css3-conditional/Overview.src.html@139db90ee7b2
css3-conditional/Overview.src.html
Thu, 07 Feb 2013 17:53:44 -0800
- author
- L. David Baron <dbaron@dbaron.org>
- date
- Thu, 07 Feb 2013 17:53:44 -0800
- changeset 7393
- 139db90ee7b2
- parent 7392
- 37491845e530
- child 7412
- 3b40faa91133
- permissions
- -rw-r--r--
[css3-conditional] Describe the AND and NOT tokens closer to the OR and SUPPORTS_SYM tokens, and make sure all the tokens are hyperlinked.
Addresses http://lists.w3.org/Archives/Public/www-style/2013Feb/0236.html
which is Issue 9 (an editorial issue) in the last call disposition of comments.
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>
16 <div class="head">
17 <!--logo-->
19 <h1>CSS Conditional Rules Module Level 3</h1>
21 <h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
22 <dl>
23 <dt>This version:
24 <dd><a href="[VERSION]">
25 http://www.w3.org/TR/[YEAR]/ED-css3-conditional-[CDATE]/</a>
27 <dt>Latest version:
28 <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>
30 <dt>Editor's draft:
31 <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
33 <dt>Previous version:
34 <dd><a href="http://www.w3.org/TR/2012/WD-css3-conditional-20120911/">http://www.w3.org/TR/2012/WD-css3-conditional-20120911/</a></dd>
36 <dt>Editors:
37 <dd class=vcard><a class=fn href="http://dbaron.org/">L. David Baron</a>,
38 <a class=org href="http://www.mozilla.org/">Mozilla</a>
40 <dt>Issues list:
41 <dd>Maintained in document (only editor's draft is current)
43 <dt>Feedback:
45 <dd><a
46 href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
47 with subject line “<kbd>[[SHORTNAME]] <var>… message topic
48 …</var></kbd>”
50 <dt>Test suite:
51 <dd><a href="https://test.csswg.org/shepherd/search/spec/css3-conditional/">submitted tests</a>; no built test suite yet
53 </dl>
55 <!--copyright-->
57 <hr title="Separator for header">
58 </div>
60 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
62 <p>CSS is a language for describing the rendering of structured documents
63 (such as HTML and XML) on screen, on paper, in speech, etc. This module
64 contains the features of CSS for conditional processing of parts of
65 style sheets, conditioned on capabilities of the processor or the
66 document the style sheet is being applied to.
67 It includes and extends the functionality of CSS level 2 [[!CSS21]],
68 which builds on CSS level 1 [[CSS1]].
69 The main extensions compared to level 2 are
70 allowing nesting of certain at-rules inside '@media',
71 and the addition of the '@supports'
72 rule for conditional processing.
74 <h2 class="no-num no-toc" id="status">Status of this document</h2>
76 <!--status-->
78 <p>The following features are at risk:
79 <ul>
80 <li>The inclusion of '@font-face' rules and
81 '@keyframes' rules as allowed within all of the @-rules in
82 this specification is at risk, though only because of the relative
83 rates of advancement of specifications. If this specification is able
84 to advance faster than one or both of the specifications defining
85 those rules, then the inclusion of those rules will move from this
86 specification to the specification defining those rules.</li>
88 <li>The addition of support for @-rules inside of conditional grouping
89 rules is at risk; if interoperable implementations are not found, it
90 may be removed to advance the other features in this specification to
91 Proposed Recommendation.</li>
93 <li>The '@supports' rule is at risk; if interoperable
94 implementations are not found, it may be removed to advance the other
95 features in this specification to Proposed Recommendation.</li>
96 </ul>
98 <p>This is a <strong>Last Call Working Draft</strong>.
99 The deadline for comments is <strong>10 January 2013</strong>.
101 <!--
103 Things to go in level 4:
105 * Create some way to put these new conditional things on an @import.
106 * The @document rule (commented out, down below).
108 -->
110 <h2 class="no-num no-toc" id="contents">Table of contents</h2>
112 <!--toc-->
114 <h2 id="introduction">Introduction</h2>
116 <h3 id="context">Background</h3>
118 <p><em>This section is not normative.</em>
120 <p>[[!CSS21]] defines one type of conditional group rule, the
121 '@media' rule, and allows only rulesets (not other @-rules)
122 inside of it. The '@media' rule provides the ability to
123 have media-specific style sheets, which is also provided by style
124 sheet linking features such as '@import' and
125 <code class="html"><link></code>. The restrictions on the contents of
126 '@media' rules made them less useful; they have forced authors
127 using CSS features involving @-rules in media-specific style sheets to
128 use separate style sheets for each medium.</p>
130 <p>This specification extends the rules for the contents of
131 conditional group rules to allow other @-rules, which enables authors
132 to combine CSS features involving @-rules with media specific style
133 sheets within a single style sheet.</p>
135 <p>This specification also defines an additional type of conditional
136 group rule, '@supports', to
137 address author and user requirements.</p>
139 <p>The '@supports' rule allows CSS to be conditioned on
140 implementation support for CSS properties and values. This rule makes
141 it much easier for authors to use new CSS features and provide good
142 fallback for implementations that do not support those features. This
143 is particularly important for CSS features that provide new layout
144 mechanisms, and for other cases where a set of related styles needs to
145 be conditioned on property support.</p>
147 <h3 id="placement">Module Interactions</h3>
149 <p>This module replaces and extends the '@media' rule
150 feature defined in [[!CSS21]] section <var>7.2.1</var> and
151 incorporates the modifications previously made non-normatively by
152 [[!MEDIAQ]] section <var>1</var>.</p>
154 <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
155 and [[!CSS3-ANIMATIONS]], but that dependency is only on the
156 assumption that those modules will advance ahead of this one. If this
157 module advances faster, then the dependency will be reversed.</p>
159 <h3 id="conventions">Document Conventions</h3>
161 <p>Conformance requirements are expressed with a combination of
162 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
163 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
164 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
165 document are to be interpreted as described in RFC 2119.
166 However, for readability, these words do not appear in all uppercase
167 letters in this specification.
169 <p>All of the text of this specification is normative except sections
170 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
172 <p>Examples in this specification are introduced with the words “for example”
173 or are set apart from the normative text with
174 <code class="html">class="example"</code>, like this:
176 <div class="example">
177 <p>This is an example of an informative example.</p>
178 </div>
180 <p>Informative notes begin with the word “Note” and are set apart from the
181 normative text with <code class="html">class="note"</code>, like this:
183 <p class="note">Note, this is an informative note.</p>
185 <h2 id="processing">Processing of conditional group rules</h2>
187 <p>This specification defines some CSS @-rules, called <dfn>conditional
188 group rules</dfn>, that associate a condition with a group of other
189 CSS rules. These different rules allow testing different types of
190 conditions, but share common behavior for how their contents are used
191 when the condition is true and when the condition is false.</p>
193 <div class="example">
194 <p>For example, this rule:</p>
195 <pre>@media print {
196 /* hide navigation controls when printing */
197 #navigation { display: none }
198 }</pre>
199 <p>causes a particular CSS rule (making elements with ID "navigation" be
200 display:none) apply only when the style sheet is used for a print
201 medium.
202 </div>
204 <p>Each conditional group rule has a condition, which at any time
205 evaluates to true or false. When the condition is true, CSS processors
206 <strong>must</strong> apply the rules inside the group rule as though
207 they were at the group rule's location; when the condition is false, CSS
208 processors <strong>must not</strong> apply any of rules inside the group
209 rule. The current state of the condition does not affect the CSS object
210 model, in which the contents of the group rule always remain within the
211 group rule.</p>
213 <p>This means that when multiple conditional group rules are nested,
214 a rule inside of both of them applies only when all of the rules'
215 conditions are true.</p>
217 <div class="example">For example, with this set of nested rules:
218 <pre>@media print { // rule (1)
219 /* hide navigation controls when printing */
220 #navigation { display: none }
221 @media (max-width: 12cm) { // rule (2)
222 /* keep notes in flow when printing to narrow pages */
223 .note { float: none }
224 }
225 }</pre>
226 the condition of the rule marked (1) is true for print media, and the
227 condition of the rule marked (2) is true when the width of the display
228 area (which for print media is the page box) is less than or equal to
229 12cm. Thus the rule ''#navigation { display: none }'' applies
230 whenever this style sheet is applied to print media, and the rule
231 ''.note { float: none }'' is applied only when the style sheet
232 is applied to print media <em>and</em> the width of the page box is less
233 than or equal to 12 centimeters.</div>
235 <p>When the condition for a conditional group rule changes, CSS
236 processors <strong>must</strong> reflect that the rules now apply or no
237 longer apply, except for properties whose definitions define effects of
238 computed values that persist past the lifetime of that value (such as
239 for some properties in [[CSS3-TRANSITIONS]] and
240 [[!CSS3-ANIMATIONS]]).</p>
242 <h2 id="contents-of">Contents of conditional group rules</h2>
244 <p>The syntax of each conditional group rule consists of some syntax
245 specific to the type of rule followed by a <dfn>group rule body</dfn>,
246 which is a block (pair of braces) containing a sequence of rules.</p>
248 <p>A group rule body is allowed to contain rulesets and any @-rules that
249 are allowed at the top level of a style sheet before and after a
250 ruleset. This means that @-rules that must occur at the beginning of
251 the style sheet (such as '@charset', '@import',
252 and '@namespace' rules) are not allowed inside of conditional group
253 rules. Conditional group rules can be nested.</p>
255 <p>In terms of the grammar, this specification defines the following
256 productions for use in the grammar of conditional group rules:</p>
258 <pre><dfn>nested_statement</dfn>
259 : ruleset | <i>media</i> | page | font_face_rule | keyframes_rule |
260 <i>supports_rule</i>
261 ;
263 <dfn>group_rule_body</dfn>
264 : '{' S* <i>nested_statement</i>* '}' S*
265 ;</pre>
266 <p>
267 in which all the productions are defined in that grammar with the
268 exception of <code>font_face_rule</code>
269 defined in [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
270 [[!CSS3-ANIMATIONS]], and <code>media</code> and <code>supports_rule</code>
271 defined in this specification.</p>
273 <p>In general, future CSS specifications that add new @-rules that are
274 not forbidden to occur after some other types of rules should modify
275 this <code>nested_statement</code> production to keep the grammar
276 accurate.</p>
278 <p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
279 conditional group rules.</p>
281 <p>CSS processors <strong>must</strong> ignore rules that are not
282 allowed within a group rule, and <strong>must</strong> handle invalid
283 rules inside of group rules as described in <a
284 href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
285 4.2 (Rules for handling parsing errors)</a>, <a
286 href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
287 (At-rules)</a>, and <a
288 href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
289 (Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>
291 <h2 id="use">Placement of conditional group rules</h2>
293 <p>Conditional group rules are allowed at the top-level of a style
294 sheet, and inside other conditional group rules. CSS processors
295 <strong>must</strong> process such rules as <a
296 href="#processing">described above</a>.</p>
298 <p>Any rules that are not allowed after a ruleset (e.g., ''@charset'',
299 ''@import'', or ''@namespace'' rules) are also not allowed after a
300 conditional group rule. Therefore, style sheets <strong>must
301 not</strong> place such rules after a conditional group rules, and CSS
302 processors <strong>must</strong> ignore such rules.</p>
304 <h2 id="at-media">Media-specific style sheets: the '@media' rule</h2>
306 <p>The <dfn>'@media' rule</dfn> is a conditional group rule whose
307 condition is a media query. It consists of the at-keyword
308 '@media' followed by a (possibly empty) media query list (as
309 defined in [[!MEDIAQ]]), followed by a group rule body. The condition
310 of the rule is the result of the media query.</p>
312 <div class="example">
313 <p>This '@media' rule:</p>
314 <pre>@media screen and (min-width: 35em),
315 print and (min-width: 40em) {
316 #section_navigation { float: left; width: 10em; }
317 }</pre>
318 <p>has the condition
319 ''screen and (min-width: 35em), print and (min-width: 40em)'',
320 which is true for screen displays
321 whose viewport is at least 35 times the initial font size
322 and for print displays
323 whose viewport is at least 40 times the initial font size.
324 When either of these is true,
325 the condition of the rule is true,
326 and the rule
327 ''#section_navigation { float: left; width: 10em; }''
328 is applied.</p>
329 </div>
331 <p>In terms of the grammar, this specification extends the
332 <code>media</code> production in the
333 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
334 ([[!CSS21]], Appendix G) into:
335 <pre><dfn>media</dfn>
336 : MEDIA_SYM S* media_query_list <i>group_rule_body</i>
337 ;</pre>
338 <p>where the <code>group_rule_body</code> production is defined in this
339 specification, the <code>media_query_list</code> production is defined
340 in [[!MEDIAQ]], and the others are defined in the <a
341 href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
342 ([[!CSS21]], Appendix G).
344 <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
346 <p>The <dfn>'@supports' rule</dfn> is a conditional group
347 rule whose condition tests whether the user agent supports CSS
348 property:value pairs. Authors can use it to write style sheets that use
349 new features when available but degrade gracefully when those features
350 are not supported. CSS has existing mechanisms for graceful
351 degradation, such as ignoring unsupported properties or values, but
352 these are not always sufficient when large groups of styles need to be
353 tied to the support for certain features, as is the case for use of new
354 layout system features.</p>
356 <p>The syntax of the condition in the '@supports' rule is
357 slightly more complicated than for the other conditional group rules
358 (though has some similarities to media queries) since:</p>
359 <ul>
360 <li>negation is needed so that the new-feature styles and the fallback
361 styles can be separated (within the forward-compatible grammar's rules
362 for the syntax of @-rules), and not required to override each other</li>
363 <li>conjunction (and) is needed so that multiple required features can
364 be tested</li>
365 <li>disjunction (or) is needed when there are multiple alternative
366 features for a set of styles, particularly when some of those
367 alternatives are vendor-prefixed properties or values</li>
368 </ul>
370 <p>Therefore, the syntax of the '@supports' rule allows
371 testing for property:value pairs, and arbitrary conjunctions (and),
372 disjunctions (or), and negations (not) of them.</p>
374 <p>This extends the lexical scanner in the
375 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
376 ([[!CSS21]], Appendix G) by adding:
377 <pre>
378 @{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn>SUPPORTS_SYM</dfn>;}
379 {O}{R} {return <dfn>OR</dfn>;}
380 </pre>
382 <p>This then extends the grammar in the
383 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>,
384 using the lexical scanner there, with the additions of
385 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and
386 <code><a href="http://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code>
387 tokens defined in the Media Queries specification [[!MEDIAQ]]
388 and the <code>OR</code> and <code>SUPPORTS_SYM</code> tokens defined above,
389 and with
390 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>,
391 <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
392 and <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code>
393 productions
394 and the <code><a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code> token
395 taken from the core syntax of CSS defined in
396 <a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1 (Tokenization)</a> of [[!CSS21]],
397 by adding:</p>
399 <pre><dfn>supports_rule</dfn>
400 : <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>
401 ;
403 <dfn>supports_condition</dfn>
404 : <i>supports_negation</i> | <i>supports_conjunction</i> | <i>supports_disjunction</i> |
405 <i>supports_condition_in_parens</i>
406 ;
408 <dfn>supports_condition_in_parens</dfn>
409 : ( '(' <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> |
410 <i>general_enclosed</i>
411 ;
413 <dfn>supports_negation</dfn>
414 : <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>
415 ;
417 <dfn>supports_conjunction</dfn>
418 : <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> )+
419 ;
421 <dfn>supports_disjunction</dfn>
422 : <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> )+
423 ;
425 <dfn>supports_declaration_condition</dfn>
426 : '(' <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> ')'
427 ;
429 <dfn>general_enclosed</dfn>
430 : ( <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> )* ')'
431 ;
432 </pre>
434 <p>
435 Implementations <strong>must</strong> parse ''@supports'' rules
436 based on the above grammar,
437 and when interpreting the above grammar,
438 <strong>must</strong> match the production before an <code>|</code> operator
439 in preference to the one after it.
440 </p>
442 <p>
443 The above grammar is purposely very loose for forwards-compatibility reasons,
444 since the <code>general_enclosed</code> production
445 allows for substantial future extensibility.
446 Any ''@supports'' rule that does not parse according to the grammar above
447 (that is, a rule that does not match this loose grammar
448 which includes the <code>general_enclosed</code> production)
449 is invalid.
450 Style sheets <strong>must not</strong> use such a rule and
451 processors <strong>must</strong> ignore such a rule (including all of its contents).
453 <p>Each of these grammar terms is associated with a boolean result, as
454 follows:</p>
455 <dl>
456 <dt>supports_condition</dt>
457 <dd>
458 The result is the result of the single child term.
459 </dd>
461 <dt>supports_condition_in_parens</dt>
462 <dd>
463 The result is the result of the single <code>supports_condition</code>
464 or <code>supports_declaration_condition</code> child term.
465 </dd>
467 <dt>supports_negation</dt>
468 <dd>
469 The result is the <em>negation</em> of the result of the
470 <code>supports_condition_in_parens</code> child term.
471 </dd>
473 <dt>supports_conjunction</dt>
474 <dd>
475 The result is true if the result of <em>all</em> of the
476 <code>supports_condition_in_parens</code> child terms is true;
477 otherwise it is false.
478 </dd>
480 <dt>supports_disjunction</dt>
481 <dd>
482 The result is true if the result of <em>any</em> of the
483 <code>supports_condition_in_parens</code> child terms is true;
484 otherwise it is false.
485 </dd>
487 <dt>supports_declaration_condition</dt>
488 <dd>
489 The result is whether the CSS processor <a href="#support-definition">supports</a> the declaration
490 within the parentheses.
491 </dd>
493 <dt>general_enclosed</dt>
494 <dd>
495 The result is always false.
496 Additionally, style sheets <strong>must not</strong>
497 write ''@supports'' rules
498 that match this grammar production.
499 (In other words, this production exists only for future extensibility,
500 and is not part of the description of a valid style sheet
501 in this level of the specification.)
502 <span class="note">Note that future levels may define functions
503 or other parenthesized expressions that can evaluate to true.</span>
504 </dd>
505 </dl>
507 <p>The condition of the '@supports' rule is the result of the
508 <code>supports_condition</code> term that is a child of the
509 <code>supports_rule</code> term.</p>
511 <div class="example">
512 <p>For example, the following rule</p>
513 <pre>@supports ( display: flexbox ) {
514 body, #navigation, #content { display: flexbox; }
515 #navigation { background: blue; color: white; }
516 #article { background: white; color: black; }
517 }</pre>
518 <p>applies the rules inside the '@supports' rule only when
519 ''display: flexbox'' is supported.</p>
520 </div>
522 <div class="example">
523 <p>The following example shows an additional '@supports' rule that can
524 be used to provide an alternative for when ''display: flexbox'' is not
525 supported:</p>
526 <pre>@supports not ( display: flexbox ) {
527 body { width: 100%; height: 100%; background: white; color: black; }
528 #navigation { width: 25%; }
529 #article { width: 75%; }
530 }</pre>
531 <p>Note that the 'width' declarations may be harmful to the
532 flexbox-based layout, so it is important that they be present only in
533 the non-flexbox styles.</p>
534 </div>
536 <div class="example">
537 <p>The following example checks for support for the 'box-shadow'
538 property, including checking for support for vendor-prefixed versions of
539 it. When the support is present, it specifies both 'box-shadow' (with
540 the prefixed versions) and 'color' in a way what would cause the text to
541 become invisible were 'box-shadow' not supported.</p>
542 <pre>@supports ( box-shadow: 2px 2px 2px black ) or
543 ( -moz-box-shadow: 2px 2px 2px black ) or
544 ( -webkit-box-shadow: 2px 2px 2px black ) or
545 ( -o-box-shadow: 2px 2px 2px black ) {
546 .outline {
547 color: white;
548 -moz-box-shadow: 2px 2px 2px black;
549 -webkit-box-shadow: 2px 2px 2px black;
550 -o-box-shadow: 2px 2px 2px black;
551 box-shadow: 2px 2px 2px black; /* unprefixed last */
552 }
553 }</pre></div>
555 <p>To avoid confusion between ''and'' and ''or'', the syntax requires
556 that both ''and'' and ''or'' be specified explicitly (rather than, say,
557 using commas or spaces for one of them). Likewise, to avoid confusion
558 caused by precedence rules, the syntax does not allow ''and'', ''or'',
559 and ''not'' operators to be mixed without a layer of parentheses.</p>
561 <div class="example">
562 <p>For example, the following rule is not valid:
563 <pre class="illegal">@supports (transition-property: color) or
564 (animation-name: foo) and
565 (transform: rotate(10deg)) {
566 // ...
567 }</pre>
568 <p>Instead, authors must write one of the following:</p>
569 <pre>@supports ((transition-property: color) or
570 (animation-name: foo)) and
571 (transform: rotate(10deg)) {
572 // ...
573 }</pre>
574 <pre>@supports (transition-property: color) or
575 ((animation-name: foo) and
576 (transform: rotate(10deg))) {
577 // ...
578 }</pre>
579 </div>
581 <p>Furthermore, whitespace is required after a ''not'' and on both
582 sides of an ''and'' or ''or''.</p>
584 <p>The declaration being tested must always occur within parentheses,
585 when it is the only thing in the expression.<p>
587 <div class="example">
588 <p>For example, the following rule is not valid:
589 <pre class="illegal">@supports display: flexbox {
590 // ...
591 }</pre>
592 <p>Instead, authors must write:</p>
593 <pre>@supports (display: flexbox) {
594 // ...
595 }</pre>
596 </div>
598 <p>The syntax allows extra parentheses when they are not needed. This
599 flexibility is sometimes useful for authors (for example, when
600 commenting out parts of an expression) and may also be useful for
601 authoring tools.</p>
603 <div class="example">
604 <p>For example, authors may write:</p>
605 <pre>@supports ((display: flexbox)) {
606 // ...
607 }</pre>
608 </div>
610 <p>A trailing ''!important'' on a declaration being tested is allowed,
611 though it won't change the validity of the declaration.
613 <div class="example">
614 <p>For example, the following rule is valid:
615 <pre>@supports (display: flexbox !important) {
616 // ...
617 }</pre>
618 </div>
620 <h3 id="support-definition">Definition of support</h3>
622 <p>For forward-compatibility,
623 <a href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
624 (Declarations and properties)</a> of [[!CSS21]]
625 defines rules for handling invalid properties and values.
626 CSS processors that
627 do not implement or partially implement a specification
628 <strong>must</strong> treat any part of a value that they
629 do not implement, or
630 do not have a usable level of support for,
631 as invalid according to this rule
632 for handling invalid properties and values,
633 and therefore <strong>must</strong> discard the declaration as a parse error.</p>
635 <p>A CSS processor is considered to <dfn id="dfn-support">support</dfn>
636 a declaration (consisting of a property and value) if it accepts that
637 declaration (rather than discarding it as a parse error).
638 If a processor does not implement, with a usable level of support,
639 the value given,
640 then it <strong>must not</strong>
641 accept the declaration or claim support for it.</p>
643 <p class="note">Note that properties or values
644 whose support is effectively disabled by user preferences
645 are still considered as supported by this definition.
646 For example, if a user has enabled a high-contrast mode
647 that causes colors to be overridden,
648 the CSS processor is still considered to support the 'color' property
649 even though declarations of the 'color' property may have no effect.
650 On the other hand, a developer-facing preference
651 whose purpose is to enable or disable support for an experimental CSS feature
652 does affect this definition of support.</p>
654 <p>These rules (and the equivalence between them) allow
655 authors to use fallback (either in the [[CSS1]] sense of declarations
656 that are overridden by later declarations or with the new capabilities
657 provided by the ''@supports'' rule in this specification) that works
658 correctly for the features implemented. This applies especially to
659 compound values; implementations must implement all parts of the value
660 in order to consider the declaration supported, either inside a ruleset
661 or in the declaration condition of an ''@supports'' rule.</p>
663 <!--
664 <h2 id="at-document">Document queries: the '@document' rule</h2>
666 <p>The <dfn>'@document' rule</dfn> is a conditional group
667 rule whose condition depends on the
668 <a href="#url-of-doc">URL of the document being styled</a>.
669 This allows style sheets, particularly user style sheets, to have styles
670 that only apply to a set of pages rather than to all pages using the
671 style sheet.</p>
673 <p class="issue">Given that this @-rule is intended primarily for user
674 style sheets, what should this specification say about its use in author
675 style sheets? Should it be forbidden? Should use instead be
676 discouraged? Or should this specification remain neutral on the
677 topic, since there are valid uses in author style sheets?</p>
679 <p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
680 the URI at which the document is located, excluding any fragment
681 identifiers. (This means, for example, that HTTP redirects have been
682 followed.) If the styles are being applied inside a complete document
683 embedded into the presentation of another (e.g., [[HTML5]]'s <code
684 class="html">iframe</code>, <code class="html">object</code>, or <code
685 class="html">img</code> elements), the relevant URI is that of the
686 frame, not of its container. However, if content from other documents
687 is mixed in via mechanisms that mix content from one document into
688 another (e.g., [[SVG11]]'s <code>use</code> element), then the
689 address of the container document is used.</p>
691 <p class="note">Note: In [[HTML5]], this is the
692 <a href="http://dev.w3.org/html5/spec/dom.html#documents">document's address</a>
693 of a document in a
694 <a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>
696 <div class="issue">What form of normalization is done on URLs and domains
697 before matching? In particular, this specification needs to describe:
698 <ul>
699 <li>what form is used for the <a href="#url-of-doc">URL of the document
700 being styled</a> (and what has been normalized in that form)</li>
701 <li>what normalization (if any) happens to the argument of each of the match
702 functions before the comparison that they describe and</li>
703 <li>whether the
704 comparison algorithm used is string comparison or some other URL
705 comparison algorithm.</li></ul></div>
707 <p>The '@document' rule's condition is written as a
708 comma-separated list of <dfn>URL matching functions</dfn>, and the
709 condition evaluates to true whenever any one of those functions
710 evaluates to true. The following URL matching functions are
711 permitted:</p>
713 <dl>
714 <dt><dfn id="url-exact" title="url()|URL matching functions::exact"><url></dfn></dt>
716 <dd>
717 <p>The 'url()' function is the <dfn>exact url matching
718 function</dfn>. It evaluates to true whenever the <a
719 href="#url-of-doc">URL of the document being styled</a> is exactly
720 the URL given.</p>
722 <p class="Note">The 'url()' function, since it is a core syntax
723 element in CSS, is allowed (subject to different character
724 limitations and thus escaping requirements) to contain an unquoted
725 value (in addition to the string values that are allowed as
726 arguments for all four functions).</p>
728 <div class="example">
729 <p>For example, this rule:</p>
730 <pre>@document url("http://www.w3.org/Style/CSS/") {
731 #summary { background: yellow; color: black}
732 }</pre>
733 <p>styles the <code class="html">summary</code> element on the page
734 <code>http://www.w3.org/Style/CSS/</code>, but not on any other
735 pages.</p>
736 </div>
737 </dd>
739 <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(<string>)</dfn></dt>
741 <dd>
742 <p>The 'url-prefix()' function is the <dfn>url prefix
743 matching function</dfn>. It evaluates to true whenever the
744 <a href="#url-of-doc">URL of the document being styled</a>
745 has the argument to the function as an
746 initial substring (which is true when the two strings are equal).
747 When the argument is the empty string, it evaluates to true for all
748 documents.</p>
749 <div class="example">
750 <p>For example, this rule:</p>
751 <pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
752 #summary { background: yellow; color: black}
753 }</pre>
754 <p>styles the <code class="html">summary</code> element on the page
755 <code>http://www.w3.org/Style/CSS/</code> and on the page
756 <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
757 affect the page <code>http://www.w3.org/</code> or the page
758 <code>http://www.example.com/Style/CSS/</code>.</p>
759 </div>
760 </dd>
762 <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(<string>)</dfn></dt>
764 <dd>
765 <p>The 'domain()' function is the <dfn>domain
766 matching function</dfn>. It evaluates to true whenever
767 the <a href="#url-of-doc">URL of the document being styled</a>
768 has a host subcomponent (as defined in [[!URI]])
769 and that host subcomponent is exactly the argument to the
770 'domain()' function or a final substring of the host
771 component is a period (U+002E) immediately followed by the argument
772 to the 'domain()' function.</p>
773 <div class="example">
774 <p>For example, this rule:</p>
775 <pre>@document domain("w3.org") {
776 body { font-size: 16px ! important }
777 }</pre>
778 <p>changes the font size of the body element for pages such as
779 <code>http://www.w3.org/Style/CSS/</code> and
780 <code>http://w3.org/Style/CSS/</code> and
781 <code>http://lists.w3.org/Archives/Public/www-style/</code>
782 but it does not affect the page
783 <code>http://www.example.com/Style/CSS/</code>.</p>
784 </div>
785 </dd>
787 <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(<string>)</dfn></dt>
789 <dd>
790 <p>The contents of the <string> argument <strong>must</strong>
791 match the JavaScript <code>Pattern</code> production
792 ([[!ECMA-262-5.1]], section 15.10.1). However,
793 failing to do so is not a CSS syntax error and does not trigger any
794 error handling for CSS syntax errors.</p>
796 <p>The ''regexp()'' function evaluates to true whenever the string
797 argument compiled as a JavaScript regular expression with the
798 <code>global</code>, <code>ignoreCase</code> and
799 <code>multiline</code> flags <em>disabled</em>
800 (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
801 compiles successfully and the resulting regular expression matches
802 the entirety of the
803 <a href="#url-of-doc">URL of the document being styled</a>.</p>
805 <p class="note">Note that regular expression must match the entire
806 URL, not just a part of it.</p>
808 <p class="note">Note that this definition intentionally matches the
809 behavior of the <a
810 href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
811 attribute</a> on the <code class="html">input</code> element
812 in [[HTML5]].</p>
814 <div class="example">
815 <p>For example, this rule:</p>
816 <pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
817 body { font-size: 20px ! important }
818 }</pre>
819 <p>changes the font size of the body element for pages such as
820 <code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
821 <p class="note">Note that the backslashes in the regular
822 expression require CSS escaping as ''\\''.</p>
823 </div>
824 </dd>
826 </dl>
828 <p>Implementations <strong>must</strong> treat any unknown URL matching
829 functions as a syntax error, and thus ignore the '@document' rule.
830 <span class="issue">Should we instead have more complicated error
831 handling rules to make forward-compatibility work differently, or is
832 this rule the best solution for such future expansion anyway?</span></p>
834 <div class="issue">This syntax doesn't offer any ability to do negations,
835 which has been requested in <a
836 href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
837 349813</a>. Use cases that people have wanted negations for
838 include:
839 <ul>
840 <li>User style sheets that want a particular rule in general, but know
841 that that rule does more harm than good on specific sites.</li>
842 <li>Authors who have a rule that they want to apply to most of their
843 pages, but wish to make a few exceptions for.</li>
844 </ul>
845 </div>
847 <p>This extends the lexical scanner in the
848 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
849 ([[!CSS21]], Appendix G) by adding:
850 <pre>@{D}{O}{C}{U}{M}{E}{N}{T} {return DOCUMENT_SYM;}</pre>
851 <p>and the grammar by adding</p>
852 <pre><dfn>document_rule</dfn>
853 : DOCUMENT_SYM S+ <i>url_match_fn</i> ( "," S* <i>url_match_fn</i> )* <i>group_rule_body</i>
854 ;
856 <dfn>url_match_fn</dfn>
857 : (URI | FUNCTION S* STRING S* ')' ) S*
858 ;</pre>
859 -->
862 <h2 id="apis">APIs</h2>
864 <h3 id='extentions-to-cssrule-interface'>
865 Extensions to the <code>CSSRule</code> interface</h3>
867 <p>The <code>CSSRule</code> interface is extended as follows:
869 <pre class='idl'>partial interface CSSRule {
870 const unsigned short SUPPORTS_RULE = 12;
871 <!--
872 const unsigned short DOCUMENT_RULE = 13;
873 -->
874 }</pre>
877 <h3 id='the-cssgroupingrule-interface'>
878 The <code>CSSGroupingRule</code> interface</h3>
880 <p>The <dfn><code>CSSGroupingRule</code></dfn> interface represents an at-rule that contains other rules nested inside itself.
882 <pre class='idl'>interface CSSGroupingRule : CSSRule {
883 readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
884 unsigned long insertRule (DOMString rule, unsigned long index);
885 void deleteRule (unsigned long index);
886 }</pre>
888 <dl class='idl-attributes'>
889 <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
890 <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
891 object for the list of CSS rules nested inside the grouping rule.
892 </dl>
894 <dl class='idl-methods'>
895 <dt><code>insertRule(DOMString rule, unsigned long index)</code>, returns
896 <code>unsigned long</code>
897 <dd>
898 The <code>insertRule</code> operation must
899 insert a CSS rule <var>rule</var>
900 into the CSS rule list returned by <code>cssRules</code>,
901 such that the inserted rule will be at position <var>index</var>,
902 and any rules previously at <var>index</var> or higher
903 will increase their index by one.
904 It must throw INDEX_SIZE_ERR
905 if index is greater than <code>cssRules.length</code>.
906 It must throw SYNTAX_ERR
907 if the rule has a syntax error and is unparseable;
908 this does not include syntax errors handled by error handling rules
909 for constructs inside of the rule.
910 It must throw HIERARCHY_REQUEST_ERR
911 if the rule cannot be inserted at the location specified,
912 for example, if an ''@import'' rule is inserted inside a group rule.
914 <span class="issue">This needs to specify what to do
915 if <code>rule</code> is the empty string,
916 if it contains more than one CSS rule,
917 or if it contains garbage after a valid rule.</span>
919 <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
920 <dd>
921 The <code>deleteRule</code> operation must
922 remove a CSS rule from
923 the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
924 It must throw INDEX_SIZE_ERR
925 if index is greater than or equal to <code>cssRules.length</code>.
926 </dl>
929 <h3 id="the-cssconditionrule-interface">
930 The <code>CSSConditionRule</code> interface</h3>
932 <p>The <dfn><code>CSSConditionRule</code></dfn> interface represents all the "conditional" at-rules,
933 which consist of a condition and a statement block.
935 <pre class='idl'>interface CSSConditionRule : CSSGroupingRule {
936 attribute DOMString conditionText;
937 }</pre>
939 <dl class='idl-attributes'>
941 <dt><code>conditionText</code> of type <code>DOMString</code>
942 <dd>
943 <p>The <code>conditionText</code> attribute represents
944 the condition of the rule.
945 Since what this condition does
946 varies between the derived interfaces of <code>CSSConditionRule</code>,
947 those derived interfaces
948 may specify different behavior for this attribute
949 (see, for example, <code>CSSMediaRule</code> below).
950 In the absence of such rule-specific behavior,
951 the following rules apply:</p>
953 <p>The <code>conditionText</code> attribute, on getting, must return
954 the result of serializing the associated condition.
956 <p>On setting the <code>conditionText</code> attribute these steps
957 must be run:
959 <ol>
960 <li>Trim the given value of white space.
961 <li>If the given value matches the grammar of the
962 appropriate condition production for the given rule,
963 replace the associated CSS condition with the given value.
964 <li>Otherwise, do nothing.
965 </ol>
966 </dl>
969 <h3 id="the-cssmediarule-interface">
970 The <code>CSSMediaRule</code> interface</h3>
972 <p>The <dfn><code>CSSMediaRule</code></dfn> interface represents a ''@media'' rule:
974 <pre class='idl'>interface CSSMediaRule : CSSConditionRule {
975 readonly attribute <a href="http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html#StyleSheets-MediaList">MediaList</a> media;
976 }</pre>
978 <dl class='idl-attributes'>
979 <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
980 <dd>The <code>media</code> attribute must return a <code>MediaList</code> object
981 for the list of media queries specified with the ''@media'' rule.
983 <dt><code>conditionText</code> of type <code>DOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
984 <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
985 on getting, must return the value of <code>media.mediaText</code> on the rule.
987 <p>Setting the <code>conditionText</code> attribute
988 must set the <code>media.mediaText</code> attribute on the rule.
989 </dl>
992 <h3 id="the-csssupportsrule-interface">
993 The <code>CSSSupportsRule</code> interface</h3>
995 <p>The <dfn><code>CSSSupportsRule</code></dfn> interface represents a ''@supports'' rule.</p>
997 <pre class='idl'>interface CSSSupportsRule : CSSConditionRule {
998 }</pre>
1000 <dl class='idl-attributes'>
1001 <dt><code>conditionText</code> of type <code>DOMString</code> (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
1002 <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
1003 on getting, must return the condition that was specified,
1004 without any logical simplifications,
1005 so that the returned condition will evaluate to the same result
1006 as the specified condition
1007 in any conformant implementation of this specification
1008 (including implementations that implement future extensions
1009 allowed by the <i>general_enclosed</i> exensibility mechanism in this specification).
1010 In other words,
1011 token stream simplifications are allowed
1012 (such as reducing whitespace to a single space
1013 or omitting it in cases where it is known to be optional),
1014 but logical simplifications (such as removal of unneeded parentheses,
1015 or simplification based on evaluating results) are not allowed.
1017 </dl>
1019 <!--
1020 <h3 id="the-cssdocumentrule-interface">
1021 The <code>CSSDocumentRule</code> interface</h3>
1023 <p>The <dfn><code>CSSDocumentRule</code></dfn> interface represents a ''@document'' rule.</p>
1025 <pre class='idl'>interface CSSDocumentRule : CSSConditionRule {
1026 }</pre>
1027 -->
1030 <h3 id='the-css-interface'>
1031 The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>
1033 <p>The <dfn id='CSS-interface'><code>CSS</code></dfn> interface holds useful CSS-related functions that do not belong elsewhere.
1035 <pre class='idl'>interface CSS {
1036 static boolean supports(DOMString property, DOMString value);
1037 static boolean supports(DOMString conditionText);
1038 }</pre>
1040 <dl class='idl-methods'>
1041 <dt><code>supports(DOMString property, DOMString value)</code>,
1042 returns <code>boolean</code>
1043 <dt><code>supports(DOMString conditionText)</code>,
1044 returns <code>boolean</code>
1045 <dd>
1046 When the <code title=''>supports()</code> method is invoked with two arguments <var>property</var> and <var>value</var>,
1047 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,
1048 and <var>value</var> would be successfully parsed as a supported value for that property.
1049 (Literal match means that no CSS escape processing is performed,
1050 and leading and trailing whitespace are not stripped,
1051 so any leading whitespace, trailing whitespace,
1052 or CSS escapes equivalent to the name of a property
1053 would cause the method to return <code>false</code>.)
1054 Otherwise, it must return <code>false</code>.
1056 <p>
1057 When invoked with a single <var>conditionText</var> argument,
1058 it must return <code>true</code> if <var>conditionText</var>,
1059 when parsed and evaluated as a <code>supports_condition</code>,
1060 would return true.
1061 Otherwise, it must return <code>false</code>.
1062 </dl>
1065 <h2 class=no-num id="grammar">Grammar</h2>
1067 <p>In order to allow these new @-rules in CSS style sheets, this
1068 specification modifies the <code>stylesheet</code> production in the <a
1069 href="http://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
1070 [[!CSS21]] by replacing the <code>media</code> production defined in
1071 [[!CSS21]] with the <code>media</code> production defined in this one,
1072 and additionally inserting <code>| supports_rule</code>
1073 alongside <code>ruleset | media | page</code>.</p>
1076 <h2 id="conformance">Conformance</h2>
1078 <h3 id="base-modules">Base Modules</h3>
1080 <p>This specification defines conformance in terms of base modules,
1081 which are modules that this specification builds on top of. The base
1082 modules of this module are:</p>
1084 <ul>
1085 <li>[[!CSS21]]</li>
1086 </ul>
1088 <p>All of the conformance requirements of all base modules are
1089 incorporated as conformance requirements of this module, except where
1090 overridden by this module.</p>
1092 <p>Additionally, all conformance requirements related to validity of
1093 syntax in this module and all of its base modules are to be interpreted
1094 as though all syntax in all of those modules is valid.</p>
1096 <div class="example"><p>For example, this means that grammar presented
1097 in modules other than [[!CSS21]] must obey the requirements that
1098 [[!CSS21]] defines for the parsing of properties, and that requirements
1099 for handling invalid syntax in [[!CSS21]] do not treat syntax added by
1100 other modules as invalid.</p></div>
1102 <p>Additionally, the set of valid syntax can be increased by the
1103 conformance of a style sheet or processor to additional modules; use of
1104 such syntax does not make a style sheet nonconformant and failure to
1105 treat such syntax as invalid does not make a processor
1106 nonconformant.</p>
1108 <h3 id="conformance-classes">Conformance Classes</h3>
1110 <p>Conformance to the CSS Conditional Rules Module is defined for three
1111 conformance classes:
1112 <dl>
1113 <dt><dfn title="conformance::style sheet" id="conform-style-sheet">style sheet</dfn>
1114 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
1115 style sheet</a>.</dd>
1116 <dt><dfn title="conformance::processor" id="conform-processor">processor</dfn></dt>
1117 <dd>A tool that reads CSS style sheets: it may be a renderer or
1118 <a
1119 href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user-agent</a>
1120 that interprets the semantics of a style sheet and renders
1121 documents that use style sheets, or it may be a validator that
1122 checks style sheets.</dd>
1123 <dt><dfn title="conformance::authoring tool" id="conform-authoring-tool">authoring tool</dfn></dt>
1124 <dd>A tool that writes a style sheet.</dd>
1125 </dl>
1127 <p>A style sheet is conformant to the CSS Conditional Rules Module
1128 if it meets all of the conformance requirements in the module that are
1129 described as requirements of style sheets.</p>
1131 <p>A processor is conformant to the CSS Conditional Rules Module if it
1132 meets all applicable conformance requirements in the module that are
1133 described as requirements of processors. In general, all requirements
1134 are applicable to renderers. Requirements concerning a part of CSS
1135 not performed by a processor are not applicable, e.g., requirements
1136 related to rendering are not applicable to a validator. The inability
1137 of a processor to correctly render a document due to limitations of
1138 the device does not make it non-conformant. (For example, a renderer
1139 is not required to render color on a monochrome monitor.)</p>
1141 <p>An authoring tool is conformant to the CSS Conditional Rules Module
1142 if it writes style sheets that conform to the module and (if it reads
1143 CSS) it is a conformant processor.</p>
1145 <h3 id="partial">
1146 Partial Implementations</h3>
1148 <p>So that authors can exploit the forward-compatible parsing rules to
1149 assign fallback values, CSS renderers <strong>must</strong>
1150 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
1151 as appropriate</a>) any at-rules, properties, property values, keywords,
1152 and other syntactic constructs for which they have no usable level of
1153 support. In particular, user agents <strong>must not</strong> selectively
1154 ignore unsupported component values and honor supported values in a single
1155 multi-value property declaration: if any value is considered invalid
1156 (as unsupported values must be), CSS requires that the entire declaration
1157 be ignored.</p>
1159 <h3 id="experimental">Experimental Implementations</h3>
1161 <p>To avoid clashes with future CSS features, the CSS specifications
1162 reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
1163 syntax</a> for proprietary property and value extensions to CSS. The CSS
1164 Working Group recommends that experimental implementations of features in
1165 CSS Working Drafts also use vendor-prefixed property or value names. This
1166 avoids any incompatibilities with future changes in the draft. Once a
1167 specification reaches the Candidate Recommendation stage, implementors
1168 should implement the non-prefixed syntax for any feature they consider to
1169 be correctly implemented according to spec.</p>
1171 <h3 id="cr-exit-criteria">CR Exit Criteria</h3>
1173 <p>For this specification to be advanced to Proposed Recommendation,
1174 there must be at least two independent, interoperable implementations
1175 of each feature. Each feature may be implemented by a different set of
1176 products, there is no requirement that all features be implemented by
1177 a single product. For the purposes of this criterion, we define the
1178 following terms:
1180 <dl>
1181 <dt>independent <dd>each implementation must be developed by a
1182 different party and cannot share, reuse, or derive from code
1183 used by another qualifying implementation. Sections of code that
1184 have no bearing on the implementation of this specification are
1185 exempt from this requirement.
1187 <dt>interoperable <dd>passing the respective test case(s) in the
1188 official CSS test suite, or, if the implementation is not a Web
1189 browser, an equivalent test. Every relevant test in the test
1190 suite should have an equivalent test created if such a user
1191 agent (UA) is to be used to claim interoperability. In addition
1192 if such a UA is to be used to claim interoperability, then there
1193 must one or more additional UAs which can also pass those
1194 equivalent tests in the same way for the purpose of
1195 interoperability. The equivalent tests must be made publicly
1196 available for the purposes of peer review.
1198 <dt>implementation <dd>a user agent which:
1200 <ol class=inline>
1201 <li>implements the specification.
1203 <li>is available to the general public. The implementation may
1204 be a shipping product or other publicly available version
1205 (i.e., beta version, preview release, or “nightly build”).
1206 Non-shipping product releases must have implemented the
1207 feature(s) for a period of at least one month in order to
1208 demonstrate stability.
1210 <li>is not experimental (i.e., a version specifically designed
1211 to pass the test suite and is not intended for normal usage
1212 going forward).
1213 </ol>
1214 </dl>
1216 <p>The specification will remain Candidate Recommendation for at least
1217 six months.
1219 <h2 id="changes">
1220 Changes</h2>
1222 <p>The following (non-editorial) changes were made to this specification since the
1223 <a href="http://www.w3.org/TR/2012/WD-css3-conditional-20120911/">11 September 2012 Working Draft</a>:
1225 <ul>
1226 <li>Removed ''@document'' rule; it has been deferred to Level 4.
1227 <li>Allow functional notation in ''@supports'' queries to be valid (to allow for future extensions),
1228 but treat such notations as always being false.
1229 <li>Corrected the grammar as follows:
1230 <pre>
1231 - : SUPPORTS_SYM S+ supports_condition group_rule_body
1232 + : SUPPORTS_SYM S* supports_condition group_rule_body
1233 </pre>
1234 <pre>
1235 - : (URI | FUNCTION) S*
1236 + : (URI | FUNCTION S* STRING S* ')' ) S*
1237 </pre>
1238 <li>Switched "and", "or", and "not" keywords to use appropriate productions rather than literals.
1239 <li>Clarified definition of support used for interpreting ''@support'' rules
1240 and its relationship to CSS forwards-compatible parsing rules.
1241 </ul>
1243 <h2 class=no-num id="acknowledgments">Acknowledgments</h2>
1245 <p>
1246 Thanks to the ideas and feedback from
1247 Tab Atkins,
1248 Arthur Barstow,
1249 Ben Callahan,
1250 <span lang="tr">Tantek Çelik</span>,
1251 Alex Danilo,
1252 Elika Etemad,
1253 Pascal Germroth,
1254 <span lang="de">Björn Höhrmann</span>,
1255 Paul Irish,
1256 <span lang="nl">Anne van Kesteren</span>,
1257 Vitor Menezes,
1258 Alex Mogilevsky,
1259 Chris Moschini,
1260 James Nurthen,
1261 Simon Pieters,
1262 <span lang="fr">Florian Rivoal</span>,
1263 <span lang="fr">Simon Sapin</span>,
1264 Nicholas Shanks,
1265 Ben Ward,
1266 Zack Weinberg,
1267 Estelle Weyl,
1268 Boris Zbarsky,
1269 and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
1271 </p>
1273 <h2 class=no-num id="references">References</h2>
1276 <h3 class="no-num" id="normative-references">Normative references</h3>
1277 <!--normative-->
1279 <h3 class="no-num" id="other-references">Other references</h3>
1280 <!--informative-->
1282 <h2 class="no-num" id="index">Index</h2>
1283 <!--index-->
1285 </body>
1286 </html>
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1306 End:
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