css3-conditional/Overview.src.html@9d25222da9cb
css3-conditional/Overview.src.html
Thu, 21 Feb 2013 14:36:22 -0800
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- Tantek Çelik <tantek@cs.stanford.edu>
- date
- Thu, 21 Feb 2013 14:36:22 -0800
- changeset 7538
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[css3-conditional] add h-card h-entry
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>
34 <dt>Previous version:
35 <dd><a rel="previous" href="http://www.w3.org/TR/2012/WD-css3-conditional-20120911/">http://www.w3.org/TR/2012/WD-css3-conditional-20120911/</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:
46 <dd><a
47 href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
48 with subject line “<kbd>[[SHORTNAME]] <var>… message topic
49 …</var></kbd>”
51 <dt>Test suite:
52 <dd><a href="https://test.csswg.org/shepherd/search/spec/css3-conditional/">submitted tests</a>; no built test suite yet
54 </dl>
56 <!--copyright-->
58 <hr title="Separator for header">
59 </div>
61 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
63 <p>CSS is a language for describing the rendering of structured documents
64 (such as HTML and XML) on screen, on paper, in speech, etc.
65 <span class="p-summary">
66 This module
67 contains the features of CSS for conditional processing of parts of
68 style sheets, conditioned on capabilities of the processor or the
69 document the style sheet is being applied to.
70 It includes and extends the functionality of CSS level 2 [[!CSS21]],
71 which builds on CSS level 1 [[CSS1]].
72 The main extensions compared to level 2 are
73 allowing nesting of certain at-rules inside '@media',
74 and the addition of the '@supports'
75 rule for conditional processing.
76 </span>
78 <h2 class="no-num no-toc" id="status">Status of this document</h2>
80 <!--status-->
82 <p>The following features are at risk:
83 <ul>
84 <li>The inclusion of '@font-face' rules and
85 '@keyframes' rules as allowed within all of the @-rules in
86 this specification is at risk, though only because of the relative
87 rates of advancement of specifications. If this specification is able
88 to advance faster than one or both of the specifications defining
89 those rules, then the inclusion of those rules will move from this
90 specification to the specification defining those rules.</li>
92 <li>The addition of support for @-rules inside of conditional grouping
93 rules is at risk; if interoperable implementations are not found, it
94 may be removed to advance the other features in this specification to
95 Proposed Recommendation.</li>
97 <li>The '@supports' rule is at risk; if interoperable
98 implementations are not found, it may be removed to advance the other
99 features in this specification to Proposed Recommendation.</li>
100 </ul>
102 <p>This is a <strong>Last Call Working Draft</strong>.
103 The deadline for comments is <strong>10 January 2013</strong>.
105 <!--
107 Things to go in level 4:
109 * Create some way to put these new conditional things on an @import.
110 * The @document rule (commented out, down below).
112 -->
114 <h2 class="no-num no-toc" id="contents">Table of contents</h2>
116 <!--toc-->
118 <h2 id="introduction">Introduction</h2>
120 <h3 id="context">Background</h3>
122 <p><em>This section is not normative.</em>
124 <p>[[!CSS21]] defines one type of conditional group rule, the
125 '@media' rule, and allows only rulesets (not other @-rules)
126 inside of it. The '@media' rule provides the ability to
127 have media-specific style sheets, which is also provided by style
128 sheet linking features such as '@import' and
129 <code class="html"><link></code>. The restrictions on the contents of
130 '@media' rules made them less useful; they have forced authors
131 using CSS features involving @-rules in media-specific style sheets to
132 use separate style sheets for each medium.</p>
134 <p>This specification extends the rules for the contents of
135 conditional group rules to allow other @-rules, which enables authors
136 to combine CSS features involving @-rules with media specific style
137 sheets within a single style sheet.</p>
139 <p>This specification also defines an additional type of conditional
140 group rule, '@supports', to
141 address author and user requirements.</p>
143 <p>The '@supports' rule allows CSS to be conditioned on
144 implementation support for CSS properties and values. This rule makes
145 it much easier for authors to use new CSS features and provide good
146 fallback for implementations that do not support those features. This
147 is particularly important for CSS features that provide new layout
148 mechanisms, and for other cases where a set of related styles needs to
149 be conditioned on property support.</p>
151 <h3 id="placement">Module Interactions</h3>
153 <p>This module replaces and extends the '@media' rule
154 feature defined in [[!CSS21]] section <var>7.2.1</var> and
155 incorporates the modifications previously made non-normatively by
156 [[!MEDIAQ]] section <var>1</var>.</p>
158 <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
159 and [[!CSS3-ANIMATIONS]], but that dependency is only on the
160 assumption that those modules will advance ahead of this one. If this
161 module advances faster, then the dependency will be reversed.</p>
163 <h3 id="conventions">Document Conventions</h3>
165 <p>Conformance requirements are expressed with a combination of
166 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
167 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
168 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
169 document are to be interpreted as described in RFC 2119.
170 However, for readability, these words do not appear in all uppercase
171 letters in this specification.
173 <p>All of the text of this specification is normative except sections
174 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
176 <p>Examples in this specification are introduced with the words “for example”
177 or are set apart from the normative text with
178 <code class="html">class="example"</code>, like this:
180 <div class="example">
181 <p>This is an example of an informative example.</p>
182 </div>
184 <p>Informative notes begin with the word “Note” and are set apart from the
185 normative text with <code class="html">class="note"</code>, like this:
187 <p class="note">Note, this is an informative note.</p>
189 <h2 id="processing">Processing of conditional group rules</h2>
191 <p>This specification defines some CSS @-rules, called <dfn>conditional
192 group rules</dfn>, that associate a condition with a group of other
193 CSS rules. These different rules allow testing different types of
194 conditions, but share common behavior for how their contents are used
195 when the condition is true and when the condition is false.</p>
197 <div class="example">
198 <p>For example, this rule:</p>
199 <pre>@media print {
200 /* hide navigation controls when printing */
201 #navigation { display: none }
202 }</pre>
203 <p>causes a particular CSS rule (making elements with ID "navigation" be
204 display:none) apply only when the style sheet is used for a print
205 medium.
206 </div>
208 <p>Each conditional group rule has a condition, which at any time
209 evaluates to true or false. When the condition is true, CSS processors
210 <strong>must</strong> apply the rules inside the group rule as though
211 they were at the group rule's location; when the condition is false, CSS
212 processors <strong>must not</strong> apply any of rules inside the group
213 rule. The current state of the condition does not affect the CSS object
214 model, in which the contents of the group rule always remain within the
215 group rule.</p>
217 <p>This means that when multiple conditional group rules are nested,
218 a rule inside of both of them applies only when all of the rules'
219 conditions are true.</p>
221 <div class="example">For example, with this set of nested rules:
222 <pre>@media print { // rule (1)
223 /* hide navigation controls when printing */
224 #navigation { display: none }
225 @media (max-width: 12cm) { // rule (2)
226 /* keep notes in flow when printing to narrow pages */
227 .note { float: none }
228 }
229 }</pre>
230 the condition of the rule marked (1) is true for print media, and the
231 condition of the rule marked (2) is true when the width of the display
232 area (which for print media is the page box) is less than or equal to
233 12cm. Thus the rule ''#navigation { display: none }'' applies
234 whenever this style sheet is applied to print media, and the rule
235 ''.note { float: none }'' is applied only when the style sheet
236 is applied to print media <em>and</em> the width of the page box is less
237 than or equal to 12 centimeters.</div>
239 <p>When the condition for a conditional group rule changes, CSS
240 processors <strong>must</strong> reflect that the rules now apply or no
241 longer apply, except for properties whose definitions define effects of
242 computed values that persist past the lifetime of that value (such as
243 for some properties in [[CSS3-TRANSITIONS]] and
244 [[!CSS3-ANIMATIONS]]).</p>
246 <h2 id="contents-of">Contents of conditional group rules</h2>
248 <p>The syntax of each conditional group rule consists of some syntax
249 specific to the type of rule followed by a <dfn>group rule body</dfn>,
250 which is a block (pair of braces) containing a sequence of rules.</p>
252 <p>A group rule body is allowed to contain rulesets and any @-rules that
253 are allowed at the top level of a style sheet before and after a
254 ruleset. This means that @-rules that must occur at the beginning of
255 the style sheet (such as '@charset', '@import',
256 and '@namespace' rules) are not allowed inside of conditional group
257 rules. Conditional group rules can be nested.</p>
259 <p>In terms of the grammar, this specification defines the following
260 productions for use in the grammar of conditional group rules:</p>
262 <pre><dfn>nested_statement</dfn>
263 : ruleset | <i>media</i> | page | font_face_rule | keyframes_rule |
264 <i>supports_rule</i>
265 ;
267 <dfn>group_rule_body</dfn>
268 : '{' S* <i>nested_statement</i>* '}' S*
269 ;</pre>
270 <p>
271 in which all the productions are defined in that grammar with the
272 exception of <code>font_face_rule</code>
273 defined in [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
274 [[!CSS3-ANIMATIONS]], and <code>media</code> and <code>supports_rule</code>
275 defined in this specification.</p>
277 <p>In general, future CSS specifications that add new @-rules that are
278 not forbidden to occur after some other types of rules should modify
279 this <code>nested_statement</code> production to keep the grammar
280 accurate.</p>
282 <p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
283 conditional group rules.</p>
285 <p>CSS processors <strong>must</strong> ignore rules that are not
286 allowed within a group rule, and <strong>must</strong> handle invalid
287 rules inside of group rules as described in <a
288 href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
289 4.2 (Rules for handling parsing errors)</a>, <a
290 href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
291 (At-rules)</a>, and <a
292 href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
293 (Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>
295 <h2 id="use">Placement of conditional group rules</h2>
297 <p>Conditional group rules are allowed at the top-level of a style
298 sheet, and inside other conditional group rules. CSS processors
299 <strong>must</strong> process such rules as <a
300 href="#processing">described above</a>.</p>
302 <p>Any rules that are not allowed after a ruleset (e.g., ''@charset'',
303 ''@import'', or ''@namespace'' rules) are also not allowed after a
304 conditional group rule. Therefore, style sheets <strong>must
305 not</strong> place such rules after a conditional group rules, and CSS
306 processors <strong>must</strong> ignore such rules.</p>
308 <h2 id="at-media">Media-specific style sheets: the '@media' rule</h2>
310 <p>The <dfn>'@media' rule</dfn> is a conditional group rule whose
311 condition is a media query. It consists of the at-keyword
312 '@media' followed by a (possibly empty) media query list (as
313 defined in [[!MEDIAQ]]), followed by a group rule body. The condition
314 of the rule is the result of the media query.</p>
316 <div class="example">
317 <p>This '@media' rule:</p>
318 <pre>@media screen and (min-width: 35em),
319 print and (min-width: 40em) {
320 #section_navigation { float: left; width: 10em; }
321 }</pre>
322 <p>has the condition
323 ''screen and (min-width: 35em), print and (min-width: 40em)'',
324 which is true for screen displays
325 whose viewport is at least 35 times the initial font size
326 and for print displays
327 whose viewport is at least 40 times the initial font size.
328 When either of these is true,
329 the condition of the rule is true,
330 and the rule
331 ''#section_navigation { float: left; width: 10em; }''
332 is applied.</p>
333 </div>
335 <p>In terms of the grammar, this specification extends the
336 <code>media</code> production in the
337 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
338 ([[!CSS21]], Appendix G) into:
339 <pre><dfn>media</dfn>
340 : MEDIA_SYM S* media_query_list <i>group_rule_body</i>
341 ;</pre>
342 <p>where the <code>group_rule_body</code> production is defined in this
343 specification, the <code>media_query_list</code> production is defined
344 in [[!MEDIAQ]], and the others are defined in the <a
345 href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
346 ([[!CSS21]], Appendix G).
348 <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
350 <p>The <dfn>'@supports' rule</dfn> is a conditional group
351 rule whose condition tests whether the user agent supports CSS
352 property:value pairs. Authors can use it to write style sheets that use
353 new features when available but degrade gracefully when those features
354 are not supported. CSS has existing mechanisms for graceful
355 degradation, such as ignoring unsupported properties or values, but
356 these are not always sufficient when large groups of styles need to be
357 tied to the support for certain features, as is the case for use of new
358 layout system features.</p>
360 <p>The syntax of the condition in the '@supports' rule is
361 slightly more complicated than for the other conditional group rules
362 (though has some similarities to media queries) since:</p>
363 <ul>
364 <li>negation is needed so that the new-feature styles and the fallback
365 styles can be separated (within the forward-compatible grammar's rules
366 for the syntax of @-rules), and not required to override each other</li>
367 <li>conjunction (and) is needed so that multiple required features can
368 be tested</li>
369 <li>disjunction (or) is needed when there are multiple alternative
370 features for a set of styles, particularly when some of those
371 alternatives are vendor-prefixed properties or values</li>
372 </ul>
374 <p>Therefore, the syntax of the '@supports' rule allows
375 testing for property:value pairs, and arbitrary conjunctions (and),
376 disjunctions (or), and negations (not) of them.</p>
378 <p>This extends the lexical scanner in the
379 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
380 ([[!CSS21]], Appendix G) by adding:
381 <pre>
382 @{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn>SUPPORTS_SYM</dfn>;}
383 {O}{R} {return <dfn>OR</dfn>;}
384 </pre>
386 <p>This then extends the grammar in the
387 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>,
388 using the lexical scanner there, with the additions of
389 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and
390 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code>
391 tokens defined in the Media Queries specification [[!MEDIAQ]]
392 and the <code>OR</code> and <code>SUPPORTS_SYM</code> tokens defined above,
393 and with
394 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>,
395 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
396 and <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code>
397 productions
398 and the <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code> token
399 taken from the core syntax of CSS defined in
400 <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1 (Tokenization)</a> of [[!CSS21]],
401 by adding:</p>
403 <pre><dfn>supports_rule</dfn>
404 : <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>
405 ;
407 <dfn>supports_condition</dfn>
408 : <i>supports_negation</i> | <i>supports_conjunction</i> | <i>supports_disjunction</i> |
409 <i>supports_condition_in_parens</i>
410 ;
412 <dfn>supports_condition_in_parens</dfn>
413 : ( '(' <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> |
414 <i>general_enclosed</i>
415 ;
417 <dfn>supports_negation</dfn>
418 : <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>
419 ;
421 <dfn>supports_conjunction</dfn>
422 : <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> )+
423 ;
425 <dfn>supports_disjunction</dfn>
426 : <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> )+
427 ;
429 <dfn>supports_declaration_condition</dfn>
430 : '(' <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> ')'
431 ;
433 <dfn>general_enclosed</dfn>
434 : ( <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> )* ')'
435 ;
436 </pre>
438 <p>
439 Implementations <strong>must</strong> parse ''@supports'' rules
440 based on the above grammar,
441 and when interpreting the above grammar,
442 <strong>must</strong> match the production before an <code>|</code> operator
443 in preference to the one after it.
444 </p>
446 <p>
447 The above grammar is purposely very loose for forwards-compatibility reasons,
448 since the <code>general_enclosed</code> production
449 allows for substantial future extensibility.
450 Any ''@supports'' rule that does not parse according to the grammar above
451 (that is, a rule that does not match this loose grammar
452 which includes the <code>general_enclosed</code> production)
453 is invalid.
454 Style sheets <strong>must not</strong> use such a rule and
455 processors <strong>must</strong> ignore such a rule (including all of its contents).
457 <p>Each of these grammar terms is associated with a boolean result, as
458 follows:</p>
459 <dl>
460 <dt>supports_condition</dt>
461 <dd>
462 The result is the result of the single child term.
463 </dd>
465 <dt>supports_condition_in_parens</dt>
466 <dd>
467 The result is the result of the single <code>supports_condition</code>
468 or <code>supports_declaration_condition</code> child term.
469 </dd>
471 <dt>supports_negation</dt>
472 <dd>
473 The result is the <em>negation</em> of the result of the
474 <code>supports_condition_in_parens</code> child term.
475 </dd>
477 <dt>supports_conjunction</dt>
478 <dd>
479 The result is true if the result of <em>all</em> of the
480 <code>supports_condition_in_parens</code> child terms is true;
481 otherwise it is false.
482 </dd>
484 <dt>supports_disjunction</dt>
485 <dd>
486 The result is true if the result of <em>any</em> of the
487 <code>supports_condition_in_parens</code> child terms is true;
488 otherwise it is false.
489 </dd>
491 <dt>supports_declaration_condition</dt>
492 <dd>
493 The result is whether the CSS processor <a href="#support-definition">supports</a> the declaration
494 within the parentheses.
495 </dd>
497 <dt>general_enclosed</dt>
498 <dd>
499 The result is always false.
500 Additionally, style sheets <strong>must not</strong>
501 write ''@supports'' rules
502 that match this grammar production.
503 (In other words, this production exists only for future extensibility,
504 and is not part of the description of a valid style sheet
505 in this level of the specification.)
506 <span class="note">Note that future levels may define functions
507 or other parenthesized expressions that can evaluate to true.</span>
508 </dd>
509 </dl>
511 <p>The condition of the '@supports' rule is the result of the
512 <code>supports_condition</code> term that is a child of the
513 <code>supports_rule</code> term.</p>
515 <div class="example">
516 <p>For example, the following rule</p>
517 <pre>@supports ( display: flexbox ) {
518 body, #navigation, #content { display: flexbox; }
519 #navigation { background: blue; color: white; }
520 #article { background: white; color: black; }
521 }</pre>
522 <p>applies the rules inside the '@supports' rule only when
523 ''display: flexbox'' is supported.</p>
524 </div>
526 <div class="example">
527 <p>The following example shows an additional '@supports' rule that can
528 be used to provide an alternative for when ''display: flexbox'' is not
529 supported:</p>
530 <pre>@supports not ( display: flexbox ) {
531 body { width: 100%; height: 100%; background: white; color: black; }
532 #navigation { width: 25%; }
533 #article { width: 75%; }
534 }</pre>
535 <p>Note that the 'width' declarations may be harmful to the
536 flexbox-based layout, so it is important that they be present only in
537 the non-flexbox styles.</p>
538 </div>
540 <div class="example">
541 <p>The following example checks for support for the 'box-shadow'
542 property, including checking for support for vendor-prefixed versions of
543 it. When the support is present, it specifies both 'box-shadow' (with
544 the prefixed versions) and 'color' in a way what would cause the text to
545 become invisible were 'box-shadow' not supported.</p>
546 <pre>@supports ( box-shadow: 2px 2px 2px black ) or
547 ( -moz-box-shadow: 2px 2px 2px black ) or
548 ( -webkit-box-shadow: 2px 2px 2px black ) or
549 ( -o-box-shadow: 2px 2px 2px black ) {
550 .outline {
551 color: white;
552 -moz-box-shadow: 2px 2px 2px black;
553 -webkit-box-shadow: 2px 2px 2px black;
554 -o-box-shadow: 2px 2px 2px black;
555 box-shadow: 2px 2px 2px black; /* unprefixed last */
556 }
557 }</pre></div>
559 <p>To avoid confusion between ''and'' and ''or'', the syntax requires
560 that both ''and'' and ''or'' be specified explicitly (rather than, say,
561 using commas or spaces for one of them). Likewise, to avoid confusion
562 caused by precedence rules, the syntax does not allow ''and'', ''or'',
563 and ''not'' operators to be mixed without a layer of parentheses.</p>
565 <div class="example">
566 <p>For example, the following rule is not valid:
567 <pre class="illegal">@supports (transition-property: color) or
568 (animation-name: foo) and
569 (transform: rotate(10deg)) {
570 // ...
571 }</pre>
572 <p>Instead, authors must write one of the following:</p>
573 <pre>@supports ((transition-property: color) or
574 (animation-name: foo)) and
575 (transform: rotate(10deg)) {
576 // ...
577 }</pre>
578 <pre>@supports (transition-property: color) or
579 ((animation-name: foo) and
580 (transform: rotate(10deg))) {
581 // ...
582 }</pre>
583 </div>
585 <p>Furthermore, whitespace is required after a ''not'' and on both
586 sides of an ''and'' or ''or''.</p>
588 <p>The declaration being tested must always occur within parentheses,
589 when it is the only thing in the expression.<p>
591 <div class="example">
592 <p>For example, the following rule is not valid:
593 <pre class="illegal">@supports display: flexbox {
594 // ...
595 }</pre>
596 <p>Instead, authors must write:</p>
597 <pre>@supports (display: flexbox) {
598 // ...
599 }</pre>
600 </div>
602 <p>The syntax allows extra parentheses when they are not needed. This
603 flexibility is sometimes useful for authors (for example, when
604 commenting out parts of an expression) and may also be useful for
605 authoring tools.</p>
607 <div class="example">
608 <p>For example, authors may write:</p>
609 <pre>@supports ((display: flexbox)) {
610 // ...
611 }</pre>
612 </div>
614 <p>A trailing ''!important'' on a declaration being tested is allowed,
615 though it won't change the validity of the declaration.
617 <div class="example">
618 <p>For example, the following rule is valid:
619 <pre>@supports (display: flexbox !important) {
620 // ...
621 }</pre>
622 </div>
624 <h3 id="support-definition">Definition of support</h3>
626 <p>For forward-compatibility,
627 <a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
628 (Declarations and properties)</a> of [[!CSS21]]
629 defines rules for handling invalid properties and values.
630 CSS processors that
631 do not implement or partially implement a specification
632 <strong>must</strong> treat any part of a value that they
633 do not implement, or
634 do not have a usable level of support for,
635 as invalid according to this rule
636 for handling invalid properties and values,
637 and therefore <strong>must</strong> discard the declaration as a parse error.</p>
639 <p>A CSS processor is considered to <dfn id="dfn-support">support</dfn>
640 a declaration (consisting of a property and value) if it accepts that
641 declaration (rather than discarding it as a parse error).
642 If a processor does not implement, with a usable level of support,
643 the value given,
644 then it <strong>must not</strong>
645 accept the declaration or claim support for it.</p>
647 <p class="note">Note that properties or values
648 whose support is effectively disabled by user preferences
649 are still considered as supported by this definition.
650 For example, if a user has enabled a high-contrast mode
651 that causes colors to be overridden,
652 the CSS processor is still considered to support the 'color' property
653 even though declarations of the 'color' property may have no effect.
654 On the other hand, a developer-facing preference
655 whose purpose is to enable or disable support for an experimental CSS feature
656 does affect this definition of support.</p>
658 <p>These rules (and the equivalence between them) allow
659 authors to use fallback (either in the [[CSS1]] sense of declarations
660 that are overridden by later declarations or with the new capabilities
661 provided by the ''@supports'' rule in this specification) that works
662 correctly for the features implemented. This applies especially to
663 compound values; implementations must implement all parts of the value
664 in order to consider the declaration supported, either inside a ruleset
665 or in the declaration condition of an ''@supports'' rule.</p>
667 <!--
668 <h2 id="at-document">Document queries: the '@document' rule</h2>
670 <p>The <dfn>'@document' rule</dfn> is a conditional group
671 rule whose condition depends on the
672 <a href="#url-of-doc">URL of the document being styled</a>.
673 This allows style sheets, particularly user style sheets, to have styles
674 that only apply to a set of pages rather than to all pages using the
675 style sheet.</p>
677 <p class="issue">Given that this @-rule is intended primarily for user
678 style sheets, what should this specification say about its use in author
679 style sheets? Should it be forbidden? Should use instead be
680 discouraged? Or should this specification remain neutral on the
681 topic, since there are valid uses in author style sheets?</p>
683 <p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
684 the URI at which the document is located, excluding any fragment
685 identifiers. (This means, for example, that HTTP redirects have been
686 followed.) If the styles are being applied inside a complete document
687 embedded into the presentation of another (e.g., [[HTML5]]'s <code
688 class="html">iframe</code>, <code class="html">object</code>, or <code
689 class="html">img</code> elements), the relevant URI is that of the
690 frame, not of its container. However, if content from other documents
691 is mixed in via mechanisms that mix content from one document into
692 another (e.g., [[SVG11]]'s <code>use</code> element), then the
693 address of the container document is used.</p>
695 <p class="note">Note: In [[HTML5]], this is the
696 <a href="http://dev.w3.org/html5/spec/dom.html#documents">document's address</a>
697 of a document in a
698 <a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>
700 <div class="issue">What form of normalization is done on URLs and domains
701 before matching? In particular, this specification needs to describe:
702 <ul>
703 <li>what form is used for the <a href="#url-of-doc">URL of the document
704 being styled</a> (and what has been normalized in that form)</li>
705 <li>what normalization (if any) happens to the argument of each of the match
706 functions before the comparison that they describe and</li>
707 <li>whether the
708 comparison algorithm used is string comparison or some other URL
709 comparison algorithm.</li></ul></div>
711 <p>The '@document' rule's condition is written as a
712 comma-separated list of <dfn>URL matching functions</dfn>, and the
713 condition evaluates to true whenever any one of those functions
714 evaluates to true. The following URL matching functions are
715 permitted:</p>
717 <dl>
718 <dt><dfn id="url-exact" title="url()|URL matching functions::exact"><url></dfn></dt>
720 <dd>
721 <p>The 'url()' function is the <dfn>exact url matching
722 function</dfn>. It evaluates to true whenever the <a
723 href="#url-of-doc">URL of the document being styled</a> is exactly
724 the URL given.</p>
726 <p class="Note">The 'url()' function, since it is a core syntax
727 element in CSS, is allowed (subject to different character
728 limitations and thus escaping requirements) to contain an unquoted
729 value (in addition to the string values that are allowed as
730 arguments for all four functions).</p>
732 <div class="example">
733 <p>For example, this rule:</p>
734 <pre>@document url("http://www.w3.org/Style/CSS/") {
735 #summary { background: yellow; color: black}
736 }</pre>
737 <p>styles the <code class="html">summary</code> element on the page
738 <code>http://www.w3.org/Style/CSS/</code>, but not on any other
739 pages.</p>
740 </div>
741 </dd>
743 <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(<string>)</dfn></dt>
745 <dd>
746 <p>The 'url-prefix()' function is the <dfn>url prefix
747 matching function</dfn>. It evaluates to true whenever the
748 <a href="#url-of-doc">URL of the document being styled</a>
749 has the argument to the function as an
750 initial substring (which is true when the two strings are equal).
751 When the argument is the empty string, it evaluates to true for all
752 documents.</p>
753 <div class="example">
754 <p>For example, this rule:</p>
755 <pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
756 #summary { background: yellow; color: black}
757 }</pre>
758 <p>styles the <code class="html">summary</code> element on the page
759 <code>http://www.w3.org/Style/CSS/</code> and on the page
760 <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
761 affect the page <code>http://www.w3.org/</code> or the page
762 <code>http://www.example.com/Style/CSS/</code>.</p>
763 </div>
764 </dd>
766 <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(<string>)</dfn></dt>
768 <dd>
769 <p>The 'domain()' function is the <dfn>domain
770 matching function</dfn>. It evaluates to true whenever
771 the <a href="#url-of-doc">URL of the document being styled</a>
772 has a host subcomponent (as defined in [[!URI]])
773 and that host subcomponent is exactly the argument to the
774 'domain()' function or a final substring of the host
775 component is a period (U+002E) immediately followed by the argument
776 to the 'domain()' function.</p>
777 <div class="example">
778 <p>For example, this rule:</p>
779 <pre>@document domain("w3.org") {
780 body { font-size: 16px ! important }
781 }</pre>
782 <p>changes the font size of the body element for pages such as
783 <code>http://www.w3.org/Style/CSS/</code> and
784 <code>http://w3.org/Style/CSS/</code> and
785 <code>http://lists.w3.org/Archives/Public/www-style/</code>
786 but it does not affect the page
787 <code>http://www.example.com/Style/CSS/</code>.</p>
788 </div>
789 </dd>
791 <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(<string>)</dfn></dt>
793 <dd>
794 <p>The contents of the <string> argument <strong>must</strong>
795 match the JavaScript <code>Pattern</code> production
796 ([[!ECMA-262-5.1]], section 15.10.1). However,
797 failing to do so is not a CSS syntax error and does not trigger any
798 error handling for CSS syntax errors.</p>
800 <p>The ''regexp()'' function evaluates to true whenever the string
801 argument compiled as a JavaScript regular expression with the
802 <code>global</code>, <code>ignoreCase</code> and
803 <code>multiline</code> flags <em>disabled</em>
804 (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
805 compiles successfully and the resulting regular expression matches
806 the entirety of the
807 <a href="#url-of-doc">URL of the document being styled</a>.</p>
809 <p class="note">Note that regular expression must match the entire
810 URL, not just a part of it.</p>
812 <p class="note">Note that this definition intentionally matches the
813 behavior of the <a
814 href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
815 attribute</a> on the <code class="html">input</code> element
816 in [[HTML5]].</p>
818 <div class="example">
819 <p>For example, this rule:</p>
820 <pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
821 body { font-size: 20px ! important }
822 }</pre>
823 <p>changes the font size of the body element for pages such as
824 <code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
825 <p class="note">Note that the backslashes in the regular
826 expression require CSS escaping as ''\\''.</p>
827 </div>
828 </dd>
830 </dl>
832 <p>Implementations <strong>must</strong> treat any unknown URL matching
833 functions as a syntax error, and thus ignore the '@document' rule.
834 <span class="issue">Should we instead have more complicated error
835 handling rules to make forward-compatibility work differently, or is
836 this rule the best solution for such future expansion anyway?</span></p>
838 <div class="issue">This syntax doesn't offer any ability to do negations,
839 which has been requested in <a
840 href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
841 349813</a>. Use cases that people have wanted negations for
842 include:
843 <ul>
844 <li>User style sheets that want a particular rule in general, but know
845 that that rule does more harm than good on specific sites.</li>
846 <li>Authors who have a rule that they want to apply to most of their
847 pages, but wish to make a few exceptions for.</li>
848 </ul>
849 </div>
851 <p>This extends the lexical scanner in the
852 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
853 ([[!CSS21]], Appendix G) by adding:
854 <pre>@{D}{O}{C}{U}{M}{E}{N}{T} {return DOCUMENT_SYM;}</pre>
855 <p>and the grammar by adding</p>
856 <pre><dfn>document_rule</dfn>
857 : DOCUMENT_SYM S+ <i>url_match_fn</i> ( "," S* <i>url_match_fn</i> )* <i>group_rule_body</i>
858 ;
860 <dfn>url_match_fn</dfn>
861 : (URI | FUNCTION S* STRING S* ')' ) S*
862 ;</pre>
863 -->
866 <h2 id="apis">APIs</h2>
868 <h3 id='extentions-to-cssrule-interface'>
869 Extensions to the <code>CSSRule</code> interface</h3>
871 <p>The <code>CSSRule</code> interface is extended as follows:
873 <pre class='idl'>partial interface CSSRule {
874 const unsigned short SUPPORTS_RULE = 12;
875 <!--
876 const unsigned short DOCUMENT_RULE = 13;
877 -->
878 }</pre>
881 <h3 id='the-cssgroupingrule-interface'>
882 The <code>CSSGroupingRule</code> interface</h3>
884 <p>The <dfn><code>CSSGroupingRule</code></dfn> interface represents an at-rule that contains other rules nested inside itself.
886 <pre class='idl'>interface CSSGroupingRule : CSSRule {
887 readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
888 unsigned long insertRule (DOMString rule, unsigned long index);
889 void deleteRule (unsigned long index);
890 }</pre>
892 <dl class='idl-attributes'>
893 <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
894 <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
895 object for the list of CSS rules nested inside the grouping rule.
896 </dl>
898 <dl class='idl-methods'>
899 <dt><code>insertRule(DOMString rule, unsigned long index)</code>, returns
900 <code>unsigned long</code>
901 <dd>
902 The <code>insertRule</code> operation must
903 insert a CSS rule <var>rule</var>
904 into the CSS rule list returned by <code>cssRules</code>,
905 such that the inserted rule will be at position <var>index</var>,
906 and any rules previously at <var>index</var> or higher
907 will increase their index by one.
908 It must throw INDEX_SIZE_ERR
909 if index is greater than <code>cssRules.length</code>.
910 It must throw SYNTAX_ERR
911 if the rule has a syntax error and is unparseable;
912 this does not include syntax errors handled by error handling rules
913 for constructs inside of the rule,
914 but this does include cases where the string given
915 does not parse into a single CSS rule (such as when the string is empty)
916 or where there is anything other than whitespace or comments
917 after that single CSS rule.
918 It must throw HIERARCHY_REQUEST_ERR
919 if the rule cannot be inserted at the location specified,
920 for example, if an ''@import'' rule is inserted inside a group rule.
922 <p>The return value is the <var>index</var> parameter.
924 <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
925 <dd>
926 The <code>deleteRule</code> operation must
927 remove a CSS rule from
928 the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
929 It must throw INDEX_SIZE_ERR
930 if index is greater than or equal to <code>cssRules.length</code>.
931 </dl>
934 <h3 id="the-cssconditionrule-interface">
935 The <code>CSSConditionRule</code> interface</h3>
937 <p>The <dfn><code>CSSConditionRule</code></dfn> interface represents all the "conditional" at-rules,
938 which consist of a condition and a statement block.
940 <pre class='idl'>interface CSSConditionRule : CSSGroupingRule {
941 attribute DOMString conditionText;
942 }</pre>
944 <dl class='idl-attributes'>
946 <dt><code>conditionText</code> of type <code>DOMString</code>
947 <dd>
948 <p>The <code>conditionText</code> attribute represents
949 the condition of the rule.
950 Since what this condition does
951 varies between the derived interfaces of <code>CSSConditionRule</code>,
952 those derived interfaces
953 may specify different behavior for this attribute
954 (see, for example, <code>CSSMediaRule</code> below).
955 In the absence of such rule-specific behavior,
956 the following rules apply:</p>
958 <p>The <code>conditionText</code> attribute, on getting, must return
959 the result of serializing the associated condition.
961 <p>On setting the <code>conditionText</code> attribute these steps
962 must be run:
964 <ol>
965 <li>Trim the given value of white space.
966 <li>If the given value matches the grammar of the
967 appropriate condition production for the given rule,
968 replace the associated CSS condition with the given value.
969 <li>Otherwise, do nothing.
970 </ol>
971 </dl>
974 <h3 id="the-cssmediarule-interface">
975 The <code>CSSMediaRule</code> interface</h3>
977 <p>The <dfn><code>CSSMediaRule</code></dfn> interface represents a ''@media'' rule:
979 <pre class='idl'>interface CSSMediaRule : CSSConditionRule {
980 readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a> media;
981 }</pre>
983 <dl class='idl-attributes'>
984 <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
985 <dd>The <code>media</code> attribute must return a <code>MediaList</code> object
986 for the list of media queries specified with the ''@media'' rule.
988 <dt><code>conditionText</code> of type <code>DOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
989 <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
990 on getting, must return the value of <code>media.mediaText</code> on the rule.
992 <p>Setting the <code>conditionText</code> attribute
993 must set the <code>media.mediaText</code> attribute on the rule.
994 </dl>
997 <h3 id="the-csssupportsrule-interface">
998 The <code>CSSSupportsRule</code> interface</h3>
1000 <p>The <dfn><code>CSSSupportsRule</code></dfn> interface represents a ''@supports'' rule.</p>
1002 <pre class='idl'>interface CSSSupportsRule : CSSConditionRule {
1003 }</pre>
1005 <dl class='idl-attributes'>
1006 <dt><code>conditionText</code> of type <code>DOMString</code> (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
1007 <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
1008 on getting, must return the condition that was specified,
1009 without any logical simplifications,
1010 so that the returned condition will evaluate to the same result
1011 as the specified condition
1012 in any conformant implementation of this specification
1013 (including implementations that implement future extensions
1014 allowed by the <i>general_enclosed</i> exensibility mechanism in this specification).
1015 In other words,
1016 token stream simplifications are allowed
1017 (such as reducing whitespace to a single space
1018 or omitting it in cases where it is known to be optional),
1019 but logical simplifications (such as removal of unneeded parentheses,
1020 or simplification based on evaluating results) are not allowed.
1022 </dl>
1024 <!--
1025 <h3 id="the-cssdocumentrule-interface">
1026 The <code>CSSDocumentRule</code> interface</h3>
1028 <p>The <dfn><code>CSSDocumentRule</code></dfn> interface represents a ''@document'' rule.</p>
1030 <pre class='idl'>interface CSSDocumentRule : CSSConditionRule {
1031 }</pre>
1032 -->
1035 <h3 id='the-css-interface'>
1036 The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>
1038 <p>The <dfn id='CSS-interface'><code>CSS</code></dfn> interface holds useful CSS-related functions that do not belong elsewhere.
1040 <pre class='idl'>interface CSS {
1041 static boolean supports(DOMString property, DOMString value);
1042 static boolean supports(DOMString conditionText);
1043 }</pre>
1045 <dl class='idl-methods'>
1046 <dt><code>supports(DOMString property, DOMString value)</code>,
1047 returns <code>boolean</code>
1048 <dt><code>supports(DOMString conditionText)</code>,
1049 returns <code>boolean</code>
1050 <dd>
1051 When the <code title=''>supports()</code> method is invoked with two arguments <var>property</var> and <var>value</var>,
1052 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,
1053 and <var>value</var> would be successfully parsed as a supported value for that property.
1054 (Literal match means that no CSS escape processing is performed,
1055 and leading and trailing whitespace are not stripped,
1056 so any leading whitespace, trailing whitespace,
1057 or CSS escapes equivalent to the name of a property
1058 would cause the method to return <code>false</code>.)
1059 Otherwise, it must return <code>false</code>.
1061 <p>
1062 When invoked with a single <var>conditionText</var> argument,
1063 it must return <code>true</code> if <var>conditionText</var>,
1064 when parsed and evaluated as a <code>supports_condition</code>,
1065 would return true.
1066 Otherwise, it must return <code>false</code>.
1067 </dl>
1070 <h2 class=no-num id="grammar">Grammar</h2>
1072 <p>In order to allow these new @-rules in CSS style sheets, this
1073 specification modifies the <code>stylesheet</code> production in the <a
1074 href="http://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
1075 [[!CSS21]] by replacing the <code>media</code> production defined in
1076 [[!CSS21]] with the <code>media</code> production defined in this one,
1077 and additionally inserting <code>| supports_rule</code>
1078 alongside <code>ruleset | media | page</code>.</p>
1081 <h2 id="conformance">Conformance</h2>
1083 <h3 id="base-modules">Base Modules</h3>
1085 <p>This specification defines conformance in terms of base modules,
1086 which are modules that this specification builds on top of. The base
1087 modules of this module are:</p>
1089 <ul>
1090 <li>[[!CSS21]]</li>
1091 </ul>
1093 <p>All of the conformance requirements of all base modules are
1094 incorporated as conformance requirements of this module, except where
1095 overridden by this module.</p>
1097 <p>Additionally, all conformance requirements related to validity of
1098 syntax in this module and all of its base modules are to be interpreted
1099 as though all syntax in all of those modules is valid.</p>
1101 <div class="example"><p>For example, this means that grammar presented
1102 in modules other than [[!CSS21]] must obey the requirements that
1103 [[!CSS21]] defines for the parsing of properties, and that requirements
1104 for handling invalid syntax in [[!CSS21]] do not treat syntax added by
1105 other modules as invalid.</p></div>
1107 <p>Additionally, the set of valid syntax can be increased by the
1108 conformance of a style sheet or processor to additional modules; use of
1109 such syntax does not make a style sheet nonconformant and failure to
1110 treat such syntax as invalid does not make a processor
1111 nonconformant.</p>
1113 <h3 id="conformance-classes">Conformance Classes</h3>
1115 <p>Conformance to the CSS Conditional Rules Module is defined for three
1116 conformance classes:
1117 <dl>
1118 <dt><dfn title="conformance::style sheet" id="conform-style-sheet">style sheet</dfn>
1119 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
1120 style sheet</a>.</dd>
1121 <dt><dfn title="conformance::processor" id="conform-processor">processor</dfn></dt>
1122 <dd>A tool that reads CSS style sheets: it may be a renderer or
1123 <a
1124 href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user-agent</a>
1125 that interprets the semantics of a style sheet and renders
1126 documents that use style sheets, or it may be a validator that
1127 checks style sheets.</dd>
1128 <dt><dfn title="conformance::authoring tool" id="conform-authoring-tool">authoring tool</dfn></dt>
1129 <dd>A tool that writes a style sheet.</dd>
1130 </dl>
1132 <p>A style sheet is conformant to the CSS Conditional Rules Module
1133 if it meets all of the conformance requirements in the module that are
1134 described as requirements of style sheets.</p>
1136 <p>A processor is conformant to the CSS Conditional Rules Module if it
1137 meets all applicable conformance requirements in the module that are
1138 described as requirements of processors. In general, all requirements
1139 are applicable to renderers. Requirements concerning a part of CSS
1140 not performed by a processor are not applicable, e.g., requirements
1141 related to rendering are not applicable to a validator. The inability
1142 of a processor to correctly render a document due to limitations of
1143 the device does not make it non-conformant. (For example, a renderer
1144 is not required to render color on a monochrome monitor.)</p>
1146 <p>An authoring tool is conformant to the CSS Conditional Rules Module
1147 if it writes style sheets that conform to the module and (if it reads
1148 CSS) it is a conformant processor.</p>
1150 <h3 id="partial">
1151 Partial Implementations</h3>
1153 <p>So that authors can exploit the forward-compatible parsing rules to
1154 assign fallback values, CSS renderers <strong>must</strong>
1155 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
1156 as appropriate</a>) any at-rules, properties, property values, keywords,
1157 and other syntactic constructs for which they have no usable level of
1158 support. In particular, user agents <strong>must not</strong> selectively
1159 ignore unsupported component values and honor supported values in a single
1160 multi-value property declaration: if any value is considered invalid
1161 (as unsupported values must be), CSS requires that the entire declaration
1162 be ignored.</p>
1164 <h3 id="experimental">Experimental Implementations</h3>
1166 <p>To avoid clashes with future CSS features, the CSS specifications
1167 reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
1168 syntax</a> for proprietary property and value extensions to CSS. The CSS
1169 Working Group recommends that experimental implementations of features in
1170 CSS Working Drafts also use vendor-prefixed property or value names. This
1171 avoids any incompatibilities with future changes in the draft. Once a
1172 specification reaches the Candidate Recommendation stage, implementors
1173 should implement the non-prefixed syntax for any feature they consider to
1174 be correctly implemented according to spec.</p>
1176 <h3 id="cr-exit-criteria">CR Exit Criteria</h3>
1178 <p>For this specification to be advanced to Proposed Recommendation,
1179 there must be at least two independent, interoperable implementations
1180 of each feature. Each feature may be implemented by a different set of
1181 products, there is no requirement that all features be implemented by
1182 a single product. For the purposes of this criterion, we define the
1183 following terms:
1185 <dl>
1186 <dt>independent <dd>each implementation must be developed by a
1187 different party and cannot share, reuse, or derive from code
1188 used by another qualifying implementation. Sections of code that
1189 have no bearing on the implementation of this specification are
1190 exempt from this requirement.
1192 <dt>interoperable <dd>passing the respective test case(s) in the
1193 official CSS test suite, or, if the implementation is not a Web
1194 browser, an equivalent test. Every relevant test in the test
1195 suite should have an equivalent test created if such a user
1196 agent (UA) is to be used to claim interoperability. In addition
1197 if such a UA is to be used to claim interoperability, then there
1198 must one or more additional UAs which can also pass those
1199 equivalent tests in the same way for the purpose of
1200 interoperability. The equivalent tests must be made publicly
1201 available for the purposes of peer review.
1203 <dt>implementation <dd>a user agent which:
1205 <ol class=inline>
1206 <li>implements the specification.
1208 <li>is available to the general public. The implementation may
1209 be a shipping product or other publicly available version
1210 (i.e., beta version, preview release, or “nightly build”).
1211 Non-shipping product releases must have implemented the
1212 feature(s) for a period of at least one month in order to
1213 demonstrate stability.
1215 <li>is not experimental (i.e., a version specifically designed
1216 to pass the test suite and is not intended for normal usage
1217 going forward).
1218 </ol>
1219 </dl>
1221 <p>The specification will remain Candidate Recommendation for at least
1222 six months.
1224 <h2 id="changes">
1225 Changes</h2>
1227 <p>The following (non-editorial) changes were made to this specification since the
1228 <a href="http://www.w3.org/TR/2012/WD-css3-conditional-20120911/">11 September 2012 Working Draft</a>:
1230 <ul>
1231 <li>Removed ''@document'' rule; it has been deferred to Level 4.
1232 <li>Allow functional notation in ''@supports'' queries to be valid (to allow for future extensions),
1233 but treat such notations as always being false.
1234 <li>Corrected the grammar as follows:
1235 <pre>
1236 - : SUPPORTS_SYM S+ supports_condition group_rule_body
1237 + : SUPPORTS_SYM S* supports_condition group_rule_body
1238 </pre>
1239 <pre>
1240 - : (URI | FUNCTION) S*
1241 + : (URI | FUNCTION S* STRING S* ')' ) S*
1242 </pre>
1243 <li>Switched "and", "or", and "not" keywords to use appropriate productions rather than literals.
1244 <li>Clarified definition of support used for interpreting ''@support'' rules
1245 and its relationship to CSS forwards-compatible parsing rules.
1246 </ul>
1248 <h2 class=no-num id="acknowledgments">Acknowledgments</h2>
1250 <p>
1251 Thanks to the ideas and feedback from
1252 Tab Atkins,
1253 Arthur Barstow,
1254 Ben Callahan,
1255 <span lang="tr">Tantek Çelik</span>,
1256 Alex Danilo,
1257 Elika Etemad,
1258 Pascal Germroth,
1259 <span lang="de">Björn Höhrmann</span>,
1260 Paul Irish,
1261 <span lang="nl">Anne van Kesteren</span>,
1262 Vitor Menezes,
1263 Alex Mogilevsky,
1264 Chris Moschini,
1265 James Nurthen,
1266 Simon Pieters,
1267 <span lang="fr">Florian Rivoal</span>,
1268 <span lang="fr">Simon Sapin</span>,
1269 Nicholas Shanks,
1270 Ben Ward,
1271 Zack Weinberg,
1272 Estelle Weyl,
1273 Boris Zbarsky,
1274 and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
1276 </p>
1278 <h2 class=no-num id="references">References</h2>
1281 <h3 class="no-num" id="normative-references">Normative references</h3>
1282 <!--normative-->
1284 <h3 class="no-num" id="other-references">Other references</h3>
1285 <!--informative-->
1287 <h2 class="no-num" id="index">Index</h2>
1288 <!--index-->
1290 </body>
1291 </html>
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