drafts: css-conditional/Overview.src.html@7e80b7d0224f (annotated)

css-conditional/Overview.src.html@7e80b7d0224f (annotated)

css-conditional/Overview.src.html

Sat, 08 Jun 2013 14:19:52 +0900

author: Simon Sapin <simon.sapin@exyr.org>
date: Sat, 08 Jun 2013 14:19:52 +0900
changeset 8381: 7e80b7d0224f
parent 8326: 9421e8cccd2e
child 8599: 18bc8956c979
permissions: -rw-r--r--

[css-*] Replace s/rule ?set/style rule/, except in grammars.

Having three names for the same concept is silly.
"Rule set" is confusing as it implies multiple rules,
while it in fact refers to a single rule.
CSS OM and other specs already use "style rule".

This changes "rule set" and "ruleset" to "style rule",
except when used in a grammar.
(The normative definiton of the Core Grammar is in 2.1
which is harder to change.)

This is missing a few changes in css-fonts,
since its source is not version-controlled in this repository.

 <!DOCTYPE html public '-//W3C//DTD HTML 4.01//EN'
   'http://www.w3.org/TR/html4/strict.dtd'>
 <html lang="en">
 <head profile="http://www.w3.org/2006/03/hcard">
   <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
   <title>CSS Conditional Rules Module Level 3</title>
   <link rel="stylesheet" type="text/css" href="../default.css">
   <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
   <link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
   <script src="http://test.csswg.org/harness/annotate.js#CSS3-CONDITIONAL_DEV"
         type="text/javascript" defer></script>
 </head>
 <body class="h-entry">
 <div class="head">
 <!--logo-->
 <h1 class="p-name">CSS Conditional Rules Module Level 3</h1>
 <h2 class="no-num no-toc">[LONGSTATUS] <span class="dt-updated"><span class="value-title" title="[CDATE]">[DATE]</span></span></h2>
 <dl>
   <dt>This version:
     <dd><a class="u-url" href="[VERSION]">
     http://www.w3.org/TR/[YEAR]/ED-css3-conditional-[CDATE]/</a>
   <dt>Latest version:
     <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>
   <dt>Editor's draft:
     <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
     (<a href="https://dvcs.w3.org/hg/csswg/log/tip/css-conditional/Overview.src.html">change log</a>,
     <a href="https://dvcs.w3.org/hg/csswg/log/tip/css3-conditional/Overview.src.html">older change log</a>)
   <dt>Previous version:
     <dd><a rel="previous" href="http://www.w3.org/TR/2013/CR-css3-conditional-20130404/">http://www.w3.org/TR/2013/CR-css3-conditional-20130404/</a></dd>
   <dt>Editors:
     <dd class="p-author h-card vcard"><a class="p-name fn u-url url" rel="author" href="http://dbaron.org/">L. David Baron</a>,
       <a class="p-org org h-org" href="http://www.mozilla.org/">Mozilla</a>
   <dt>Issues list:
     <dd>Maintained in document (only editor's draft is current)
   <dt>Feedback:</dt>
     <dd><a href="mailto:www-style@w3.org?subject=%5Bcss3-conditional%5D%20feedback"
          >www-style@w3.org</a>
          with subject line &ldquo;<kbd>[css3-conditional]
          <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
          (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/"
            >archives</a>)
   <dt>Test suite:
     <dd><a href="https://test.csswg.org/shepherd/search/spec/css3-conditional/">submitted tests</a>,
         <a href="https://test.csswg.org/harness/suite/CSS3-CONDITIONAL_DEV/">nightly test suite</a>
 </dl>
 <!--copyright-->
 <hr title="Separator for header">
 </div>
 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
   <p>CSS is a language for describing the rendering of structured documents
   (such as HTML and XML) on screen, on paper,  in speech, etc.
   <span class="p-summary">
   This module
   contains the features of CSS for conditional processing of parts of
   style sheets, conditioned on capabilities of the processor or the
   document the style sheet is being applied to.
   It includes and extends the functionality of CSS level&nbsp;2 [[!CSS21]],
   which builds on CSS level&nbsp;1 [[CSS1]].
   The main extensions compared to level&nbsp;2 are
   allowing nesting of certain at-rules inside '@media',
   and the addition of the '@supports'
   rule for conditional processing.
   </span>
 <h2 class="no-num no-toc" id="status">Status of this document</h2>
 <!--status-->
 <p>See the section <a href="#cr-exit-criteria">“CR Exit Criteria”</a>
 for details on advancing this specification to W3C Recommendation. The
 specification will remain Candidate Recommendation at least until 2
 October 2013. A <a
 href="http://test.csswg.org/suites/css3-conditional/nightly-unstable/report/"
 >test suite and implementation report</a> are under development.
 <p>See the section <a href="#changes">“Changes”</a> for changes since
 the last Working Draft.
 <p>The following features are at risk:
 <ul>
   <li>The inclusion of '@font-face' rules and
   '@keyframes' rules as allowed within all of the @-rules in
   this specification is at risk, though only because of the relative
   rates of advancement of specifications.  If this specification is able
   to advance faster than one or both of the specifications defining
   those rules, then the inclusion of those rules will move from this
   specification to the specification defining those rules.</li>
   <li>The addition of support for @-rules inside of conditional grouping
   rules is at risk; if interoperable implementations are not found, it
   may be removed to advance the other features in this specification to
   Proposed Recommendation.</li>
   <li>The '@supports' rule is at risk; if interoperable
   implementations are not found, it may be removed to advance the other
   features in this specification to Proposed Recommendation.</li>
 </ul>
 <!--
   Things to go in level 4:
   * Create some way to put these new conditional things on an @import.
   * The @document rule (commented out, down below).
 -->
 <h2 class="no-num no-toc" id="contents">Table of contents</h2>
 <!--toc-->
 <h2 id="introduction">Introduction</h2>
 <h3 id="context">Background</h3>
   <p><em>This section is not normative.</em>
   <p>[[!CSS21]] defines one type of conditional group rule, the
   '@media' rule, and allows only style rules (not other @-rules)
   inside of it.  The '@media' rule provides the ability to
   have media-specific style sheets, which is also provided by style
   sheet linking features such as '@import' and
   <code class="html">&lt;link&gt;</code>.  The restrictions on the contents of
   '@media' rules made them less useful; they have forced authors
   using CSS features involving @-rules in media-specific style sheets to
   use separate style sheets for each medium.</p>
   <p>This specification extends the rules for the contents of
   conditional group rules to allow other @-rules, which enables authors
   to combine CSS features involving @-rules with media specific style
   sheets within a single style sheet.</p>
   <p>This specification also defines an additional type of conditional
   group rule, '@supports', to
   address author and user requirements.</p>
   <p>The '@supports' rule allows CSS to be conditioned on
   implementation support for CSS properties and values.  This rule makes
   it much easier for authors to use new CSS features and provide good
   fallback for implementations that do not support those features.  This
   is particularly important for CSS features that provide new layout
   mechanisms, and for other cases where a set of related styles needs to
   be conditioned on property support.</p>
 <h3 id="placement">Module Interactions</h3>
   <p>This module replaces and extends the '@media' rule
   feature defined in [[!CSS21]] section <var>7.2.1</var> and
   incorporates the modifications previously made non-normatively by
   [[!MEDIAQ]] section <var>1</var>.</p>
   <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
   and [[!CSS3-ANIMATIONS]], but that dependency is only on the
   assumption that those modules will advance ahead of this one.  If this
   module advances faster, then the dependency will be reversed.</p>
 <h3 id="conventions">Document Conventions</h3>
   <p>Conformance requirements are expressed with a combination of
   descriptive assertions and RFC 2119 terminology. The key words “MUST”,
   “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
   “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
   document are to be interpreted as described in RFC 2119.
   However, for readability, these words do not appear in all uppercase
   letters in this specification.
   <p>All of the text of this specification is normative except sections
   explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
   <p>Examples in this specification are introduced with the words “for example”
   or are set apart from the normative text with
   <code class="html">class="example"</code>, like this:
   <div class="example">
     <p>This is an example of an informative example.</p>
   </div>
   <p>Informative notes begin with the word “Note” and are set apart from the
   normative text with <code class="html">class="note"</code>, like this:
   <p class="note">Note, this is an informative note.</p>
 <h2 id="processing">Processing of conditional group rules</h2>
 <p>This specification defines some CSS @-rules, called <dfn>conditional
 group rules</dfn>, that associate a condition with a group of other
 CSS rules.  These different rules allow testing different types of
 conditions, but share common behavior for how their contents are used
 when the condition is true and when the condition is false.</p>
 <div class="example">
 <p>For example, this rule:</p>
 <pre>@media print {
   /* hide navigation controls when printing */
   #navigation { display: none }
 }</pre>
 <p>causes a particular CSS rule (making elements with ID "navigation" be
 display:none) apply only when the style sheet is used for a print
 medium.
 </div>
 <p>Each conditional group rule has a condition, which at any time
 evaluates to true or false.  When the condition is true, CSS processors
 <strong>must</strong> apply the rules inside the group rule as though
 they were at the group rule's location; when the condition is false, CSS
 processors <strong>must not</strong> apply any of rules inside the group
 rule.  The current state of the condition does not affect the CSS object
 model, in which the contents of the group rule always remain within the
 group rule.</p>
 <p>This means that when multiple conditional group rules are nested,
 a rule inside of both of them applies only when all of the rules'
 conditions are true.</p>
 <div class="example">For example, with this set of nested rules:
 <pre>@media print { // rule (1)
   /* hide navigation controls when printing */
   #navigation { display: none }
   @media (max-width: 12cm) { // rule (2)
     /* keep notes in flow when printing to narrow pages */
     .note { float: none }
   }
 }</pre>
 the condition of the rule marked (1) is true for print media, and the
 condition of the rule marked (2) is true when the width of the display
 area (which for print media is the page box) is less than or equal to
 cm.  Thus the rule ''#navigation { display: none }'' applies
 whenever this style sheet is applied to print media, and the rule
 ''.note { float: none }'' is applied only when the style sheet
 is applied to print media <em>and</em> the width of the page box is less
 than or equal to 12 centimeters.</div>
 <p>When the condition for a conditional group rule changes, CSS
 processors <strong>must</strong> reflect that the rules now apply or no
 longer apply, except for properties whose definitions define effects of
 computed values that persist past the lifetime of that value (such as
 for some properties in [[CSS3-TRANSITIONS]] and
 [[!CSS3-ANIMATIONS]]).</p>
 <h2 id="contents-of">Contents of conditional group rules</h2>
 <p>The syntax of each conditional group rule consists of some syntax
 specific to the type of rule followed by a <dfn>group rule body</dfn>,
 which is a block (pair of braces) containing a sequence of rules.</p>
 <p>A group rule body is allowed to contain style rules and any @-rules that
 are allowed at the top level of a style sheet before and after a
 style rule.  This means that @-rules that must occur at the beginning of
 the style sheet (such as '@charset', '@import',
 and '@namespace' rules) are not allowed inside of conditional group
 rules.  Conditional group rules can be nested.</p>
 <p>In terms of the grammar, this specification defines the following
 productions for use in the grammar of conditional group rules:</p>
 <p class=note>Note: Style rules are defined in grammars
 by the <code>ruleset</code> production.</p>
 <pre><dfn>nested_statement</dfn>
   : ruleset | <i>media</i> | page | font_face_rule | keyframes_rule |
     <i>supports_rule</i>
   ;
 <dfn>group_rule_body</dfn>
   : '{' S* <i>nested_statement</i>* '}' S*
   ;</pre>
 <p>
 in which all the productions are defined in that grammar with the
 exception of <code>font_face_rule</code>
 defined in [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
 [[!CSS3-ANIMATIONS]], and <code>media</code> and <code>supports_rule</code>
 defined in this specification.</p>
 <p>In general, future CSS specifications that add new @-rules that are
 not forbidden to occur after some other types of rules should modify
 this <code>nested_statement</code> production to keep the grammar
 accurate.</p>
 <p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
 conditional group rules.</p>
 <p>CSS processors <strong>must</strong> ignore rules that are not
 allowed within a group rule, and <strong>must</strong> handle invalid
 rules inside of group rules as described in <a
 href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
 .2 (Rules for handling parsing errors)</a>, <a
 href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
 (At-rules)</a>, and <a
 href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
 (Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>
 <h2 id="use">Placement of conditional group rules</h2>
 <p>Conditional group rules are allowed at the top-level of a style
 sheet, and inside other conditional group rules.  CSS processors
 <strong>must</strong> process such rules as <a
 href="#processing">described above</a>.</p>
 <p>Any rules that are not allowed after a style rule (e.g., ''@charset'',
 ''@import'', or ''@namespace'' rules) are also not allowed after a
 conditional group rule.  Therefore, style sheets <strong>must
 not</strong> place such rules after a conditional group rules, and CSS
 processors <strong>must</strong> ignore such rules.</p>
 <h2 id="at-media">Media-specific style sheets:  the '@media' rule</h2>
 <p>The <dfn id="atmedia-rule">'@media' rule</dfn> is a conditional group rule whose
 condition is a media query.  It consists of the at-keyword
 '@media' followed by a (possibly empty) media query list (as
 defined in [[!MEDIAQ]]), followed by a group rule body.  The condition
 of the rule is the result of the media query.</p>
 <div class="example">
 <p>This '@media' rule:</p>
 <pre>@media screen and (min-width: 35em),
        print and (min-width: 40em) {
   #section_navigation { float: left; width: 10em; }
 }</pre>
 <p>has the condition
 ''screen and (min-width: 35em), print and (min-width: 40em)'',
 which is true for screen displays
 whose viewport is at least 35 times the initial font size
 and for print displays
 whose viewport is at least 40 times the initial font size.
 When either of these is true,
 the condition of the rule is true,
 and the rule
 ''#section_navigation { float: left; width: 10em; }''
 is applied.</p>
 </div>
 <p>In terms of the grammar, this specification extends the
 <code>media</code> production in the
 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
 ([[!CSS21]], Appendix G) into:
 <pre><dfn>media</dfn>
   : MEDIA_SYM S* media_query_list <i>group_rule_body</i>
   ;</pre>
 <p>where the <code>group_rule_body</code> production is defined in this
 specification, the <code>media_query_list</code> production is defined
 in [[!MEDIAQ]], and the others are defined in the <a
 href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
 ([[!CSS21]], Appendix G).
 <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
 <p>The <dfn id="atsupports-rule">'@supports' rule</dfn> is a conditional group
 rule whose condition tests whether the user agent supports CSS
 property:value pairs.  Authors can use it to write style sheets that use
 new features when available but degrade gracefully when those features
 are not supported.  CSS has existing mechanisms for graceful
 degradation, such as ignoring unsupported properties or values, but
 these are not always sufficient when large groups of styles need to be
 tied to the support for certain features, as is the case for use of new
 layout system features.</p>
 <p>The syntax of the condition in the '@supports' rule is
 slightly more complicated than for the other conditional group rules
 (though has some similarities to media queries) since:</p>
 <ul>
   <li>negation is needed so that the new-feature styles and the fallback
   styles can be separated (within the forward-compatible grammar's rules
   for the syntax of @-rules), and not required to override each other</li>
   <li>conjunction (and) is needed so that multiple required features can
   be tested</li>
   <li>disjunction (or) is needed when there are multiple alternative
   features for a set of styles, particularly when some of those
   alternatives are vendor-prefixed properties or values</li>
 </ul>
 <p>Therefore, the syntax of the '@supports' rule allows
 testing for property:value pairs, and arbitrary conjunctions (and),
 disjunctions (or), and negations (not) of them.</p>
 <p>This extends the lexical scanner in the
 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
 ([[!CSS21]], Appendix G) by adding:
 <pre>
 @{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn>SUPPORTS_SYM</dfn>;}
 {O}{R}                    {return <dfn>OR</dfn>;}
 </pre>
 <p>This then extends the grammar in the
 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>,
 using the lexical scanner there, with the additions of
 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and
 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code>
 tokens defined in the Media Queries specification [[!MEDIAQ]]
 and the <code>OR</code> and <code>SUPPORTS_SYM</code> tokens defined above,
 and with
 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>,
 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
 and <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code>
 productions
 and the <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code> token
 taken from the core syntax of CSS defined in
 <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1 (Tokenization)</a> of [[!CSS21]],
 by adding:</p>
 <pre><dfn>supports_rule</dfn>
   : <i>SUPPORTS_SYM</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>supports_condition</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>group_rule_body</i>
   ;
 <dfn>supports_condition</dfn>
   : <i>supports_negation</i> | <i>supports_conjunction</i> | <i>supports_disjunction</i> |
     <i>supports_condition_in_parens</i>
   ;
 <dfn>supports_condition_in_parens</dfn>
   : ( '(' <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <i>supports_condition</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* ')' ) | <i>supports_declaration_condition</i> |
     <i>general_enclosed</i>
   ;
 <dfn>supports_negation</dfn>
   : <a href="http://www.w3.org/TR/css3-mediaqueries/#syntax"><i>NOT</i></a> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>supports_condition_in_parens</i>
   ;
 <dfn>supports_conjunction</dfn>
   : <i>supports_condition_in_parens</i> ( <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <a href="http://www.w3.org/TR/css3-mediaqueries/#syntax"><i>AND</i></a> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>supports_condition_in_parens</i> )+
   ;
 <dfn>supports_disjunction</dfn>
   : <i>supports_condition_in_parens</i> ( <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>OR</i> <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <i>supports_condition_in_parens</i> )+
   ;
 <dfn>supports_declaration_condition</dfn>
   : '(' <a href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>declaration</i></a> ')'
   ;
 <dfn>general_enclosed</dfn>
   : ( <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>FUNCTION</i></a> | '(' ) ( <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>any</i></a> | <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization"><i>unused</i></a> )* ')'
   ;
 </pre>
 <p>
   Implementations <strong>must</strong> parse ''@supports'' rules
   based on the above grammar,
   and when interpreting the above grammar,
   <strong>must</strong> match the production before an <code>|</code> operator
   in preference to the one after it.
 </p>
 <p>
   The above grammar is purposely very loose for forwards-compatibility reasons,
   since the <code>general_enclosed</code> production
   allows for substantial future extensibility.
   Any ''@supports'' rule that does not parse according to the grammar above
   (that is, a rule that does not match this loose grammar
   which includes the <code>general_enclosed</code> production)
   is invalid.
   Style sheets <strong>must not</strong> use such a rule and
   processors <strong>must</strong> ignore such a rule (including all of its contents).
 <p>Each of these grammar terms is associated with a boolean result, as
 follows:</p>
 <dl>
 <dt>supports_condition</dt>
 <dd>
   The result is the result of the single child term.
 </dd>
 <dt>supports_condition_in_parens</dt>
 <dd>
   The result is the result of the single <code>supports_condition</code>
   or <code>supports_declaration_condition</code> child term.
 </dd>
 <dt>supports_negation</dt>
 <dd>
   The result is the <em>negation</em> of the result of the
   <code>supports_condition_in_parens</code> child term.
 </dd>
 <dt>supports_conjunction</dt>
 <dd>
   The result is true if the result of <em>all</em> of the
   <code>supports_condition_in_parens</code> child terms is true;
   otherwise it is false.
 </dd>
 <dt>supports_disjunction</dt>
 <dd>
   The result is true if the result of <em>any</em> of the
   <code>supports_condition_in_parens</code> child terms is true;
   otherwise it is false.
 </dd>
 <dt>supports_declaration_condition</dt>
 <dd>
   The result is whether the CSS processor <a href="#support-definition">supports</a> the declaration
   within the parentheses.
 </dd>
 <dt>general_enclosed</dt>
 <dd>
   The result is always false.
   Additionally, style sheets <strong>must not</strong>
   write ''@supports'' rules
   that match this grammar production.
   (In other words, this production exists only for future extensibility,
   and is not part of the description of a valid style sheet
   in this level of the specification.)
   <span class="note">Note that future levels may define functions
   or other parenthesized expressions that can evaluate to true.</span>
 </dd>
 </dl>
 <p>The condition of the '@supports' rule is the result of the
 <code>supports_condition</code> term that is a child of the
 <code>supports_rule</code> term.</p>
 <div class="example">
 <p>For example, the following rule</p>
 <pre>@supports ( display: flexbox ) {
   body, #navigation, #content { display: flexbox; }
   #navigation { background: blue; color: white; }
   #article { background: white; color: black; }
 }</pre>
 <p>applies the rules inside the '@supports' rule only when
 ''display: flexbox'' is supported.</p>
 </div>
 <div class="example">
 <p>The following example shows an additional '@supports' rule that can
 be used to provide an alternative for when ''display: flexbox'' is not
 supported:</p>
 <pre>@supports not ( display: flexbox ) {
   body { width: 100%; height: 100%; background: white; color: black; }
   #navigation { width: 25%; }
   #article { width: 75%; }
 }</pre>
 <p>Note that the 'width' declarations may be harmful to the
 flexbox-based layout, so it is important that they be present only in
 the non-flexbox styles.</p>
 </div>
 <div class="example">
 <p>The following example checks for support for the 'box-shadow'
 property, including checking for support for vendor-prefixed versions of
 it.  When the support is present, it specifies both 'box-shadow' (with
 the prefixed versions) and 'border' in a way what would cause the box to
 become invisible were 'box-shadow' not supported.</p>
 <pre>.noticebox {
   border: 1px solid black;
   padding: 1px;
 }
 @supports ( box-shadow: 0 0 2px black inset ) or
           ( -moz-box-shadow: 0 0 2px black inset ) or
           ( -webkit-box-shadow: 0 0 2px black inset ) or
           ( -o-box-shadow: 0 0 2px black inset ) {
   .noticebox {
     -moz-box-shadow: 0 0 2px black inset;
     -webkit-box-shadow: 0 0 2px black inset;
     -o-box-shadow: 0 0 2px black inset;
     box-shadow: 0 0 2px black inset; /* unprefixed last */
     /* override the rule above the @supports rule */
     border: none;
     padding: 2px;
   }
 }</pre></div>
 <p>To avoid confusion between ''and'' and ''or'', the syntax requires
 that both ''and'' and ''or'' be specified explicitly (rather than, say,
 using commas or spaces for one of them).  Likewise, to avoid confusion
 caused by precedence rules, the syntax does not allow ''and'', ''or'',
 and ''not'' operators to be mixed without a layer of parentheses.</p>
 <div class="example">
 <p>For example, the following rule is not valid:
 <pre class="illegal">@supports (transition-property: color) or
           (animation-name: foo) and
           (transform: rotate(10deg)) {
   // ...
 }</pre>
 <p>Instead, authors must write one of the following:</p>
 <pre>@supports ((transition-property: color) or
            (animation-name: foo)) and
           (transform: rotate(10deg)) {
   // ...
 }</pre>
 <pre>@supports (transition-property: color) or
           ((animation-name: foo) and
            (transform: rotate(10deg))) {
   // ...
 }</pre>
 </div>
 <p>Furthermore, whitespace is required after a ''not'' and on both
 sides of an ''and'' or ''or''.</p>
 <p>The declaration being tested must always occur within parentheses,
 when it is the only thing in the expression.<p>
 <div class="example">
 <p>For example, the following rule is not valid:
 <pre class="illegal">@supports display: flexbox {
   // ...
 }</pre>
 <p>Instead, authors must write:</p>
 <pre>@supports (display: flexbox) {
   // ...
 }</pre>
 </div>
 <p>The syntax allows extra parentheses when they are not needed.  This
 flexibility is sometimes useful for authors (for example, when
 commenting out parts of an expression) and may also be useful for
 authoring tools.</p>
 <div class="example">
 <p>For example, authors may write:</p>
 <pre>@supports ((display: flexbox)) {
   // ...
 }</pre>
 </div>
 <p>A trailing ''!important'' on a declaration being tested is allowed,
 though it won't change the validity of the declaration.
 <div class="example">
 <p>For example, the following rule is valid:
 <pre>@supports (display: flexbox !important) {
   // ...
 }</pre>
 </div>
 <h3 id="support-definition">Definition of support</h3>
 <p>For forward-compatibility,
 <a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
 (Declarations and properties)</a> of [[!CSS21]]
 defines rules for handling invalid properties and values.
 CSS processors that
 do not implement or partially implement a specification
 <strong>must</strong> treat any part of a value that they
 do not implement, or
 do not have a usable level of support for,
 as invalid according to this rule
 for handling invalid properties and values,
 and therefore <strong>must</strong> discard the declaration as a parse error.</p>
 <p>A CSS processor is considered to <dfn id="dfn-support">support</dfn>
 a declaration (consisting of a property and value) if it accepts that
 declaration (rather than discarding it as a parse error).
 If a processor does not implement, with a usable level of support,
 the value given,
 then it <strong>must not</strong>
 accept the declaration or claim support for it.</p>
 <p class="note">Note that properties or values
 whose support is effectively disabled by user preferences
 are still considered as supported by this definition.
 For example, if a user has enabled a high-contrast mode
 that causes colors to be overridden,
 the CSS processor is still considered to support the 'color' property
 even though declarations of the 'color' property may have no effect.
 On the other hand, a developer-facing preference
 whose purpose is to enable or disable support for an experimental CSS feature
 does affect this definition of support.</p>
 <p>These rules (and the equivalence between them) allow
 authors to use fallback (either in the [[CSS1]] sense of declarations
 that are overridden by later declarations or with the new capabilities
 provided by the ''@supports'' rule in this specification) that works
 correctly for the features implemented.  This applies especially to
 compound values; implementations must implement all parts of the value
 in order to consider the declaration supported, either inside a style rule
 or in the declaration condition of an ''@supports'' rule.</p>
 <!--
 <h2 id="at-document">Document queries: the '@document' rule</h2>
 <p>The <dfn>'@document' rule</dfn> is a conditional group
 rule whose condition depends on the
 <a href="#url-of-doc">URL of the document being styled</a>.
 This allows style sheets, particularly user style sheets, to have styles
 that only apply to a set of pages rather than to all pages using the
 style sheet.</p>
 <p class="issue">Given that this @-rule is intended primarily for user
 style sheets, what should this specification say about its use in author
 style sheets?  Should it be forbidden?  Should use instead be
 discouraged?  Or should this specification remain neutral on the
 topic, since there are valid uses in author style sheets?</p>
 <p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
 the URI at which the document is located, excluding any fragment
 identifiers.  (This means, for example, that HTTP redirects have been
 followed.)  If the styles are being applied inside a complete document
 embedded into the presentation of another (e.g., [[HTML5]]&#39;s <code
 class="html">iframe</code>, <code class="html">object</code>, or <code
 class="html">img</code> elements), the relevant URI is that of the
 frame, not of its container.  However, if content from other documents
 is mixed in via mechanisms that mix content from one document into
 another (e.g., [[SVG11]]&#39;s <code>use</code> element), then the
 address of the container document is used.</p>
 <p class="note">Note:  In [[HTML5]], this is the
 <a href="http://dev.w3.org/html5/spec/dom.html#documents">document's address</a>
 of a document in a
 <a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>
 <div class="issue">What form of normalization is done on URLs and domains
 before matching?  In particular, this specification needs to describe:
 <ul>
 <li>what form is used for the <a href="#url-of-doc">URL of the document
 being styled</a> (and what has been normalized in that form)</li>
 <li>what normalization (if any) happens to the argument of each of the match
 functions before the comparison that they describe and</li>
 <li>whether the
 comparison algorithm used is string comparison or some other URL
 comparison algorithm.</li></ul></div>
 <p>The '@document' rule's condition is written as a
 comma-separated list of <dfn>URL matching functions</dfn>, and the
 condition evaluates to true whenever any one of those functions
 evaluates to true.  The following URL matching functions are
 permitted:</p>
 <dl>
   <dt><dfn id="url-exact" title="url()|URL matching functions::exact">&lt;url&gt;</dfn></dt>
   <dd>
     <p>The 'url()' function is the <dfn>exact url matching
     function</dfn>.  It evaluates to true whenever the <a
     href="#url-of-doc">URL of the document being styled</a> is exactly
     the URL given.</p>
     <p class="Note">The 'url()' function, since it is a core syntax
     element in CSS, is allowed (subject to different character
     limitations and thus escaping requirements) to contain an unquoted
     value (in addition to the string values that are allowed as
     arguments for all four functions).</p>
     <div class="example">
       <p>For example, this rule:</p>
 <pre>@document url("http://www.w3.org/Style/CSS/") {
   #summary { background: yellow; color: black}
 }</pre>
       <p>styles the <code class="html">summary</code> element on the page
       <code>http://www.w3.org/Style/CSS/</code>, but not on any other
       pages.</p>
     </div>
   </dd>
   <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(&lt;string&gt;)</dfn></dt>
   <dd>
     <p>The 'url-prefix()' function is the <dfn>url prefix
     matching function</dfn>.  It evaluates to true whenever the
     <a href="#url-of-doc">URL of the document being styled</a>
     has the argument to the function as an
     initial substring (which is true when the two strings are equal).
     When the argument is the empty string, it evaluates to true for all
     documents.</p>
     <div class="example">
       <p>For example, this rule:</p>
 <pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
   #summary { background: yellow; color: black}
 }</pre>
       <p>styles the <code class="html">summary</code> element on the page
       <code>http://www.w3.org/Style/CSS/</code> and on the page
       <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
       affect the page <code>http://www.w3.org/</code> or the page
       <code>http://www.example.com/Style/CSS/</code>.</p>
     </div>
   </dd>
   <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(&lt;string&gt;)</dfn></dt>
   <dd>
     <p>The 'domain()' function is the <dfn>domain
     matching function</dfn>.  It evaluates to true whenever
     the <a href="#url-of-doc">URL of the document being styled</a>
     has a host subcomponent (as defined in [[!URI]])
     and that host subcomponent is exactly the argument to the
     'domain()' function or a final substring of the host
     component is a period (U+002E) immediately followed by the argument
     to the 'domain()' function.</p>
     <div class="example">
       <p>For example, this rule:</p>
 <pre>@document domain("w3.org") {
   body { font-size: 16px ! important }
 }</pre>
       <p>changes the font size of the body element for pages such as
       <code>http://www.w3.org/Style/CSS/</code> and
       <code>http://w3.org/Style/CSS/</code> and
       <code>http://lists.w3.org/Archives/Public/www-style/</code>
       but it does not affect the page
       <code>http://www.example.com/Style/CSS/</code>.</p>
     </div>
   </dd>
   <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(&lt;string&gt;)</dfn></dt>
   <dd>
     <p>The contents of the &lt;string&gt; argument <strong>must</strong>
     match the JavaScript <code>Pattern</code> production
     ([[!ECMA-262-5.1]], section 15.10.1).  However,
     failing to do so is not a CSS syntax error and does not trigger any
     error handling for CSS syntax errors.</p>
     <p>The ''regexp()'' function evaluates to true whenever the string
     argument compiled as a JavaScript regular expression with the
     <code>global</code>, <code>ignoreCase</code> and
     <code>multiline</code> flags <em>disabled</em>
     (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
     compiles successfully and the resulting regular expression matches
     the entirety of the
     <a href="#url-of-doc">URL of the document being styled</a>.</p>
     <p class="note">Note that regular expression must match the entire
     URL, not just a part of it.</p>
     <p class="note">Note that this definition intentionally matches the
     behavior of the <a
     href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
     attribute</a> on the <code class="html">input</code> element
     in [[HTML5]].</p>
     <div class="example">
       <p>For example, this rule:</p>
 <pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
   body { font-size: 20px ! important }
 }</pre>
       <p>changes the font size of the body element for pages such as
       <code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
       <p class="note">Note that the backslashes in the regular
       expression require CSS escaping as ''\\''.</p>
     </div>
   </dd>
 </dl>
 <p>Implementations <strong>must</strong> treat any unknown URL matching
 functions as a syntax error, and thus ignore the '@document' rule.
 <span class="issue">Should we instead have more complicated error
 handling rules to make forward-compatibility work differently, or is
 this rule the best solution for such future expansion anyway?</span></p>
 <div class="issue">This syntax doesn't offer any ability to do negations,
 which has been requested in <a
 href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
 </a>.  Use cases that people have wanted negations for
 include:
 <ul>
   <li>User style sheets that want a particular rule in general, but know
   that that rule does more harm than good on specific sites.</li>
   <li>Authors who have a rule that they want to apply to most of their
   pages, but wish to make a few exceptions for.</li>
 </ul>
 </div>
 <p>This extends the lexical scanner in the
 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
 ([[!CSS21]], Appendix G) by adding:
 <pre>@{D}{O}{C}{U}{M}{E}{N}{T}	{return DOCUMENT_SYM;}</pre>
 <p>and the grammar by adding</p>
 <pre><dfn>document_rule</dfn>
   : DOCUMENT_SYM S+ <i>url_match_fn</i> ( "," S* <i>url_match_fn</i> )* <i>group_rule_body</i>
   ;
 <dfn>url_match_fn</dfn>
   : (URI | FUNCTION S* STRING S* ')' ) S*
   ;</pre>
 -->
 <h2 id="apis">APIs</h2>
 <h3 id='extentions-to-cssrule-interface'>
 Extensions to the <code>CSSRule</code> interface</h3>
 <p>The <code>CSSRule</code> interface is extended as follows:
 <pre class='idl'>partial interface CSSRule {
     const unsigned short SUPPORTS_RULE = 12;
     <!--
     const unsigned short DOCUMENT_RULE = 13;
     -->
 }</pre>
 <h3 id='the-cssgroupingrule-interface'>
 The <code>CSSGroupingRule</code> interface</h3>
 <p>The <dfn><code>CSSGroupingRule</code></dfn> interface represents an at-rule that contains other rules nested inside itself.
 <pre class='idl'>interface CSSGroupingRule : CSSRule {
     readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
     unsigned long insertRule (DOMString rule, unsigned long index);
     void deleteRule (unsigned long index);
 }</pre>
 <dl class='idl-attributes'>
   <dt><code>cssRules</code> of type <code><a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a></code>, readonly
   <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
     object for the list of CSS rules nested inside the grouping rule.
 </dl>
 <dl class='idl-methods'>
   <dt><code>insertRule(DOMString rule, unsigned long index)</code>, returns
     <code>unsigned long</code>
   <dd>
     The <code>insertRule</code> operation must
     insert a CSS rule <var>rule</var>
     into the CSS rule list returned by <code>cssRules</code>,
     such that the inserted rule will be at position <var>index</var>,
     and any rules previously at <var>index</var> or higher
     will increase their index by one.
     It must throw INDEX_SIZE_ERR
     if index is greater than <code>cssRules.length</code>.
     It must throw SYNTAX_ERR
     if the rule has a syntax error and is unparseable;
     this does not include syntax errors handled by error handling rules
     for constructs inside of the rule,
     but this does include cases where the string given
     does not parse into a single CSS rule (such as when the string is empty)
     or where there is anything other than whitespace or comments
     after that single CSS rule.
     It must throw HIERARCHY_REQUEST_ERR
     if the rule cannot be inserted at the location specified,
     for example, if an ''@import'' rule is inserted inside a group rule.
     <p>The return value is the <var>index</var> parameter.
   <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
   <dd>
     The <code>deleteRule</code> operation must
     remove a CSS rule from
     the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
     It must throw INDEX_SIZE_ERR
     if index is greater than or equal to <code>cssRules.length</code>.
 </dl>
 <h3 id="the-cssconditionrule-interface">
 The <code>CSSConditionRule</code> interface</h3>
 <p>The <dfn><code>CSSConditionRule</code></dfn> interface represents all the "conditional" at-rules,
   which consist of a condition and a statement block.
 <pre class='idl'>interface CSSConditionRule : CSSGroupingRule {
     attribute DOMString conditionText;
 }</pre>
 <dl class='idl-attributes'>
   <dt><code>conditionText</code> of type <code>DOMString</code>
   <dd>
     <p>The <code>conditionText</code> attribute represents
     the condition of the rule.
     Since what this condition does
     varies between the derived interfaces of <code>CSSConditionRule</code>,
     those derived interfaces
     may specify different behavior for this attribute
     (see, for example, <code>CSSMediaRule</code> below).
     In the absence of such rule-specific behavior,
     the following rules apply:</p>
     <p>The <code>conditionText</code> attribute, on getting, must return
     the result of serializing the associated condition.
     <p>On setting the <code>conditionText</code> attribute these steps
       must be run:
     <ol>
       <li>Trim the given value of white space.
       <li>If the given value matches the grammar of the
         appropriate condition production for the given rule,
         replace the associated CSS condition with the given value.
       <li>Otherwise, do nothing.
     </ol>
 </dl>
 <h3 id="the-cssmediarule-interface">
 The <code>CSSMediaRule</code> interface</h3>
 <p>The <dfn><code>CSSMediaRule</code></dfn> interface represents a ''@media'' rule:
 <pre class='idl'>interface CSSMediaRule : CSSConditionRule {
     readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a> media;
 }</pre>
 <dl class='idl-attributes'>
   <dt><code>media</code> of type <code><a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a></code>, readonly
   <dd>The <code>media</code> attribute must return a <code>MediaList</code> object
     for the list of media queries specified with the ''@media'' rule.
   <dt><code>conditionText</code> of type <code>DOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
   <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
     on getting, must return the value of <code>media.mediaText</code> on the rule.
     <p>Setting the <code>conditionText</code> attribute
       must set the <code>media.mediaText</code> attribute on the rule.
 </dl>
 <h3 id="the-csssupportsrule-interface">
 The <code>CSSSupportsRule</code> interface</h3>
 <p>The <dfn><code>CSSSupportsRule</code></dfn> interface represents a ''@supports'' rule.</p>
 <pre class='idl'>interface CSSSupportsRule : CSSConditionRule {
 }</pre>
 <dl class='idl-attributes'>
   <dt><code>conditionText</code> of type <code>DOMString</code> (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
   <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
     on getting, must return the condition that was specified,
     without any logical simplifications,
     so that the returned condition will evaluate to the same result
     as the specified condition
     in any conformant implementation of this specification
     (including implementations that implement future extensions
     allowed by the <i>general_enclosed</i> exensibility mechanism in this specification).
     In other words,
     token stream simplifications are allowed
     (such as reducing whitespace to a single space
     or omitting it in cases where it is known to be optional),
     but logical simplifications (such as removal of unneeded parentheses,
     or simplification based on evaluating results) are not allowed.
 </dl>
 <!--
 <h3 id="the-cssdocumentrule-interface">
 The <code>CSSDocumentRule</code> interface</h3>
 <p>The <dfn><code>CSSDocumentRule</code></dfn> interface represents a ''@document'' rule.</p>
 <pre class='idl'>interface CSSDocumentRule : CSSConditionRule {
 }</pre>
 -->
 <h3 id='the-css-interface'>
 The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>
 <p>The <dfn id='CSS-interface'><code>CSS</code></dfn> interface holds useful CSS-related functions that do not belong elsewhere.
 <pre class='idl'>interface CSS {
   static boolean supports(DOMString property, DOMString value);
   static boolean supports(DOMString conditionText);
 }</pre>
 <dl class='idl-methods'>
   <dt><code>supports(DOMString property, DOMString value)</code>,
     returns <code>boolean</code>
   <dt><code>supports(DOMString conditionText)</code>,
     returns <code>boolean</code>
   <dd>
     When the <code title=''>supports()</code> method is invoked with two arguments <var>property</var> and <var>value</var>,
     it must return <code>true</code> if <var>property</var> is a literal match for the name of a CSS property that the UA supports,
     and <var>value</var> would be successfully parsed as a supported value for that property.
     (Literal match means that no CSS escape processing is performed,
     and leading and trailing whitespace are not stripped,
     so any leading whitespace, trailing whitespace,
     or CSS escapes equivalent to the name of a property
     would cause the method to return <code>false</code>.)
     Otherwise, it must return <code>false</code>.
     <p>
       When invoked with a single <var>conditionText</var> argument,
       it must return <code>true</code> if <var>conditionText</var>,
       when parsed and evaluated as a <code>supports_condition</code>,
       would return true.
       Otherwise, it must return <code>false</code>.
   </dl>
 <h2 class=no-num id="grammar">Grammar</h2>
 <p>In order to allow these new @-rules in CSS style sheets, this
 specification modifies the <code>stylesheet</code> production in the <a
 href="http://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
 [[!CSS21]] by replacing the <code>media</code> production defined in
 [[!CSS21]] with the <code>media</code> production defined in this one,
 and additionally inserting <code>| supports_rule</code>
 alongside <code>ruleset | media | page</code>.</p>
 <h2 id="conformance">Conformance</h2>
 <h3 id="base-modules">Base Modules</h3>
 <p>This specification defines conformance in terms of base modules,
 which are modules that this specification builds on top of.  The base
 modules of this module are:</p>
 <ul>
 <li>[[!CSS21]]</li>
 </ul>
 <p>All of the conformance requirements of all base modules are
 incorporated as conformance requirements of this module, except where
 overridden by this module.</p>
 <p>Additionally, all conformance requirements related to validity of
 syntax in this module and all of its base modules are to be interpreted
 as though all syntax in all of those modules is valid.</p>
 <div class="example"><p>For example, this means that grammar presented
 in modules other than [[!CSS21]] must obey the requirements that
 [[!CSS21]] defines for the parsing of properties, and that requirements
 for handling invalid syntax in [[!CSS21]] do not treat syntax added by
 other modules as invalid.</p></div>
 <p>Additionally, the set of valid syntax can be increased by the
 conformance of a style sheet or processor to additional modules; use of
 such syntax does not make a style sheet nonconformant and failure to
 treat such syntax as invalid does not make a processor
 nonconformant.</p>
 <h3 id="conformance-classes">Conformance Classes</h3>
   <p>Conformance to the CSS Conditional Rules Module is defined for three
   conformance classes:
   <dl>
     <dt><dfn title="conformance::style sheet" id="conform-style-sheet">style sheet</dfn>
       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
       style sheet</a>.</dd>
     <dt><dfn title="conformance::processor" id="conform-processor">processor</dfn></dt>
       <dd>A tool that reads CSS style sheets:  it may be a renderer or
       <a
       href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user-agent</a>
       that interprets the semantics of a style sheet and renders
       documents that use style sheets, or it may be a validator that
       checks style sheets.</dd>
     <dt><dfn title="conformance::authoring tool" id="conform-authoring-tool">authoring tool</dfn></dt>
       <dd>A tool that writes a style sheet.</dd>
   </dl>
   <p>A style sheet is conformant to the CSS Conditional Rules Module
   if it meets all of the conformance requirements in the module that are
   described as requirements of style sheets.</p>
   <p>A processor is conformant to the CSS Conditional Rules Module if it
   meets all applicable conformance requirements in the module that are
   described as requirements of processors.  In general, all requirements
   are applicable to renderers.  Requirements concerning a part of CSS
   not performed by a processor are not applicable, e.g., requirements
   related to rendering are not applicable to a validator.  The inability
   of a processor to correctly render a document due to limitations of
   the device does not make it non-conformant. (For example, a renderer
   is not required to render color on a monochrome monitor.)</p>
   <p>An authoring tool is conformant to the CSS Conditional Rules Module
   if it writes style sheets that conform to the module and (if it reads
   CSS) it is a conformant processor.</p>
 <h3 id="partial">
 Partial Implementations</h3>
   <p>So that authors can exploit the forward-compatible parsing rules to
   assign fallback values, CSS renderers <strong>must</strong>
   treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
   as appropriate</a>) any at-rules, properties, property values, keywords,
   and other syntactic constructs for which they have no usable level of
   support. In particular, user agents <strong>must not</strong> selectively
   ignore unsupported component values and honor supported values in a single
   multi-value property declaration: if any value is considered invalid
   (as unsupported values must be), CSS requires that the entire declaration
   be ignored.</p>
 <h3 id="experimental">Experimental Implementations</h3>
   <p>To avoid clashes with future CSS features, the CSS specifications
   reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
   syntax</a> for proprietary property and value extensions to CSS. The CSS
   Working Group recommends that experimental implementations of features in
   CSS Working Drafts also use vendor-prefixed property or value names. This
   avoids any incompatibilities with future changes in the draft. Once a
   specification reaches the Candidate Recommendation stage, implementors
   should implement the non-prefixed syntax for any feature they consider to
   be correctly implemented according to spec.</p>
 <h3 id="cr-exit-criteria">CR Exit Criteria</h3>
   <p>For this specification to be advanced to Proposed Recommendation,
   there must be at least two independent, interoperable implementations
   of each feature. Each feature may be implemented by a different set of
   products, there is no requirement that all features be implemented by
   a single product. For the purposes of this criterion, we define the
   following terms:
   <dl>
     <dt>independent <dd>each implementation must be developed by a
     different party and cannot share, reuse, or derive from code
     used by another qualifying implementation. Sections of code that
     have no bearing on the implementation of this specification are
     exempt from this requirement.
     <dt>interoperable <dd>passing the respective test case(s) in the
     official CSS test suite, or, if the implementation is not a Web
     browser, an equivalent test. Every relevant test in the test
     suite should have an equivalent test created if such a user
     agent (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.
     <dt>implementation <dd>a user agent which:
     <ol class=inline>
       <li>implements the specification.
       <li>is available to the general public. The implementation may
       be a shipping product or other publicly available version
       (i.e., beta version, preview release, or “nightly build”).
       Non-shipping product releases must have implemented the
       feature(s) for a period of at least one month in order to
       demonstrate stability.
       <li>is not experimental (i.e., a version specifically designed
       to pass the test suite and is not intended for normal usage
       going forward).
     </ol>
   </dl>
   <p>The specification will remain Candidate Recommendation for at least
   six months.
 <h2 id="changes">
 Changes</h2>
 <p>The following (non-editorial) changes were made to this specification since the
 <a href="http://www.w3.org/TR/2012/WD-css3-conditional-20121213/">13 December 2012 Working Draft</a>:
 <ul>
   <li>Require whitespace around ''and'' and ''or'' and after ''not''.
   <li>Add note explaining that user preferences that effectively disable a property (e.g., high-contrast mode disabling colors) do not effect the definition of support.
   <li>Describe requirements for conditionText getter on CSSSupportsRule.
   <li>Clarify the definition of "literal match" in CSS.supports().
   <li>Specify behavior of CSSGroupingRule.insertRule when given an empty string or more than one syntactically valid rule.
 </ul>
 <h2 class=no-num id="acknowledgments">Acknowledgments</h2>
 <p>
 Thanks to the ideas and feedback from
 Tab Atkins,
 Arthur Barstow,
 Ben Callahan,
 <span lang="tr">Tantek Çelik</span>,
 Alex Danilo,
 Elika Etemad,
 Pascal Germroth,
 <span lang="de">Björn Höhrmann</span>,
 Paul Irish,
 Brad Kemper,
 <span lang="nl">Anne van Kesteren</span>,
 Vitor Menezes,
 Alex Mogilevsky,
 Chris Moschini,
 James Nurthen,
 Simon Pieters,
 <span lang="fr">Florian Rivoal</span>,
 <span lang="fr">Simon Sapin</span>,
 Nicholas Shanks,
 Ben Ward,
 Zack Weinberg,
 Estelle Weyl,
 Boris Zbarsky,
 and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
 </p>
 <h2 class=no-num id="references">References</h2>
 <h3 class="no-num" id="normative-references">Normative references</h3>
 <!--normative-->
 <h3 class="no-num" id="other-references">Other references</h3>
 <!--informative-->
 <h2 class="no-num" id="index">Index</h2>
 <!--index-->
 </body>
 </html>
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