
* lower-roman, 1
* mapping elements to table parts, 1
* margin edge, 1
* 'margin', 1
* 'margin-bottom', 1
* 'margin-left', 1
* 'margin-right', 1
* 'margin-top', 1
* <margin-width>
+ definition of, 1
* margin
+ horizontal, 1
+ of a box, 1
+ vertical, 1
* match, 1
* 'max-height', 1
* 'max-width', 1
* MAY, 1
* media, 1
* media group, 1
* media-dependent import, 1
* message entity, 1
* 'min-height', 1
* 'min-width', 1
* multiple declarations, 1
* MUST, 1
* MUST NOT, 1
* newline, 1
* no-close-quote, 1, 2
* no-open-quote, 1, 2
* none, 1
* 'none'
+ as border style, 1, 2
+ as display value, 1
* normal, 1
* <number>, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
+ definition of, 1
* open-quote, 1, 2
* OPTIONAL, 1
* 'orphans', 1
* outer edge, 1
* outline, 1
* 'outline', 1
* 'outline-color', 1
* 'outline-style', 1
* 'outline-width', 1
* 'outset', 1, 2
* overflow, 1
* 'overflow', 1
* padding edge, 1
* 'padding', 1
* 'padding-bottom', 1
* 'padding-left', 1
* 'padding-right', 1
* 'padding-top', 1
* <padding-width>
+ definition of, 1
* padding
+ of a box, 1
* page area, 1
* page box, 1
* page model, 1
* page selector, 1
* 'page-break-after', 1
* 'page-break-before', 1
* 'page-break-inside', 1
* page-context, 1
* 'paged' media group, 1
* parent, 1
* 'pause', 1
* 'pause-after', 1
* 'pause-before', 1
* <percentage>, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
+ definition of, 1
* 'pitch', 1
* 'pitch-range', 1
* pixel, 1
* 'play-during', 1
* 'position', 1
* positioned element/box, 1
* positioning scheme, 1
* preceding element, 1
* principal block box, 1
* property, 1
* 'property-name', 1
* pseudo-class
+ :first, 1
+ :left, 1
+ :right, 1
* pseudo-classes, 1
+ :active, 1
+ :focus, 1
+ :hover, 1
+ :lang, 1
+ :link, 1
+ :visited, 1
* pseudo-elements, 1
+ :after, 1, 2
+ :before, 1, 2
+ :first-letter, 1
+ :first-line, 1, 2
* quad width, 1
* 'quotes', 1
* RECOMMENDED, 1
* reference pixel, 1
* relative positioning, 1
* relative units, 1
* rendered content, 1
* replaced element, 1
* REQUIRED, 1
* Resource Identifier (URI), 1
* 'richness', 1
* 'ridge', 1, 2
* <right>
+ definition of, 1
* 'right', 1
* root, 1
* root stacking context, 1
* rule sets, 1
* run-in, 1
* run-in box, 1
* 'run-in', definition of, 1
* scope, 1
* screen reader, 1
* selector, 1, 2, 3, 4
+ match, 1
+ subject of, 1
* separated borders, 1
* SHALL, 1
* SHALL NOT, 1
* <shape>
+ definition of, 1
* sheet, 1
* shorthand property, 1, 2, 3
* SHOULD, 1
* SHOULD NOT, 1
* sibling, 1
* simple selector, 1
* 'solid', 1, 2
* source document, 1
* space-separated matching, 1
* 'speak', 1
* 'speak-header', 1
* 'speak-numeral', 1
* 'speak-punctuation', 1
* <specific-voice>
+ definition of, 1
* specified value, 1
* 'speech' media group, 1
* 'speech-rate', 1
* square, 1
* stack level, 1
* stacking context, 1
* statements, 1
* 'static' media group, 1
* 'stress', 1
* string, 1
* <string>, 1, 2, 3
* <string>, definition of, 1
* style sheet, 1
* subject (of selector), 1
* system fonts, 1
* table, 1
* table element, 1
+ internal, 1
* 'table', 1
* table-caption, 1
* 'table-caption', 1
* table-cell, 1
* 'table-cell', 1
* table-column, 1
* 'table-column', 1
* table-column-group, 1
* 'table-column-group', 1
* table-footer-group, 1
* 'table-footer-group', 1
* table-header-group, 1
* 'table-header-group', 1
* 'table-layout', 1
* table-row, 1
* 'table-row', 1
* table-row-group, 1
* 'table-row-group', 1
* tables, 1
* 'tactile' media group, 1
* 'text-align', 1
* 'text-decoration', 1
* 'text-indent', 1
* 'text-transform', 1
* text/css, 1
* <time>, 1
+ definition of, 1
* tokenizer, 1
* <top>
+ definition of, 1
* 'top', 1
* type selector, 1
* UA, 1
* unicode, 1
* 'unicode-bidi', 1
* Uniform Resource Locator (URL), 1
* Uniform Resource Name (URN), 1
* universal selector, 1
* upper-latin, 1
* upper-roman, 1
* URI (Uniform Resource Identifier), 1
* <uri>, 1, 2, 3, 4, 5, 6, 7
+ definition of, 1
* URL (Uniform Resource Locator), 1
* URN (Uniform Resource Name), 1
* user agent, 1
* valid style sheet, 1
* value, 1
* vertical margin, 1
* 'vertical-align', 1
* viewport, 1
* 'visibility', 1
* visited (pseudo-class), 1
* visual formatting model, 1
* 'visual' media group, 1
* 'voice-family', 1
* volume, 1
* 'volume', 1
* 'white-space', 1
* 'widows', 1
* 'width', 1
* 'word-spacing', 1
* x-height, 1
* 'z-index', 1
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