#next contents index first
     _________________________________________________________________

   W3C 

Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification

W3C Candidate Recommendation 19 July 2007

   This version:
          http://www.w3.org/TR/2007/CR-CSS21-20070719

   Latest version:
          http://www.w3.org/TR/CSS21

   Previous version:
          http://www.w3.org/TR/2006/WD-CSS21-20061106

   Editors:
          Bert Bos <bert @w3.org>
          Tantek Çelik <tantek @cs.stanford.edu>
          Ian Hickson <ian @hixie.ch>
          Håkon Wium Lie <howcome @opera.com>

   Please refer to the errata for this document.

   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 © 2006 W3C^® (MIT, ERCIM, Keio), All Rights Reserved. W3C
   liability, trademark and document use 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 and spacing) 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), and adds a few highly requested features which have
   already been widely implemented. But most of all CSS 2.1 represents a
   "snapshot" of CSS usage: it consists of all CSS features that are
   implemented interoperably at the date of publication of the
   Recommendation.

   CSS 2.1 is derived from and is intended to replace CSS2. Some parts of
   CSS2 are unchanged in CSS 2.1, some parts have been altered, and some
   parts removed. The removed portions may be used in a future CSS3
   specification. Future specs should refer to CSS 2.1 (unless they need
   features from CSS2 which have been dropped in CSS 2.1, and then they
   should only reference CSS2 for those features, or preferably reference
   such feature(s) in the respective CSS3 Module that includes those
   feature(s)).

Status of this document

   This section describes the status of this document at the time of its
   publication. Other documents may supersede this document. A list of
   current W3C publications and the latest revision of this technical
   report can be found in the W3C technical reports index at
   http://www.w3.org/TR/.

   This is a W3C Candidate Recommendation, which means the specification
   has been widely reviewed and W3C recommends that it be implemented. It
   will remain Candidate Recommendation at least until 20 December 2007.
   A test suite and a report on implementations will be provided before
   the document becomes a Proposed Recommendation.

   Publication as a Candidate Recommendation does not imply endorsement
   by the W3C Membership. This is a draft document and may be updated,
   replaced or obsoleted by other documents at any time. It is
   inappropriate to cite this document as other than work in progress.

   The (archived) public mailing list www-style@w3.org (see instructions)
   is preferred for discussion of this specification. When sending
   e-mail, please put the text "[CSS21]" in the subject, preferably like
   this: "[CSS21] ...summary of comment..."

   A list of changes since the last Working Draft is available in the
   Disposition of Comments.

   This document was produced by the CSS Working Group (part of the Style
   Activity).

   This document was produced by a group operating under the 5 February
   2004 W3C Patent Policy. W3C maintains a public list of any patent
   disclosures made in connection with the deliverables of the group;
   that page also includes instructions for disclosing a patent. An
   individual who has actual knowledge of a patent which the individual
   believes contains Essential Claim(s) must disclose the information in
   accordance with section 6 of the W3C Patent Policy.

  Candidate Recommendation Exit Criteria

   For this specification to exit the CR stage, the following conditions
   must be met:
    1. There must be at least two interoperable implementations for every
       feature. For the purposes of this criterion, we define the
       following terms:

        feature
                A section or subsection of the specification.

        interoperable
                passing the respective test cases in the test suite, or,
                if the implementation is not a web browser, equivalent
                tests. Every relevant test in the test suite should have
                an equivalent test created if such a UA is to be used to
                claim interoperability. In addition if such a UA is to be
                used to claim interoperability, then there must one or
                more additional UAs which can also pass those equivalent
                tests in the same way for the purpose of
                interoperability. The equivalent tests must be made
                publicly available for the purposes of peer review.

        implementation
                a user agent which:

              1. implements the feature.
              2. is available (i.e. publicly downloadable or available
                 through some other public point of sale mechanism). This
                 is the "show me" requirement.
              3. is shipping (i.e. development, private or unofficial
                 versions are insufficient).
              4. is not experimental (i.e. is intended for a wide
                 audience and could be used on a daily basis).

    2. A minimum of six months of the CR period must have elapsed. This
       is to ensure that enough time is given for any remaining major
       errors to be caught.
    3. The CR period will be extended if implementations are slow to
       appear.
    4. Features that were not in CSS1 will be dropped (thus reducing the
       list of "all" features mentioned above) if two or more
       interoperable implementations of those features are not found by
       the end of the CR period.
    5. Features will also be dropped if sufficient and adequate tests (by
       judgment of the working group) have not been produced for those
       features by the end of the CR period.

  Features at risk

   The working group has identified the following features as being
   currently poorly implemented by UAs. They are therefore most at risk
   of being removed from CSS 2.1 when exiting CR. (Any changes of this
   nature will still result in the specification being returned to last
   call.) Implementors are urged to implement these features, or correct
   bugs in their implementations, if they wish to see these features
   remain in this specification.

   New 'list-style-type' values

          + 'armenian'
          + 'georgian'
          + 'lower-greek'

          Implementors are advised to look at CSS3 Lists instead, where
          these and many other new values not found in CSS1 are defined
          in detail. [CSS3LIST]

   Support for multiple ID attributes for the ID selector
          Because implementations are not expected to support multiple
          IDs per element soon, this feature may be made informative. The
          W3C Selectors specification will continue to have this feature
          normatively. (Section 5.9.)

   Automatic table layout algorithm
          The input to the suggested (non-normative) automatic layout
          algorithm for tables is restricted to (1) the containing block
          width and (2) the content and properties of the table and its
          children. This restriction may be lifted.

   Quotes
          The 'quotes' property and the 'open-quote', 'close-quote',
          'no-open-quote' and 'no-close-quote' keywords may be dropped.

   BODY element in XHTML
          The effect of 'overflow' and 'background' is different on BODY
          elements in HTML than on other elements. It may be that the
          exceptional handling of BODY in HTML is extended to BODY in
          XHTML1.

Quick Table of Contents

     * 1 About the CSS 2.1 Specification
     * 2 Introduction to CSS 2.1
     * 3 Conformance: Requirements and Recommendations
     * 4 Syntax and basic data types
     * 5 Selectors
     * 6 Assigning property values, Cascading, and Inheritance
     * 7 Media types
     * 8 Box model
     * 9 Visual formatting model
     * 10 Visual formatting model details
     * 11 Visual effects
     * 12 Generated content, automatic numbering, and lists
     * 13 Paged media
     * 14 Colors and Backgrounds
     * 15 Fonts
     * 16 Text
     * 17 Tables
     * 18 User interface
     * Appendix A. Aural style sheets
     * Appendix B. Bibliography
     * Appendix C. Changes
     * Appendix D. Default style sheet for HTML 4
     * Appendix E. Elaborate description of Stacking Contexts
     * Appendix F. Full property table
     * Appendix G. Grammar of CSS 2.1
     * Appendix I. Index

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
                    # 1.4.2.1 Value
                    # 1.4.2.2 Initial
                    # 1.4.2.3 Applies to
                    # 1.4.2.4 Inherited
                    # 1.4.2.5 Percentage values
                    # 1.4.2.6 Media groups
                    # 1.4.2.7 Computed value
               o 1.4.3 Shorthand properties
               o 1.4.4 Notes and examples
               o 1.4.5 Images and long descriptions
          + 1.5 Acknowledgments
     * 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 UA Conformance
          + 3.3 Error conditions
          + 3.4 The text/css content type
     * 4 Syntax and basic data types
          + 4.1 Syntax
               o 4.1.1 Tokenization
               o 4.1.2 Keywords
                    # 4.1.2.1 Vendor-specific extensions
                    # 4.1.2.2 Informative Historical Notes
               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 URLs and URIs
               o 4.3.5 Counters
               o 4.3.6 Colors
               o 4.3.7 Strings
               o 4.3.8 Unsupported Values
          + 4.4 CSS style sheet 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 Used values
               o 6.1.4 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-right', 'border-bottom', 'border-left', and
                 'border'
          + 8.6 The box model for inline elements in bidirection context
     * 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
                    # 9.2.1.1 Anonymous block boxes
               o 9.2.2 Inline-level elements and inline boxes
                    # 9.2.2.1 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 contexts
               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 Calculating widths and margins
               o 10.3.1 Inline, non-replaced elements
               o 10.3.2 Inline, replaced elements
               o 10.3.3 Block-level, non-replaced elements in normal flow
               o 10.3.4 Block-level, replaced elements in normal flow
               o 10.3.5 Floating, non-replaced elements
               o 10.3.6 Floating, replaced elements
               o 10.3.7 Absolutely positioned, non-replaced elements
               o 10.3.8 Absolutely positioned, replaced elements
               o 10.3.9 'Inline-block', non-replaced elements in normal
                 flow
               o 10.3.10 'Inline-block', replaced elements in normal flow
          + 10.4 Minimum and maximum widths: 'min-width' and 'max-width'
          + 10.5 Content height: the 'height' property
          + 10.6 Calculating heights and margins
               o 10.6.1 Inline, non-replaced elements
               o 10.6.2 Inline replaced elements, block-level replaced
                 elements in normal flow, 'inline-block' replaced
                 elements in normal flow and floating replaced elements
               o 10.6.3 Block-level non-replaced elements in normal flow
                 when 'overflow' computes to 'visible'
               o 10.6.4 Absolutely positioned, non-replaced elements
               o 10.6.5 Absolutely positioned, replaced elements
               o 10.6.6 Complicated cases
               o 10.6.7 'Auto' heights for block formatting context roots
          + 10.7 Minimum and maximum heights: 'min-height' and
            'max-height'
          + 10.8 Line height calculations: the 'line-height' and
            'vertical-align' properties
               o 10.8.1 Leading and half-leading
     * 11 Visual effects
          + 11.1 Overflow and clipping
               o 11.1.1 Overflow: the 'overflow' property
               o 11.1.2 Clipping: the 'clip' property
          + 11.2 Visibility: the 'visibility' property
     * 12 Generated content, automatic numbering, and lists
          + 12.1 The :before and :after pseudo-elements
          + 12.2 The 'content' property
          + 12.3 Quotation marks
               o 12.3.1 Specifying quotes with the 'quotes' property
               o 12.3.2 Inserting quotes with the 'content' property
          + 12.4 Automatic counters and numbering
               o 12.4.1 Nested counters and scope
               o 12.4.2 Counter styles
               o 12.4.3 Counters in elements with 'display: none'
          + 12.5 Lists
               o 12.5.1 Lists: the 'list-style-type', 'list-style-image',
                 'list-style-position', and 'list-style' properties
     * 13 Paged media
          + 13.1 Introduction to paged media
          + 13.2 Page boxes: the @page rule
               o 13.2.1 Page margins
                    # 13.2.1.1 Rendering page boxes that do not fit a
                      target sheet
                    # 13.2.1.2 Positioning the page box on the sheet
               o 13.2.2 Page selectors: selecting left, right, and first
                 pages
               o 13.2.3 Content outside the page box
          + 13.3 Page breaks
               o 13.3.1 Page break properties: 'page-break-before',
                 'page-break-after', 'page-break-inside'
               o 13.3.2 Breaks inside elements: 'orphans', 'widows'
               o 13.3.3 Allowed page breaks
               o 13.3.4 Forced page breaks
               o 13.3.5 "Best" page breaks
          + 13.4 Cascading in the page context
     * 14 Colors and Backgrounds
          + 14.1 Foreground color: the 'color' property
          + 14.2 The background
               o 14.2.1 Background properties: 'background-color',
                 'background-image', 'background-repeat',
                 'background-attachment', 'background-position', and
                 'background'
          + 14.3 Gamma correction
     * 15 Fonts
          + 15.1 Introduction
          + 15.2 Font matching algorithm
          + 15.3 Font family: the 'font-family' property
               o 15.3.1 Generic font families
                    # 15.3.1.1 serif
                    # 15.3.1.2 sans-serif
                    # 15.3.1.3 cursive
                    # 15.3.1.4 fantasy
                    # 15.3.1.5 monospace
          + 15.4 Font styling: the 'font-style' property
          + 15.5 Small-caps: the 'font-variant' property
          + 15.6 Font boldness: the 'font-weight' property
          + 15.7 Font size: the 'font-size' property
          + 15.8 Shorthand font property: the 'font' property
     * 16 Text
          + 16.1 Indentation: the 'text-indent' property
          + 16.2 Alignment: the 'text-align' property
          + 16.3 Decoration
               o 16.3.1 Underlining, overlining, striking, and blinking:
                 the 'text-decoration' property
          + 16.4 Letter and word spacing: the 'letter-spacing' and
            'word-spacing' properties
          + 16.5 Capitalization: the 'text-transform' property
          + 16.6 Whitespace: the 'white-space' property
               o 16.6.1 The 'white-space' processing model
               o 16.6.2 Example of bidirectionality with white-space
                 collapsing
               o 16.6.3 Control and combining characters' details
     * 17 Tables
          + 17.1 Introduction to tables
          + 17.2 The CSS table model
               o 17.2.1 Anonymous table objects
          + 17.3 Columns
          + 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
                    # 17.5.2.1 Fixed table layout
                    # 17.5.2.2 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
                    # 17.6.1.1 Borders and Backgrounds around empty
                      cells: the 'empty-cells' property
               o 17.6.2 The collapsing border model
                    # 17.6.2.1 Border conflict resolution
               o 17.6.3 Border styles
     * 18 User interface
          + 18.1 Cursors: the 'cursor' property
          + 18.2 System Colors
          + 18.3 User preferences for fonts
          + 18.4 Dynamic outlines: the 'outline' property
               o 18.4.1 Outlines and the focus
          + 18.5 Magnification
     * Appendix A. Aural style sheets
          + A.1 The media types 'aural' and 'speech'
          + A.2 Introduction to aural style sheets
               o A.2.1 Angles
               o A.2.2 Times
               o A.2.3 Frequencies
          + A.3 Volume properties: 'volume'
          + A.4 Speaking properties: 'speak'
          + A.5 Pause properties: 'pause-before', 'pause-after', and
            'pause'
          + A.6 Cue properties: 'cue-before', 'cue-after', and 'cue'
          + A.7 Mixing properties: 'play-during'
          + A.8 Spatial properties: 'azimuth' and 'elevation'
          + A.9 Voice characteristic properties: 'speech-rate',
            'voice-family', 'pitch', 'pitch-range', 'stress', and
            'richness'
          + A.10 Speech properties: 'speak-punctuation' and
            'speak-numeral'
          + A.11 Audio rendering of tables
               o A.11.1 Speaking headers: the 'speak-header' property
          + A.12 Sample style sheet for HTML
          + A.13 Emacspeak
     * Appendix B. Bibliography
          + B.1 Normative references
          + B.2 Informative references
     * Appendix C. Changes
          + C.1 Additional property values
               o C.1.1 Section 4.3.6 Colors
               o C.1.2 Section 9.2.4 The 'display' property
               o C.1.3 Section 12.2 The 'content' property
               o C.1.4 Section 16.6 Whitespace: the 'white-space'
                 property
               o C.1.5 Section 18.1 Cursors: the 'cursor' property
          + C.2 Changes
               o C.2.1 Section 1.1 CSS 2.1 vs CSS 2
               o C.2.2 Section 1.2 Reading the specification
               o C.2.3 Section 1.3 How the specification is organized
               o C.2.4 Section 1.4.2.1 Value
               o C.2.5 Section 1.4.2.6 Media groups
               o C.2.6 Section 1.4.2.7 Computed value
               o C.2.7 Section 1.4.4 Notes and examples
               o C.2.8 Section 1.5 Acknowledgements
               o C.2.9 Section 3.2 Conformance
               o C.2.10 Section 3.3 Error Conditions
               o C.2.11 4.1.1 Tokenization
               o C.2.12 4.1.3 Characters and case
               o C.2.13 Section 4.2 Rules for handling parsing errors
               o C.2.14 Section 4.3 Values
               o C.2.15 Section 4.3.2 Lengths
               o C.2.16 Section 4.3.4 URLs and URIs
               o C.2.17 Section 4.3.6 Colors
               o C.2.18 Section 4.3.8 Unsupported Values
               o C.2.19 Section 4.4 CSS style sheet representation
               o C.2.20 Section 5.8.1 Matching attributes and attribute
                 values
               o C.2.21 Section 5.8.3 Class selectors
               o C.2.22 Section 5.9 ID selectors
               o C.2.23 Section 5.10 Pseudo-elements and pseudo-classes
               o C.2.24 5.11.2 The link pseudo-classes: :link and
                 :visited
               o C.2.25 Section 5.11.4 The language pseudo-class: :lang
               o C.2.26 Section 5.12.1 The :first-line pseudo-element
               o C.2.27 Section 5.12.2 The :first-letter pseudo-element
               o C.2.28 Section 6.1 Specified, computed, and actual
                 values
               o C.2.29 Section 6.4.1 Cascading order
               o C.2.30 Section 6.4.3 Calculating a selector's
                 specificity
               o C.2.31 Section 6.4.4 Precedence of non-CSS
                 presentational hints
               o C.2.32 Section 7.3 Recognized Media Types
               o C.2.33 Section 7.3.1 Media Groups
               o C.2.34 Section 8.3 Margin properties
               o C.2.35 Section 8.3.1 Collapsing margins
               o C.2.36 Section 8.4 Padding properties
               o C.2.37 Section 8.5.2 Border color
               o C.2.38 Section 8.5.3 Border style
               o C.2.39 Section 8.6 The box model for inline elements in
                 bidirection context
               o C.2.40 Section 9.1.2 Containing blocks
               o C.2.41 Section 9.2.1.1 Anonymous block boxes
               o C.2.42 Section 9.2.2.1 Anonymous inline boxes
               o C.2.43 Section 9.2.3 Run-in boxes
               o C.2.44 Section 9.2.4 The 'display' property
               o C.2.45 Section 9.3.1 Choosing a positioning scheme
               o C.2.46 Section 9.3.2 Box offsets
               o C.2.47 Section 9.4.1 Block formatting contexts
               o C.2.48 Section 9.4.2 Inline formatting context
               o C.2.49 Section 9.4.3 Relative positioning
               o C.2.50 Section 9.5 Floats
               o C.2.51 Section 9.5.1 Positioning the float
               o C.2.52 Section 9.5.2 Controlling flow next to floats
               o C.2.53 Section 9.7 Relationships between 'display',
                 'position', and 'float'
               o C.2.54 Section 9.9 Layered presentation
               o C.2.55 Section 9.10 Text direction
               o C.2.56 Chapter Visual formatting model details
               o C.2.57 Section 10.1 Definition of "containing block"
               o C.2.58 Section 10.2 Content width
               o C.2.59 Section 10.3 Calculating widths and margins
               o C.2.60 Section 10.3.2 Inline, replaced elements
               o C.2.61 Section 10.3.3 Block-level, non-replaced elements
                 in normal flow
               o C.2.62 Section 10.3.4 Block-level, replaced elements in
                 normal flow
               o C.2.63 Section 10.3.5 Floating, non-replaced elements
               o C.2.64 Section 10.3.6 Floating, replaced elements
               o C.2.65 Section 10.3.7 Absolutely positioned,
                 non-replaced elements
               o C.2.66 Section 10.3.8 Absolutely positioned, replaced
                 elements
               o C.2.67 Section 10.4 Minimum and maximum widths
               o C.2.68 Section 10.5 Content height
               o C.2.69 Section 10.6 Calculating heights and margins
               o C.2.70 Section 10.6.1 Inline, non-replaced elements
               o C.2.71 Section 10.6.2 Inline replaced elements,
                 block-level replaced elements in normal flow,
                 'inline-block' replaced elements in normal flow and
                 floating replaced elements
               o C.2.72 Section 10.6.3 Block-level non-replaced elements
                 in normal flow when 'overflow' computes to 'visible'
               o C.2.73 Section 10.6.4 Absolutely positioned,
                 non-replaced elements
               o C.2.74 Section 10.6.5 Absolutely positioned, replaced
                 elements
               o C.2.75 Section 10.7 Minimum and maximum heights
               o C.2.76 Section 10.8 Line height calculations
               o C.2.77 Section 10.8.1 Leading and half-leading
               o C.2.78 Section 11.1 Overflow and clipping
               o C.2.79 Section 11.1.1 Overflow
               o C.2.80 Section 11.1.2 Clipping: the 'clip' property
               o C.2.81 Section 11.2 Visibility
               o C.2.82 Chapter 12 Generated content, automatic
                 numbering, and lists
               o C.2.83 Section 12.1 The :before and :after
                 pseudo-elements
               o C.2.84 Section 12.2 The 'content' property
               o C.2.85 Section 12.3.2 Inserting quotes with the
                 'content' property
               o C.2.86 Section 12.4 Automatic counters and numbering
               o C.2.87 Section 12.4.1 Nested counters and scope
               o C.2.88 Section 12.5 Lists
               o C.2.89 Section 12.5.1 Lists
               o C.2.90 Chapter 13 Paged media
               o C.2.91 Section 13.2.2 Page selectors
               o C.2.92 Section 13.3.1 Page break properties
               o C.2.93 Section 13.3.3 Allowed page breaks
               o C.2.94 Section 14.2.1 Background properties
               o C.2.95 Section 14.3 Gamma correction
               o C.2.96 Chapter 15 Fonts
               o C.2.97 Section 15.2 Font matching algorithm
               o C.2.98 Section 15.2.2 Font family
               o C.2.99 Section 15.5 Small-caps
               o C.2.100 Section 15.6 Font boldness
               o C.2.101 Section 15.7 Font size
               o C.2.102 Chapter 16 Text
               o C.2.103 Section 16.2 Alignment
               o C.2.104 Section 16.3.1 Underlining, over lining,
                 striking, and blinking
               o C.2.105 Section 16.4 Letter and word spacing
               o C.2.106 Section 16.5 Capitalization
               o C.2.107 Section 16.6 Whitespace
               o C.2.108 Chapter 17 Tables
               o C.2.109 17.2 The CSS table model
               o C.2.110 Section 17.2.1 Anonymous table objects
               o C.2.111 Section 17.4 Tables in the visual formatting
                 model
               o C.2.112 Section 17.4.1 Caption position and alignment
               o C.2.113 Section 17.5 Visual layout of table contents
               o C.2.114 Section 17.5.1 Table layers and transparency
               o C.2.115 Section 17.5.2.1 Fixed table layout
               o C.2.116 Section 17.5.2.2 Automatic table layout
               o C.2.117 Section 17.5.3 Table height algorithms
               o C.2.118 17.5.4 Horizontal alignment in a column
               o C.2.119 Section 17.6 Borders
               o C.2.120 Section 17.6.1 The separated borders model
               o C.2.121 Section 17.6.1.1 Borders and Backgrounds around
                 empty cells
               o C.2.122 Section 17.6.2 The collapsing border model
               o C.2.123 Section 17.6.2.1 Border conflict resolution
               o C.2.124 Section 18.4 Dynamic outlines
               o C.2.125 Chapter 12 Generated content, automatic
                 numbering, and lists
               o C.2.126 Appendix A. Aural style sheets
               o C.2.127 Appendix A Section 5 Pause properties
               o C.2.128 Appendix A Section 6 Cue properties
               o C.2.129 Appendix A Section 7 Mixing properties
               o C.2.130 Appendix B Bibliography
               o C.2.131 Other
          + C.3 Errors
               o C.3.1 Shorthand properties
               o C.3.2 Applies to
               o C.3.3 Section 4.1.1 (and G2)
               o C.3.4 Section 4.1.3 Characters and case
               o C.3.5 Section 4.3 (Double sign problem)
               o C.3.6 Section 4.3.2 Lengths
               o C.3.7 Section 4.3.3 Percentages
               o C.3.8 Section 4.3.4 URLs and URIs
               o C.3.9 Section 4.3.5 Counters
               o C.3.10 Section 4.3.6
               o C.3.11 Section 4.3.7 Strings
               o C.3.12 Section 5.10 Pseudo-elements and pseudo-classes
               o C.3.13 Section 6.4 The cascade
               o C.3.14 Section 8.1 Box Dimensions
               o C.3.15 8.2 Example of margins, padding, and borders
               o C.3.16 Section 8.5.4 Border shorthand properties
               o C.3.17 Section 9.2.1 Block-level elements and block
                 boxes
               o C.3.18 Section 9.3.1 Choosing a positioning scheme
               o C.3.19 Section 9.3.2 Box offsets
               o C.3.20 Section 9.4.1 Block formatting contexts
               o C.3.21 Section 9.4.2 Inline formatting context
               o C.3.22 Section 9.4.3 Relative positioning
               o C.3.23 Section 9.5 Floats
               o C.3.24 Section 9.5.1 Positioning the float
               o C.3.25 Section 9.5.2 Controlling flow next to floats
               o C.3.26 Section 9.6 Absolute positioning
               o C.3.27 Section 9.7 Relationships between 'display',
                 'position', and 'float'
               o C.3.28 Section 9.10 Text direction
               o C.3.29 Section 10.1 Definition of "containing block"
               o C.3.30 Section 10.3.3
               o C.3.31 Section 10.4 Minimum and maximum widths
               o C.3.32 Section 10.6.3 Block-level non-replaced elements
                 in normal flow when 'overflow' computes to 'visible'
               o C.3.33 Section 10.7 Minimum and maximum heights
               o C.3.34 Section 11.1.1 Overflow
               o C.3.35 Section 11.1.2 Clipping: the 'clip' property
               o C.3.36 Section 11.2 Visibility
               o C.3.37 12.6.2 Lists
               o C.3.38 14.2 The background
               o C.3.39 14.2.1 Background properties
               o C.3.40 Section 15.2
               o C.3.41 Section 15.7 Font size
               o C.3.42 Section 16.1 Indentation
               o C.3.43 Section 16.2 Alignment
               o C.3.44 Section 17.2 The CSS table model
               o C.3.45 Section 17.2.1 Anonymous table objects 
               o C.3.46 Section 17.4 Tables in the visual formatting
                 model
               o C.3.47 Section 17.5 Visual layout of table contents
               o C.3.48 Section 17.5.1 Table layers and transparency
               o C.3.49 Section 17.6.1 The separated borders model
               o C.3.50 Section 18.2
          + C.4 Clarifications
               o C.4.1 Section 2.1 A brief CSS 2.1 tutorial for HTML
               o C.4.2 Section 2.2 A brief CSS 2.1 tutorial for XML
               o C.4.3 Section 3.1 Definitions
               o C.4.4 Section 4.1 Syntax
               o C.4.5 Section 4.1.1 Tokenization
               o C.4.6 Section 4.1.3 Characters and case
               o C.4.7 Section 4.1.7 Rule sets, declaration blocks, and
                 selectors
               o C.4.8 Section 4.2 Rules for handling parsing errors
               o C.4.9 Section 4.3.1 Integers and real numbers
               o C.4.10 Section 4.3.2 Lengths
               o C.4.11 Section 4.3.4 URLs and URIs
               o C.4.12 Section 5.1 Pattern matching
               o C.4.13 Section 5.7 Adjacent sibling selectors
               o C.4.14 Section 5.8.2 Default attribute values in DTDs
               o C.4.15 Section 5.9 ID selectors
               o C.4.16 Section 5.11.3 The dynamic pseudo-classes:
                 :hover, :active, and :focus
               o C.4.17 Section 5.11.4 The language pseudo-class: :lang
               o C.4.18 Section 5.12.2 The :first-letter pseudo-element
               o C.4.19 Section 6.2 Inheritance
               o C.4.20 Section 6.2.1 The 'inherit' value
               o C.4.21 Section 6.3 The @import rule
               o C.4.22 6.4 The Cascade
               o C.4.23 6.4.1 Cascading order
               o C.4.24 Section 6.4.3 Calculating a selector's
                 specificity
               o C.4.25 Section 7.2.1 The @media rule
               o C.4.26 Section 7.3 Recognized media types
               o C.4.27 Section 7.3.1 Media groups
               o C.4.28 Section 8.1 Box dimensions
               o C.4.29 Section 8.3 Margin properties
               o C.4.30 Section 8.3.1
               o C.4.31 Section 8.5.3 Border style
               o C.4.32 Section 9.1.1 The viewport
               o C.4.33 Section 9.3.1 Choosing a positioning scheme
               o C.4.34 Section 9.3.2 Box offsets
               o C.4.35 Section 9.4.2 Inline formatting context
               o C.4.36 Section 9.4.3 Relative positioning
               o C.4.37 Section 9.5 Floats
               o C.4.38 Section 9.5.1 Positioning the float
               o C.4.39 Section 9.5.2 Controlling flow next to floats
               o C.4.40 Section 9.8 Comparison of normal flow, floats,
                 and absolute positioning
               o C.4.41 Section 10.1 Definition of "containing block"
               o C.4.42 Section 10.2 Content width
               o C.4.43 Section 10.3.3 Block-level, non-replaced elements
                 in normal flow
               o C.4.44 Section 10.4 Minimum and maximum widths
               o C.4.45 Section 10.6 Calculating heights and margins
               o C.4.46 Section 10.7 Minimum and maximum heights
               o C.4.47 Section 10.8 Line height calculations
               o C.4.48 Section 11.1 Overflow and clipping
               o C.4.49 Section 11.1.1 Overflow
               o C.4.50 Section 11.1.2 Clipping
               o C.4.51 Section 11.2 Visibility
               o C.4.52 Section 12.1 The :before and :after
                 pseudo-elements
               o C.4.53 Section 12.2 The 'content' property
               o C.4.54 Section 12.3.2 Inserting quotes with the
                 'content' property
               o C.4.55 Section 12.4 Automatic counters and numbering
               o C.4.56 Section 12.4.3 Counters in elements with
                 'display: none'
               o C.4.57 Section 14.2 The background
               o C.4.58 Section 15.1 Fonts Introduction
               o C.4.59 Section 15.2 Font matching algorithm
               o C.4.60 Section 15.2.2 Font family
               o C.4.61 Section 15.3.1 Generic font families
               o C.4.62 Section 15.4 Font styling
               o C.4.63 Section 15.5 Small-caps
               o C.4.64 Section 15.6 Font boldness
               o C.4.65 Section 15.7 Font size
               o C.4.66 Section 16.1 Indentation
               o C.4.67 16.2 Alignment
               o C.4.68 Section 16.3.1 Underlining, over lining,
                 striking, and blinking
               o C.4.69 Section 16.5 Capitalization
               o C.4.70 Section 16.6 Whitespace
               o C.4.71 Section 17.1 Introduction to tables
               o C.4.72 Section 17.2 The CSS table model
               o C.4.73 Section 17.2.1 Anonymous table objects
               o C.4.74 Section 17.4 Tables in the visual formatting
                 model
               o C.4.75 Section 17.5 Visual layout of table contents
               o C.4.76 Section 17.5.1 Table layers and transparency
               o C.4.77 Section 17.5.2 Table width algorithms
               o C.4.78 Section 17.5.2.1 Fixed table layout
               o C.4.79 Section 17.5.2.2 Automatic table layout
               o C.4.80 Section 17.5.5 Dynamic row and column effects
               o C.4.81 17.6.1 The separated borders model
               o C.4.82 Section 17.6.2 The collapsing borders model
               o C.4.83 Section 18.2 System Colors
               o C.4.84 Section 18.4 Dynamic outlines
               o C.4.85 Section 18.4.1 Outlines and the focus
               o C.4.86 Appendix D
          + C.5 Code Diffs for Appendices D and G
               o C.5.1 Appendix D Default style sheet for HTML4
               o C.5.2 Section G.1 Grammar
               o C.5.3 Section G.2 Lexical scanner
     * Appendix D. Default style sheet for HTML 4
     * Appendix E. Elaborate description of Stacking Contexts
          + E.1 Definitions
          + E.2 Painting order
          + E.3 Notes
     * Appendix F. Full property table
     * 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
     * Appendix I. Index
     _________________________________________________________________
     _________________________________________________________________

                       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 1.4.2.1 Value
               o 1.4.2.2 Initial
               o 1.4.2.3 Applies to
               o 1.4.2.4 Inherited
               o 1.4.2.5 Percentage values
               o 1.4.2.6 Media groups
               o 1.4.2.7 Computed value
          + 1.4.3 Shorthand properties
          + 1.4.4 Notes and examples
          + 1.4.5 Images and long descriptions
     * 1.5 Acknowledgments

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 CSS2 features which, by virtue of not having been
       implemented, have been rejected by the CSS community. CSS 2.1 aims
       to reflect what CSS features are reasonably widely implemented for
       HTML and XML languages in general (rather than only for a
       particular XML language, or only for HTML).
     * Removing CSS2 features that will be obsoleted by CSS3, thus
       encouraging adoption of the proposed CSS3 features in their place.
     * Adding a (very) small number of new property values, when
       implementation experience has shown that they are needed for
       implementing CSS2.

   Thus, while it is not the case that a CSS2 style sheet is necessarily
   forwards-compatible with CSS 2.1, it is the case that a style sheet
   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.

   CSS 2.1 is derived from and is intended to replace CSS2. Some parts of
   CSS2 are unchanged in CSS 2.1, some parts have been altered, and some
   parts removed. The removed portions may be used in a future CSS3
   specification. Future specs should refer to CSS 2.1 (unless they need
   features from CSS2 which have been dropped in CSS 2.1, and then they
   should only reference CSS2 for those features, or preferably reference
   such feature(s) in the respective CSS3 Module that includes those
   feature(s)).

1.2 Reading the specification

   This section is non-normative.

   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

   This section is non-normative.

   The specification is organized into the following sections:

   Section 2: An introduction to CSS 2.1
          The introduction includes a brief tutorial on CSS 2.1 and a
          discussion of design principles behind CSS 2.1.

   Sections 3 - 20: CSS 2.1 reference manual.
          The bulk of the reference manual consists of the CSS 2.1
          language reference. This reference defines what may go into a
          CSS 2.1 style sheet (syntax, properties, property values) and
          how user agents must interpret these style sheets in order to
          claim conformance.

   Appendixes:
          Appendixes contain information about aural properties
          (non-normative), a sample style sheet for HTML 4, changes from
          CSS2, the grammar of CSS 2.1, a list of normative and
          informative references, and two indexes: one for properties and
          one general index.

1.4 Conventions

  1.4.1 Document language elements and attributes

     * CSS property and pseudo-class names are delimited by single
       quotes.
     * CSS values are delimited by single quotes.
     * Document language attribute names are in lowercase letters and
       delimited by double quotes.

  1.4.2 CSS property definitions

   Each CSS property definition begins with a summary of key information
   that resembles the following:

   'property-name'

          Value:            legal values & syntax
          Initial:          initial value
          Applies to:       elements this property applies to
          Inherited:        whether the property is inherited
          Percentages:      how percentage values are interpreted
          Media:            which media groups the property applies to
          Computed value:   how to compute the computed value

    1.4.2.1 Value

   This part specifies the set of valid values for the property whose
   name is 'property-name'. Value types may be designated in several
   ways:
    1. keyword values (e.g., auto, disc, etc.)
    2. basic data types, which appear between "<" and ">" (e.g.,
       <length>, <percentage>, etc.). In the electronic version of the
       document, each instance of a basic data type links to its
       definition.
    3. types that have the same range of values as a property bearing the
       same name (e.g., <'border-width'> <'background-attachment'>,
       etc.). In this case, the type name is the property name (complete
       with quotes) between "<" and ">" (e.g., <'border-width'>). Such a
       type does not include the value 'inherit'. In the electronic
       version of the document, each instance of this type of
       non-terminal links to the corresponding property definition.
    4. non-terminals that do not share the same name as a property. In
       this case, the non-terminal name appears between "<" and ">", as
       in <border-width>. Notice the distinction between <border-width>
       and <'border-width'>; the latter is defined in terms of the
       former. The definition of a non-terminal is located near its first
       appearance in the specification. In the electronic version of the
       document, each instance of this type of value links to the
       corresponding value definition.

   Other words in these definitions are keywords that must appear
   literally, without quotes (e.g., red). The slash (/) and the comma (,)
   must also appear literally.

   Values may be arranged as follows:
     * Several juxtaposed words mean that all of them must occur, in the
       given order.
     * A bar (|) separates two or more alternatives: exactly one of them
       must occur.
     * A double bar (||) separates two or more options: one or more of
       them must occur, in any order.
     * Brackets ([ ]) are for grouping.

   Juxtaposition is stronger than the double bar, and the double bar is
   stronger than the bar. Thus, the following lines are equivalent:
    a b   |   c || d e
  [ a b ] | [ c || [ d e ]]

   Every type, keyword, or bracketed group may be followed by one of the
   following modifiers:
     * An asterisk (*) indicates that the preceding type, word, or group
       occurs zero or more times.
     * A plus (+) indicates that the preceding type, word, or group
       occurs one or more times.
     * A question mark (?) indicates that the preceding type, word, or
       group is optional.
     * A pair of numbers in curly braces ({A,B}) indicates that the
       preceding type, word, or group occurs at least A and at most B
       times.

   The following examples illustrate different value types:

     Value: N | NW | NE
     Value: [ <length> | thick | thin ]{1,4}
     Value: [<family-name> , ]* <family-name>
     Value: <uri>? <color> [ / <color> ]?
     Value: <uri> || <color>

   Value types are specified in terms of tokens, as described in Appendix
   G.2. As the grammar allows spaces between tokens in the components of
   the expr production, spaces may appear between tokens in values.

   Note: In many cases, spaces will in fact be required between tokens in
   order to distinguish them from each other. For example, the value
   '1em2em' would be parsed as a single DIMEN token with the number '1'
   and the identifier 'em2em', which is an invalid unit. In this case, a
   space would be required before the '2' to get this parsed as the two
   lengths '1em' and '2em'.

    1.4.2.2 Initial

   This part specifies the property's initial value. Please consult the
   section on the cascade for information about the interaction between
   style sheet-specified, inherited, and initial values.

    1.4.2.3 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, the
   'clear' property only affects block-level elements.

    1.4.2.4 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.

    1.4.2.5 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.

    1.4.2.6 Media groups

   This part indicates the media groups to which the property applies.
   Information about media groups is non-normative.

    1.4.2.7 Computed value

   This part describes the computed value for the property. See the
   section on computed values for how this definition is used.

  1.4.3 Shorthand properties

   Some properties are shorthand properties, meaning that they allow
   authors to specify the values of several properties with a single
   property.

   For instance, the 'font' property is a shorthand property for setting
   'font-style', 'font-variant', 'font-weight', 'font-size',
   'line-height', and 'font-family' all at once.

   When values are omitted from a shorthand form, each "missing" property
   is assigned its initial value (see the section on the cascade).

   Example(s):

   The multiple style rules of this example:
h1 {
  font-weight: bold;
  font-size: 12pt;
  line-height: 14pt;
  font-family: Helvetica;
  font-variant: normal;
  font-style: normal;
}

   may be rewritten with a single shorthand property:
h1 { font: bold 12pt/14pt Helvetica }

   In this example, 'font-variant', and 'font-style' take their initial
   values.

  1.4.4 Notes and examples

   All examples that illustrate illegal usage are clearly marked as
   "ILLEGAL EXAMPLE".

   HTML examples lacking DOCTYPE declarations are SGML Text Entities
   conforming to the HTML 4.01 Strict DTD [HTML4]. Other HTML examples
   conform to the DTDs given in the examples.

   All notes are informative only.

   Examples and notes are marked within the source HTML for the
   specification and CSS 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]" after the image.

   Images and long descriptions are informative only.

1.5 Acknowledgments

   This section is non-normative.

   CSS 2.1 is based on CSS2. See the acknowledgments section of CSS2 for
   the people that contributed to CSS2.

   We would like to thank the following people who, through their input
   and feedback on the www-style mailing list, have helped us with the
   creation of this specification: Andrew Clover, Bernd Mielke, C.
   Bottelier, Christian Roth, Christoph Päper, Claus Färber, Coises,
   Craig Saila, Darren Ferguson, Dylan Schiemann, Etan Wexler, George
   Lund, James Craig, Jan Eirik Olufsen, Jan Roland Eriksson, Joris
   Huizer, Joshua Prowse, Kai Lahmann, Kevin Smith, Lachlan Cannon, Lars
   Knoll, Lauri Raittila, Mark Gallagher, Michael Day, Peter Sheerin,
   Rijk van Geijtenbeek, Robin Berjon, Scott Montgomery, Shelby Moore,
   Stuart Ballard, Tom Gilder, Vadim Plessky, and the Open eBook
   Publication Structure Working Group Editors. We would also like to
   thank Glenn Adams and Susan Lesch who helped proofread this document.

   In addition, we would like to extend special thanks to fantasai, Ada
   Chan and Boris Zbarsky who have contributed significant time to
   CSS 2.1, and to Kimberly Blessing for help with the editing.
     _________________________________________________________________
     _________________________________________________________________

                           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

   This section is non-normative.

   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
   [HTML4]) and some basic desktop publishing terminology.

   We begin with a small HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

   To set the text color of the H1 elements to red, you can write the
   following CSS rules:
  h1 { color: red }

   A CSS rule consists of two main parts: selector ('h1') and declaration
   ('color: red'). In HTML, element names are case-insensitive so 'h1'
   works just as well as 'H1'. The declaration has two parts: property
   ('color') and value ('red'). While the example above tries to
   influence only one of the properties needed for rendering an HTML
   document, it qualifies as a style sheet on its own. Combined with
   other style sheets (one fundamental feature of CSS is that style
   sheets are combined) it will determine the final presentation of the
   document.

   The HTML 4 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 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  <STYLE type="text/css">
    h1 { color: red }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

   For maximum flexibility, we recommend that authors specify external
   style sheets; they may be changed without modifying the source HTML
   document, and they may be shared among several documents. To link to
   an external style sheet, you can use the LINK element:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//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 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  <STYLE type="text/css">
    body { color: black; background: white }
    h1 { color: red; background: white }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

   The style sheet now contains four rules: the first two set the color
   and background of the BODY element (it's a good idea to set the text
   color and background color together), while the last two set the color
   and the background of the H1 element. Since no color has been
   specified for the P element, it will inherit the color from its parent
   element, namely BODY. The H1 element is also a child element of BODY
   but the second rule overrides the inherited value. In CSS there are
   often such conflicts between different values, and this specification
   describes how to resolve them.

   CSS 2.1 has more than 90 properties, including 'color'. Let's look at
   some of the others:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//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

   This section is non-normative.

   CSS can be used with any structured document format, for example with
   applications of the eXtensible Markup Language [XML10]. In fact, XML
   depends more on style sheets than HTML, since authors can make up
   their own elements that user agents don't know how to display.

   Here is a simple XML fragment:
<ARTICLE>
  <HEADLINE>Fredrick the Great meets Bach</HEADLINE>
  <AUTHOR>Johann Nikolaus Forkel</AUTHOR>
  <PARA>
    One evening, just as he was getting his
    <INSTRUMENT>flute</INSTRUMENT> ready and his
    musicians were assembled, an officer brought him a list of
    the strangers who had arrived.
  </PARA>
</ARTICLE>

   To display this fragment in a document-like fashion, we must first
   declare which elements are inline-level (i.e., do not cause line
   breaks) and which are block-level (i.e., cause line breaks).
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }

   The first rule declares INSTRUMENT to be inline and the second rule,
   with its comma-separated list of selectors, declares all the other
   elements to be block-level. Element names in XML are case-sensitive,
   so a selector written in lowercase (e.g. 'instrument') is different
   from uppercase (e.g. 'INSTRUMENT').

   One way of linking a style sheet to an XML document is to use a
   processing instruction:
<?xml-stylesheet type="text/css" href="bach.css"?>
<ARTICLE>
  <HEADLINE>Fredrick the Great meets Bach</HEADLINE>
  <AUTHOR>Johann Nikolaus Forkel</AUTHOR>
  <PARA>
    One evening, just as he was getting his
    <INSTRUMENT>flute</INSTRUMENT> ready and his
    musicians were assembled, an officer brought him a list of
    the strangers who had arrived.
  </PARA>
</ARTICLE>

   A visual user agent could format the above example as:

   Example rendering    [D]

   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    [D]

   Adding more rules to the style sheet will allow you to further
   describe the presentation of the document.

2.3 The CSS 2.1 processing model

   This section presents one possible model of how user agents that
   support CSS work. This is only a conceptual model; real
   implementations may vary.

   In this model, a user agent processes a source by going through the
   following steps:
    1. Parse the source document and create a document tree.
    2. Identify the target media type.
    3. Retrieve all style sheets associated with the document that are
       specified for the target media type.
    4. Annotate every element of the document tree by assigning a single
       value to every property that is applicable to the target media
       type. Properties are assigned values according to the mechanisms
       described in the section on cascading and inheritance.
       Part of the calculation of values depends on the formatting
       algorithm appropriate for the target media type. For example, if
       the target medium is the screen, user agents apply the visual
       formatting model.
    5. From the annotated document tree, generate a formatting structure.
       Often, the formatting structure closely resembles the document
       tree, but it may also differ significantly, notably when authors
       make use of pseudo-elements and generated content. First, the
       formatting structure need not be "tree-shaped" at all -- the
       nature of the structure depends on the implementation. Second, the
       formatting structure may contain more or less information than the
       document tree. For instance, if an element in the document tree
       has a value of 'none' for the 'display' property, that element
       will generate nothing in the formatting structure. A list element,
       on the other hand, may generate more information in the formatting
       structure: the list element's content and list style information
       (e.g., a bullet image).
       Note that the CSS user agent does not alter the document tree
       during this phase. In particular, content generated due to style
       sheets is not fed back to the document language processor (e.g.,
       for reparsing).
    6. Transfer the formatting structure to the target medium (e.g.,
       print the results, display them on the screen, render them as
       speech, etc.).

   Step 1 lies outside the scope of this specification (see, for example,
   [DOM]).

   Steps 2-5 are addressed by the bulk of this specification.

   Step 6 lies outside the scope of this specification.

  2.3.1 The canvas

   For all media, the term canvas describes "the space where the
   formatting structure is rendered." The canvas is infinite for each
   dimension of the space, but rendering generally occurs within a finite
   region of the canvas, established by the user agent according to the
   target medium. For instance, user agents rendering to a screen
   generally impose a minimum width and choose an initial width based on
   the dimensions of the viewport. User agents rendering to a page
   generally impose width and height constraints. Aural user agents may
   impose limits in audio space, but not in time.

  2.3.2 CSS 2.1 addressing model

   CSS 2.1 selectors and properties allow style sheets to refer to the
   following parts of a document or user agent:
     * Elements in the document tree and certain relationships between
       them (see the section on selectors).
     * Attributes of elements in the document tree, and values of those
       attributes (see the section on attribute selectors).
     * Some parts of element content (see the :first-line and
       :first-letter pseudo-elements).
     * Elements of the document tree when they are in a certain state
       (see the section on pseudo-classes).
     * Some aspects of the canvas where the document will be rendered.
     * Some system information (see the section on user interface).

2.4 CSS design principles

   This section is non-normative.

   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 UA Conformance
     * 3.3 Error conditions
     * 3.4 The text/css content type

3.1 Definitions

   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.

   The fact that a feature is marked as deprecated (namely the 'aural'
   keyword) or going to be deprecated in CSS3 (namely the system colors)
   also has no influence on conformance. (For example, 'aural' is marked
   as non-normative, so UAs do not need to support it; the system colors
   are normative, so UAs must support them.)

   All sections of this specification, including appendices, are
   normative unless otherwise noted.

   Examples and notes are not normative.

   Example(s):

   Examples usually have the word "example" near their start ("Example:",
   "The following example...," "For example," etc.) and are shown in the
   color maroon, like this paragraph.

   Notes start with the word "Note," are indented and shown in green,
   like this paragraph.

   Figures are for illustration only, they are not reference renderings,
   unless explicitly stated.

   Style sheet
          A set of statements that specify presentation of a document.

          Style sheets may have three different origins: author, user,
          and user agent. The interaction of these sources is described
          in the section on cascading and inheritance.

   Valid style sheet
          The validity of a style sheet depends on the level of CSS used
          for the style sheet. All valid CSS1 style sheets are valid
          CSS 2.1 style sheets, but some changes from CSS1 mean that a
          few CSS1 style sheets will have slightly different semantics in
          CSS 2.1. Some features in CSS2 are not part of CSS 2.1, so not
          all CSS2 style sheets are valid CSS 2.1 style sheets.

          A valid CSS 2.1 style sheet must be written according to the
          grammar of CSS 2.1. Furthermore, it must contain only at-rules,
          property names, and property values defined in this
          specification. An illegal (invalid) at-rule, property name, or
          property value is one that is not valid.

   Source document
          The document to which one or more style sheets apply. This is
          encoded in some language that represents the document as a tree
          of elements. Each element consists of a name that identifies
          the type of element, optionally a number of attributes, and a
          (possibly empty) content. For example, the source document
          could be an XML or SGML instance.

   Document language
          The encoding language of the source document (e.g., HTML, XHTML
          or SVG). CSS is used to describe the presentation of document
          languages and CSS does not change the underlying semantics of
          the document languages.

   Element
          (An SGML term, see [ISO8879].) The primary syntactic constructs
          of the document language. Most CSS style sheet rules use the
          names of these elements (such as P, TABLE, and OL in HTML) to
          specify how the elements should be rendered.

   Replaced element
          An element whose content is outside the scope of the CSS
          formatting model, such as an image, embedded document, or
          applet. For example, the content of the HTML IMG element is
          often replaced by the image that its "src" attribute
          designates. Replaced elements often have intrinsic dimensions:
          an intrinsic width, an intrinsic height, and an intrinsic
          ratio. For example, a bitmap image has an intrinsic width and
          an intrinsic height specified in absolute units (from which the
          intrinsic ratio can obviously be determined). On the other
          hand, other documents may not have any intrinsic dimensions
          (for example a blank HTML document).

          User agents may consider a replaced element to not have any
          intrinsic dimensions if it is believed that those dimensions
          could leak sensitive information to a third party. For example,
          if an HTML document changed intrinsic size depending on the
          user's bank balance, then the UA might want to act as if that
          resource had no intrinsic dimensions.

   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 only replaced
          elements can come with intrinsic dimensions.

   Attribute
          A value associated with an element, consisting of a name, and
          an associated (textual) value.

   Content
          The content associated with an element in the source document.
          Some elements have no content, in which case they are called
          empty. The content of an element may include text, and it may
          include a number of sub-elements, in which case the element is
          called the parent of those sub-elements.

   Ignore
          This term has two slightly different meanings in this
          specification. First, a CSS parser must follow certain rules
          when it discovers unknown or illegal syntax in a style sheet.
          The parser must then ignore certain parts of the style sheets.
          The exact rules for what parts must be ignored is given in
          these section: Declarations and properties, Rules for handling
          parsing errors, Unsupported Values, or may be explained in the
          text where the term "ignore" appears. Second, a user agent may
          (and, in some cases must) disregard certain properties or
          values in the style sheet even if the syntax is legal. For
          example, table-column elements can't affect the font of the
          column, so the font properties must be ignored.

   Rendered content
          The content of an element after the rendering that applies to
          it according to the relevant style sheets has been applied. How
          a replaced element's content is rendered is not defined by this
          specification. 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 is a User Agent that generates 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.

   Property
          CSS defines a finite set of parameters, called properties, that
          direct the rendering of a document. Each property has a name
          (e.g., 'color', 'font' or border') and a value (e.g., 'red',
          '12pt Times' or 'dotted'). Properties are attached to various
          parts of the document and to the page on which the document is
          to be displayed by the mechanisms of specificity, cascading and
          inheritance (see the chapter on Assigning property values,
          Cascading, and Inheritance).

   Here is an example of a source document written in HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//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    [D]

   According to the definition of HTML 4, 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 UA 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 recognize 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 recognized
       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 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 [HTML4]), the UA must
       allow the user to select which style sheet set the UA should
       apply.
    6. The UA must allow the user to turn off the influence of author
       style sheets.

   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 1-6 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.

   CSS 2.1 does not define which properties apply to form controls and
   frames, or how CSS can be used to style them. User agents may apply
   CSS properties to these elements. Authors are recommended to treat
   such support as experimental. A future level of CSS may specify this
   further.

3.3 Error conditions

   In general, this document specifies error handling behavior throughout
   the specification. For example, see the rules for handling parsing
   errors.

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 2616 (see [RFC2045] and [RFC2616]). A message
   entity with a content type of "text/css" represents an independent CSS
   document. The "text/css" content type has been registered by RFC 2318
   ([RFC2318]).
     _________________________________________________________________
     _________________________________________________________________

                         4 Syntax and basic data types

   Contents
     * 4.1 Syntax
          + 4.1.1 Tokenization
          + 4.1.2 Keywords
               o 4.1.2.1 Vendor-specific extensions
               o 4.1.2.2 Informative Historical Notes
          + 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 URLs and URIs
          + 4.3.5 Counters
          + 4.3.6 Colors
          + 4.3.7 Strings
          + 4.3.8 Unsupported Values
     * 4.4 CSS style sheet 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 level of CSS (including CSS 2.1). Future updates
   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 G.

   In this specification, the expressions "immediately before" or
   "immediately after" mean with no intervening whitespace or comments.

  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 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}
   INVALID {invalid}
   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-z]|{nonascii}|{escape}
   nonascii [^\0-\177]
   unicode \\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])?
   escape {unicode}|\\[^\n\r\f0-9a-f]
   nmchar [_a-z0-9-]|{nonascii}|{escape}
   num [0-9]+|[0-9]*\.[0-9]+
   string {string1}|{string2}
   string1 \"([^\n\r\f\\"]|\\{nl}|{escape})*\"
   string2 \'([^\n\r\f\\']|\\{nl}|{escape})*\'
   invalid {invalid1}|{invalid2}
   invalid1 \"([^\n\r\f\\"]|\\{nl}|{escape})*
   invalid2 \'([^\n\r\f\\']|\\{nl}|{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 G describes a more restrictive grammar that is
   closer to the CSS level 2 language. Parts of style sheets that can be
   parsed according to this grammar but not according to the grammar in
   Appendix G are among the parts that will be ignored according to the
   rules for handling parsing errors.
stylesheet  : [ CDO | CDC | S | statement ]*;
statement   : ruleset | at-rule;
at-rule     : ATKEYWORD S* any* [ block | ';' S* ];
block       : '{' S* [ any | block | ATKEYWORD S* | ';' S* ]* '}' S*;
ruleset     : selector? '{' S* declaration? [ ';' S* declaration? ]* '}' S*;
selector    : any+;
declaration : DELIM? property S* ':' S* value;
property    : IDENT;
value       : [ any | block | ATKEYWORD S* ]+;
any         : [ IDENT | NUMBER | PERCENTAGE | DIMENSION | STRING
              | DELIM | URI | HASH | UNICODE-RANGE | INCLUDES
              | DASHMATCH | FUNCTION S* any* ')'
              | '(' S* any* ')' | '[' S* 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" (U+0020), "tab" (U+0009), "line feed" (U+000A),
   "carriage return" (U+000D), and "form feed" (U+000C) can occur in
   whitespace. Other space-like characters, such as "em-space" (U+2003)
   and "ideographic space" (U+3000), are never part of whitespace.

   The meaning of input that cannot be tokenized or parsed is undefined
   in CSS 2.1.

  4.1.2 Keywords

   Keywords have the form of identifiers. Keywords must not be placed
   between quotes ("..." or '...'). Thus,
red

   is a keyword, but
"red"

   is not. (It is a string.) Other illegal examples:

   Illegal example(s):
width: "auto";
border: "none";
background: "red";

    4.1.2.1 Vendor-specific extensions

   In CSS, identifiers may begin with '-' (dash) or '_' (underscore).
   Keywords and property names beginning with -' or '_' are reserved for
   vendor-specific extensions. Such vendor-specific extensions should
   have one of the following formats:
'-' + vendor identifier + '-' + meaningful name
'_' + vendor identifier + '-' + meaningful name

   Example(s):

   For example, if XYZ organization added a property to describe the
   color of the border on the East side of the display, they might call
   it -xyz-border-east-color.

   Other known examples:
-moz-box-sizing
-moz-border-radius
-wap-accesskey

   An initial dash or underscore is guaranteed never to be used in a
   property or keyword by any current or future level of CSS. Thus
   typical CSS implementations may not recognize such properties and may
   ignore them according to the rules for handling parsing errors.
   However, because the initial dash or underscore is part of the
   grammar, CSS 2.1 implementers should always be able to use a
   CSS-conforming parser, whether or not they support any vendor-specific
   extensions.

   Authors should avoid vendor-specific extensions

    4.1.2.2 Informative Historical Notes

   This section is informative.

   At the time of writing, the following prefixes are known to exist:

   prefix              organization
   -ms-   Microsoft
   mso-   Microsoft Office
   -moz-  Mozilla
   -o-    Opera Software
   -atsc- Advanced Television Standards Committee
   -wap-  The WAP Forum

  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, identifiers (including element names, classes, and IDs in
       selectors) can contain only the characters [a-z0-9] and ISO 10646
       characters U+00A1 and higher, plus the hyphen (-) and the
       underscore (_); they cannot start with a digit, or a hyphen
       followed by a digit. Identifiers 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, which
       must not be zero. (It is undefined in CSS 2.1 what happens if a
       style sheet does contain a character with Unicode codepoint zero.)
       If a character in the range [0-9a-f] 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
            (U+000D/U+000A) 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.
       If the number is outside the range allowed by Unicode (e.g.,
       "\110000" is above the maximum 10FFFF allowed in current Unicode),
       the UA may replace the escape with the "replacement character"
       (U+FFFD). If the character is to be displayed, the UA should show
       a visible symbol, such as a "missing character" glyph (cf. 15.2,
       point 5).
     * Note: Backslash escapes, where allowed, 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).
       The identifier "te\st" is exactly the same identifier as "test".

  4.1.4 Statements

   A CSS style sheet, for any level 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.

  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.

   CSS 2.1 user agents must ignore any '@import' rule that occurs inside
   a block or after any valid statement other than an @charset or an
   @import rule.

   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 CSS 2.1. The CSS 2.1
   parser ignores the whole at-rule, effectively reducing the style sheet
   to:
@import "subs.css";
h1 { color: blue }

   Illegal example(s):

   In the following example, the second '@import' rule is invalid, since
   it occurs inside a '@media' block.
@import "subs.css";
@media print {
  @import "print-main.css";
  body { font-size: 10pt }
}
h1 {color: blue }

   Instead, to achieve the effect of only importing a style sheet for
   'print' media, use the @import rule with media syntax, e.g.:
@import "subs.css";
@import "print-main.css" print;
@media print {
  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 tokens, 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 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 declaration block. When a user
   agent can't parse the selector (i.e., it is not valid CSS 2.1), it
   must ignore the declaration 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 updates 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 rule.
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 name, 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 name is an identifier. Any token may occur in the value.
   Parentheses ("( )"), brackets ("[ ]"), braces ("{ }"), single quotes
   (') and double quotes (") must come in matching pairs, and semicolons
   not in strings must be escaped. Parentheses, brackets, and braces may
   be nested. Inside the quotes, characters are parsed as a string.

   The syntax of values is specified separately for each property, but in
   any case, values are built from identifiers, strings, numbers,
   lengths, percentages, URIs, colors, etc.

   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 defined by the grammar, 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 specification ([HTML4]) 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. CSS 2.1 reserves for
   future updates of CSS all property:value combinations and @-keywords
   that do not contain an identifier beginning with dash or underscore.
   Implementations must ignore such combinations (other than those
   introduced by future updates of CSS).

   To ensure that new properties and new values for existing properties
   can be added in the future, user agents are required to obey the
   following rules when they encounter the following scenarios:
     * Unknown properties. User agents must ignore a declaration with an
       unknown property. For example, if the style sheet is:
h1 { color: red; rotation: 70minutes }
       the user agent will treat this as if the style sheet had been
h1 { color: red }
     * Illegal values. User agents must ignore a declaration with an
       illegal value. For example:
img { float: left }       /* correct CSS 2.1 */
img { float: left here }  /* "here" is not a value of 'float' */
img { background: "red" } /* keywords cannot be quoted */
img { border-width: 3 }   /* a unit must be specified for length values */
       A CSS 2.1 parser would honor the first rule and ignore the rest,
       as if the style sheet had been:
img { float: left }
img { }
img { }
img { }
       A user agent conforming to a future CSS specification may accept
       one or more of the other rules as well.
     * Malformed declarations. User agents must handle unexpected tokens
       encountered while parsing a declaration by reading until the end
       of the declaration, while observing the rules for matching pairs
       of (), [], {}, "", and '', and correctly handling escapes. For
       example, a malformed declaration may be missing a property, colon
       (:) or value. The following are all equivalent:
p { color:green }
p { color:green; color }  /* malformed declaration missing ':', value */
p { color:red;   color; color:green }  /* same with expected recovery */
p { color:green; color: } /* malformed declaration missing value */
p { color:red;   color:; color:green } /* same with expected recovery */
p { color:green; color{;color:maroon} } /* unexpected tokens { } */
p { color:red;   color{;color:maroon}; color:green } /* same with recovery */
     * 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 }
       Something inside an at-rule that is ignored because it is invalid,
       such as an invalid declaration within an @media-rule, does not
       make the entire at-rule invalid.
     * Unexpected end of style sheet.
       User agents must close all open constructs (for example: blocks,
       parentheses, brackets, rules, strings, and comments) at the end of
       the style sheet. For example:
  @media screen {
    p:before { content: 'Hello
       would be treated the same as:
  @media screen {
    p:before { content: 'Hello'; }
  }
       in a conformant UA.
     * Unexpected end of string.
       User agents must close strings upon reaching the end of a line,
       but then drop the construct (declaration or rule) in which the
       string was found. For example:
      p {
        color: green;
        font-family: 'Courier New Times
        color: red;
        color: green;
      }
       ...would be treated the same as:
      p { color: green; color: green; }
       ...because the second declaration (from 'font-family' to the
       semicolon after 'color: red') is invalid and is dropped.
     * See also Rule sets, declaration blocks, and selectors for parsing
       rules for declaration blocks.

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. -0 is equivalent to 0 and is not a
   negative number.

   Note that many properties that allow an integer or real number as a
   value actually restrict the value to some range, often to a
   non-negative value.

  4.3.2 Lengths

   Lengths refer to horizontal or vertical measurements.

   The format of a length value (denoted by <length> in this
   specification) is a <number> (with or without a decimal point)
   immediately followed by a unit identifier (e.g., px, em, etc.). After
   a zero length, the unit identifier is optional.

   Some properties allow negative length values, but this may complicate
   the formatting model and there may be implementation-specific limits.
   If a negative length value cannot be supported, it should be converted
   to the nearest value that can be supported.

   If a negative length value is set on a property that does not allow
   negative length values, the declaration is ignored.

   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 element's first available font. 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".

   The x-height of a font can be found in different ways. Some fonts
   contain reliable metrics for the x-height. If reliable font metrics
   are not available, UAs may determine the x-height from the height of a
   lowercase glyph. One possible heuristics is to look at how far the
   glyph for the lowercase "o" extends below the baseline, and subtract
   that value from the top of its bounding box. In the cases where it is
   impossible or impractical to determine the x-height, a value of 0.5em
   should be used.

   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    [D]

   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    [D]

   Child elements do not inherit the relative values specified for their
   parent; they 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 1/72nd 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 used 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 URLs and URIs

   URI values (Uniform Resource Identifiers, see [RFC3986], which
   includes URLs, URNs, etc) in this specification are denoted by <uri>.
   The functional notation used to designate URIs in property values is
   "url()", as in:

   Example(s):
body { background: url("http://www.example.com/pinkish.png") }

   The format of a URI value is 'url(' followed by optional whitespace
   followed by an optional single quote (') or double quote (") character
   followed by the URI itself, followed by an optional single quote (')
   or double quote (") character followed by optional whitespace followed
   by ')'. The two quote characters must be the same.

   Example(s):

   An example without quotes:
li { list-style: url(http://www.example.com/redball.png) disc }

   Some characters appearing in an unquoted URI, such as parentheses,
   commas, whitespace characters, single quotes (') and double quotes
   ("), must be escaped with a backslash so that the resulting URI value
   is a URI token: '\(', '\)', '\,'.

   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 [RFC3986].

   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 [RFC3986]) are resolved to full URIs
   using a base URI. RFC 3986, section 5, defines the normative algorithm
   for this process. For CSS style sheets, the base URI is that of the
   style sheet, not that of the source document.

   Example(s):

   For example, suppose the following rule:
body { background: url("yellow") }

   is located in a style sheet designated by the URI:
http://www.example.org/style/basic.css

   The background of the source document's BODY will be tiled with
   whatever image is described by the resource designated by the URI
http://www.example.org/style/yellow

   User agents may vary in how they handle invalid URIs or URIs that
   designate unavailable or inapplicable resources.

  4.3.5 Counters

   Counters are denoted by identifiers (see the 'counter-increment' and
   'counter-reset' properties). To refer to the value of a counter, the
   notation 'counter(<identifier>)' or 'counter(<identifier>,
   <'list-style-type'>)', with optional whitespace separating the tokens,
   is used. The default style is 'decimal'.

   To refer to a sequence of nested counters of the same name, the
   notation is 'counters(<identifier>, <string>)' or
   'counters(<identifier>, <string>, <'list-style-type'>)' with optional
   whitespace separating the tokens.

   See "Nested counters and scope" in the chapter on generated content
   for how user agents must determine the value or values of the counter.
   See the definition of counter values of the 'content' property for how
   it must convert these values to a string.

   In CSS 2.1, the values of counters can only be referred to from the
   'content' property. Note that 'none' is a possible
   <'list-style-type'>: 'counter(x, none)' yields an empty string.

   Example(s):

   Here is a style sheet that numbers paragraphs (p) for each chapter
   (h1). The paragraphs are numbered with roman numerals, followed by a
   period and a space:
p {counter-increment: par-num}
h1 {counter-reset: par-num}
p:before {content: counter(par-num, upper-roman) ". "}

  4.3.6 Colors

   A <color> is either a keyword or a numerical RGB specification.

   The list of color keywords 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 or mapped into the
   gamut when the gamut is known: the red, green, and blue values must be
   changed to fall within the range supported by the device. Users agents
   may perform higher quality mapping of colors from one gamut to
   another. For a typical CRT monitor, whose device gamut is the same as
   sRGB, the four rules below are equivalent:

   Example(s):
em { color: rgb(255,0,0) }       /* integer range 0 - 255 */
em { color: rgb(300,0,0) }       /* clipped to rgb(255,0,0) */
em { color: rgb(255,-10,0) }     /* clipped to rgb(255,0,0) */
em { color: rgb(110%, 0%, 0%) }  /* clipped to rgb(100%,0%,0%) */

   Other devices, such as printers, have different gamuts than sRGB; some
   colors outside the 0..255 sRGB range will be representable (inside the
   device gamut), while other colors inside the 0..255 sRGB range will be
   outside the device gamut and will thus be mapped.

   Note. Mapping or clipping of color values should be done to the actual
   device gamut if known (which may be larger or smaller than 0..255).

  4.3.7 Strings

   Strings can either be written with double quotes or with single
   quotes. Double quotes cannot occur inside double quotes, unless
   escaped (e.g., as '\"' or as '\22'). Analogously for single quotes
   (e.g., "\'" 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 an escape representing the line feed character in
   ISO-10646 (U+000A), such as "\A" or "\00000a". This character
   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"] {/*...*/}

  4.3.8 Unsupported Values

   If a UA does not support a particular value, it should ignore that
   value when parsing style sheets, as if that value was an illegal
   value. For example:

   Example(s):
  h3 {
    display: inline;
    display: run-in;
  }

   A UA that supports the 'run-in' value for the 'display' property will
   accept the first display declaration and then "write over" that value
   with the second display declaration. A UA that does not support the
   'run-in' value will process the first display declaration and ignore
   the second display declaration.

4.4 CSS style sheet 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., UTF-8, ISO 8859-x,
   SHIFT JIS, etc.). For a good introduction to character sets and
   character encodings, please consult the HTML 4 specification ([HTML4],
   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 style sheet's
   character encoding (from highest priority to lowest):
    1. An HTTP "charset" parameter in a "Content-Type" field (or similar
       parameters in other protocols)
    2. BOM and/or @charset (see below)
    3. <link charset=""> or other metadata from the linking mechanism (if
       any)
    4. charset of referring style sheet or document (if any)
    5. Assume UTF-8

   Authors using an @charset rule must place the rule at the very
   beginning of the style sheet, preceded by no characters. (If a byte
   order mark is appropriate for the encoding used, it may precede the
   @charset rule.)

   After "@charset", authors specify the name of a character encoding (in
   quotes). For example:
@charset "ISO-8859-1";

   @charset must be written literally, i.e., the 10 characters '@charset
   "' (lowercase, no backslash escapes), followed by the encoding name,
   followed by '";'.

   The name must be a charset name as described in the IANA registry. See
   [CHARSETS] for a complete list of charsets. Authors should use the
   charset names marked as "preferred MIME name" in the IANA registry.

   User agents must support at least the UTF-8 encoding.

   User agents must ignore any @charset rule not at the beginning of the
   style sheet. When user agents detect the character encoding using the
   BOM and/or the @charset rule, they should follow the following rules:
     * Except as specified in these rules, all @charset rules are
       ignored.
     * The encoding is detected based on the stream of bytes that begins
       the style sheet. The following table gives a set of possibilities
       for initial byte sequences (written in hexadecimal). The first row
       that matches the beginning of the style sheet gives the result of
       encoding detection based on the BOM and/or @charset rule. If no
       rows match, the encoding cannot be detected based on the BOM
       and/or @charset rule. The notation (...)* refers to repetition for
       which the best match is the one that repeats as few times as
       possible. The bytes marked "XX" are those used to determine the
       name of the encoding, by treating them, in the order given, as a
       sequence of ASCII characters. Bytes marked "YY" are similar, but
       need to be transcoded into ASCII as noted. User agents may ignore
       entries in the table if they do not support any encodings relevant
       to the entry.

   Initial Bytes Result
   EF BB BF 40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B as specified
   EF BB BF UTF-8
   40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B as specified
   FE FF 00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00
   XX)* 00 22 00 3B as specified (with BE endianness if not specified)
   00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00 XX)*
   00 22 00 3B as specified (with BE endianness if not specified)
   FF FE 40 00 63 00 68 00 61 00 72 00 73 0