css3-conditional/Overview.src.html@66482bcc960b
css3-conditional/Overview.src.html
Fri, 31 Aug 2012 13:49:19 -0700
- author
- Tab Atkins Jr. <jackalmage@gmail.com>
- date
- Fri, 31 Aug 2012 13:49:19 -0700
- changeset 6625
- 66482bcc960b
- parent 6622
- 44bf9c71914a
- child 6655
- 8c75e6e6f34d
- permissions
- -rwxr-xr-x
[css3-conditional] Fix the one-arg CSS.supports() overload.
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 rel="stylesheet" type="text/css"
9 href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
10 </head>
12 <div class="head">
13 <!--logo-->
15 <h1>CSS Conditional Rules Module Level 3</h1>
17 <h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
18 <dl>
19 <dt>This version:
20 <dd><a href="[VERSION]">
21 http://www.w3.org/TR/[YEAR]/ED-css3-conditional-[CDATE]/</a>
23 <dt>Latest version:
24 <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>
26 <dt>Editor's draft:
27 <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
29 <dt>Previous version:
30 <dd><a href="http://www.w3.org/TR/2011/WD-css3-conditional-20110901/">http://www.w3.org/TR/2011/WD-css3-conditional-20110901/</a></dd>
32 <dt>Editors:
33 <dd class=vcard><a class=fn href="http://dbaron.org/">L. David Baron</a>,
34 <a class=org href="http://www.mozilla.org/">Mozilla</a>
36 <dt>Issues list:
37 <dd>Maintained in document (only editor's draft is current)
39 <dt>Send feedback to:
40 <dd>public mailing list <a href="www-style@w3.org">www-style@w3.org</a> (<a href="http://lists.w3.org/Archives/Public/www-style/">archive</a>, <a href="http://www.w3.org/Mail/Request">instructions</a>) with [[SHORTNAME]] at start of the subject
42 <dt>Test suite:
43 <dd><a href="https://test.csswg.org/shepherd/search/spec/css3-conditional/">submitted tests</a>; no built test suite yet
45 </dl>
47 <!--copyright-->
49 <hr title="Separator for header">
50 </div>
52 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
54 <p>CSS is a language for describing the rendering of structured documents
55 (such as HTML and XML) on screen, on paper, in speech, etc. This module
56 contains the features of CSS for conditional processing of parts of
57 style sheets, conditioned on capabilities of the processor or the
58 document the style sheet is being applied to.
59 It includes and extends the functionality of CSS level 2 [[!CSS21]],
60 which builds on CSS level 1 [[CSS1]].
61 The main extensions compared to level 2 are
62 allowing nesting of certain at-rules inside '@media',
63 the addition of the '@supports' and '@document'
64 rules for conditional processing.
66 <h2 class="no-num no-toc" id="status">Status of this document</h2>
68 <!--status-->
70 <p>The following features are at risk:
71 <ul>
72 <li>The inclusion of '@font-face' rules and
73 '@keyframes' rules as allowed within all of the @-rules in
74 this specification is at risk, though only because of the relative
75 rates of advancement of specifications. If this specification is able
76 to advance faster than one or both of the specifications defining
77 those rules, then the inclusion of those rules will move from this
78 specification to the specification defining those rules.</li>
80 <li>The addition of support for @-rules inside of conditional grouping
81 rules is at risk; if interoperable implementations are not found, it
82 may be removed to advance the other features in this specification to
83 Proposed Recommendation.</li>
85 <li>The '@supports' rule is at risk; if interoperable
86 implementations are not found, it may be removed to advance the other
87 features in this specification to Proposed Recommendation.</li>
89 <li>The '@document' rule is at risk; if interoperable
90 implementations are not found, it may be removed to advance the other
91 features in this specification to Proposed Recommendation.</li>
93 </ul>
95 <h2 class="no-num no-toc" id="contents">Table of contents</h2>
97 <!--toc-->
99 <h2 id="introduction">Introduction</h2>
101 <h3 id="context">Background</h3>
103 <p><em>This section is not normative.</em>
105 <p>[[!CSS21]] defines one type of conditional group rule, the
106 '@media' rule, and allows only rulesets (not other @-rules)
107 inside of it. The '@media' rule provides the ability to
108 have media-specific style sheets, which is also provided by style
109 sheet linking features such as '@import' and
110 <code class="html"><link></code>. The restrictions on the contents of
111 '@media' rules made them less useful; they have forced authors
112 using CSS features involving @-rules in media-specific style sheets to
113 use separate style sheets for each medium.</p>
115 <p>This specification extends the rules for the contents of
116 conditional group rules to allow other @-rules, which enables authors
117 to combine CSS features involving @-rules with media specific style
118 sheets within a single style sheet.</p>
120 <p>This specification also defines additional types of conditional
121 group rules, '@supports' and '@document', to
122 address author and user requirements.</p>
124 <p>The '@supports' rule allows CSS to be conditioned on
125 implementation support for CSS properties and values. This rule makes
126 it much easier for authors to use new CSS features and provide good
127 fallback for implementations that do not support those features. This
128 is particularly important for CSS features that provide new layout
129 mechanisms, and for other cases where a set of related styles needs to
130 be conditioned on property support.</p>
132 <p>The '@document' rule allows CSS to be conditioned on the
133 page to which the style sheet is being applied. This allows users to
134 apply styles to a particular page or group of pages, which greatly
135 increases the power of user style sheets.</p>
137 <h3 id="placement">Module Interactions</h3>
139 <p>This module replaces and extends the '@media' rule
140 feature defined in [[!CSS21]] section <var>7.2.1</var> and
141 incorporates the modifications previously made non-normatively by
142 [[!MEDIAQ]] section <var>1</var>.</p>
144 <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
145 and [[!CSS3-ANIMATIONS]], but that dependency is only on the
146 assumption that those modules will advance ahead of this one. If this
147 module advances faster, then the dependency will be reversed.</p>
149 <h3 id="conventions">Document Conventions</h3>
151 <p>Conformance requirements are expressed with a combination of
152 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
153 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
154 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
155 document are to be interpreted as described in RFC 2119.
156 However, for readability, these words do not appear in all uppercase
157 letters in this specification.
159 <p>All of the text of this specification is normative except sections
160 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
162 <p>Examples in this specification are introduced with the words “for example”
163 or are set apart from the normative text with
164 <code class="html">class="example"</code>, like this:
166 <div class="example">
167 <p>This is an example of an informative example.</p>
168 </div>
170 <p>Informative notes begin with the word “Note” and are set apart from the
171 normative text with <code class="html">class="note"</code>, like this:
173 <p class="note">Note, this is an informative note.</p>
175 <h2 id="processing">Processing of conditional group rules</h2>
177 <p>This specification defines some CSS @-rules, called <dfn>conditional
178 group rules</dfn>, that associate a condition with a group of other
179 CSS rules. These different rules allow testing different types of
180 conditions, but share common behavior for how their contents are used
181 when the condition is true and when the condition is false.</p>
183 <div class="example">
184 <p>For example, this rule:</p>
185 <pre>@media print {
186 #navigation { display: none }
187 }</pre>
188 <p>causes a particular CSS rule (making elements with ID "navigation" be
189 display:none) apply only when the style sheet is used for a print
190 medium. Likewise, this CSS rule:</p>
191 <pre>@document url("http://www.example.com/") {
192 #example1 { display: none }
193 }</pre>
194 <p>does the same type of conditional application, but using a different
195 condition: whether the style sheet is being applied to the page
196 <code>http://www.example.com/</code>.</p>
197 </div>
199 <p>Each conditional group rule has a condition, which at any time
200 evaluates to true or false. When the condition is true, CSS processors
201 <strong>must</strong> apply the rules inside the group rule as though
202 they were at the group rule's location; when the condition is false, CSS
203 processors <strong>must not</strong> apply any of rules inside the group
204 rule. The current state of the condition does not affect the CSS object
205 model, in which the contents of the group rule always remain within the
206 group rule.</p>
208 <p>This means that when multiple conditional group rules are nested,
209 a rule inside of both of them applies only when all of the rules'
210 conditions are true.</p>
212 <div class="example">For example, with this set of nested rules:
213 <pre>@media print { // rule (1)
214 #navigation { display: none }
215 @media (max-width: 12cm) { // rule (2)
216 .note { float: none }
217 }
218 }</pre>
219 the condition of the rule marked (1) is true for print media, and the
220 condition of the rule marked (2) is true when the width of the display
221 area (which for print media is the page box) is less than or equal to
222 12cm. Thus the rule ''#navigation { display: none }'' applies
223 whenever this style sheet is applied to print media, and the rule
224 ''.note { float: none }'' is applied only when the style sheet
225 is applied to print media <em>and</em> the width of the page box is less
226 than or equal to 12 centimeters.</div>
228 <p>When the condition for a conditional group rule changes, CSS
229 processors <strong>must</strong> reflect that the rules now apply or no
230 longer apply, except for properties whose definitions define effects of
231 computed values that persist past the lifetime of that value (such as
232 for some properties in [[CSS3-TRANSITIONS]] and
233 [[!CSS3-ANIMATIONS]]).</p>
235 <h2 id="contents-of">Contents of conditional group rules</h2>
237 <p class="issue">There is also likely demand for using these
238 conditions with '@import'. We should see if we can
239 come up with sensible syntax for that, perhaps functional notation
240 at the end of the '@import' rule.</p>
242 <p>The syntax of each conditional group rule consists of some syntax
243 specific to the type of rule followed by a <dfn>group rule body</dfn>,
244 which is a block (pair of braces) containing a sequence of rules.</p>
246 <p>A group rule body is allowed to contain rulesets and any @-rules that
247 are allowed at the top level of a style sheet before and after a
248 ruleset. This means that @-rules that must occur at the beginning of
249 the style sheet (such as '@charset', '@import',
250 and '@namespace' rules) are not allowed inside of conditional group
251 rules. Conditional group rules can be nested.</p>
253 <p>In terms of the grammar, this specification defines the following
254 productions for use in the grammar of conditional group rules:</p>
256 <pre>nested_statement
257 : ruleset | media | page | font_face_rule | keyframes_rule |
258 supports_rule | document_rule
259 ;
261 group_rule_body
262 : '{' S* nested_statement* '}' S*
263 ;</pre>
264 <p>
265 in which all the productions are defined in that grammar with the
266 exception of <code>font_face_rule</code> <span class="issue">not</span>
267 defined in [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
268 [[!CSS3-ANIMATIONS]], and <code>media</code>, <code>supports_rule</code>
269 and <code>document_rule</code> defined in this specification.</p>
271 <p>In general, future CSS specifications that add new @-rules that are
272 not forbidden to occur after some other types of rules should modify
273 this <code>nested_statement</code> production to keep the grammar
274 accurate.</p>
276 <p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
277 conditional group rules.</p>
279 <p>CSS processors <strong>must</strong> ignore rules that are not
280 allowed within a group rule, and <strong>must</strong> handle invalid
281 rules inside of group rules as described in <a
282 href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
283 4.2 (Rules for handling parsing errors)</a>, <a
284 href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
285 (At-rules)</a>, and <a
286 href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
287 (Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>
289 <h2 id="use">Placement of conditional group rules</h2>
291 <p>Conditional group rules are allowed at the top-level of a style
292 sheet, and inside other conditional group rules. CSS processors
293 <strong>must</strong> process such rules as <a
294 href="#processing">described above</a>.</p>
296 <p>Any rules that are not allowed after a ruleset (e.g., ''@charset'',
297 ''@import'', or ''@namespace'' rules) are also not allowed after a
298 conditional group rule. Therefore, style sheets <strong>must
299 not</strong> place such rules after a conditional group rules, and CSS
300 processors <strong>must</strong> ignore such rules.</p>
302 <h2 id="at-media">Media-specific style sheets: the '@media' rule</h2>
304 <p>The <dfn>'@media' rule</dfn> is a conditional group rule whose
305 condition is a media query. It consists of the at-keyword
306 '@media' followed by a (possibly empty) media query (as
307 defined in [[!MEDIAQ]]), followed by a group rule body. The condition
308 of the rule is the result of the media query.</p>
310 <div class="example">
311 <p>This '@media' rule:</p>
312 <pre>@media print, (max-width: 600px) {
313 #extra_navigation { display: none }
314 }</pre>
315 <p>has the condition ''print, (max-width: 600px)'', which is
316 true for print media and for devices whose width is at most 600px. When
317 either of these is true, the condition of the rule is true, and the rule
318 ''#extra_navigation { display: none }'' is applied.
319 </div>
321 <p>In terms of the grammar, this specification extends the
322 <code>media</code> production in the
323 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
324 ([[!CSS21]], Appendix G) into:
325 <pre>media
326 : MEDIA_SYM S* media_query_list group_rule_body
327 ;</pre>
328 <p>where the <code>group_rule_body</code> production is defined in this
329 specification, the <code>media_query_list</code> production is defined
330 in [[!MEDIAQ]], and the others are defined in the <a
331 href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
332 ([[!CSS21]], Appendix G).
334 <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
336 <p>The <dfn>'@supports' rule</dfn> is a conditional group
337 rule whose condition tests whether the user agent supports CSS
338 property:value pairs. Authors can use it to write style sheets that use
339 new features when available but degrade gracefully when those features
340 are not supported. CSS has existing mechanisms for graceful
341 degradation, such as ignoring unsupported properties or values, but
342 these are not always sufficient when large groups of styles need to be
343 tied to the support for certain features, as is the case for use of new
344 layout system features.</p>
346 <p>The syntax of the condition in the '@supports' rule is
347 slightly more complicated than for the other conditional group rules
348 (though has some similarities to media queries) since:</p>
349 <ul>
350 <li>negation is needed so that the new-feature styles and the fallback
351 styles can be separated (within the forward-compatible grammar's rules
352 for the syntax of @-rules), and not required to override each other</li>
353 <li>conjunction (and) is needed so that multiple required features can
354 be tested</li>
355 <li>disjunction (or) is needed when there are multiple alternative
356 features for a set of styles, particularly when some of those
357 alternatives are vendor-prefixed properties or values</li>
358 </ul>
360 <p>Therefore, the syntax of the '@supports' rule allows
361 testing for property:value pairs, and arbitrary conjunctions (and),
362 disjunctions (or), and negations (not) of them.</p>
364 <p>This extends the lexical scanner in the
365 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
366 ([[!CSS21]], Appendix G) by adding:
367 <pre>@{S}{U}{P}{P}{O}{R}{T}{S} {return SUPPORTS_SYM;}</pre>
368 <p>and the grammar by adding</p>
369 <pre><dfn>supports_rule</dfn>
370 : SUPPORTS_SYM S+ supports_condition group_rule_body
371 ;
373 <dfn>supports_condition</dfn>
374 : supports_negation | supports_conjunction | supports_disjunction |
375 supports_condition_in_parens
376 ;
378 <dfn>supports_condition_in_parens</dfn>
379 : ( '(' S* supports_condition ')' S* ) | supports_declaration_condition
380 ;
382 <dfn>supports_negation</dfn>
383 : 'not' S* supports_condition_in_parens
384 ;
386 <dfn>supports_conjunction</dfn>
387 : supports_condition_in_parens ( 'and' S* supports_condition_in_parens )+
388 ;
390 <dfn>supports_disjunction</dfn>
391 : supports_condition_in_parens ( 'or' S* supports_condition_in_parens )+
392 ;
394 <dfn>supports_declaration_condition</dfn>
395 : '(' S* core_declaration ')' S*
396 ;</pre>
397 <p>in which <code>core_declaration</code> is the production
398 <code>declaration</code> in the core syntax of CSS defined in <a
399 href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section
400 4.1.1 (Tokenization)</a> of [[!CSS21]].</p>
402 <p>Any ''@supports'' rule that does not parse according to the grammar
403 above is invalid. Style sheets <strong>must not</strong> use such a
404 rule and processors <strong>must</strong> ignore such a rule.</p>
406 <p class="note">Note that this means that declarations that meet the
407 forward-compatible syntax for declarations are permitted (and support
408 for them is then tested by the ''@supports'' rule), but declarations
409 that do not meet the forward-compatible syntax for declarations cause
410 the entire ''@supports'' rule to be ignored.</p>
412 <p class="issue">Is any further allowance for forward-compatible parsing
413 needed, for example, to allow additional features (such as, say,
414 selector tests) to be added to the ''@supports'' rule? Or are these
415 forward-compatible parsing rules the best solution for such future
416 expansion anyway?</p>
418 <p>Each of these grammar terms is associated with a boolean result, as
419 follows:</p>
420 <dl>
421 <dt>supports_condition</dt>
422 <dd>
423 The result is the result of the single child term.
424 </dd>
426 <dt>supports_condition_in_parens</dt>
427 <dd>
428 The result is the result of the single <code>supports_condition</code>
429 or <code>supports_declaration_condition</code> child term.
430 </dd>
432 <dt>supports_negation</dt>
433 <dd>
434 The result is the <em>negation</em> of the result of the
435 <code>supports_condition_in_parens</code> child term.
436 </dd>
438 <dt>supports_conjunction</dt>
439 <dd>
440 The result is true if the result of <em>all</em> of the
441 <code>supports_condition_in_parens</code> child terms is true;
442 otherwise it is false.
443 </dd>
445 <dt>supports_disjunction</dt>
446 <dd>
447 The result is true if the result of <em>any</em> of the
448 <code>supports_condition_in_parens</code> child terms is true;
449 otherwise it is false.
450 </dd>
452 <dt>supports_declaration_condition</dt>
453 <dd>
454 The result is whether the CSS processor <a
455 href="#support-definition">supports</a> the declaration.
456 </dd>
457 </dl>
459 <p>The condition of the '@supports' rule is the result of the
460 <code>supports_condition</code> term that is a child of the
461 <code>supports_rule</code> term.</p>
463 <div class="example">
464 <p>For example, the following rule</p>
465 <pre>@supports ( display: flexbox ) {
466 body, #navigation, #content { display: flexbox; }
467 #navigation { background: blue; color: white; }
468 #article { background: white; color: black; }
469 }</pre>
470 <p>applies the rules inside the '@supports' rule only when
471 ''display: flexbox'' is supported.</p>
472 </div>
474 <div class="example">
475 <p>The following example shows an additional '@supports' rule that can
476 be used to provide an alternative for when ''display: flexbox'' is not
477 supported:</p>
478 <pre>@supports not ( display: flexbox ) {
479 body { width: 100%; height: 100%; background: white; color: black; }
480 #navigation { width: 25%; }
481 #article { width: 75%; }
482 }</pre>
483 <p>Note that the 'width' declarations may be harmful to the
484 flexbox-based layout, so it is important that they be present only in
485 the non-flexbox styles.</p>
486 </div>
488 <div class="example">
489 <p>The following example checks for support for the 'box-shadow'
490 property, including checking for support for vendor-prefixed versions of
491 it. When the support is present, it specifies both 'box-shadow' (with
492 the prefixed versions) and 'color' in a way what would cause the text to
493 become invisible were 'box-shadow' not supported.</p>
494 <pre>@supports ( box-shadow: 2px 2px 2px black ) or
495 ( -moz-box-shadow: 2px 2px 2px black ) or
496 ( -webkit-box-shadow: 2px 2px 2px black ) or
497 ( -o-box-shadow: 2px 2px 2px black ) {
498 .outline {
499 color: white;
500 -moz-box-shadow: 2px 2px 2px black;
501 -webkit-box-shadow: 2px 2px 2px black;
502 -o-box-shadow: 2px 2px 2px black;
503 box-shadow: 2px 2px 2px black; /* unprefixed last */
504 }
505 }</pre></div>
507 <p>To avoid confusion between ''and'' and ''or'', the syntax requires
508 that both ''and'' and ''or'' be specified explicitly (rather than, say,
509 using commas or spaces for one of them). Likewise, to avoid confusion
510 caused by precedence rules, the syntax does not allow ''and'', ''or'',
511 and ''not'' operators to be mixed without a layer of parentheses.</p>
513 <div class="example">
514 <p>For example, the following rule is not valid:
515 <pre class="illegal">@supports (transition-property: color) or
516 (animation-name: foo) and
517 (transform: rotate(10deg)) {
518 // ...
519 }</pre>
520 <p>Instead, authors must write one of the following:</p>
521 <pre>@supports ((transition-property: color) or
522 (animation-name: foo)) and
523 (transform: rotate(10deg)) {
524 // ...
525 }</pre>
526 <pre>@supports (transition-property: color) or
527 ((animation-name: foo) and
528 (transform: rotate(10deg))) {
529 // ...
530 }</pre>
531 </div>
533 <p>The declaration being tested must always occur within parentheses,
534 when it is the only thing in the expression.<p>
536 <div class="example">
537 <p>For example, the following rule is not valid:
538 <pre class="illegal">@supports display: flexbox {
539 // ...
540 }</pre>
541 <p>Instead, authors must write:</p>
542 <pre>@supports (display: flexbox) {
543 // ...
544 }</pre>
545 </div>
547 <p>The syntax allows extra parentheses when they are not needed. This
548 flexibility is sometimes useful for authors (for example, when
549 commenting out parts of an expression) and may also be useful for
550 authoring tools.</p>
552 <div class="example">
553 <p>For example, authors may write:</p>
554 <pre>@supports ((display: flexbox)) {
555 // ...
556 }</pre>
557 </div>
559 <p class="issue">Add an example involving <code>!important</code>.</p>
561 <h3 id="support-definition">Definition of support</h3>
563 <p>A CSS processor is considered to <dfn id="dfn-support">support</dfn>
564 a declaration (consisting of a property and value) if it implements the
565 given value of the given property.</p>
567 <h3 id="partial-implementations">Partial implementations</h3>
569 <p>For forward-compatibility, <a
570 href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
571 (Declarations and properties)</a> of [[!CSS21]] defines rules for
572 handling invalid properties and values. CSS processors that partially
573 implement a specification <strong>must</strong> treat any part of a
574 value that they:</p>
575 <ul>
576 <li>do not implement, or</li>
577 <li>do not have a usable level of support for,</li>
578 </ul>
579 <p>as invalid according to this rule
580 for handling invalid properties and values,
581 and <strong>must not</strong>
582 accept or support a declaration containing such a value. This allows
583 authors to use fallback (either in the [[CSS1]] sense of declarations
584 that are overridden by later declarations or with the new capabilities
585 provided by the ''@supports'' rule in this specification) that works
586 correctly for the features implemented. This applies especially to
587 compound values; implementations must implement all parts of the value
588 in order to consider the declaration supported, either inside a ruleset
589 or in the declaration condition of an ''@supports'' rule.</p>
591 <h2 id="at-document">Document queries: the '@document' rule</h2>
593 <p>The <dfn>'@document' rule</dfn> is a conditional group
594 rule whose condition depends on the
595 <a href="#url-of-doc">URL of the document being styled</a>.
596 This allows style sheets, particularly user style sheets, to have styles
597 that only apply to a set of pages rather than to all pages using the
598 style sheet.</p>
600 <p class="issue">Given that this @-rule is intended primarily for user
601 style sheets, what should this specification say about its use in author
602 style sheets? Should it be forbidden? Should use instead be
603 discouraged? Or should this specification remain neutral on the
604 topic, since there are valid uses in author style sheets?</p>
606 <p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
607 the URI at which the document is located, excluding any fragment
608 identifiers. (This means, for example, that HTTP redirects have been
609 followed.) If the styles are being applied inside a complete document
610 embedded into the presentation of another (e.g., [[HTML5]]'s <code
611 class="html">iframe</code>, <code class="html">object</code>, or <code
612 class="html">img</code> elements), the relevant URI is that of the
613 frame, not of its container. However, if content from other documents
614 is mixed in via mechanisms that mix content from one document into
615 another (e.g., [[SVG11]]'s <code>use</code> element), then the
616 address of the container document is used.</p>
618 <p class="note">Note: In [[HTML5]], this is the
619 <a href="http://dev.w3.org/html5/spec/dom.html#the-document-s-address">document's address</a>
620 of a document in a
621 <a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>
623 <div class="issue">What form of normalization is done on URLs and domains
624 before matching? In particular, this specification needs to describe:
625 <ul>
626 <li>what form is used for the <a href="#url-of-doc">URL of the document
627 being styled</a> (and what has been normalized in that form)</li>
628 <li>what normalization (if any) happens to the argument of each of the match
629 functions before the comparison that they describe and</li>
630 <li>whether the
631 comparison algorithm used is string comparison or some other URL
632 comparison algorithm.</li></ul></div>
634 <p>The '@document' rule's condition is written as a
635 comma-separated list of <dfn>URL matching functions</dfn>, and the
636 condition evaluates to true whenever any one of those functions
637 evaluates to true. The following URL matching functions are
638 permitted:</p>
640 <dl>
641 <dt><dfn id="url-exact" title="url()|URL matching functions::exact"><url></dfn></dt>
643 <dd>
644 <p>The 'url()' function is the <dfn>exact url matching
645 function</dfn>. It evaluates to true whenever the <a
646 href="#url-of-doc">URL of the document being styled</a> is exactly
647 the URL given.</p>
649 <p class="Note">The 'url()' function, since it is a core syntax
650 element in CSS, is allowed (subject to different character
651 limitations and thus escaping requirements) to contain an unquoted
652 value (in addition to the string values that are allowed as
653 arguments for all four functions).</p>
655 <div class="example">
656 <p>For example, this rule:</p>
657 <pre>@document url("http://www.w3.org/Style/CSS/") {
658 #summary { background: yellow; color: black}
659 }</pre>
660 <p>styles the <code class="html">summary</code> element on the page
661 <code>http://www.w3.org/Style/CSS/</code>, but not on any other
662 pages.</p>
663 </div>
664 </dd>
666 <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(<string>)</dfn></dt>
668 <dd>
669 <p>The 'url-prefix()' function is the <dfn>url prefix
670 matching function</dfn>. It evaluates to true whenever the
671 <a href="#url-of-doc">URL of the document being styled</a>
672 has the argument to the function as an
673 initial substring (which is true when the two strings are equal).
674 When the argument is the empty string, it evaluates to true for all
675 documents.</p>
676 <div class="example">
677 <p>For example, this rule:</p>
678 <pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
679 #summary { background: yellow; color: black}
680 }</pre>
681 <p>styles the <code class="html">summary</code> element on the page
682 <code>http://www.w3.org/Style/CSS/</code> and on the page
683 <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
684 affect the page <code>http://www.w3.org/</code> or the page
685 <code>http://www.example.com/Style/CSS/</code>.</p>
686 </div>
687 </dd>
689 <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(<string>)</dfn></dt>
691 <dd>
692 <p>The 'domain()' function is the <dfn>domain
693 matching function</dfn>. It evaluates to true whenever
694 the <a href="#url-of-doc">URL of the document being styled</a>
695 has a host subcomponent (as defined in [[!URI]])
696 and that host subcomponent is exactly the argument to the
697 'domain()' function or a final substring of the host
698 component is a period (U+002E) immediately followed by the argument
699 to the 'domain()' function.</p>
700 <div class="example">
701 <p>For example, this rule:</p>
702 <pre>@document domain("w3.org") {
703 body { font-size: 16px ! important }
704 }</pre>
705 <p>changes the font size of the body element for pages such as
706 <code>http://www.w3.org/Style/CSS/</code> and
707 <code>http://w3.org/Style/CSS/</code> and
708 <code>http://lists.w3.org/Archives/Public/www-style/</code>
709 but it does not affect the page
710 <code>http://www.example.com/Style/CSS/</code>.</p>
711 </div>
712 </dd>
714 <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(<string>)</dfn></dt>
716 <dd>
717 <p>The contents of the <string> argument <strong>must</strong>
718 match the JavaScript <code>Pattern</code> production
719 ([[!ECMA-262-5.1]], section 15.10.1). However,
720 failing to do so is not a CSS syntax error and does not trigger any
721 error handling for CSS syntax errors.</p>
723 <p>The ''regexp()'' function evaluates to true whenever the string
724 argument compiled as a JavaScript regular expression with the
725 <code>global</code>, <code>ignoreCase</code> and
726 <code>multiline</code> flags <em>disabled</em>
727 (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
728 compiles successfully and the resulting regular expression matches
729 the entirety of the
730 <a href="#url-of-doc">URL of the document being styled</a>.</p>
732 <p class="note">Note that regular expression must match the entire
733 URL, not just a part of it.</p>
735 <p class="note">Note that this definition intentionally matches the
736 behavior of the <a
737 href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
738 attribute</a> on the <code class="html">input</code> element
739 in [[HTML5]].</p>
741 <div class="example">
742 <p>For example, this rule:</p>
743 <pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
744 body { font-size: 20px ! important }
745 }</pre>
746 <p>changes the font size of the body element for pages such as
747 <code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
748 <p class="note">Note that the backslashes in the regular
749 expression require CSS escaping as ''\\''.</p>
750 </div>
751 </dd>
753 </dl>
755 <p>Implementations <strong>must</strong> treat any unknown URL matching
756 functions as a syntax error, and thus ignore the '@document' rule.
757 <span class="issue">Should we instead have more complicated error
758 handling rules to make forward-compatibility work differently, or is
759 this rule the best solution for such future expansion anyway?</span></p>
761 <div class="issue">This syntax doesn't offer any ability to do negations,
762 which has been requested in <a
763 href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
764 349813</a>. Use cases that people have wanted negations for
765 include:
766 <ul>
767 <li>User style sheets that want a particular rule in general, but know
768 that that rule does more harm than good on specific sites.</li>
769 <li>Authors who have a rule that they want to apply to most of their
770 pages, but wish to make a few exceptions for.</li>
771 </ul>
772 </div>
774 <p>This extends the lexical scanner in the
775 <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
776 ([[!CSS21]], Appendix G) by adding:
777 <pre>@{D}{O}{C}{U}{M}{E}{N}{T} {return DOCUMENT_SYM;}</pre>
778 <p>and the grammar by adding</p>
779 <pre>document_rule
780 : DOCUMENT_SYM S+ url_match_fn ( "," S* url_match_fn )* group_rule_body
781 ;
783 url_match_fn
784 : (URI | FUNCTION) S*
785 ;</pre>
787 <h2 id="apis">APIs</h2>
789 <h3 id='extentions-to-cssrule-interface'>
790 Extensions to the <code>CSSRule</code> interface</h3>
792 <p>The <code>CSSRule</code> interface is extended as follows:
794 <pre class='idl'>partial interface CSSRule {
795 const unsigned short SUPPORTS_RULE = 12;
796 const unsigned short DOCUMENT_RULE = 13;
797 }</pre>
800 <h3 id="the-cssconditionrule-interface">
801 The <code>CSSConditionRule</code> interface</h3>
803 <p>The <dfn><code>CSSConditionRule</code></dfn> interface represents all the "conditional" at-rules,
804 which consist of a condition and a statement block.
806 <pre class='idl'>interface CSSConditionRule : CSSRule {
807 attribute DOMString conditionText;
808 readonly attribute CSSRuleList cssRules;
809 unsigned long insertRule (DOMString rule, unsigned long index);
810 void deleteRule (unsigned long index);
811 }</pre>
813 <dl class='idl-attributes'>
814 <dt><code>conditionText</code> of type <code>DOMString</code>
815 <dd>The <code>conditionText</code> attribute, on getting, must return
816 the result of serializing the associated condition.
818 <p>On setting the <code>conditionText</code> attribute these steps
819 must be run:
821 <ol>
822 <li>Trim the given value of white space.
823 <li>If the given value matches the grammar of the
824 appropriate condition production for the given rule,
825 replace the associated CSS condition with the given value.
826 <li>Otherwise, do nothing.
827 </ol>
829 <dt><code>cssRules</code> of type <code>CSSRuleList</code>, readonly
830 <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
831 object for the list of CSS rules nested inside the condition rule.
832 </dl>
834 <dl class='idl-methods'>
835 <dt><code>insertRule(DOMString rule, unsigned long index)</code>, returns
836 <code>unsigned long</code>
837 <dd>The <code>insertRule</code> operation must insert a CSS rule <var>rule</var>
838 into the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
840 <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
841 <dd>The <code>deleteRule</code> operation must remove a CSS rule from the
842 CSS rule list returned by <code>cssRules</code> at <var>index</var>.
843 </dl>
846 <h3 id="the-cssmediarule-interface">
847 The <code>CSSMediaRule</code> interface</h3>
849 <p>The <dfn><code>CSSMediaRule</code></dfn> interface represents a ''@media'' rule:
851 <pre class='idl'>interface CSSMediaRule : CSSConditionRule {
852 readonly attribute MediaList media;
853 }</pre>
855 <dl class='idl-attributes'>
856 <dt><code>media</code> of type <code>MediaList</code>, readonly
857 <dd>The <code>media</code> attribute must return a <code>MediaList</code> object
858 for the list of media queries specified with the ''@media'' rule.
860 <dt><code>conditionText</code> of type <code>DOMString</code>
861 <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
862 on getting, must return the value of <code>media.mediaText</code> on the rule.
864 <p>Setting the <code>conditionText</code> attribute
865 must set the <code>media.mediaText</code> attribute on the rule.
866 </dl>
869 <h3 id="the-csssupportsrule-interface">
870 The <code>CSSSupportsRule</code> interface</h3>
872 <p>The <dfn><code>CSSSupportsRule</code></dfn> interface represents a ''@supports'' rule.</p>
874 <pre class='idl'>interface CSSSupportsRule : CSSConditionRule {
875 }</pre>
878 <h3 id="the-cssdocumentrule-interface">
879 The <code>CSSDocumentRule</code> interface</h3>
881 <p>The <dfn><code>CSSDocumentRule</code></dfn> interface represents a ''@document'' rule.</p>
883 <pre class='idl'>interface CSSDocumentRule : CSSConditionRule {
884 }</pre>
887 <h3 id='the-css-interface'>
888 The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>
890 <p>The <dfn id='CSS-interface'><code>CSS</code></dfn> interface holds useful CSS-related functions that do not belong elsewhere.
892 <pre class='idl'>interface CSS {
893 boolean supports(DOMString property, DOMString value);
894 boolean supports(DOMString declaration);
895 }</pre>
897 <dl class='idl-methods'>
898 <dt><code>supports(DOMString property, DOMString value)</code>,
899 returns <code>boolean</code>
900 <dt><code>supports(DOMString conditionText)</code>,
901 returns <code>boolean</code>
902 <dd>
903 When the <code title=''>supports()</code> method is invoked with two arguments <var>property</var> and <var>value</var>,
904 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,
905 and <var>value</var> would be successfully parsed as a supported value for that property.
906 Otherwise, it must return <code>false</code>.
908 <p>
909 When invoked with a single <var>conditionText</var> argument,
910 it must return <code>true</code> if <var>conditionText</var>,
911 when parsed and evaluated as a <code>supports_condition</code>,
912 would return true.
913 Otherwise, it must return <code>false</code>.
914 </dl>
917 <h2 id="conformance">Conformance</h2>
919 <h3 id="base-modules">Base Modules</h3>
921 <p>This specification defines conformance in terms of base modules,
922 which are modules that this specification builds on top of. The base
923 modules of this module are:</p>
925 <ul>
926 <li>[[!CSS21]]</li>
927 </ul>
929 <p>All of the conformance requirements of all base modules are
930 incorporated as conformance requirements of this module, except where
931 overridden by this module.</p>
933 <p>Additionally, all conformance requirements related to validity of
934 syntax in this module and all of its base modules are to be interpreted
935 as though all syntax in all of those modules is valid.</p>
937 <div class="example"><p>For example, this means that grammar presented
938 in modules other than [[!CSS21]] must obey the requirements that
939 [[!CSS21]] defines for the parsing of properties, and that requirements
940 for handling invalid syntax in [[!CSS21]] do not treat syntax added by
941 other modules as invalid.</p></div>
943 <p>Additionally, the set of valid syntax can be increased by the
944 conformance of a style sheet or processor to additional modules; use of
945 such syntax does not make a style sheet nonconformant and failure to
946 treat such syntax as invalid does not make a processor
947 nonconformant.</p>
949 <h3 id="conformance-classes">Conformance Classes</h3>
951 <p>Conformance to the CSS Conditional Rules Module is defined for three
952 conformance classes:
953 <dl>
954 <dt><dfn title="conformance::style sheet" id="conform-style-sheet">style sheet</dfn>
955 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
956 style sheet</a>.</dd>
957 <dt><dfn title="conformance::processor" id="conform-processor">processor</dfn></dt>
958 <dd>A tool that reads CSS style sheets: it may be a renderer or
959 <a
960 href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user-agent</a>
961 that interprets the semantics of a style sheet and renders
962 documents that use style sheets, or it may be a validator that
963 checks style sheets.</dd>
964 <dt><dfn title="conformance::authoring tool" id="conform-authoring-tool">authoring tool</dfn></dt>
965 <dd>A tool that writes a style sheet.</dd>
966 </dl>
968 <p>A style sheet is conformant to the CSS Conditional Rules Module
969 if it meets all of the conformance requirements in the module that are
970 described as requirements of style sheets.</p>
972 <p>A processor is conformant to the CSS Conditional Rules Module if it
973 meets all applicable conformance requirements in the module that are
974 described as requirements of processors. In general, all requirements
975 are applicable to renderers. Requirements concerning a part of CSS
976 not performed by a processor are not applicable, e.g., requirements
977 related to rendering are not applicable to a validator. The inability
978 of a processor to correctly render a document due to limitations of
979 the device does not make it non-conformant. (For example, a renderer
980 is not required to render color on a monochrome monitor.)</p>
982 <p>An authoring tool is conformant to the CSS Conditional Rules Module
983 if it writes style sheets that conform to the module and (if it reads
984 CSS) it is a conformant processor.</p>
986 <h3 id="partial">
987 Partial Implementations</h3>
989 <p>So that authors can exploit the forward-compatible parsing rules to
990 assign fallback values, CSS renderers <strong>must</strong>
991 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
992 as appropriate</a>) any at-rules, properties, property values, keywords,
993 and other syntactic constructs for which they have no usable level of
994 support. In particular, user agents <strong>must not</strong> selectively
995 ignore unsupported component values and honor supported values in a single
996 multi-value property declaration: if any value is considered invalid
997 (as unsupported values must be), CSS requires that the entire declaration
998 be ignored.</p>
1000 <h3 id="experimental">Experimental Implementations</h3>
1002 <p>To avoid clashes with future CSS features, the CSS specifications
1003 reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
1004 syntax</a> for proprietary property and value extensions to CSS. The CSS
1005 Working Group recommends that experimental implementations of features in
1006 CSS Working Drafts also use vendor-prefixed property or value names. This
1007 avoids any incompatibilities with future changes in the draft. Once a
1008 specification reaches the Candidate Recommendation stage, implementors
1009 should implement the non-prefixed syntax for any feature they consider to
1010 be correctly implemented according to spec.</p>
1012 <h3 id="cr-exit-criteria">CR Exit Criteria</h3>
1014 <p>For this specification to be advanced to Proposed Recommendation,
1015 there must be at least two independent, interoperable implementations
1016 of each feature. Each feature may be implemented by a different set of
1017 products, there is no requirement that all features be implemented by
1018 a single product. For the purposes of this criterion, we define the
1019 following terms:
1021 <dl>
1022 <dt>independent <dd>each implementation must be developed by a
1023 different party and cannot share, reuse, or derive from code
1024 used by another qualifying implementation. Sections of code that
1025 have no bearing on the implementation of this specification are
1026 exempt from this requirement.
1028 <dt>interoperable <dd>passing the respective test case(s) in the
1029 official CSS test suite, or, if the implementation is not a Web
1030 browser, an equivalent test. Every relevant test in the test
1031 suite should have an equivalent test created if such a user
1032 agent (UA) is to be used to claim interoperability. In addition
1033 if such a UA is to be used to claim interoperability, then there
1034 must one or more additional UAs which can also pass those
1035 equivalent tests in the same way for the purpose of
1036 interoperability. The equivalent tests must be made publicly
1037 available for the purposes of peer review.
1039 <dt>implementation <dd>a user agent which:
1041 <ol class=inline>
1042 <li>implements the specification.
1044 <li>is available to the general public. The implementation may
1045 be a shipping product or other publicly available version
1046 (i.e., beta version, preview release, or “nightly build”).
1047 Non-shipping product releases must have implemented the
1048 feature(s) for a period of at least one month in order to
1049 demonstrate stability.
1051 <li>is not experimental (i.e., a version specifically designed
1052 to pass the test suite and is not intended for normal usage
1053 going forward).
1054 </ol>
1055 </dl>
1057 <p>The specification will remain Candidate Recommendation for at least
1058 six months.
1060 <h2 class=no-num id="grammar">Grammar</h2>
1062 <p>In order to allow these new @-rules in CSS style sheets, this
1063 specification modifies the <code>stylesheet</code> production in the <a
1064 href="http://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
1065 [[!CSS21]] by replacing the <code>media</code> production defined in
1066 [[!CSS21]] with the <code>media</code> production defined in this one,
1067 and additionally inserting <code>| supports_rule | document_rule</code>
1068 alongside <code>ruleset | media | page</code>.</p>
1070 <h2 class=no-num id="acknowledgments">Acknowledgments</h2>
1072 <p>
1073 Thanks to the ideas and feedback from
1074 Tab Atkins,
1075 <span lang="tr">Tantek Çelik</span>,
1076 Alex Danilo,
1077 Elika Etemad,
1078 Pascal Germroth,
1079 <span lang="de">Björn Höhrmann</span>,
1080 Paul Irish,
1081 Vitor Menezes,
1082 Alex Mogilevsky,
1083 Chris Moschini,
1084 Ben Ward,
1085 Zack Weinberg,
1086 Estelle Weyl,
1087 Boris Zbarsky,
1088 and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
1090 </p>
1092 <h2 class=no-num id="references">References</h2>
1095 <h3 class="no-num" id="normative-references">Normative references</h3>
1096 <!--normative-->
1098 <h3 class="no-num" id="other-references">Other references</h3>
1099 <!--informative-->
1101 <h2 class="no-num" id="index">Index</h2>
1102 <!--index-->
1104 </body>
1105 </html>
1106 <!-- Keep this comment at the end of the file
1107 Local variables:
1108 mode: sgml
1109 sgml-declaration:"~/SGML/HTML4.decl"
1110 sgml-default-doctype-name:"html"
1111 sgml-minimize-attributes:t
1112 sgml-nofill-elements:("pre" "style" "br")
1113 sgml-live-element-indicator:t
1114 sgml-omittag:nil
1115 sgml-shorttag:nil
1116 sgml-namecase-general:t
1117 sgml-general-insert-case:lower
1118 sgml-always-quote-attributes:t
1119 sgml-indent-step:nil
1120 sgml-indent-data:t
1121 sgml-parent-document:nil
1122 sgml-exposed-tags:nil
1123 sgml-local-catalogs:nil
1124 sgml-local-ecat-files:nil
1125 End:
1126 -->
