css-conditional/Overview.src.html@99e6228da0b8
css-conditional/Overview.src.html
Tue, 09 Apr 2013 16:08:00 -0700
- author
- Peter Linss <peter.linss@hp.com>
- date
- Tue, 09 Apr 2013 16:08:00 -0700
- changeset 7933
- 99e6228da0b8
- parent 7907
- 4cf8a925226d
- child 8324
- 6a24538736c9
- permissions
- -rw-r--r--
[css-conditional] uncomment test annotation script
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 <script src="http://test.csswg.org/harness/annotate.js#CSS3-CONDITIONAL_DEV"
11 type="text/javascript" defer></script
12 </head>
13 <body class="h-entry">
15 <div class="head">
16 <!--logo-->
18 <h1 class="p-name">CSS Conditional Rules Module Level 3</h1>
20 <h2 class="no-num no-toc">[LONGSTATUS] <span class="dt-updated"><span class="value-title" title="[CDATE]">[DATE]</span></span></h2>
21 <dl>
22 <dt>This version:
23 <dd><a class="u-url" href="[VERSION]">
24 http://www.w3.org/TR/[YEAR]/ED-css3-conditional-[CDATE]/</a>
26 <dt>Latest version:
27 <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>
29 <dt>Editor's draft:
30 <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
31 (<a href="https://dvcs.w3.org/hg/csswg/log/tip/css-conditional/Overview.src.html">change log</a>,
32 <a href="https://dvcs.w3.org/hg/csswg/log/tip/css3-conditional/Overview.src.html">older change log</a>)
34 <dt>Previous version:
35 <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>
37 <dt>Editors:
38 <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>,
39 <a class="p-org org h-org" href="http://www.mozilla.org/">Mozilla</a>
41 <dt>Issues list:
42 <dd>Maintained in document (only editor's draft is current)
44 <dt>Feedback:</dt>
45 <dd><a href="mailto:www-style@w3.org?subject=%5Bcss3-conditional%5D%20feedback"
46 >www-style@w3.org</a>
47 with subject line “<kbd>[css3-conditional]
48 <var>… message topic …</var></kbd>”
49 (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/"
50 >archives</a>)
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 2 [[!CSS21]],
73 which builds on CSS level 1 [[CSS1]].
74 The main extensions compared to level 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>See the section <a href="#cr-exit-criteria">“CR Exit Criteria”</a>
85 for details on advancing this specification to W3C Recommendation. The
86 specification will remain Candidate Recommendation at least until 2
87 October 2013. A <a
88 href="http://test.csswg.org/suites/css3-conditional/nightly-unstable/report/"
89 >test suite and implementation report</a> are under development.
91 <p>See the section <a href="#changes">“Changes”</a> for changes since
92 the last Working Draft.
94 <p>The following features are at risk:
95 <ul>
96 <li>The inclusion of '@font-face' rules and
97 '@keyframes' rules as allowed within all of the @-rules in
98 this specification is at risk, though only because of the relative
99 rates of advancement of specifications. If this specification is able
100 to advance faster than one or both of the specifications defining
101 those rules, then the inclusion of those rules will move from this
102 specification to the specification defining those rules.</li>
104 <li>The addition of support for @-rules inside of conditional grouping
105 rules is at risk; if interoperable implementations are not found, it
106 may be removed to advance the other features in this specification to
107 Proposed Recommendation.</li>
109 <li>The '@supports' rule is at risk; if interoperable
110 implementations are not found, it may be removed to advance the other
111 features in this specification to Proposed Recommendation.</li>
112 </ul>
114 <!--
116 Things to go in level 4:
118 * Create some way to put these new conditional things on an @import.
119 * The @document rule (commented out, down below).
121 -->
123 <h2 class="no-num no-toc" id="contents">Table of contents</h2>
125 <!--toc-->
127 <h2 id="introduction">Introduction</h2>
129 <h3 id="context">Background</h3>
131 <p><em>This section is not normative.</em>
133 <p>[[!CSS21]] defines one type of conditional group rule, the
134 '@media' rule, and allows only rulesets (not other @-rules)
135 inside of it. The '@media' rule provides the ability to
136 have media-specific style sheets, which is also provided by style
137 sheet linking features such as '@import' and
138 <code class="html"><link></code>. The restrictions on the contents of
139 '@media' rules made them less useful; they have forced authors
140 using CSS features involving @-rules in media-specific style sheets to
141 use separate style sheets for each medium.</p>
143 <p>This specification extends the rules for the contents of
144 conditional group rules to allow other @-rules, which enables authors
145 to combine CSS features involving @-rules with media specific style
146 sheets within a single style sheet.</p>
148 <p>This specification also defines an additional type of conditional
149 group rule, '@supports', to
150 address author and user requirements.</p>
152 <p>The '@supports' rule allows CSS to be conditioned on
153 implementation support for CSS properties and values. This rule makes
154 it much easier for authors to use new CSS features and provide good
155 fallback for implementations that do not support those features. This
156 is particularly important for CSS features that provide new layout
157 mechanisms, and for other cases where a set of related styles needs to
158 be conditioned on property support.</p>
160 <h3 id="placement">Module Interactions</h3>
162 <p>This module replaces and extends the '@media' rule
163 feature defined in [[!CSS21]] section <var>7.2.1</var> and
164 incorporates the modifications previously made non-normatively by
165 [[!MEDIAQ]] section <var>1</var>.</p>
167 <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
168 and [[!CSS3-ANIMATIONS]], but that dependency is only on the
169 assumption that those modules will advance ahead of this one. If this
170 module advances faster, then the dependency will be reversed.</p>
172 <h3 id="conventions">Document Conventions</h3>
174 <p>Conformance requirements are expressed with a combination of
175 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
176 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
177 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
178 document are to be interpreted as described in RFC 2119.
179 However, for readability, these words do not appear in all uppercase
180 letters in this specification.
182 <p>All of the text of this specification is normative except sections
183 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
185 <p>Examples in this specification are introduced with the words “for example”
186 or are set apart from the normative text with
187 <code class="html">class="example"</code>, like this:
189 <div class="example">
190 <p>This is an example of an informative example.</p>
191 </div>
193 <p>Informative notes begin with the word “Note” and are set apart from the
194 normative text with <code class="html">class="note"</code>, like this:
196 <p class="note">Note, this is an informative note.</p>
198 <h2 id="processing">Processing of conditional group rules</h2>
200 <p>This specification defines some CSS @-rules, called <dfn>conditional
201 group rules</dfn>, that associate a condition with a group of other
202 CSS rules. These different rules allow testing different types of
203 conditions, but share common behavior for how their contents are used
204 when the condition is true and when the condition is false.</p>
206 <div class="example">
207 <p>For example, this rule:</p>
208 <pre>@media print {
209 /* hide navigation controls when printing */
210 #navigation { display: none }
211 }</pre>
212 <p>causes a particular CSS rule (making elements with ID "navigation" be
213 display:none) apply only when the style sheet is used for a print
214 medium.
215 </div>
217 <p>Each conditional group rule has a condition, which at any time
218 evaluates to true or false. When the condition is true, CSS processors
219 <strong>must</strong> apply the rules inside the group rule as though
220 they were at the group rule's location; when the condition is false, CSS
221 processors <strong>must not</strong> apply any of rules inside the group
222 rule. The current state of the condition does not affect the CSS object
223 model, in which the contents of the group rule always remain within the
224 group rule.</p>
226 <p>This means that when multiple conditional group rules are nested,
227 a rule inside of both of them applies only when all of the rules'
228 conditions are true.</p>
230 <div class="example">For example, with this set of nested rules:
231 <pre>@media print { // rule (1)
232 /* hide navigation controls when printing */
233 #navigation { display: none }
234 @media (max-width: 12cm) { // rule (2)
235 /* keep notes in flow when printing to narrow pages */
236 .note { float: none }
237 }
238 }</pre>
239 the condition of the rule marked (1) is true for print media, and the
240 condition of the rule marked (2) is true when the width of the display
241 area (which for print media is the page box) is less than or equal to
242 12cm. Thus the rule ''#navigation { display: none }'' applies
243 whenever this style sheet is applied to print media, and the rule
244 ''.note { float: none }'' is applied only when the style sheet
245 is applied to print media <em>and</em> the width of the page box is less
246 than or equal to 12 centimeters.</div>
248 <p>When the condition for a conditional group rule changes, CSS
249 processors <strong>must</strong> reflect that the rules now apply or no
250 longer apply, except for properties whose definitions define effects of
251 computed values that persist past the lifetime of that value (such as
252 for some properties in [[CSS3-TRANSITIONS]] and
253 [[!CSS3-ANIMATIONS]]).</p>
255 <h2 id="contents-of">Contents of conditional group rules</h2>
257 <p>The syntax of each conditional group rule consists of some syntax
258 specific to the type of rule followed by a <dfn>group rule body</dfn>,
259 which is a block (pair of braces) containing a sequence of rules.</p>
261 <p>A group rule body is allowed to contain rulesets and any @-rules that
262 are allowed at the top level of a style sheet before and after a
263 ruleset. This means that @-rules that must occur at the beginning of
264 the style sheet (such as '@charset', '@import',
265 and '@namespace' rules) are not allowed inside of conditional group
266 rules. Conditional group rules can be nested.</p>
268 <p>In terms of the grammar, this specification defines the following
269 productions for use in the grammar of conditional group rules:</p>
271 <pre><dfn>nested_statement</dfn>
272 : ruleset | <i>media</i> | page | font_face_rule | keyframes_rule |
273 <i>supports_rule</i>
274 ;
276 <dfn>group_rule_body</dfn>
277 : '{' S* <i>nested_statement</i>* '}' S*
278 ;</pre>
279 <p>
280 in which all the productions are defined in that grammar with the
281 exception of <code>font_face_rule</code>
282 defined in [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
283 [[!CSS3-ANIMATIONS]], and <code>media</code> and <code>supports_rule</code>
284 defined in this specification.</p>
286 <p>In general, future CSS specifications that add new @-rules that are
287 not forbidden to occur after some other types of rules should modify
288 this <code>nested_statement</code> production to keep the grammar
289 accurate.</p>
291 <p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
292 conditional group rules.</p>
294 <p>CSS processors <strong>must</strong> ignore rules that are not
295 allowed within a group rule, and <strong>must</strong> handle invalid
296 rules inside of group rules as described in <a
297 href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
298 4.2 (Rules for handling parsing errors)</a>, <a
299 href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
300 (At-rules)</a>, and <a
301 href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
302 (Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>
304 <h2 id="use">Placement of conditional group rules</h2>
306 <p>Conditional group rules are allowed at the top-level of a style
307 sheet, and inside other conditional group rules. CSS processors
308 <strong>must</strong> process such rules as <a
309 href="#processing">described above</a>.</p>
311 <p>Any rules that are not allowed after a ruleset (e.g., ''@charset'',
312 ''@import'', or ''@namespace'' rules) are also not allowed after a
313 conditional group rule. Therefore, style sheets <strong>must
314 not</strong> place such rules after a conditional group rules, and CSS
315 processors <strong>must</strong> ignore such rules.</p>
317 <h2 id="at-media">Media-specific style sheets: the '@media' rule</h2>
319 <p>The <dfn id="atmedia-rule">'@media' rule</dfn> is a conditional group rule whose
320 condition is a media query. It consists of the at-keyword
321 '@media' followed by a (possibly empty) media query list (as
322 defined in [[!MEDIAQ]]), followed by a group rule body. The condition
323 of the rule is the result of the media query.</p>
325 <div class="example">
326 <p>This '@media' rule:</p>
327 <pre>@media screen and (min-width: 35em),
328 print and (min-width: 40em) {
329 #section_navigation { float: left; width: 10em; }
330 }</pre>
331 <p>has the condition
332 ''screen and (min-width: 35em), print and (min-width: 40em)'',
333 which is true for screen displays
334 whose viewport is at least 35 times the initial font size
335 and for print displays
336 whose viewport is at least 40 times the initial font size.
337 When either of these is true,
338 the condition of the rule is true,
339 and the rule
340 ''#section_navigation { float: left; width: 10em; }''
341 is applied.</p>
342 </div>
344 <p>In terms of the grammar, this specification extends the
345 <code>media</code> production in the
346 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
347 ([[!CSS21]], Appendix G) into:
348 <pre><dfn>media</dfn>
349 : MEDIA_SYM S* media_query_list <i>group_rule_body</i>
350 ;</pre>
351 <p>where the <code>group_rule_body</code> production is defined in this
352 specification, the <code>media_query_list</code> production is defined
353 in [[!MEDIAQ]], and the others are defined in the <a
354 href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
355 ([[!CSS21]], Appendix G).
357 <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
359 <p>The <dfn id="atsupports-rule">'@supports' rule</dfn> is a conditional group
360 rule whose condition tests whether the user agent supports CSS
361 property:value pairs. Authors can use it to write style sheets that use
362 new features when available but degrade gracefully when those features
363 are not supported. CSS has existing mechanisms for graceful
364 degradation, such as ignoring unsupported properties or values, but
365 these are not always sufficient when large groups of styles need to be
366 tied to the support for certain features, as is the case for use of new
367 layout system features.</p>
369 <p>The syntax of the condition in the '@supports' rule is
370 slightly more complicated than for the other conditional group rules
371 (though has some similarities to media queries) since:</p>
372 <ul>
373 <li>negation is needed so that the new-feature styles and the fallback
374 styles can be separated (within the forward-compatible grammar's rules
375 for the syntax of @-rules), and not required to override each other</li>
376 <li>conjunction (and) is needed so that multiple required features can
377 be tested</li>
378 <li>disjunction (or) is needed when there are multiple alternative
379 features for a set of styles, particularly when some of those
380 alternatives are vendor-prefixed properties or values</li>
381 </ul>
383 <p>Therefore, the syntax of the '@supports' rule allows
384 testing for property:value pairs, and arbitrary conjunctions (and),
385 disjunctions (or), and negations (not) of them.</p>
387 <p>This extends the lexical scanner in the
388 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
389 ([[!CSS21]], Appendix G) by adding:
390 <pre>
391 @{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn>SUPPORTS_SYM</dfn>;}
392 {O}{R} {return <dfn>OR</dfn>;}
393 </pre>
395 <p>This then extends the grammar in the
396 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>,
397 using the lexical scanner there, with the additions of
398 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and
399 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code>
400 tokens defined in the Media Queries specification [[!MEDIAQ]]
401 and the <code>OR</code> and <code>SUPPORTS_SYM</code> tokens defined above,
402 and with
403 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>,
404 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
405 and <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code>
406 productions
407 and the <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code> token
408 taken from the core syntax of CSS defined in
409 <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1 (Tokenization)</a> of [[!CSS21]],
410 by adding:</p>
412 <pre><dfn>supports_rule</dfn>
413 : <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>
414 ;
416 <dfn>supports_condition</dfn>
417 : <i>supports_negation</i> | <i>supports_conjunction</i> | <i>supports_disjunction</i> |
418 <i>supports_condition_in_parens</i>
419 ;
421 <dfn>supports_condition_in_parens</dfn>
422 : ( '(' <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> |
423 <i>general_enclosed</i>
424 ;
426 <dfn>supports_negation</dfn>
427 : <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>
428 ;
430 <dfn>supports_conjunction</dfn>
431 : <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> )+
432 ;
434 <dfn>supports_disjunction</dfn>
435 : <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> )+
436 ;
438 <dfn>supports_declaration_condition</dfn>
439 : '(' <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> ')'
440 ;
442 <dfn>general_enclosed</dfn>
443 : ( <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> )* ')'
444 ;
445 </pre>
447 <p>
448 Implementations <strong>must</strong> parse ''@supports'' rules
449 based on the above grammar,
450 and when interpreting the above grammar,
451 <strong>must</strong> match the production before an <code>|</code> operator
452 in preference to the one after it.
453 </p>
455 <p>
456 The above grammar is purposely very loose for forwards-compatibility reasons,
457 since the <code>general_enclosed</code> production
458 allows for substantial future extensibility.
459 Any ''@supports'' rule that does not parse according to the grammar above
460 (that is, a rule that does not match this loose grammar
461 which includes the <code>general_enclosed</code> production)
462 is invalid.
463 Style sheets <strong>must not</strong> use such a rule and
464 processors <strong>must</strong> ignore such a rule (including all of its contents).
466 <p>Each of these grammar terms is associated with a boolean result, as
467 follows:</p>
468 <dl>
469 <dt>supports_condition</dt>
470 <dd>
471 The result is the result of the single child term.
472 </dd>
474 <dt>supports_condition_in_parens</dt>
475 <dd>
476 The result is the result of the single <code>supports_condition</code>
477 or <code>supports_declaration_condition</code> child term.
478 </dd>
480 <dt>supports_negation</dt>
481 <dd>
482 The result is the <em>negation</em> of the result of the
483 <code>supports_condition_in_parens</code> child term.
484 </dd>
486 <dt>supports_conjunction</dt>
487 <dd>
488 The result is true if the result of <em>all</em> of the
489 <code>supports_condition_in_parens</code> child terms is true;
490 otherwise it is false.
491 </dd>
493 <dt>supports_disjunction</dt>
494 <dd>
495 The result is true if the result of <em>any</em> of the
496 <code>supports_condition_in_parens</code> child terms is true;
497 otherwise it is false.
498 </dd>
500 <dt>supports_declaration_condition</dt>
501 <dd>
502 The result is whether the CSS processor <a href="#support-definition">supports</a> the declaration
503 within the parentheses.
504 </dd>
506 <dt>general_enclosed</dt>
507 <dd>
508 The result is always false.
509 Additionally, style sheets <strong>must not</strong>
510 write ''@supports'' rules
511 that match this grammar production.
512 (In other words, this production exists only for future extensibility,
513 and is not part of the description of a valid style sheet
514 in this level of the specification.)
515 <span class="note">Note that future levels may define functions
516 or other parenthesized expressions that can evaluate to true.</span>
517 </dd>
518 </dl>
520 <p>The condition of the '@supports' rule is the result of the
521 <code>supports_condition</code> term that is a child of the
522 <code>supports_rule</code> term.</p>
524 <div class="example">
525 <p>For example, the following rule</p>
526 <pre>@supports ( display: flexbox ) {
527 body, #navigation, #content { display: flexbox; }
528 #navigation { background: blue; color: white; }
529 #article { background: white; color: black; }
530 }</pre>
531 <p>applies the rules inside the '@supports' rule only when
532 ''display: flexbox'' is supported.</p>
533 </div>
535 <div class="example">
536 <p>The following example shows an additional '@supports' rule that can
537 be used to provide an alternative for when ''display: flexbox'' is not
538 supported:</p>
539 <pre>@supports not ( display: flexbox ) {
540 body { width: 100%; height: 100%; background: white; color: black; }
541 #navigation { width: 25%; }
542 #article { width: 75%; }
543 }</pre>
544 <p>Note that the 'width' declarations may be harmful to the
545 flexbox-based layout, so it is important that they be present only in
546 the non-flexbox styles.</p>
547 </div>
549 <div class="example">
550 <p>The following example checks for support for the 'box-shadow'
551 property, including checking for support for vendor-prefixed versions of
552 it. When the support is present, it specifies both 'box-shadow' (with
553 the prefixed versions) and 'color' in a way what would cause the text to
554 become invisible were 'box-shadow' not supported.</p>
555 <pre>@supports ( box-shadow: 2px 2px 2px black ) or
556 ( -moz-box-shadow: 2px 2px 2px black ) or
557 ( -webkit-box-shadow: 2px 2px 2px black ) or
558 ( -o-box-shadow: 2px 2px 2px black ) {
559 .outline {
560 color: white;
561 -moz-box-shadow: 2px 2px 2px black;
562 -webkit-box-shadow: 2px 2px 2px black;
563 -o-box-shadow: 2px 2px 2px black;
564 box-shadow: 2px 2px 2px black; /* unprefixed last */
565 }
566 }</pre></div>
568 <p>To avoid confusion between ''and'' and ''or'', the syntax requires
569 that both ''and'' and ''or'' be specified explicitly (rather than, say,
570 using commas or spaces for one of them). Likewise, to avoid confusion
571 caused by precedence rules, the syntax does not allow ''and'', ''or'',
572 and ''not'' operators to be mixed without a layer of parentheses.</p>
574 <div class="example">
575 <p>For example, the following rule is not valid:
576 <pre class="illegal">@supports (transition-property: color) or
577 (animation-name: foo) and
578 (transform: rotate(10deg)) {
579 // ...
580 }</pre>
581 <p>Instead, authors must write one of the following:</p>
582 <pre>@supports ((transition-property: color) or
583 (animation-name: foo)) and
584 (transform: rotate(10deg)) {
585 // ...
586 }</pre>
587 <pre>@supports (transition-property: color) or
588 ((animation-name: foo) and
589 (transform: rotate(10deg))) {
590 // ...
591 }</pre>
592 </div>
594 <p>Furthermore, whitespace is required after a ''not'' and on both
595 sides of an ''and'' or ''or''.</p>
597 <p>The declaration being tested must always occur within parentheses,
598 when it is the only thing in the expression.<p>
600 <div class="example">
601 <p>For example, the following rule is not valid:
602 <pre class="illegal">@supports display: flexbox {
603 // ...
604 }</pre>
605 <p>Instead, authors must write:</p>
606 <pre>@supports (display: flexbox) {
607 // ...
608 }</pre>
609 </div>
611 <p>The syntax allows extra parentheses when they are not needed. This
612 flexibility is sometimes useful for authors (for example, when
613 commenting out parts of an expression) and may also be useful for
614 authoring tools.</p>
616 <div class="example">
617 <p>For example, authors may write:</p>
618 <pre>@supports ((display: flexbox)) {
619 // ...
620 }</pre>
621 </div>
623 <p>A trailing ''!important'' on a declaration being tested is allowed,
624 though it won't change the validity of the declaration.
626 <div class="example">
627 <p>For example, the following rule is valid:
628 <pre>@supports (display: flexbox !important) {
629 // ...
630 }</pre>
631 </div>
633 <h3 id="support-definition">Definition of support</h3>
635 <p>For forward-compatibility,
636 <a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
637 (Declarations and properties)</a> of [[!CSS21]]
638 defines rules for handling invalid properties and values.
639 CSS processors that
640 do not implement or partially implement a specification
641 <strong>must</strong> treat any part of a value that they
642 do not implement, or
643 do not have a usable level of support for,
644 as invalid according to this rule
645 for handling invalid properties and values,
646 and therefore <strong>must</strong> discard the declaration as a parse error.</p>
648 <p>A CSS processor is considered to <dfn id="dfn-support">support</dfn>
649 a declaration (consisting of a property and value) if it accepts that
650 declaration (rather than discarding it as a parse error).
651 If a processor does not implement, with a usable level of support,
652 the value given,
653 then it <strong>must not</strong>
654 accept the declaration or claim support for it.</p>
656 <p class="note">Note that properties or values
657 whose support is effectively disabled by user preferences
658 are still considered as supported by this definition.
659 For example, if a user has enabled a high-contrast mode
660 that causes colors to be overridden,
661 the CSS processor is still considered to support the 'color' property
662 even though declarations of the 'color' property may have no effect.
663 On the other hand, a developer-facing preference
664 whose purpose is to enable or disable support for an experimental CSS feature
665 does affect this definition of support.</p>
667 <p>These rules (and the equivalence between them) allow
668 authors to use fallback (either in the [[CSS1]] sense of declarations
669 that are overridden by later declarations or with the new capabilities
670 provided by the ''@supports'' rule in this specification) that works
671 correctly for the features implemented. This applies especially to
672 compound values; implementations must implement all parts of the value
673 in order to consider the declaration supported, either inside a ruleset
674 or in the declaration condition of an ''@supports'' rule.</p>
676 <!--
677 <h2 id="at-document">Document queries: the '@document' rule</h2>
679 <p>The <dfn>'@document' rule</dfn> is a conditional group
680 rule whose condition depends on the
681 <a href="#url-of-doc">URL of the document being styled</a>.
682 This allows style sheets, particularly user style sheets, to have styles
683 that only apply to a set of pages rather than to all pages using the
684 style sheet.</p>
686 <p class="issue">Given that this @-rule is intended primarily for user
687 style sheets, what should this specification say about its use in author
688 style sheets? Should it be forbidden? Should use instead be
689 discouraged? Or should this specification remain neutral on the
690 topic, since there are valid uses in author style sheets?</p>
692 <p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
693 the URI at which the document is located, excluding any fragment
694 identifiers. (This means, for example, that HTTP redirects have been
695 followed.) If the styles are being applied inside a complete document
696 embedded into the presentation of another (e.g., [[HTML5]]'s <code
697 class="html">iframe</code>, <code class="html">object</code>, or <code
698 class="html">img</code> elements), the relevant URI is that of the
699 frame, not of its container. However, if content from other documents
700 is mixed in via mechanisms that mix content from one document into
701 another (e.g., [[SVG11]]'s <code>use</code> element), then the
702 address of the container document is used.</p>
704 <p class="note">Note: In [[HTML5]], this is the
705 <a href="http://dev.w3.org/html5/spec/dom.html#documents">document's address</a>
706 of a document in a
707 <a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>
709 <div class="issue">What form of normalization is done on URLs and domains
710 before matching? In particular, this specification needs to describe:
711 <ul>
712 <li>what form is used for the <a href="#url-of-doc">URL of the document
713 being styled</a> (and what has been normalized in that form)</li>
714 <li>what normalization (if any) happens to the argument of each of the match
715 functions before the comparison that they describe and</li>
716 <li>whether the
717 comparison algorithm used is string comparison or some other URL
718 comparison algorithm.</li></ul></div>
720 <p>The '@document' rule's condition is written as a
721 comma-separated list of <dfn>URL matching functions</dfn>, and the
722 condition evaluates to true whenever any one of those functions
723 evaluates to true. The following URL matching functions are
724 permitted:</p>
726 <dl>
727 <dt><dfn id="url-exact" title="url()|URL matching functions::exact"><url></dfn></dt>
729 <dd>
730 <p>The 'url()' function is the <dfn>exact url matching
731 function</dfn>. It evaluates to true whenever the <a
732 href="#url-of-doc">URL of the document being styled</a> is exactly
733 the URL given.</p>
735 <p class="Note">The 'url()' function, since it is a core syntax
736 element in CSS, is allowed (subject to different character
737 limitations and thus escaping requirements) to contain an unquoted
738 value (in addition to the string values that are allowed as
739 arguments for all four functions).</p>
741 <div class="example">
742 <p>For example, this rule:</p>
743 <pre>@document url("http://www.w3.org/Style/CSS/") {
744 #summary { background: yellow; color: black}
745 }</pre>
746 <p>styles the <code class="html">summary</code> element on the page
747 <code>http://www.w3.org/Style/CSS/</code>, but not on any other
748 pages.</p>
749 </div>
750 </dd>
752 <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(<string>)</dfn></dt>
754 <dd>
755 <p>The 'url-prefix()' function is the <dfn>url prefix
756 matching function</dfn>. It evaluates to true whenever the
757 <a href="#url-of-doc">URL of the document being styled</a>
758 has the argument to the function as an
759 initial substring (which is true when the two strings are equal).
760 When the argument is the empty string, it evaluates to true for all
761 documents.</p>
762 <div class="example">
763 <p>For example, this rule:</p>
764 <pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
765 #summary { background: yellow; color: black}
766 }</pre>
767 <p>styles the <code class="html">summary</code> element on the page
768 <code>http://www.w3.org/Style/CSS/</code> and on the page
769 <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
770 affect the page <code>http://www.w3.org/</code> or the page
771 <code>http://www.example.com/Style/CSS/</code>.</p>
772 </div>
773 </dd>
775 <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(<string>)</dfn></dt>
777 <dd>
778 <p>The 'domain()' function is the <dfn>domain
779 matching function</dfn>. It evaluates to true whenever
780 the <a href="#url-of-doc">URL of the document being styled</a>
781 has a host subcomponent (as defined in [[!URI]])
782 and that host subcomponent is exactly the argument to the
783 'domain()' function or a final substring of the host
784 component is a period (U+002E) immediately followed by the argument
785 to the 'domain()' function.</p>
786 <div class="example">
787 <p>For example, this rule:</p>
788 <pre>@document domain("w3.org") {
789 body { font-size: 16px ! important }
790 }</pre>
791 <p>changes the font size of the body element for pages such as
792 <code>http://www.w3.org/Style/CSS/</code> and
793 <code>http://w3.org/Style/CSS/</code> and
794 <code>http://lists.w3.org/Archives/Public/www-style/</code>
795 but it does not affect the page
796 <code>http://www.example.com/Style/CSS/</code>.</p>
797 </div>
798 </dd>
800 <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(<string>)</dfn></dt>
802 <dd>
803 <p>The contents of the <string> argument <strong>must</strong>
804 match the JavaScript <code>Pattern</code> production
805 ([[!ECMA-262-5.1]], section 15.10.1). However,
806 failing to do so is not a CSS syntax error and does not trigger any
807 error handling for CSS syntax errors.</p>
809 <p>The ''regexp()'' function evaluates to true whenever the string
810 argument compiled as a JavaScript regular expression with the
811 <code>global</code>, <code>ignoreCase</code> and
812 <code>multiline</code> flags <em>disabled</em>
813 (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
814 compiles successfully and the resulting regular expression matches
815 the entirety of the
816 <a href="#url-of-doc">URL of the document being styled</a>.</p>
818 <p class="note">Note that regular expression must match the entire
819 URL, not just a part of it.</p>
821 <p class="note">Note that this definition intentionally matches the
822 behavior of the <a
823 href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
824 attribute</a> on the <code class="html">input</code> element
825 in [[HTML5]].</p>
827 <div class="example">
828 <p>For example, this rule:</p>
829 <pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
830 body { font-size: 20px ! important }
831 }</pre>
832 <p>changes the font size of the body element for pages such as
833 <code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
834 <p class="note">Note that the backslashes in the regular
835 expression require CSS escaping as ''\\''.</p>
836 </div>
837 </dd>
839 </dl>
841 <p>Implementations <strong>must</strong> treat any unknown URL matching
842 functions as a syntax error, and thus ignore the '@document' rule.
843 <span class="issue">Should we instead have more complicated error
844 handling rules to make forward-compatibility work differently, or is
845 this rule the best solution for such future expansion anyway?</span></p>
847 <div class="issue">This syntax doesn't offer any ability to do negations,
848 which has been requested in <a
849 href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
850 349813</a>. Use cases that people have wanted negations for
851 include:
852 <ul>
853 <li>User style sheets that want a particular rule in general, but know
854 that that rule does more harm than good on specific sites.</li>
855 <li>Authors who have a rule that they want to apply to most of their
856 pages, but wish to make a few exceptions for.</li>
857 </ul>
858 </div>
860 <p>This extends the lexical scanner in the
861 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
862 ([[!CSS21]], Appendix G) by adding:
863 <pre>@{D}{O}{C}{U}{M}{E}{N}{T} {return DOCUMENT_SYM;}</pre>
864 <p>and the grammar by adding</p>
865 <pre><dfn>document_rule</dfn>
866 : DOCUMENT_SYM S+ <i>url_match_fn</i> ( "," S* <i>url_match_fn</i> )* <i>group_rule_body</i>
867 ;
869 <dfn>url_match_fn</dfn>
870 : (URI | FUNCTION S* STRING S* ')' ) S*
871 ;</pre>
872 -->
875 <h2 id="apis">APIs</h2>
877 <h3 id='extentions-to-cssrule-interface'>
878 Extensions to the <code>CSSRule</code> interface</h3>
880 <p>The <code>CSSRule</code> interface is extended as follows:
882 <pre class='idl'>partial interface CSSRule {
883 const unsigned short SUPPORTS_RULE = 12;
884 <!--
885 const unsigned short DOCUMENT_RULE = 13;
886 -->
887 }</pre>
890 <h3 id='the-cssgroupingrule-interface'>
891 The <code>CSSGroupingRule</code> interface</h3>
893 <p>The <dfn><code>CSSGroupingRule</code></dfn> interface represents an at-rule that contains other rules nested inside itself.
895 <pre class='idl'>interface CSSGroupingRule : CSSRule {
896 readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
897 unsigned long insertRule (DOMString rule, unsigned long index);
898 void deleteRule (unsigned long index);
899 }</pre>
901 <dl class='idl-attributes'>
902 <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
903 <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
904 object for the list of CSS rules nested inside the grouping rule.
905 </dl>
907 <dl class='idl-methods'>
908 <dt><code>insertRule(DOMString rule, unsigned long index)</code>, returns
909 <code>unsigned long</code>
910 <dd>
911 The <code>insertRule</code> operation must
912 insert a CSS rule <var>rule</var>
913 into the CSS rule list returned by <code>cssRules</code>,
914 such that the inserted rule will be at position <var>index</var>,
915 and any rules previously at <var>index</var> or higher
916 will increase their index by one.
917 It must throw INDEX_SIZE_ERR
918 if index is greater than <code>cssRules.length</code>.
919 It must throw SYNTAX_ERR
920 if the rule has a syntax error and is unparseable;
921 this does not include syntax errors handled by error handling rules
922 for constructs inside of the rule,
923 but this does include cases where the string given
924 does not parse into a single CSS rule (such as when the string is empty)
925 or where there is anything other than whitespace or comments
926 after that single CSS rule.
927 It must throw HIERARCHY_REQUEST_ERR
928 if the rule cannot be inserted at the location specified,
929 for example, if an ''@import'' rule is inserted inside a group rule.
931 <p>The return value is the <var>index</var> parameter.
933 <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
934 <dd>
935 The <code>deleteRule</code> operation must
936 remove a CSS rule from
937 the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
938 It must throw INDEX_SIZE_ERR
939 if index is greater than or equal to <code>cssRules.length</code>.
940 </dl>
943 <h3 id="the-cssconditionrule-interface">
944 The <code>CSSConditionRule</code> interface</h3>
946 <p>The <dfn><code>CSSConditionRule</code></dfn> interface represents all the "conditional" at-rules,
947 which consist of a condition and a statement block.
949 <pre class='idl'>interface CSSConditionRule : CSSGroupingRule {
950 attribute DOMString conditionText;
951 }</pre>
953 <dl class='idl-attributes'>
955 <dt><code>conditionText</code> of type <code>DOMString</code>
956 <dd>
957 <p>The <code>conditionText</code> attribute represents
958 the condition of the rule.
959 Since what this condition does
960 varies between the derived interfaces of <code>CSSConditionRule</code>,
961 those derived interfaces
962 may specify different behavior for this attribute
963 (see, for example, <code>CSSMediaRule</code> below).
964 In the absence of such rule-specific behavior,
965 the following rules apply:</p>
967 <p>The <code>conditionText</code> attribute, on getting, must return
968 the result of serializing the associated condition.
970 <p>On setting the <code>conditionText</code> attribute these steps
971 must be run:
973 <ol>
974 <li>Trim the given value of white space.
975 <li>If the given value matches the grammar of the
976 appropriate condition production for the given rule,
977 replace the associated CSS condition with the given value.
978 <li>Otherwise, do nothing.
979 </ol>
980 </dl>
983 <h3 id="the-cssmediarule-interface">
984 The <code>CSSMediaRule</code> interface</h3>
986 <p>The <dfn><code>CSSMediaRule</code></dfn> interface represents a ''@media'' rule:
988 <pre class='idl'>interface CSSMediaRule : CSSConditionRule {
989 readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a> media;
990 }</pre>
992 <dl class='idl-attributes'>
993 <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
994 <dd>The <code>media</code> attribute must return a <code>MediaList</code> object
995 for the list of media queries specified with the ''@media'' rule.
997 <dt><code>conditionText</code> of type <code>DOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
998 <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
999 on getting, must return the value of <code>media.mediaText</code> on the rule.
1001 <p>Setting the <code>conditionText</code> attribute
1002 must set the <code>media.mediaText</code> attribute on the rule.
1003 </dl>
1006 <h3 id="the-csssupportsrule-interface">
1007 The <code>CSSSupportsRule</code> interface</h3>
1009 <p>The <dfn><code>CSSSupportsRule</code></dfn> interface represents a ''@supports'' rule.</p>
1011 <pre class='idl'>interface CSSSupportsRule : CSSConditionRule {
1012 }</pre>
1014 <dl class='idl-attributes'>
1015 <dt><code>conditionText</code> of type <code>DOMString</code> (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
1016 <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
1017 on getting, must return the condition that was specified,
1018 without any logical simplifications,
1019 so that the returned condition will evaluate to the same result
1020 as the specified condition
1021 in any conformant implementation of this specification
1022 (including implementations that implement future extensions
1023 allowed by the <i>general_enclosed</i> exensibility mechanism in this specification).
1024 In other words,
1025 token stream simplifications are allowed
1026 (such as reducing whitespace to a single space
1027 or omitting it in cases where it is known to be optional),
1028 but logical simplifications (such as removal of unneeded parentheses,
1029 or simplification based on evaluating results) are not allowed.
1031 </dl>
1033 <!--
1034 <h3 id="the-cssdocumentrule-interface">
1035 The <code>CSSDocumentRule</code> interface</h3>
1037 <p>The <dfn><code>CSSDocumentRule</code></dfn> interface represents a ''@document'' rule.</p>
1039 <pre class='idl'>interface CSSDocumentRule : CSSConditionRule {
1040 }</pre>
1041 -->
1044 <h3 id='the-css-interface'>
1045 The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>
1047 <p>The <dfn id='CSS-interface'><code>CSS</code></dfn> interface holds useful CSS-related functions that do not belong elsewhere.
1049 <pre class='idl'>interface CSS {
1050 static boolean supports(DOMString property, DOMString value);
1051 static boolean supports(DOMString conditionText);
1052 }</pre>
1054 <dl class='idl-methods'>
1055 <dt><code>supports(DOMString property, DOMString value)</code>,
1056 returns <code>boolean</code>
1057 <dt><code>supports(DOMString conditionText)</code>,
1058 returns <code>boolean</code>
1059 <dd>
1060 When the <code title=''>supports()</code> method is invoked with two arguments <var>property</var> and <var>value</var>,
1061 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,
1062 and <var>value</var> would be successfully parsed as a supported value for that property.
1063 (Literal match means that no CSS escape processing is performed,
1064 and leading and trailing whitespace are not stripped,
1065 so any leading whitespace, trailing whitespace,
1066 or CSS escapes equivalent to the name of a property
1067 would cause the method to return <code>false</code>.)
1068 Otherwise, it must return <code>false</code>.
1070 <p>
1071 When invoked with a single <var>conditionText</var> argument,
1072 it must return <code>true</code> if <var>conditionText</var>,
1073 when parsed and evaluated as a <code>supports_condition</code>,
1074 would return true.
1075 Otherwise, it must return <code>false</code>.
1076 </dl>
1079 <h2 class=no-num id="grammar">Grammar</h2>
1081 <p>In order to allow these new @-rules in CSS style sheets, this
1082 specification modifies the <code>stylesheet</code> production in the <a
1083 href="http://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
1084 [[!CSS21]] by replacing the <code>media</code> production defined in
1085 [[!CSS21]] with the <code>media</code> production defined in this one,
1086 and additionally inserting <code>| supports_rule</code>
1087 alongside <code>ruleset | media | page</code>.</p>
1090 <h2 id="conformance">Conformance</h2>
1092 <h3 id="base-modules">Base Modules</h3>
1094 <p>This specification defines conformance in terms of base modules,
1095 which are modules that this specification builds on top of. The base
1096 modules of this module are:</p>
1098 <ul>
1099 <li>[[!CSS21]]</li>
1100 </ul>
1102 <p>All of the conformance requirements of all base modules are
1103 incorporated as conformance requirements of this module, except where
1104 overridden by this module.</p>
1106 <p>Additionally, all conformance requirements related to validity of
1107 syntax in this module and all of its base modules are to be interpreted
1108 as though all syntax in all of those modules is valid.</p>
1110 <div class="example"><p>For example, this means that grammar presented
1111 in modules other than [[!CSS21]] must obey the requirements that
1112 [[!CSS21]] defines for the parsing of properties, and that requirements
1113 for handling invalid syntax in [[!CSS21]] do not treat syntax added by
1114 other modules as invalid.</p></div>
1116 <p>Additionally, the set of valid syntax can be increased by the
1117 conformance of a style sheet or processor to additional modules; use of
1118 such syntax does not make a style sheet nonconformant and failure to
1119 treat such syntax as invalid does not make a processor
1120 nonconformant.</p>
1122 <h3 id="conformance-classes">Conformance Classes</h3>
1124 <p>Conformance to the CSS Conditional Rules Module is defined for three
1125 conformance classes:
1126 <dl>
1127 <dt><dfn title="conformance::style sheet" id="conform-style-sheet">style sheet</dfn>
1128 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
1129 style sheet</a>.</dd>
1130 <dt><dfn title="conformance::processor" id="conform-processor">processor</dfn></dt>
1131 <dd>A tool that reads CSS style sheets: it may be a renderer or
1132 <a
1133 href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user-agent</a>
1134 that interprets the semantics of a style sheet and renders
1135 documents that use style sheets, or it may be a validator that
1136 checks style sheets.</dd>
1137 <dt><dfn title="conformance::authoring tool" id="conform-authoring-tool">authoring tool</dfn></dt>
1138 <dd>A tool that writes a style sheet.</dd>
1139 </dl>
1141 <p>A style sheet is conformant to the CSS Conditional Rules Module
1142 if it meets all of the conformance requirements in the module that are
1143 described as requirements of style sheets.</p>
1145 <p>A processor is conformant to the CSS Conditional Rules Module if it
1146 meets all applicable conformance requirements in the module that are
1147 described as requirements of processors. In general, all requirements
1148 are applicable to renderers. Requirements concerning a part of CSS
1149 not performed by a processor are not applicable, e.g., requirements
1150 related to rendering are not applicable to a validator. The inability
1151 of a processor to correctly render a document due to limitations of
1152 the device does not make it non-conformant. (For example, a renderer
1153 is not required to render color on a monochrome monitor.)</p>
1155 <p>An authoring tool is conformant to the CSS Conditional Rules Module
1156 if it writes style sheets that conform to the module and (if it reads
1157 CSS) it is a conformant processor.</p>
1159 <h3 id="partial">
1160 Partial Implementations</h3>
1162 <p>So that authors can exploit the forward-compatible parsing rules to
1163 assign fallback values, CSS renderers <strong>must</strong>
1164 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
1165 as appropriate</a>) any at-rules, properties, property values, keywords,
1166 and other syntactic constructs for which they have no usable level of
1167 support. In particular, user agents <strong>must not</strong> selectively
1168 ignore unsupported component values and honor supported values in a single
1169 multi-value property declaration: if any value is considered invalid
1170 (as unsupported values must be), CSS requires that the entire declaration
1171 be ignored.</p>
1173 <h3 id="experimental">Experimental Implementations</h3>
1175 <p>To avoid clashes with future CSS features, the CSS specifications
1176 reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
1177 syntax</a> for proprietary property and value extensions to CSS. The CSS
1178 Working Group recommends that experimental implementations of features in
1179 CSS Working Drafts also use vendor-prefixed property or value names. This
1180 avoids any incompatibilities with future changes in the draft. Once a
1181 specification reaches the Candidate Recommendation stage, implementors
1182 should implement the non-prefixed syntax for any feature they consider to
1183 be correctly implemented according to spec.</p>
1185 <h3 id="cr-exit-criteria">CR Exit Criteria</h3>
1187 <p>For this specification to be advanced to Proposed Recommendation,
1188 there must be at least two independent, interoperable implementations
1189 of each feature. Each feature may be implemented by a different set of
1190 products, there is no requirement that all features be implemented by
1191 a single product. For the purposes of this criterion, we define the
1192 following terms:
1194 <dl>
1195 <dt>independent <dd>each implementation must be developed by a
1196 different party and cannot share, reuse, or derive from code
1197 used by another qualifying implementation. Sections of code that
1198 have no bearing on the implementation of this specification are
1199 exempt from this requirement.
1201 <dt>interoperable <dd>passing the respective test case(s) in the
1202 official CSS test suite, or, if the implementation is not a Web
1203 browser, an equivalent test. Every relevant test in the test
1204 suite should have an equivalent test created if such a user
1205 agent (UA) is to be used to claim interoperability. In addition
1206 if such a UA is to be used to claim interoperability, then there
1207 must one or more additional UAs which can also pass those
1208 equivalent tests in the same way for the purpose of
1209 interoperability. The equivalent tests must be made publicly
1210 available for the purposes of peer review.
1212 <dt>implementation <dd>a user agent which:
1214 <ol class=inline>
1215 <li>implements the specification.
1217 <li>is available to the general public. The implementation may
1218 be a shipping product or other publicly available version
1219 (i.e., beta version, preview release, or “nightly build”).
1220 Non-shipping product releases must have implemented the
1221 feature(s) for a period of at least one month in order to
1222 demonstrate stability.
1224 <li>is not experimental (i.e., a version specifically designed
1225 to pass the test suite and is not intended for normal usage
1226 going forward).
1227 </ol>
1228 </dl>
1230 <p>The specification will remain Candidate Recommendation for at least
1231 six months.
1233 <h2 id="changes">
1234 Changes</h2>
1236 <p>The following (non-editorial) changes were made to this specification since the
1237 <a href="http://www.w3.org/TR/2012/WD-css3-conditional-20121213/">13 December 2012 Working Draft</a>:
1239 <ul>
1240 <li>Require whitespace around ''and'' and ''or'' and after ''not''.
1241 <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.
1242 <li>Describe requirements for conditionText getter on CSSSupportsRule.
1243 <li>Clarify the definition of "literal match" in CSS.supports().
1244 <li>Specify behavior of CSSGroupingRule.insertRule when given an empty string or more than one syntactically valid rule.
1245 </ul>
1247 <h2 class=no-num id="acknowledgments">Acknowledgments</h2>
1249 <p>
1250 Thanks to the ideas and feedback from
1251 Tab Atkins,
1252 Arthur Barstow,
1253 Ben Callahan,
1254 <span lang="tr">Tantek Çelik</span>,
1255 Alex Danilo,
1256 Elika Etemad,
1257 Pascal Germroth,
1258 <span lang="de">Björn Höhrmann</span>,
1259 Paul Irish,
1260 <span lang="nl">Anne van Kesteren</span>,
1261 Vitor Menezes,
1262 Alex Mogilevsky,
1263 Chris Moschini,
1264 James Nurthen,
1265 Simon Pieters,
1266 <span lang="fr">Florian Rivoal</span>,
1267 <span lang="fr">Simon Sapin</span>,
1268 Nicholas Shanks,
1269 Ben Ward,
1270 Zack Weinberg,
1271 Estelle Weyl,
1272 Boris Zbarsky,
1273 and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
1275 </p>
1277 <h2 class=no-num id="references">References</h2>
1280 <h3 class="no-num" id="normative-references">Normative references</h3>
1281 <!--normative-->
1283 <h3 class="no-num" id="other-references">Other references</h3>
1284 <!--informative-->
1286 <h2 class="no-num" id="index">Index</h2>
1287 <!--index-->
1289 </body>
1290 </html>
1291 <!-- Keep this comment at the end of the file
1292 Local variables:
1293 mode: sgml
1294 sgml-declaration:"~/SGML/HTML4.decl"
1295 sgml-default-doctype-name:"html"
1296 sgml-minimize-attributes:t
1297 sgml-nofill-elements:("pre" "style" "br")
1298 sgml-live-element-indicator:t
1299 sgml-omittag:nil
1300 sgml-shorttag:nil
1301 sgml-namecase-general:t
1302 sgml-general-insert-case:lower
1303 sgml-always-quote-attributes:t
1304 sgml-indent-step:nil
1305 sgml-indent-data:t
1306 sgml-parent-document:nil
1307 sgml-exposed-tags:nil
1308 sgml-local-catalogs:nil
1309 sgml-local-ecat-files:nil
1310 End:
1311 -->
