css3-conditional/Overview.src.html

Wed, 13 Mar 2013 15:18:26 -0700

author
L. David Baron <dbaron@dbaron.org>
date
Wed, 13 Mar 2013 15:18:26 -0700
changeset 7733
ef41ff04d9a8
parent 7538
9d25222da9cb
child 7734
19616016e597
permissions
-rw-r--r--

[css3-conditional] Fix previous version link and some other data in spec header.

     1 <!DOCTYPE html public '-//W3C//DTD HTML 4.01//EN'
     2   'http://www.w3.org/TR/html4/strict.dtd'>
     3 <html lang="en">
     4 <head profile="http://www.w3.org/2006/03/hcard">
     5   <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
     6   <title>CSS Conditional Rules Module Level 3</title>
     7   <link rel="stylesheet" type="text/css" href="../default.css">
     8   <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
     9   <link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
    10 <!--
    11   <script src="http://test.csswg.org/harness/annotate.js#CSS3-CONDITIONAL_DEV" 
    12         type="text/javascript" defer></script
    13 -->
    14 </head>
    15 <body class="h-entry">
    17 <div class="head">
    18 <!--logo-->
    20 <h1 class="p-name">CSS Conditional Rules Module Level 3</h1>
    22 <h2 class="no-num no-toc">[LONGSTATUS] <span class="dt-updated"><span class="value-title" title="[CDATE]">[DATE]</span></span></h2>
    23 <dl>
    24   <dt>This version:
    25     <dd><a class="u-url" href="[VERSION]">
    26     http://www.w3.org/TR/[YEAR]/ED-css3-conditional-[CDATE]/</a>
    28   <dt>Latest version:
    29     <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>
    31   <dt>Editor's draft:
    32     <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
    33     (<a href="https://dvcs.w3.org/hg/csswg/log/tip/[SHORTNAME]/Overview.src.html">change log</a>)
    35   <dt>Previous version:
    36     <dd><a rel="previous" href="http://www.w3.org/TR/2012/WD-css3-conditional-20121213/">http://www.w3.org/TR/2012/WD-css3-conditional-20121213/</a></dd>
    38   <dt>Editors:
    39     <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>,
    40       <a class="p-org org h-org" href="http://www.mozilla.org/">Mozilla</a>
    42   <dt>Issues list:
    43     <dd>Maintained in document (only editor's draft is current)
    45     <dt>Feedback:
    47     <dd><a
    48      href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
    49      with subject line “<kbd>&#x5b;[SHORTNAME]&#x5d; <var>… message topic
    50      …</var></kbd>”
    52   <dt>Test suite:
    53     <dd><a href="https://test.csswg.org/shepherd/search/spec/css3-conditional/">submitted tests</a>,
    54         <a href="https://test.csswg.org/harness/suite/CSS3-CONDITIONAL_DEV/">nightly test suite</a>
    56 </dl>
    58 <!--copyright-->
    60 <hr title="Separator for header">
    61 </div>
    63 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
    65   <p>CSS is a language for describing the rendering of structured documents
    66   (such as HTML and XML) on screen, on paper,  in speech, etc. 
    67   <span class="p-summary">
    68   This module
    69   contains the features of CSS for conditional processing of parts of
    70   style sheets, conditioned on capabilities of the processor or the
    71   document the style sheet is being applied to.
    72   It includes and extends the functionality of CSS level&nbsp;2 [[!CSS21]],
    73   which builds on CSS level&nbsp;1 [[CSS1]].
    74   The main extensions compared to level&nbsp;2 are
    75   allowing nesting of certain at-rules inside '@media',
    76   and the addition of the '@supports'
    77   rule for conditional processing.
    78   </span>
    80 <h2 class="no-num no-toc" id="status">Status of this document</h2>
    82 <!--status-->
    84 <p>The following features are at risk:
    85 <ul>
    86   <li>The inclusion of '@font-face' rules and
    87   '@keyframes' rules as allowed within all of the @-rules in
    88   this specification is at risk, though only because of the relative
    89   rates of advancement of specifications.  If this specification is able
    90   to advance faster than one or both of the specifications defining
    91   those rules, then the inclusion of those rules will move from this
    92   specification to the specification defining those rules.</li>
    94   <li>The addition of support for @-rules inside of conditional grouping
    95   rules is at risk; if interoperable implementations are not found, it
    96   may be removed to advance the other features in this specification to
    97   Proposed Recommendation.</li>
    99   <li>The '@supports' rule is at risk; if interoperable
   100   implementations are not found, it may be removed to advance the other
   101   features in this specification to Proposed Recommendation.</li>
   102 </ul>
   104 <p>This is a <strong>Last Call Working Draft</strong>.
   105 The deadline for comments is <strong>10 January 2013</strong>.
   107 <!--
   109   Things to go in level 4:
   111   * Create some way to put these new conditional things on an @import.
   112   * The @document rule (commented out, down below).
   114 -->
   116 <h2 class="no-num no-toc" id="contents">Table of contents</h2>
   118 <!--toc-->
   120 <h2 id="introduction">Introduction</h2>
   122 <h3 id="context">Background</h3>
   124   <p><em>This section is not normative.</em>
   126   <p>[[!CSS21]] defines one type of conditional group rule, the
   127   '@media' rule, and allows only rulesets (not other @-rules)
   128   inside of it.  The '@media' rule provides the ability to
   129   have media-specific style sheets, which is also provided by style
   130   sheet linking features such as '@import' and
   131   <code class="html">&lt;link&gt;</code>.  The restrictions on the contents of
   132   '@media' rules made them less useful; they have forced authors
   133   using CSS features involving @-rules in media-specific style sheets to
   134   use separate style sheets for each medium.</p>
   136   <p>This specification extends the rules for the contents of
   137   conditional group rules to allow other @-rules, which enables authors
   138   to combine CSS features involving @-rules with media specific style
   139   sheets within a single style sheet.</p>
   141   <p>This specification also defines an additional type of conditional
   142   group rule, '@supports', to
   143   address author and user requirements.</p>
   145   <p>The '@supports' rule allows CSS to be conditioned on
   146   implementation support for CSS properties and values.  This rule makes
   147   it much easier for authors to use new CSS features and provide good
   148   fallback for implementations that do not support those features.  This
   149   is particularly important for CSS features that provide new layout
   150   mechanisms, and for other cases where a set of related styles needs to
   151   be conditioned on property support.</p>
   153 <h3 id="placement">Module Interactions</h3>
   155   <p>This module replaces and extends the '@media' rule
   156   feature defined in [[!CSS21]] section <var>7.2.1</var> and
   157   incorporates the modifications previously made non-normatively by
   158   [[!MEDIAQ]] section <var>1</var>.</p>
   160   <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
   161   and [[!CSS3-ANIMATIONS]], but that dependency is only on the
   162   assumption that those modules will advance ahead of this one.  If this
   163   module advances faster, then the dependency will be reversed.</p>
   165 <h3 id="conventions">Document Conventions</h3>
   167   <p>Conformance requirements are expressed with a combination of
   168   descriptive assertions and RFC 2119 terminology. The key words “MUST”,
   169   “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
   170   “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
   171   document are to be interpreted as described in RFC 2119.
   172   However, for readability, these words do not appear in all uppercase
   173   letters in this specification.
   175   <p>All of the text of this specification is normative except sections
   176   explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
   178   <p>Examples in this specification are introduced with the words “for example”
   179   or are set apart from the normative text with
   180   <code class="html">class="example"</code>, like this:
   182   <div class="example">
   183     <p>This is an example of an informative example.</p>
   184   </div>
   186   <p>Informative notes begin with the word “Note” and are set apart from the
   187   normative text with <code class="html">class="note"</code>, like this:
   189   <p class="note">Note, this is an informative note.</p>
   191 <h2 id="processing">Processing of conditional group rules</h2>
   193 <p>This specification defines some CSS @-rules, called <dfn>conditional
   194 group rules</dfn>, that associate a condition with a group of other
   195 CSS rules.  These different rules allow testing different types of
   196 conditions, but share common behavior for how their contents are used
   197 when the condition is true and when the condition is false.</p>
   199 <div class="example">
   200 <p>For example, this rule:</p>
   201 <pre>@media print {
   202   /* hide navigation controls when printing */
   203   #navigation { display: none }
   204 }</pre>
   205 <p>causes a particular CSS rule (making elements with ID "navigation" be
   206 display:none) apply only when the style sheet is used for a print
   207 medium.
   208 </div>
   210 <p>Each conditional group rule has a condition, which at any time
   211 evaluates to true or false.  When the condition is true, CSS processors
   212 <strong>must</strong> apply the rules inside the group rule as though
   213 they were at the group rule's location; when the condition is false, CSS
   214 processors <strong>must not</strong> apply any of rules inside the group
   215 rule.  The current state of the condition does not affect the CSS object
   216 model, in which the contents of the group rule always remain within the
   217 group rule.</p>
   219 <p>This means that when multiple conditional group rules are nested,
   220 a rule inside of both of them applies only when all of the rules'
   221 conditions are true.</p>
   223 <div class="example">For example, with this set of nested rules:
   224 <pre>@media print { // rule (1)
   225   /* hide navigation controls when printing */
   226   #navigation { display: none }
   227   @media (max-width: 12cm) { // rule (2)
   228     /* keep notes in flow when printing to narrow pages */
   229     .note { float: none }
   230   }
   231 }</pre>
   232 the condition of the rule marked (1) is true for print media, and the
   233 condition of the rule marked (2) is true when the width of the display
   234 area (which for print media is the page box) is less than or equal to
   235 12cm.  Thus the rule ''#navigation { display: none }'' applies
   236 whenever this style sheet is applied to print media, and the rule
   237 ''.note { float: none }'' is applied only when the style sheet
   238 is applied to print media <em>and</em> the width of the page box is less
   239 than or equal to 12 centimeters.</div>
   241 <p>When the condition for a conditional group rule changes, CSS
   242 processors <strong>must</strong> reflect that the rules now apply or no
   243 longer apply, except for properties whose definitions define effects of
   244 computed values that persist past the lifetime of that value (such as
   245 for some properties in [[CSS3-TRANSITIONS]] and
   246 [[!CSS3-ANIMATIONS]]).</p>
   248 <h2 id="contents-of">Contents of conditional group rules</h2>
   250 <p>The syntax of each conditional group rule consists of some syntax
   251 specific to the type of rule followed by a <dfn>group rule body</dfn>,
   252 which is a block (pair of braces) containing a sequence of rules.</p>
   254 <p>A group rule body is allowed to contain rulesets and any @-rules that
   255 are allowed at the top level of a style sheet before and after a
   256 ruleset.  This means that @-rules that must occur at the beginning of
   257 the style sheet (such as '@charset', '@import',
   258 and '@namespace' rules) are not allowed inside of conditional group
   259 rules.  Conditional group rules can be nested.</p>
   261 <p>In terms of the grammar, this specification defines the following
   262 productions for use in the grammar of conditional group rules:</p>
   264 <pre><dfn>nested_statement</dfn>
   265   : ruleset | <i>media</i> | page | font_face_rule | keyframes_rule |
   266     <i>supports_rule</i>
   267   ;
   269 <dfn>group_rule_body</dfn>
   270   : '{' S* <i>nested_statement</i>* '}' S*
   271   ;</pre>
   272 <p>
   273 in which all the productions are defined in that grammar with the
   274 exception of <code>font_face_rule</code> 
   275 defined in [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
   276 [[!CSS3-ANIMATIONS]], and <code>media</code> and <code>supports_rule</code>
   277 defined in this specification.</p>
   279 <p>In general, future CSS specifications that add new @-rules that are
   280 not forbidden to occur after some other types of rules should modify
   281 this <code>nested_statement</code> production to keep the grammar
   282 accurate.</p>
   284 <p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
   285 conditional group rules.</p>
   287 <p>CSS processors <strong>must</strong> ignore rules that are not
   288 allowed within a group rule, and <strong>must</strong> handle invalid
   289 rules inside of group rules as described in <a
   290 href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
   291 4.2 (Rules for handling parsing errors)</a>, <a
   292 href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
   293 (At-rules)</a>, and <a
   294 href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
   295 (Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>
   297 <h2 id="use">Placement of conditional group rules</h2>
   299 <p>Conditional group rules are allowed at the top-level of a style
   300 sheet, and inside other conditional group rules.  CSS processors
   301 <strong>must</strong> process such rules as <a
   302 href="#processing">described above</a>.</p>
   304 <p>Any rules that are not allowed after a ruleset (e.g., ''@charset'',
   305 ''@import'', or ''@namespace'' rules) are also not allowed after a
   306 conditional group rule.  Therefore, style sheets <strong>must
   307 not</strong> place such rules after a conditional group rules, and CSS
   308 processors <strong>must</strong> ignore such rules.</p>
   310 <h2 id="at-media">Media-specific style sheets:  the '@media' rule</h2>
   312 <p>The <dfn>'@media' rule</dfn> is a conditional group rule whose
   313 condition is a media query.  It consists of the at-keyword
   314 '@media' followed by a (possibly empty) media query list (as
   315 defined in [[!MEDIAQ]]), followed by a group rule body.  The condition
   316 of the rule is the result of the media query.</p>
   318 <div class="example">
   319 <p>This '@media' rule:</p>
   320 <pre>@media screen and (min-width: 35em),
   321        print and (min-width: 40em) {
   322   #section_navigation { float: left; width: 10em; }
   323 }</pre>
   324 <p>has the condition
   325 ''screen and (min-width: 35em), print and (min-width: 40em)'',
   326 which is true for screen displays
   327 whose viewport is at least 35 times the initial font size
   328 and for print displays
   329 whose viewport is at least 40 times the initial font size.
   330 When either of these is true,
   331 the condition of the rule is true,
   332 and the rule
   333 ''#section_navigation { float: left; width: 10em; }''
   334 is applied.</p>
   335 </div>
   337 <p>In terms of the grammar, this specification extends the
   338 <code>media</code> production in the
   339 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
   340 ([[!CSS21]], Appendix G) into:
   341 <pre><dfn>media</dfn>
   342   : MEDIA_SYM S* media_query_list <i>group_rule_body</i>
   343   ;</pre>
   344 <p>where the <code>group_rule_body</code> production is defined in this
   345 specification, the <code>media_query_list</code> production is defined
   346 in [[!MEDIAQ]], and the others are defined in the <a
   347 href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
   348 ([[!CSS21]], Appendix G).
   350 <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
   352 <p>The <dfn>'@supports' rule</dfn> is a conditional group
   353 rule whose condition tests whether the user agent supports CSS
   354 property:value pairs.  Authors can use it to write style sheets that use
   355 new features when available but degrade gracefully when those features
   356 are not supported.  CSS has existing mechanisms for graceful
   357 degradation, such as ignoring unsupported properties or values, but
   358 these are not always sufficient when large groups of styles need to be
   359 tied to the support for certain features, as is the case for use of new
   360 layout system features.</p>
   362 <p>The syntax of the condition in the '@supports' rule is
   363 slightly more complicated than for the other conditional group rules
   364 (though has some similarities to media queries) since:</p>
   365 <ul>
   366   <li>negation is needed so that the new-feature styles and the fallback
   367   styles can be separated (within the forward-compatible grammar's rules
   368   for the syntax of @-rules), and not required to override each other</li>
   369   <li>conjunction (and) is needed so that multiple required features can
   370   be tested</li>
   371   <li>disjunction (or) is needed when there are multiple alternative
   372   features for a set of styles, particularly when some of those
   373   alternatives are vendor-prefixed properties or values</li>
   374 </ul>
   376 <p>Therefore, the syntax of the '@supports' rule allows
   377 testing for property:value pairs, and arbitrary conjunctions (and),
   378 disjunctions (or), and negations (not) of them.</p>
   380 <p>This extends the lexical scanner in the
   381 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
   382 ([[!CSS21]], Appendix G) by adding:
   383 <pre>
   384 @{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn>SUPPORTS_SYM</dfn>;}
   385 {O}{R}                    {return <dfn>OR</dfn>;}
   386 </pre>
   388 <p>This then extends the grammar in the
   389 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>,
   390 using the lexical scanner there, with the additions of
   391 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and
   392 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code>
   393 tokens defined in the Media Queries specification [[!MEDIAQ]]
   394 and the <code>OR</code> and <code>SUPPORTS_SYM</code> tokens defined above,
   395 and with
   396 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>,
   397 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
   398 and <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code>
   399 productions
   400 and the <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code> token
   401 taken from the core syntax of CSS defined in
   402 <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1 (Tokenization)</a> of [[!CSS21]],
   403 by adding:</p>
   405 <pre><dfn>supports_rule</dfn>
   406   : <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>
   407   ;
   409 <dfn>supports_condition</dfn>
   410   : <i>supports_negation</i> | <i>supports_conjunction</i> | <i>supports_disjunction</i> |
   411     <i>supports_condition_in_parens</i>
   412   ;
   414 <dfn>supports_condition_in_parens</dfn>
   415   : ( '(' <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> |
   416     <i>general_enclosed</i>
   417   ;
   419 <dfn>supports_negation</dfn>
   420   : <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>
   421   ;
   423 <dfn>supports_conjunction</dfn>
   424   : <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> )+
   425   ;
   427 <dfn>supports_disjunction</dfn>
   428   : <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> )+
   429   ;
   431 <dfn>supports_declaration_condition</dfn>
   432   : '(' <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> ')'
   433   ;
   435 <dfn>general_enclosed</dfn>
   436   : ( <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> )* ')'
   437   ;
   438 </pre>
   440 <p>
   441   Implementations <strong>must</strong> parse ''@supports'' rules
   442   based on the above grammar,
   443   and when interpreting the above grammar,
   444   <strong>must</strong> match the production before an <code>|</code> operator
   445   in preference to the one after it.
   446 </p>
   448 <p>
   449   The above grammar is purposely very loose for forwards-compatibility reasons,
   450   since the <code>general_enclosed</code> production
   451   allows for substantial future extensibility.
   452   Any ''@supports'' rule that does not parse according to the grammar above
   453   (that is, a rule that does not match this loose grammar
   454   which includes the <code>general_enclosed</code> production)
   455   is invalid.
   456   Style sheets <strong>must not</strong> use such a rule and
   457   processors <strong>must</strong> ignore such a rule (including all of its contents).
   459 <p>Each of these grammar terms is associated with a boolean result, as
   460 follows:</p>
   461 <dl>
   462 <dt>supports_condition</dt>
   463 <dd>
   464   The result is the result of the single child term.
   465 </dd>
   467 <dt>supports_condition_in_parens</dt>
   468 <dd>
   469   The result is the result of the single <code>supports_condition</code>
   470   or <code>supports_declaration_condition</code> child term.
   471 </dd>
   473 <dt>supports_negation</dt>
   474 <dd>
   475   The result is the <em>negation</em> of the result of the
   476   <code>supports_condition_in_parens</code> child term.
   477 </dd>
   479 <dt>supports_conjunction</dt>
   480 <dd>
   481   The result is true if the result of <em>all</em> of the
   482   <code>supports_condition_in_parens</code> child terms is true;
   483   otherwise it is false.
   484 </dd>
   486 <dt>supports_disjunction</dt>
   487 <dd>
   488   The result is true if the result of <em>any</em> of the
   489   <code>supports_condition_in_parens</code> child terms is true;
   490   otherwise it is false.
   491 </dd>
   493 <dt>supports_declaration_condition</dt>
   494 <dd>
   495   The result is whether the CSS processor <a href="#support-definition">supports</a> the declaration
   496   within the parentheses.
   497 </dd>
   499 <dt>general_enclosed</dt>
   500 <dd>
   501   The result is always false.
   502   Additionally, style sheets <strong>must not</strong>
   503   write ''@supports'' rules
   504   that match this grammar production.
   505   (In other words, this production exists only for future extensibility,
   506   and is not part of the description of a valid style sheet
   507   in this level of the specification.)
   508   <span class="note">Note that future levels may define functions
   509   or other parenthesized expressions that can evaluate to true.</span>
   510 </dd>
   511 </dl>
   513 <p>The condition of the '@supports' rule is the result of the
   514 <code>supports_condition</code> term that is a child of the
   515 <code>supports_rule</code> term.</p>
   517 <div class="example">
   518 <p>For example, the following rule</p>
   519 <pre>@supports ( display: flexbox ) {
   520   body, #navigation, #content { display: flexbox; }
   521   #navigation { background: blue; color: white; }
   522   #article { background: white; color: black; }
   523 }</pre>
   524 <p>applies the rules inside the '@supports' rule only when
   525 ''display: flexbox'' is supported.</p>
   526 </div>
   528 <div class="example">
   529 <p>The following example shows an additional '@supports' rule that can
   530 be used to provide an alternative for when ''display: flexbox'' is not
   531 supported:</p>
   532 <pre>@supports not ( display: flexbox ) {
   533   body { width: 100%; height: 100%; background: white; color: black; }
   534   #navigation { width: 25%; }
   535   #article { width: 75%; }
   536 }</pre>
   537 <p>Note that the 'width' declarations may be harmful to the
   538 flexbox-based layout, so it is important that they be present only in
   539 the non-flexbox styles.</p>
   540 </div>
   542 <div class="example">
   543 <p>The following example checks for support for the 'box-shadow'
   544 property, including checking for support for vendor-prefixed versions of
   545 it.  When the support is present, it specifies both 'box-shadow' (with
   546 the prefixed versions) and 'color' in a way what would cause the text to
   547 become invisible were 'box-shadow' not supported.</p>
   548 <pre>@supports ( box-shadow: 2px 2px 2px black ) or
   549           ( -moz-box-shadow: 2px 2px 2px black ) or
   550           ( -webkit-box-shadow: 2px 2px 2px black ) or
   551           ( -o-box-shadow: 2px 2px 2px black ) {
   552   .outline {
   553     color: white;
   554     -moz-box-shadow: 2px 2px 2px black;
   555     -webkit-box-shadow: 2px 2px 2px black;
   556     -o-box-shadow: 2px 2px 2px black;
   557     box-shadow: 2px 2px 2px black; /* unprefixed last */
   558   }
   559 }</pre></div>
   561 <p>To avoid confusion between ''and'' and ''or'', the syntax requires
   562 that both ''and'' and ''or'' be specified explicitly (rather than, say,
   563 using commas or spaces for one of them).  Likewise, to avoid confusion
   564 caused by precedence rules, the syntax does not allow ''and'', ''or'',
   565 and ''not'' operators to be mixed without a layer of parentheses.</p>
   567 <div class="example">
   568 <p>For example, the following rule is not valid:
   569 <pre class="illegal">@supports (transition-property: color) or
   570           (animation-name: foo) and
   571           (transform: rotate(10deg)) {
   572   // ...
   573 }</pre>
   574 <p>Instead, authors must write one of the following:</p>
   575 <pre>@supports ((transition-property: color) or
   576            (animation-name: foo)) and
   577           (transform: rotate(10deg)) {
   578   // ...
   579 }</pre>
   580 <pre>@supports (transition-property: color) or
   581           ((animation-name: foo) and
   582            (transform: rotate(10deg))) {
   583   // ...
   584 }</pre>
   585 </div>
   587 <p>Furthermore, whitespace is required after a ''not'' and on both
   588 sides of an ''and'' or ''or''.</p>
   590 <p>The declaration being tested must always occur within parentheses,
   591 when it is the only thing in the expression.<p>
   593 <div class="example">
   594 <p>For example, the following rule is not valid:
   595 <pre class="illegal">@supports display: flexbox {
   596   // ...
   597 }</pre>
   598 <p>Instead, authors must write:</p>
   599 <pre>@supports (display: flexbox) {
   600   // ...
   601 }</pre>
   602 </div>
   604 <p>The syntax allows extra parentheses when they are not needed.  This
   605 flexibility is sometimes useful for authors (for example, when
   606 commenting out parts of an expression) and may also be useful for
   607 authoring tools.</p>
   609 <div class="example">
   610 <p>For example, authors may write:</p>
   611 <pre>@supports ((display: flexbox)) {
   612   // ...
   613 }</pre>
   614 </div>
   616 <p>A trailing ''!important'' on a declaration being tested is allowed,
   617 though it won't change the validity of the declaration.
   619 <div class="example">
   620 <p>For example, the following rule is valid:
   621 <pre>@supports (display: flexbox !important) {
   622   // ...
   623 }</pre>
   624 </div>
   626 <h3 id="support-definition">Definition of support</h3>
   628 <p>For forward-compatibility,
   629 <a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
   630 (Declarations and properties)</a> of [[!CSS21]]
   631 defines rules for handling invalid properties and values.
   632 CSS processors that
   633 do not implement or partially implement a specification
   634 <strong>must</strong> treat any part of a value that they
   635 do not implement, or
   636 do not have a usable level of support for,
   637 as invalid according to this rule
   638 for handling invalid properties and values,
   639 and therefore <strong>must</strong> discard the declaration as a parse error.</p>
   641 <p>A CSS processor is considered to <dfn id="dfn-support">support</dfn>
   642 a declaration (consisting of a property and value) if it accepts that
   643 declaration (rather than discarding it as a parse error).
   644 If a processor does not implement, with a usable level of support,
   645 the value given,
   646 then it <strong>must not</strong>
   647 accept the declaration or claim support for it.</p>
   649 <p class="note">Note that properties or values
   650 whose support is effectively disabled by user preferences
   651 are still considered as supported by this definition.
   652 For example, if a user has enabled a high-contrast mode
   653 that causes colors to be overridden,
   654 the CSS processor is still considered to support the 'color' property
   655 even though declarations of the 'color' property may have no effect.
   656 On the other hand, a developer-facing preference
   657 whose purpose is to enable or disable support for an experimental CSS feature
   658 does affect this definition of support.</p>
   660 <p>These rules (and the equivalence between them) allow
   661 authors to use fallback (either in the [[CSS1]] sense of declarations
   662 that are overridden by later declarations or with the new capabilities
   663 provided by the ''@supports'' rule in this specification) that works
   664 correctly for the features implemented.  This applies especially to
   665 compound values; implementations must implement all parts of the value
   666 in order to consider the declaration supported, either inside a ruleset
   667 or in the declaration condition of an ''@supports'' rule.</p>
   669 <!--
   670 <h2 id="at-document">Document queries: the '@document' rule</h2>
   672 <p>The <dfn>'@document' rule</dfn> is a conditional group
   673 rule whose condition depends on the
   674 <a href="#url-of-doc">URL of the document being styled</a>.
   675 This allows style sheets, particularly user style sheets, to have styles
   676 that only apply to a set of pages rather than to all pages using the
   677 style sheet.</p>
   679 <p class="issue">Given that this @-rule is intended primarily for user
   680 style sheets, what should this specification say about its use in author
   681 style sheets?  Should it be forbidden?  Should use instead be
   682 discouraged?  Or should this specification remain neutral on the
   683 topic, since there are valid uses in author style sheets?</p>
   685 <p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
   686 the URI at which the document is located, excluding any fragment
   687 identifiers.  (This means, for example, that HTTP redirects have been
   688 followed.)  If the styles are being applied inside a complete document
   689 embedded into the presentation of another (e.g., [[HTML5]]&#39;s <code
   690 class="html">iframe</code>, <code class="html">object</code>, or <code
   691 class="html">img</code> elements), the relevant URI is that of the
   692 frame, not of its container.  However, if content from other documents
   693 is mixed in via mechanisms that mix content from one document into
   694 another (e.g., [[SVG11]]&#39;s <code>use</code> element), then the
   695 address of the container document is used.</p>
   697 <p class="note">Note:  In [[HTML5]], this is the
   698 <a href="http://dev.w3.org/html5/spec/dom.html#documents">document's address</a>
   699 of a document in a
   700 <a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>
   702 <div class="issue">What form of normalization is done on URLs and domains
   703 before matching?  In particular, this specification needs to describe:
   704 <ul>
   705 <li>what form is used for the <a href="#url-of-doc">URL of the document
   706 being styled</a> (and what has been normalized in that form)</li>
   707 <li>what normalization (if any) happens to the argument of each of the match
   708 functions before the comparison that they describe and</li>
   709 <li>whether the
   710 comparison algorithm used is string comparison or some other URL
   711 comparison algorithm.</li></ul></div>
   713 <p>The '@document' rule's condition is written as a
   714 comma-separated list of <dfn>URL matching functions</dfn>, and the
   715 condition evaluates to true whenever any one of those functions
   716 evaluates to true.  The following URL matching functions are
   717 permitted:</p>
   719 <dl>
   720   <dt><dfn id="url-exact" title="url()|URL matching functions::exact">&lt;url&gt;</dfn></dt>
   722   <dd>
   723     <p>The 'url()' function is the <dfn>exact url matching
   724     function</dfn>.  It evaluates to true whenever the <a
   725     href="#url-of-doc">URL of the document being styled</a> is exactly
   726     the URL given.</p>
   728     <p class="Note">The 'url()' function, since it is a core syntax
   729     element in CSS, is allowed (subject to different character
   730     limitations and thus escaping requirements) to contain an unquoted
   731     value (in addition to the string values that are allowed as
   732     arguments for all four functions).</p>
   734     <div class="example">
   735       <p>For example, this rule:</p>
   736 <pre>@document url("http://www.w3.org/Style/CSS/") {
   737   #summary { background: yellow; color: black}
   738 }</pre>
   739       <p>styles the <code class="html">summary</code> element on the page
   740       <code>http://www.w3.org/Style/CSS/</code>, but not on any other
   741       pages.</p>
   742     </div>
   743   </dd>
   745   <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(&lt;string&gt;)</dfn></dt>
   747   <dd>
   748     <p>The 'url-prefix()' function is the <dfn>url prefix
   749     matching function</dfn>.  It evaluates to true whenever the
   750     <a href="#url-of-doc">URL of the document being styled</a>
   751     has the argument to the function as an
   752     initial substring (which is true when the two strings are equal).
   753     When the argument is the empty string, it evaluates to true for all
   754     documents.</p>
   755     <div class="example">
   756       <p>For example, this rule:</p>
   757 <pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
   758   #summary { background: yellow; color: black}
   759 }</pre>
   760       <p>styles the <code class="html">summary</code> element on the page
   761       <code>http://www.w3.org/Style/CSS/</code> and on the page
   762       <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
   763       affect the page <code>http://www.w3.org/</code> or the page
   764       <code>http://www.example.com/Style/CSS/</code>.</p>
   765     </div>
   766   </dd>
   768   <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(&lt;string&gt;)</dfn></dt>
   770   <dd>
   771     <p>The 'domain()' function is the <dfn>domain
   772     matching function</dfn>.  It evaluates to true whenever
   773     the <a href="#url-of-doc">URL of the document being styled</a>
   774     has a host subcomponent (as defined in [[!URI]])
   775     and that host subcomponent is exactly the argument to the
   776     'domain()' function or a final substring of the host
   777     component is a period (U+002E) immediately followed by the argument
   778     to the 'domain()' function.</p>
   779     <div class="example">
   780       <p>For example, this rule:</p>
   781 <pre>@document domain("w3.org") {
   782   body { font-size: 16px ! important }
   783 }</pre>
   784       <p>changes the font size of the body element for pages such as
   785       <code>http://www.w3.org/Style/CSS/</code> and
   786       <code>http://w3.org/Style/CSS/</code> and
   787       <code>http://lists.w3.org/Archives/Public/www-style/</code>
   788       but it does not affect the page
   789       <code>http://www.example.com/Style/CSS/</code>.</p>
   790     </div>
   791   </dd>
   793   <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(&lt;string&gt;)</dfn></dt>
   795   <dd>
   796     <p>The contents of the &lt;string&gt; argument <strong>must</strong>
   797     match the JavaScript <code>Pattern</code> production
   798     ([[!ECMA-262-5.1]], section 15.10.1).  However,
   799     failing to do so is not a CSS syntax error and does not trigger any
   800     error handling for CSS syntax errors.</p>
   802     <p>The ''regexp()'' function evaluates to true whenever the string
   803     argument compiled as a JavaScript regular expression with the
   804     <code>global</code>, <code>ignoreCase</code> and
   805     <code>multiline</code> flags <em>disabled</em>
   806     (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
   807     compiles successfully and the resulting regular expression matches
   808     the entirety of the
   809     <a href="#url-of-doc">URL of the document being styled</a>.</p>
   811     <p class="note">Note that regular expression must match the entire
   812     URL, not just a part of it.</p>
   814     <p class="note">Note that this definition intentionally matches the
   815     behavior of the <a
   816     href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
   817     attribute</a> on the <code class="html">input</code> element
   818     in [[HTML5]].</p>
   820     <div class="example">
   821       <p>For example, this rule:</p>
   822 <pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
   823   body { font-size: 20px ! important }
   824 }</pre>
   825       <p>changes the font size of the body element for pages such as
   826       <code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
   827       <p class="note">Note that the backslashes in the regular
   828       expression require CSS escaping as ''\\''.</p>
   829     </div>
   830   </dd>
   832 </dl>
   834 <p>Implementations <strong>must</strong> treat any unknown URL matching
   835 functions as a syntax error, and thus ignore the '@document' rule.
   836 <span class="issue">Should we instead have more complicated error
   837 handling rules to make forward-compatibility work differently, or is
   838 this rule the best solution for such future expansion anyway?</span></p>
   840 <div class="issue">This syntax doesn't offer any ability to do negations,
   841 which has been requested in <a
   842 href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
   843 349813</a>.  Use cases that people have wanted negations for
   844 include:
   845 <ul>
   846   <li>User style sheets that want a particular rule in general, but know
   847   that that rule does more harm than good on specific sites.</li>
   848   <li>Authors who have a rule that they want to apply to most of their
   849   pages, but wish to make a few exceptions for.</li>
   850 </ul>
   851 </div>
   853 <p>This extends the lexical scanner in the
   854 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
   855 ([[!CSS21]], Appendix G) by adding:
   856 <pre>@{D}{O}{C}{U}{M}{E}{N}{T}	{return DOCUMENT_SYM;}</pre>
   857 <p>and the grammar by adding</p>
   858 <pre><dfn>document_rule</dfn>
   859   : DOCUMENT_SYM S+ <i>url_match_fn</i> ( "," S* <i>url_match_fn</i> )* <i>group_rule_body</i>
   860   ;
   862 <dfn>url_match_fn</dfn>
   863   : (URI | FUNCTION S* STRING S* ')' ) S*
   864   ;</pre>
   865 -->
   868 <h2 id="apis">APIs</h2>
   870 <h3 id='extentions-to-cssrule-interface'>
   871 Extensions to the <code>CSSRule</code> interface</h3>
   873 <p>The <code>CSSRule</code> interface is extended as follows:
   875 <pre class='idl'>partial interface CSSRule {
   876     const unsigned short SUPPORTS_RULE = 12;
   877     <!--
   878     const unsigned short DOCUMENT_RULE = 13;
   879     -->
   880 }</pre>
   883 <h3 id='the-cssgroupingrule-interface'>
   884 The <code>CSSGroupingRule</code> interface</h3>
   886 <p>The <dfn><code>CSSGroupingRule</code></dfn> interface represents an at-rule that contains other rules nested inside itself.
   888 <pre class='idl'>interface CSSGroupingRule : CSSRule {
   889     readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
   890     unsigned long insertRule (DOMString rule, unsigned long index);
   891     void deleteRule (unsigned long index);
   892 }</pre>
   894 <dl class='idl-attributes'>
   895   <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
   896   <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
   897     object for the list of CSS rules nested inside the grouping rule.
   898 </dl>
   900 <dl class='idl-methods'>
   901   <dt><code>insertRule(DOMString rule, unsigned long index)</code>, returns
   902     <code>unsigned long</code>
   903   <dd>
   904     The <code>insertRule</code> operation must
   905     insert a CSS rule <var>rule</var>
   906     into the CSS rule list returned by <code>cssRules</code>,
   907     such that the inserted rule will be at position <var>index</var>,
   908     and any rules previously at <var>index</var> or higher
   909     will increase their index by one.
   910     It must throw INDEX_SIZE_ERR
   911     if index is greater than <code>cssRules.length</code>.
   912     It must throw SYNTAX_ERR
   913     if the rule has a syntax error and is unparseable;
   914     this does not include syntax errors handled by error handling rules
   915     for constructs inside of the rule,
   916     but this does include cases where the string given
   917     does not parse into a single CSS rule (such as when the string is empty)
   918     or where there is anything other than whitespace or comments
   919     after that single CSS rule.
   920     It must throw HIERARCHY_REQUEST_ERR
   921     if the rule cannot be inserted at the location specified,
   922     for example, if an ''@import'' rule is inserted inside a group rule.
   924     <p>The return value is the <var>index</var> parameter.
   926   <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
   927   <dd>
   928     The <code>deleteRule</code> operation must
   929     remove a CSS rule from
   930     the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
   931     It must throw INDEX_SIZE_ERR
   932     if index is greater than or equal to <code>cssRules.length</code>.
   933 </dl>
   936 <h3 id="the-cssconditionrule-interface">
   937 The <code>CSSConditionRule</code> interface</h3>
   939 <p>The <dfn><code>CSSConditionRule</code></dfn> interface represents all the "conditional" at-rules,
   940   which consist of a condition and a statement block.
   942 <pre class='idl'>interface CSSConditionRule : CSSGroupingRule {
   943     attribute DOMString conditionText;
   944 }</pre>
   946 <dl class='idl-attributes'>
   948   <dt><code>conditionText</code> of type <code>DOMString</code>
   949   <dd>
   950     <p>The <code>conditionText</code> attribute represents
   951     the condition of the rule.
   952     Since what this condition does
   953     varies between the derived interfaces of <code>CSSConditionRule</code>,
   954     those derived interfaces
   955     may specify different behavior for this attribute
   956     (see, for example, <code>CSSMediaRule</code> below).
   957     In the absence of such rule-specific behavior,
   958     the following rules apply:</p>
   960     <p>The <code>conditionText</code> attribute, on getting, must return
   961     the result of serializing the associated condition.
   963     <p>On setting the <code>conditionText</code> attribute these steps
   964       must be run:
   966     <ol>
   967       <li>Trim the given value of white space.
   968       <li>If the given value matches the grammar of the 
   969         appropriate condition production for the given rule, 
   970         replace the associated CSS condition with the given value.
   971       <li>Otherwise, do nothing.
   972     </ol>
   973 </dl>
   976 <h3 id="the-cssmediarule-interface">
   977 The <code>CSSMediaRule</code> interface</h3>
   979 <p>The <dfn><code>CSSMediaRule</code></dfn> interface represents a ''@media'' rule:
   981 <pre class='idl'>interface CSSMediaRule : CSSConditionRule {
   982     readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a> media;
   983 }</pre>
   985 <dl class='idl-attributes'>
   986   <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
   987   <dd>The <code>media</code> attribute must return a <code>MediaList</code> object
   988     for the list of media queries specified with the ''@media'' rule.
   990   <dt><code>conditionText</code> of type <code>DOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
   991   <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
   992     on getting, must return the value of <code>media.mediaText</code> on the rule.
   994     <p>Setting the <code>conditionText</code> attribute
   995       must set the <code>media.mediaText</code> attribute on the rule.
   996 </dl>
   999 <h3 id="the-csssupportsrule-interface">
  1000 The <code>CSSSupportsRule</code> interface</h3>
  1002 <p>The <dfn><code>CSSSupportsRule</code></dfn> interface represents a ''@supports'' rule.</p>
  1004 <pre class='idl'>interface CSSSupportsRule : CSSConditionRule {
  1005 }</pre>
  1007 <dl class='idl-attributes'>
  1008   <dt><code>conditionText</code> of type <code>DOMString</code> (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
  1009   <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
  1010     on getting, must return the condition that was specified,
  1011     without any logical simplifications,
  1012     so that the returned condition will evaluate to the same result
  1013     as the specified condition
  1014     in any conformant implementation of this specification
  1015     (including implementations that implement future extensions
  1016     allowed by the <i>general_enclosed</i> exensibility mechanism in this specification).
  1017     In other words,
  1018     token stream simplifications are allowed
  1019     (such as reducing whitespace to a single space
  1020     or omitting it in cases where it is known to be optional),
  1021     but logical simplifications (such as removal of unneeded parentheses,
  1022     or simplification based on evaluating results) are not allowed.
  1024 </dl>
  1026 <!--
  1027 <h3 id="the-cssdocumentrule-interface">
  1028 The <code>CSSDocumentRule</code> interface</h3>
  1030 <p>The <dfn><code>CSSDocumentRule</code></dfn> interface represents a ''@document'' rule.</p>
  1032 <pre class='idl'>interface CSSDocumentRule : CSSConditionRule {
  1033 }</pre>
  1034 -->
  1037 <h3 id='the-css-interface'>
  1038 The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>
  1040 <p>The <dfn id='CSS-interface'><code>CSS</code></dfn> interface holds useful CSS-related functions that do not belong elsewhere.
  1042 <pre class='idl'>interface CSS {
  1043   static boolean supports(DOMString property, DOMString value);
  1044   static boolean supports(DOMString conditionText);
  1045 }</pre>
  1047 <dl class='idl-methods'>
  1048   <dt><code>supports(DOMString property, DOMString value)</code>,
  1049     returns <code>boolean</code>
  1050   <dt><code>supports(DOMString conditionText)</code>,
  1051     returns <code>boolean</code>
  1052   <dd>
  1053     When the <code title=''>supports()</code> method is invoked with two arguments <var>property</var> and <var>value</var>,
  1054     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,
  1055     and <var>value</var> would be successfully parsed as a supported value for that property.
  1056     (Literal match means that no CSS escape processing is performed,
  1057     and leading and trailing whitespace are not stripped,
  1058     so any leading whitespace, trailing whitespace,
  1059     or CSS escapes equivalent to the name of a property
  1060     would cause the method to return <code>false</code>.)
  1061     Otherwise, it must return <code>false</code>.
  1063     <p>
  1064       When invoked with a single <var>conditionText</var> argument,
  1065       it must return <code>true</code> if <var>conditionText</var>,
  1066       when parsed and evaluated as a <code>supports_condition</code>,
  1067       would return true.
  1068       Otherwise, it must return <code>false</code>.
  1069   </dl>
  1072 <h2 class=no-num id="grammar">Grammar</h2>
  1074 <p>In order to allow these new @-rules in CSS style sheets, this
  1075 specification modifies the <code>stylesheet</code> production in the <a
  1076 href="http://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
  1077 [[!CSS21]] by replacing the <code>media</code> production defined in
  1078 [[!CSS21]] with the <code>media</code> production defined in this one,
  1079 and additionally inserting <code>| supports_rule</code>
  1080 alongside <code>ruleset | media | page</code>.</p>
  1083 <h2 id="conformance">Conformance</h2>
  1085 <h3 id="base-modules">Base Modules</h3>
  1087 <p>This specification defines conformance in terms of base modules,
  1088 which are modules that this specification builds on top of.  The base
  1089 modules of this module are:</p>
  1091 <ul>
  1092 <li>[[!CSS21]]</li>
  1093 </ul>
  1095 <p>All of the conformance requirements of all base modules are
  1096 incorporated as conformance requirements of this module, except where
  1097 overridden by this module.</p>
  1099 <p>Additionally, all conformance requirements related to validity of
  1100 syntax in this module and all of its base modules are to be interpreted
  1101 as though all syntax in all of those modules is valid.</p>
  1103 <div class="example"><p>For example, this means that grammar presented
  1104 in modules other than [[!CSS21]] must obey the requirements that
  1105 [[!CSS21]] defines for the parsing of properties, and that requirements
  1106 for handling invalid syntax in [[!CSS21]] do not treat syntax added by
  1107 other modules as invalid.</p></div>
  1109 <p>Additionally, the set of valid syntax can be increased by the
  1110 conformance of a style sheet or processor to additional modules; use of
  1111 such syntax does not make a style sheet nonconformant and failure to
  1112 treat such syntax as invalid does not make a processor
  1113 nonconformant.</p>
  1115 <h3 id="conformance-classes">Conformance Classes</h3>
  1117   <p>Conformance to the CSS Conditional Rules Module is defined for three
  1118   conformance classes:
  1119   <dl>
  1120     <dt><dfn title="conformance::style sheet" id="conform-style-sheet">style sheet</dfn>
  1121       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
  1122       style sheet</a>.</dd>
  1123     <dt><dfn title="conformance::processor" id="conform-processor">processor</dfn></dt>
  1124       <dd>A tool that reads CSS style sheets:  it may be a renderer or
  1125       <a
  1126       href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user-agent</a>
  1127       that interprets the semantics of a style sheet and renders
  1128       documents that use style sheets, or it may be a validator that
  1129       checks style sheets.</dd>
  1130     <dt><dfn title="conformance::authoring tool" id="conform-authoring-tool">authoring tool</dfn></dt>
  1131       <dd>A tool that writes a style sheet.</dd>
  1132   </dl>
  1134   <p>A style sheet is conformant to the CSS Conditional Rules Module
  1135   if it meets all of the conformance requirements in the module that are
  1136   described as requirements of style sheets.</p>
  1138   <p>A processor is conformant to the CSS Conditional Rules Module if it
  1139   meets all applicable conformance requirements in the module that are
  1140   described as requirements of processors.  In general, all requirements
  1141   are applicable to renderers.  Requirements concerning a part of CSS
  1142   not performed by a processor are not applicable, e.g., requirements
  1143   related to rendering are not applicable to a validator.  The inability
  1144   of a processor to correctly render a document due to limitations of
  1145   the device does not make it non-conformant. (For example, a renderer
  1146   is not required to render color on a monochrome monitor.)</p>
  1148   <p>An authoring tool is conformant to the CSS Conditional Rules Module
  1149   if it writes style sheets that conform to the module and (if it reads
  1150   CSS) it is a conformant processor.</p>
  1152 <h3 id="partial">
  1153 Partial Implementations</h3>
  1155   <p>So that authors can exploit the forward-compatible parsing rules to
  1156   assign fallback values, CSS renderers <strong>must</strong>
  1157   treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
  1158   as appropriate</a>) any at-rules, properties, property values, keywords,
  1159   and other syntactic constructs for which they have no usable level of
  1160   support. In particular, user agents <strong>must not</strong> selectively
  1161   ignore unsupported component values and honor supported values in a single
  1162   multi-value property declaration: if any value is considered invalid
  1163   (as unsupported values must be), CSS requires that the entire declaration
  1164   be ignored.</p>
  1166 <h3 id="experimental">Experimental Implementations</h3>
  1168   <p>To avoid clashes with future CSS features, the CSS specifications
  1169   reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
  1170   syntax</a> for proprietary property and value extensions to CSS. The CSS
  1171   Working Group recommends that experimental implementations of features in
  1172   CSS Working Drafts also use vendor-prefixed property or value names. This
  1173   avoids any incompatibilities with future changes in the draft. Once a
  1174   specification reaches the Candidate Recommendation stage, implementors
  1175   should implement the non-prefixed syntax for any feature they consider to
  1176   be correctly implemented according to spec.</p>
  1178 <h3 id="cr-exit-criteria">CR Exit Criteria</h3>
  1180   <p>For this specification to be advanced to Proposed Recommendation,
  1181   there must be at least two independent, interoperable implementations
  1182   of each feature. Each feature may be implemented by a different set of
  1183   products, there is no requirement that all features be implemented by
  1184   a single product. For the purposes of this criterion, we define the
  1185   following terms:
  1187   <dl>
  1188     <dt>independent <dd>each implementation must be developed by a
  1189     different party and cannot share, reuse, or derive from code
  1190     used by another qualifying implementation. Sections of code that
  1191     have no bearing on the implementation of this specification are
  1192     exempt from this requirement.
  1194     <dt>interoperable <dd>passing the respective test case(s) in the
  1195     official CSS test suite, or, if the implementation is not a Web
  1196     browser, an equivalent test. Every relevant test in the test
  1197     suite should have an equivalent test created if such a user
  1198     agent (UA) is to be used to claim interoperability. In addition
  1199     if such a UA is to be used to claim interoperability, then there
  1200     must one or more additional UAs which can also pass those
  1201     equivalent tests in the same way for the purpose of
  1202     interoperability. The equivalent tests must be made publicly
  1203     available for the purposes of peer review.
  1205     <dt>implementation <dd>a user agent which:
  1207     <ol class=inline>
  1208       <li>implements the specification.
  1210       <li>is available to the general public. The implementation may
  1211       be a shipping product or other publicly available version
  1212       (i.e., beta version, preview release, or “nightly build”). 
  1213       Non-shipping product releases must have implemented the
  1214       feature(s) for a period of at least one month in order to
  1215       demonstrate stability.
  1217       <li>is not experimental (i.e., a version specifically designed
  1218       to pass the test suite and is not intended for normal usage
  1219       going forward).
  1220     </ol>
  1221   </dl>
  1223   <p>The specification will remain Candidate Recommendation for at least
  1224   six months.
  1226 <h2 id="changes">
  1227 Changes</h2>
  1229 <p>The following (non-editorial) changes were made to this specification since the
  1230 <a href="http://www.w3.org/TR/2012/WD-css3-conditional-20120911/">11 September 2012 Working Draft</a>:
  1232 <ul>
  1233   <li>Removed ''@document'' rule; it has been deferred to Level 4.
  1234   <li>Allow functional notation in ''@supports'' queries to be valid (to allow for future extensions),
  1235       but treat such notations as always being false.
  1236   <li>Corrected the grammar as follows:
  1237 <pre>
  1238 -  : SUPPORTS_SYM S+ supports_condition group_rule_body
  1239 +  : SUPPORTS_SYM S* supports_condition group_rule_body
  1240 </pre>
  1241 <pre>
  1242 -  : (URI | FUNCTION) S*
  1243 +  : (URI | FUNCTION S* STRING S* ')' ) S*
  1244 </pre>
  1245   <li>Switched "and", "or", and "not" keywords to use appropriate productions rather than literals.
  1246   <li>Clarified definition of support used for interpreting ''@support'' rules
  1247   and its relationship to CSS forwards-compatible parsing rules.
  1248 </ul>
  1250 <h2 class=no-num id="acknowledgments">Acknowledgments</h2>
  1252 <p>
  1253 Thanks to the ideas and feedback from
  1254 Tab Atkins,
  1255 Arthur Barstow,
  1256 Ben Callahan,
  1257 <span lang="tr">Tantek Çelik</span>,
  1258 Alex Danilo,
  1259 Elika Etemad,
  1260 Pascal Germroth,
  1261 <span lang="de">Björn Höhrmann</span>,
  1262 Paul Irish,
  1263 <span lang="nl">Anne van Kesteren</span>,
  1264 Vitor Menezes,
  1265 Alex Mogilevsky,
  1266 Chris Moschini,
  1267 James Nurthen,
  1268 Simon Pieters,
  1269 <span lang="fr">Florian Rivoal</span>,
  1270 <span lang="fr">Simon Sapin</span>,
  1271 Nicholas Shanks,
  1272 Ben Ward,
  1273 Zack Weinberg,
  1274 Estelle Weyl,
  1275 Boris Zbarsky,
  1276 and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
  1278 </p>
  1280 <h2 class=no-num id="references">References</h2>
  1283 <h3 class="no-num" id="normative-references">Normative references</h3>
  1284 <!--normative-->
  1286 <h3 class="no-num" id="other-references">Other references</h3>
  1287 <!--informative-->
  1289 <h2 class="no-num" id="index">Index</h2>
  1290 <!--index-->
  1292 </body>
  1293 </html>
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