#next contents index
   
   W3C 
   
Cascading Style Sheets, level 2 revision 1
CSS 2.1 Specification

W3C Working Draft 2 August 2002

   This version:
          http://www.w3.org/TR/2002/WD-CSS21-20020802
          
   Latest version:
          http://www.w3.org/TR/CSS21
          
   Previous version:
          http://www.w3.org/TR/1998/REC-CSS2-19980512
          
   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 © 2002 W3C^® (MIT, INRIA, 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). 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). This is a W3C Last Call Working Draft. "Last
   call" means that the working group believes that this specification is
   ready and therefore wishes this to be the last call for comments. If
   the feedback is positive, the working group plans to submit it for
   consideration as a W3C Candidate Recommendation. Comments can be sent
   until the 30th of August, 2002.
   
   The (archived) public mailing list www-style@w3.org (see instructions)
   is preferred for discussion of this and other drafts in the Style
   area.
   
   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 CSS 2.1 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 and lists
     * 13 Page breaks
     * 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
               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 CSS 2.1 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 Colors
               o 4.3.6 Strings
          + 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'
     * 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
          + 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, and floating replaced elements
               o 10.6.3 Block-level, 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 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 Lists
               o 12.5.1 Lists: the 'list-style-type', 'list-style-image',
                 'list-style-position', and 'list-style' properties
     * 13 Page breaks
          + 13.1 Page break properties: 'page-break-before',
            'page-break-after', 'page-break-inside'
          + 13.2 Allowed page breaks
          + 13.3 Forced page breaks
          + 13.4 "Best" page breaks
     * 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
     * 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 Introduction to aural style sheets
               o A.1.1 Angles
               o A.1.2 Times
               o A.1.3 Frequencies
          + A.2 Volume properties: 'volume'
          + A.3 Speaking properties: 'speak'
          + A.4 Pause properties: 'pause-before', 'pause-after', and
            'pause'
          + A.5 Cue properties: 'cue-before', 'cue-after', and 'cue'
          + A.6 Mixing properties: 'play-during'
          + A.7 Spatial properties: 'azimuth' and 'elevation'
          + A.8 Voice characteristic properties: 'speech-rate',
            'voice-family', 'pitch', 'pitch-range', 'stress', and
            'richness'
          + A.9 Speech properties: 'speak-punctuation' and
            'speak-numeral'
          + A.10 Audio rendering of tables
               o A.10.1 Speaking headers: the 'speak-header' property
          + A.11 Sample style sheet for HTML
          + A.12 Emacspeak
     * Appendix B. Bibliography
          + B.1 Normative references
          + B.2 Informative references
     * Appendix C. Changes
          + C.1 Changes from CSS2
               o C.1.1 Errors
                    # Shorthand properties
                    # Section 4.1.1 (and G2)
                    # 4.1.3 Characters and case
                    # Section 4.3 (Double sign problem)
                    # Section 4.3.2 Lengths
                    # Section 4.3.6
                    # 5.10 Pseudo-elements and pseudo-classes
                    # 8.2 Example of margins, padding, and borders
                    # Section 8.5.2 Border color: 'border-top-color',
                      'border-right-color', 'border-bottom-color',
                      'border-left-color', and 'border-color'
                    # Section 8.4 Padding properties
                    # 8.5.3 Border style
                    # Section 8.5.4 Border shorthand properties:
                      'border-top', 'border-bottom', 'border-right',
                      'border-left', and 'border'
                    # 8.5.4 Border shorthand properties: 'border-top',
                      'border-bottom', 'border-right', 'border-left', and
                      'border'
                    # Section 9.3.1
                    # Section 9.3.2
                    # Section 9.4.3
                    # Section 9.7 Relationships between 'display',
                      'position', and 'float'
                    # Section 10.3.2 Inline, replaced elements (and
                      10.3.4, 10.3.6, and 10.3.8)
                    # Section 10.3.3
                    # Section 10.6.2 Inline, replaced elements ... (and
                      10.6.5)
                    # Section 10.6.3
                    # Section 11.1.1
                    # 11.2 Visibility: the 'visibility' property
                    # 12.6.2 Lists
                    # Section 15.2.6
                    # Section 15.5
                    # Section 16.6 Whitespace: the 'white-space' property
                    # Section 17.2 The CSS table model
                    # 17.2.1 Anonymous table objects 
                    # 17.5 Visual layout of table contents
                    # 17.5 Visual layout of table contents
                    # Section 17.6.1 The separated borders model
                    # Appendix D.2 Lexical scanner
               o C.1.2 Clarifications
                    # 2.2 A brief CSS2 tutorial for XML
                    # Section 4.1.1
                    # Section 5.5
                    # Section 5.9 ID selectors
                    # Section 5.12.1 The :first-line pseudo-element
                    # Section 6.2.1
                    # 6.4 The Cascade 
                    # Section 6.4.3 Calculating a selector's specificity
                    # Section 7.3 Recognized media types
                    # Section 8.1
                    # Section 8.3.1
                    # Section 9.4.2
                    # Section 9.4.3
                    # Section 9.10
                    # 10.3.3 Block-level, non-replaced elements in normal
                      flow
                    # Section 10.5 Content height: the 'height' property
                    # Section 10.8.1
                    # Section 11.1
                    # Section 11.1.1
                    # Section 11.1.2
                    # 12.1 The :before and :after pseudo-elements
                    # Section 12.4.2 Inserting quotes with the 'content'
                      property
                    # Lists 12.6.2
                    # 14.2 The background
                    # 14.2.1 Background properties
                    # Section 16.1
                    # 16.2 Alignment: the 'text-align' property
                    # Section 17.5.1 Table layers and transparency
                    # Section 17.5.2 Table width algorithms
                    # Borders around empty cells: the 'empty-cells'
                      property
                    # Section 17.6.2 The collapsing borders model
                    # Section 18.2
                    # Section A.3
                    # Appendix G.2 Lexical scanner
                    # Appendix E. References
               o C.1.3 Changes
                    # Section 6.4.3 Calculating a selector's specificity
                    # Section 6.4.4 Precedence of non-CSS presentational
                      hints
                    # Chapter 9 Visual formatting model
                    # Section 10.3.7 Absolutely positioned, non-replaced
                      elements
                    # Section 10.6.4 Absolutely positioned, non-replaced
                      elements
                    # Section 11.1.2
                    # 17.4.1 Caption position and alignment
                    # Section 17.6 Borders
                    # Chapter 12 Generated content, automatic numbering,
                      and lists
                    # Section 12.2 The 'content' property
                    # Chapter 15 Fonts
                    # Section 18.1 Cursors: the 'cursor' property
                    # Chapter 16 Text
                    # Appendix A. Aural style sheets
                    # Page breaks
                    # Other
     * 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
          + 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. Thus,
       compliant implementations of CSS 2.1 need not be burdened with
       these constraints.
     * Removing CSS2 features that will be obsoleted by CSS3, thus
       encouraging adoption of the proposed CSS3 features in their place.
       
   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
          The introduction includes a brief tutorial on CSS2 and a
          discussion of design principles behind CSS2.
          
   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 a sample style sheet for
          HTML 4.0, changes from CSS1 , 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
   
    Value
    
   This part specifies the set of valid values for the property. 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,
   'white-space' only affects block-level 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.
   The conformance conditions state that user agents must support this
   property if they support rendering to the media types included in
   these media groups.
   
  1.4.3 Shorthand properties
  
   Some properties are shorthand properties, meaning 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

   The following people deserve special mention:
   
   T. V. Raman, for the information about implementation status of aural
   properties.
   
   CSS 2.1 is based on CSS2. See the acknowledgments section of CSS2 for
   the people that contributed to CSS2.
   
1.6 Copyright Notice

   Copyright © 1997-2002 W3C^® (MIT, INRIA, Keio), All Rights Reserved.
   W3C liability, trademark, document use and software licensing rules
   apply.
   
   Documents on the W3C site are provided by the copyright holders under
   the following license. By obtaining, using and/or copying this
   document, or the W3C document from which this statement is linked, you
   agree that you have read, understood, and will comply with the
   following terms and conditions:
   
   Public documents on the W3C site are provided by the copyright holders
   under the following license. The software or Document Type Definitions
   (DTDs) associated with W3C specifications are governed by the Software
   Notice. By using and/or copying this document, or the W3C document
   from which this statement is linked, you (the licensee) agree that you
   have read, understood, and will comply with the following terms and
   conditions:
   
   Permission to use, copy, and distribute the contents of this document,
   or the W3C document from which this statement is linked, in any medium
   for any purpose and without fee or royalty is hereby granted, provided
   that you include the following on ALL copies of the document, or
   portions thereof, that you use:
    1. A link or URL to the original W3C document.
    2. The pre-existing copyright notice of the original author, or if it
       doesn't exist, a notice of the form: "Copyright ©
       [$date-of-document] World Wide Web Consortium, (Massachusetts
       Institute of Technology, Institut National de Recherche en
       Informatique et en Automatique, Keio University). All Rights
       Reserved. http://www.w3.org/Consortium/Legal/" (Hypertext is
       preferred, but a textual representation is permitted.)
    3. If it exists, the STATUS of the W3C document.
       
   When space permits, inclusion of the full text of this NOTICE should
   be provided. We request that authorship attribution be provided in any
   software, documents, or other items or products that you create
   pursuant to the implementation of the contents of this document, or
   any portion thereof.
   
   No right to create modifications or derivatives of W3C documents is
   granted pursuant to this license. However, if additional requirements
   (documented in the Copyright FAQ) are satisfied, the right to create
   modifications or derivatives is sometimes granted by the W3C to
   individuals complying with those requirements.
   
   THIS DOCUMENT IS PROVIDED "AS IS," AND COPYRIGHT HOLDERS MAKE NO
   REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, BUT NOT
   LIMITED TO, WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
   PURPOSE, NON-INFRINGEMENT, OR TITLE; THAT THE CONTENTS OF THE DOCUMENT
   ARE SUITABLE FOR ANY PURPOSE; NOR THAT THE IMPLEMENTATION OF SUCH
   CONTENTS WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS,
   TRADEMARKS OR OTHER RIGHTS.
   
   COPYRIGHT HOLDERS WILL NOT BE LIABLE FOR ANY DIRECT, INDIRECT, SPECIAL
   OR CONSEQUENTIAL DAMAGES ARISING OUT OF ANY USE OF THE DOCUMENT OR THE
   PERFORMANCE OR IMPLEMENTATION OF THE CONTENTS THEREOF.
   
   The name and trademarks of copyright holders may NOT be used in
   advertising or publicity pertaining to this document or its contents
   without specific, written prior permission. Title to copyright in this
   document will at all times remain with copyright holders.
     _________________________________________________________________
     _________________________________________________________________
   
                           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 blue, you can write the
   following CSS rule:
  h1 { color: blue }

   A CSS rule consists of two main parts: selector ('h1') and declaration
   ('color: blue'). In HTML, element names are case-insensitive so 'h1'
   works just as well as 'H1'. The declaration has two parts: property
   ('color') and value ('blue'). 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: blue }
  </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: red }
    h1 { color: blue }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

   The style sheet now contains two rules: the first one sets the color
   of the BODY element to 'red', while the second one sets the color of
   the H1 element to 'blue'. Since no color value 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 different 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 proposal for 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. If the destination medium is the printed page,
       user agents apply the page model. If the destination medium is an
       aural rendering device (e.g., speech synthesizer), user agents
       apply the aural rendering 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 refer. 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).
          
   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;
          not all elements have 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.
          
   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 CSS 2.1 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 Colors
          + 4.3.6 Strings
     * 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}[ \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";
font-family: "serif";
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
       digit or letter 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. 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, but
   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, colors, angles, times, and frequencies.
   
   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 in CSS 2.1 */
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, deg, etc.). After
   the '0' 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 specified 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. An expected new 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.bg.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.redballs.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.myorg.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.myorg.org/style/yellow

   User agents may vary in how they handle URIs that designate
   unavailable or inapplicable resources.
   
  4.3.5 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 three 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 to 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.
   
  4.3.6 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"] {/*...*/}

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: L&#252;beck">...</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 HTML only. The same as DIV[class~="warning"]. Class
   selectors
   E#myid Matches any E element 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.
   
   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 rightmost 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
  
   For style sheets used with HTML, authors may use the dot (.) notation
   as an alternative to the "~=" notation when matching on the "class"
   attribute. Thus, for HTML, "DIV.value" and "DIV[class~=value]" have
   the same meaning. The attribute value must immediately follow the ".".
   
   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-elements 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.
   
  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).
       
   These pseudo-classes are not mutually exclusive. An element may match
   several of them 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 firs