csswg-drafts/css-conditional/Overview.html at 4ad330baf01bc7828f2834f08b1f3aef2ccd555e · w3c/csswg-drafts · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

<html lang=en>
 <head
  profile="http://dublincore.org/documents/2008/08/04/dc-html/ http://www.w3.org/2006/03/hcard">
  <meta content="text/html; charset=utf-8" http-equiv=Content-Type>
  <title>CSS Conditional Rules Module Level 3</title>

  <link href="http://purl.org/dc/terms/" rel=schema.dcterms>
  <link href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
   rel=dcterms.rights>
  <meta content="CSS Conditional Rules Module Level 3" name=dcterms.title>
  <meta content=text name=dcterms.type>
  <meta content=2014-05-02 name=dcterms.date>
  <meta content="L. David Baron" name=dcterms.creator>
  <meta content=W3C name=dcterms.publisher>
  <meta content="http://dev.w3.org/csswg/css3-conditional/"
   name=dcterms.identifier>
  <link href="../default.css" rel=stylesheet type="text/css">
  <style>.css::before, .css::after, .property::before, .property::after { content: none !important;}</style>
  <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
  <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
   type="text/css">
  <script defer=defer
   src="http://test.csswg.org/harness/annotate.js#css-conditional-3_dev"
   type="text/javascript"></script>

 <body class=h-entry>
  <div class=head> <!--begin-logo-->
   <p><a href="http://www.w3.org/"><img alt=W3C height=48
    src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->

   <h1 class=p-name>CSS Conditional Rules Module Level 3</h1>

   <h2 class="no-num no-toc">Editor's Draft <span class=dt-updated><span
    class=value-title title=20140502> 2 May 2014</span></span></h2>

   <dl>
    <dt>This version:

    <dd><a class=u-url href="http://dev.w3.org/csswg/css3-conditional/">
     http://www.w3.org/TR/2014/ED-css3-conditional-20140502/</a>

    <dt>Latest version:

    <dd><a
     href="http://www.w3.org/TR/css3-conditional/">http://www.w3.org/TR/css3-conditional/</a>

    <dt>Editor's draft:

    <dd><a
     href="http://dev.w3.org/csswg/css3-conditional/">http://dev.w3.org/csswg/css3-conditional/</a>
     (<a
     href="https://dvcs.w3.org/hg/csswg/log/tip/css-conditional/Overview.src.html">change
     log</a>, <a
     href="https://dvcs.w3.org/hg/csswg/log/tip/css3-conditional/Overview.src.html">older
     change log</a>)

    <dt>Previous version:

    <dd><a href="http://www.w3.org/TR/2013/CR-css3-conditional-20130404/"
     rel=previous>http://www.w3.org/TR/2013/CR-css3-conditional-20130404/</a>

    <dt>Editors:

    <dd class="p-author h-card vcard"><a class="p-name fn u-url url"
     href="http://dbaron.org/" rel=author>L. David Baron</a>, <a
     class="p-org org h-org" href="http://www.mozilla.org/">Mozilla</a>

    <dt>Issues list:

    <dd>Maintained in document (only editor's draft is current)

    <dt>Feedback:

    <dd><a
     href="mailto:www-style@w3.org?subject=%5Bcss3-conditional%5D%20feedback">www-style@w3.org</a>
     with subject line “<kbd>[css3-conditional] <var>… message topic
     …</var></kbd>” (<a
     href="http://lists.w3.org/Archives/Public/www-style/"
     rel=discussion>archives</a>)

    <dt>Test suite:

    <dd><a
     href="http://test.csswg.org/suites/css3-conditional/nightly-unstable/">http://test.csswg.org/suites/css3-conditional/nightly-unstable/</a>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> © 2014 <a href="http://www.w3.org/"><abbr
    title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
    href="http://www.csail.mit.edu/"><abbr
    title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
    href="http://www.ercim.eu/"><abbr
    title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
    <a href="http://www.keio.ac.jp/">Keio</a>, <a
    href="http://ev.buaa.edu.cn/">Beihang</a>), All Rights Reserved. W3C <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
    <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
    and <a
    href="http://www.w3.org/Consortium/Legal/copyright-documents">document
    use</a> rules apply.</p>
   <!--end-copyright-->
   <hr title="Separator for header">
  </div>

  <h2 class="no-num no-toc" id=abstract>Abstract</h2>

  <p>CSS is a language for describing the rendering of structured documents
   (such as HTML and XML) on screen, on paper, in speech, etc. <span
   class=p-summary> This module contains the features of CSS for conditional
   processing of parts of style sheets, conditioned on capabilities of the
   processor or the document the style sheet is being applied to. It includes
   and extends the functionality of CSS level 2 <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS level 1
   <a href="#ref-CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>. The main
   extensions compared to level 2 are allowing nesting of certain at-rules
   inside ‘<a href="#at-ruledef-media"><code
   class=css>@media</code></a>’, and the addition of the ‘<a
   href="#at-ruledef-supports"><code class=css>@supports</code></a>’ rule
   for conditional processing. </span>

  <h2 class="no-num no-toc" id=status>Status of this document</h2>
  <!--begin-status-->

  <p>This is a public copy of the editors' draft. It is provided for
   discussion only and may change at any moment. Its publication here does
   not imply endorsement of its contents by W3C. Don't cite this document
   other than as work in progress.

  <p>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a
   href="mailto:www-style@w3.org?Subject=%5Bcss3-conditional%5D%20PUT%20SUBJECT%20HERE">
   www-style@w3.org</a> (see <a
   href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
   discussion of this specification. When sending e-mail, please put the text
   “css3-conditional” in the subject, preferably like this:
   “[<!---->css3-conditional<!---->] <em>…summary of comment…</em>”

  <p>This document was produced by the <a
   href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of
   the <a href="http://www.w3.org/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
   2004 W3C Patent Policy</a>. W3C maintains a <a
   href="http://www.w3.org/2004/01/pp-impl/32061/status"
   rel=disclosure>public list of any patent disclosures</a> made in
   connection with the deliverables of the group; that page also includes
   instructions for disclosing a patent. An individual who has actual
   knowledge of a patent which the individual believes contains <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
   6 of the W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p>See the section <a href="#cr-exit-criteria">“CR Exit Criteria”</a>
   for details on advancing this specification to W3C Recommendation. The
   specification will remain Candidate Recommendation at least until 2
   October 2013. A <a
   href="http://test.csswg.org/suites/css3-conditional/nightly-unstable/report/">test
   suite and implementation report</a> are under development.

  <p>See the section <a href="#changes">“Changes”</a> for changes since
   the last Working Draft.

  <p>The following features are at risk:

  <ul>
   <li>The inclusion of ‘<code class=css>@font-face</code>’ rules and
    ‘<code class=css>@keyframes</code>’ rules as allowed within all of
    the @-rules in this specification is at risk, though only because of the
    relative rates of advancement of specifications. If this specification is
    able to advance faster than one or both of the specifications defining
    those rules, then the inclusion of those rules will move from this
    specification to the specification defining those rules.

   <li>The addition of support for @-rules inside of conditional grouping
    rules is at risk; if interoperable implementations are not found, it may
    be removed to advance the other features in this specification to
    Proposed Recommendation.

   <li>The ‘<a href="#at-ruledef-supports"><code
    class=css>@supports</code></a>’ rule is at risk; if interoperable
    implementations are not found, it may be removed to advance the other
    features in this specification to Proposed Recommendation.
  </ul>
  <!--

  Things to go in level 4:

  * Create some way to put these new conditional things on an @import.
  * The @document rule (commented out, down below).

-->

  <h2 class="no-num no-toc" id=contents>Table of contents</h2>
  <!--begin-toc-->

  <ul class=toc>
   <li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
    <ul class=toc>
     <li><a href="#context"><span class=secno>1.1. </span>Background</a>

     <li><a href="#placement"><span class=secno>1.2. </span>Module
      Interactions</a>

     <li><a href="#conventions"><span class=secno>1.3. </span>Document
      Conventions</a>
    </ul>

   <li><a href="#processing"><span class=secno>2. </span>Processing of
    conditional group rules</a>

   <li><a href="#contents-of"><span class=secno>3. </span>Contents of
    conditional group rules</a>

   <li><a href="#use"><span class=secno>4. </span>Placement of conditional
    group rules</a>

   <li><a href="#at-media"><span class=secno>5. </span>Media-specific style
    sheets: the ‘<code class=css>@media</code>’ rule</a>

   <li><a href="#at-supports"><span class=secno>6. </span>Feature queries:
    the ‘<code class=css>@supports</code>’ rule</a>
    <ul class=toc>
     <li><a href="#support-definition"><span class=secno>6.1.
      </span>Definition of support</a>
    </ul>

   <li><a href="#apis"><span class=secno>7. </span>APIs</a>
    <ul class=toc>
     <li><a href="#extentions-to-cssrule-interface"><span class=secno>7.1.
      </span> Extensions to the <code>CSSRule</code> interface</a>

     <li><a href="#the-cssgroupingrule-interface"><span class=secno>7.2.
      </span> The <code>CSSGroupingRule</code> interface</a>

     <li><a href="#the-cssconditionrule-interface"><span class=secno>7.3.
      </span> The <code>CSSConditionRule</code> interface</a>

     <li><a href="#the-cssmediarule-interface"><span class=secno>7.4. </span>
      The <code>CSSMediaRule</code> interface</a>

     <li><a href="#the-csssupportsrule-interface"><span class=secno>7.5.
      </span> The <code>CSSSupportsRule</code> interface</a>

     <li><a href="#the-css-interface"><span class=secno>7.6. </span> The
      <code>CSS</code> interface, and the <code title="">supports()</code>
      function</a>
    </ul>

   <li class=no-num><a href="#grammar">Grammar</a>

   <li><a href="#conformance"><span class=secno>8. </span>Conformance</a>
    <ul class=toc>
     <li><a href="#base-modules"><span class=secno>8.1. </span>Base
      Modules</a>

     <li><a href="#conformance-classes"><span class=secno>8.2.
      </span>Conformance Classes</a>

     <li><a href="#partial"><span class=secno>8.3. </span> Partial
      Implementations</a>

     <li><a href="#experimental"><span class=secno>8.4. </span>Experimental
      Implementations</a>

     <li><a href="#cr-exit-criteria"><span class=secno>8.5. </span>CR Exit
      Criteria</a>
    </ul>

   <li><a href="#changes"><span class=secno>9. </span> Changes</a>

   <li class=no-num><a href="#acknowledgments">Acknowledgments</a>

   <li class=no-num><a href="#references">References</a>
    <ul class=toc>
     <li class=no-num><a href="#normative-references">Normative
      references</a>

     <li class=no-num><a href="#other-references">Other references</a>
    </ul>

   <li class=no-num><a href="#index">Index</a>
  </ul>
  <!--end-toc-->

  <h2 id=introduction><span class=secno>1. </span>Introduction</h2>

  <h3 id=context><span class=secno>1.1. </span>Background</h3>

  <p><em>This section is not normative.</em>

  <p><a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
   defines one type of conditional group rule, the ‘<a
   href="#at-ruledef-media"><code class=css>@media</code></a>’ rule, and
   allows only style rules (not other @-rules) inside of it. The ‘<a
   href="#at-ruledef-media"><code class=css>@media</code></a>’ rule
   provides the ability to have media-specific style sheets, which is also
   provided by style sheet linking features such as ‘<code
   class=css>@import</code>’ and <code class=html>&lt;link&gt;</code>. The
   restrictions on the contents of ‘<a href="#at-ruledef-media"><code
   class=css>@media</code></a>’ rules made them less useful; they have
   forced authors using CSS features involving @-rules in media-specific
   style sheets to use separate style sheets for each medium.

  <p>This specification extends the rules for the contents of conditional
   group rules to allow other @-rules, which enables authors to combine CSS
   features involving @-rules with media specific style sheets within a
   single style sheet.

  <p>This specification also defines an additional type of conditional group
   rule, ‘<a href="#at-ruledef-supports"><code
   class=css>@supports</code></a>’, to address author and user
   requirements.

  <p>The ‘<a href="#at-ruledef-supports"><code
   class=css>@supports</code></a>’ rule allows CSS to be conditioned on
   implementation support for CSS properties and values. This rule makes it
   much easier for authors to use new CSS features and provide good fallback
   for implementations that do not support those features. This is
   particularly important for CSS features that provide new layout
   mechanisms, and for other cases where a set of related styles needs to be
   conditioned on property support.

  <h3 id=placement><span class=secno>1.2. </span>Module Interactions</h3>

  <p>This module replaces and extends the ‘<a
   href="#at-ruledef-media"><code class=css>@media</code></a>’ rule feature
   defined in <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> section <var>7.2.1</var> and
   incorporates the modifications previously made non-normatively by <a
   href="#ref-MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a> section
   <var>1</var>.

  <p>Its current definition depends on @-rules defined in <a
   href="#ref-CSS3-FONTS"
   rel=biblioentry>[CSS3-FONTS]<!--{{!CSS3-FONTS}}--></a> and <a
   href="#ref-CSS3-ANIMATIONS"
   rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>, but that
   dependency is only on the assumption that those modules will advance ahead
   of this one. If this module advances faster, then the dependency will be
   reversed.

  <h3 id=conventions><span class=secno>1.3. </span>Document Conventions</h3>

  <p>Conformance requirements are expressed with a combination of descriptive
   assertions and RFC 2119 terminology. The key words “MUST”, “MUST
   NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
   “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the
   normative parts of this document are to be interpreted as described in RFC
   2119. However, for readability, these words do not appear in all uppercase
   letters in this specification.

  <p>All of the text of this specification is normative except sections
   explicitly marked as non-normative, examples, and notes. <a
   href="#ref-RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>

  <p>Examples in this specification are introduced with the words “for
   example” or are set apart from the normative text with <code
   class=html>class="example"</code>, like this:

  <div class=example>
   <p>This is an example of an informative example.
  </div>

  <p>Informative notes begin with the word “Note” and are set apart from
   the normative text with <code class=html>class="note"</code>, like this:

  <p class=note>Note, this is an informative note.

  <h2 id=processing><span class=secno>2. </span>Processing of conditional
   group rules</h2>

  <p>This specification defines some CSS @-rules, called <dfn
   id=conditional-group-rules>conditional group rules</dfn>, that associate a
   condition with a group of other CSS rules. These different rules allow
   testing different types of conditions, but share common behavior for how
   their contents are used when the condition is true and when the condition
   is false.

  <div class=example>
   <p>For example, this rule:

   <pre>@media print {
  /* hide navigation controls when printing */
  #navigation { display: none }
}</pre>

   <p>causes a particular CSS rule (making elements with ID "navigation" be
    display:none) apply only when the style sheet is used for a print medium.
  </div>

  <p>Each conditional group rule has a condition, which at any time evaluates
   to true or false. When the condition is true, CSS processors
   <strong>must</strong> apply the rules inside the group rule as though they
   were at the group rule's location; when the condition is false, CSS
   processors <strong>must not</strong> apply any of rules inside the group
   rule. The current state of the condition does not affect the CSS object
   model, in which the contents of the group rule always remain within the
   group rule.

  <p>This means that when multiple conditional group rules are nested, a rule
   inside of both of them applies only when all of the rules' conditions are
   true.

  <div class=example>For example, with this set of nested rules:
   <pre>@media print { // rule (1)
  /* hide navigation controls when printing */
  #navigation { display: none }
  @media (max-width: 12cm) { // rule (2)
    /* keep notes in flow when printing to narrow pages */
    .note { float: none }
  }
}</pre>
   the condition of the rule marked (1) is true for print media, and the
   condition of the rule marked (2) is true when the width of the display
   area (which for print media is the page box) is less than or equal to
   12cm. Thus the rule ‘<code class=css>#navigation { display: none
   }</code>’ applies whenever this style sheet is applied to print media,
   and the rule ‘<code class=css>.note { float: none }</code>’ is applied
   only when the style sheet is applied to print media <em>and</em> the width
   of the page box is less than or equal to 12 centimeters.</div>

  <p>When the condition for a conditional group rule changes, CSS processors
   <strong>must</strong> reflect that the rules now apply or no longer apply,
   except for properties whose definitions define effects of computed values
   that persist past the lifetime of that value (such as for some properties
   in <a href="#ref-CSS3-TRANSITIONS"
   rel=biblioentry>[CSS3-TRANSITIONS]<!--{{CSS3-TRANSITIONS}}--></a> and <a
   href="#ref-CSS3-ANIMATIONS"
   rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>).

  <h2 id=contents-of><span class=secno>3. </span>Contents of conditional
   group rules</h2>

  <p>The syntax of each conditional group rule consists of some syntax
   specific to the type of rule followed by a <dfn id=group-rule-body>group
   rule body</dfn>, which is a block (pair of braces) containing a sequence
   of rules.

  <p>A group rule body is allowed to contain style rules and any @-rules that
   are allowed at the top level of a style sheet before and after a style
   rule. This means that @-rules that must occur at the beginning of the
   style sheet (such as ‘<code class=css>@charset</code>’, ‘<code
   class=css>@import</code>’, and ‘<code class=css>@namespace</code>’
   rules) are not allowed inside of conditional group rules. Conditional
   group rules can be nested.

  <p>In terms of the grammar, this specification defines the following
   productions for use in the grammar of conditional group rules:

  <p class=note>Note: Style rules are defined in grammars by the
   <code>ruleset</code> production.

  <pre><dfn id="nested_statement">nested_statement</dfn>
  : ruleset | <a
   href="#media"><i>media</i></a> | page | font_face_rule | keyframes_rule |
    <a
   href="#supports_rule"><i>supports_rule</i></a>
  ;

<dfn
   id="group_rule_body">group_rule_body</dfn>
  : '{' S* <a
   href="#nested_statement"><i>nested_statement</i></a>* '}' S*
  ;</pre>

  <p> in which all the productions are defined in that grammar with the
   exception of <code>font_face_rule</code> defined in <a
   href="#ref-CSS3-FONTS"
   rel=biblioentry>[CSS3-FONTS]<!--{{!CSS3-FONTS}}--></a>,
   <code>keyframes_rule</code> defined in <a href="#ref-CSS3-ANIMATIONS"
   rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>, and <a
   href="#media"><code>media</code></a> and <a
   href="#supports_rule"><code>supports_rule</code></a> defined in this
   specification.

  <p>In general, future CSS specifications that add new @-rules that are not
   forbidden to occur after some other types of rules should modify this <a
   href="#nested_statement"><code>nested_statement</code></a> production to
   keep the grammar accurate.

  <p>Style sheets <strong>must not</strong> use rules other than the allowed
   ones inside conditional group rules.

  <p>CSS processors <strong>must</strong> ignore rules that are not allowed
   within a group rule, and <strong>must</strong> handle invalid rules inside
   of group rules as described in <a
   href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section 4.2
   (Rules for handling parsing errors)</a>, <a
   href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
   (At-rules)</a>, and <a
   href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
   (Rule sets, declaration blocks, and selectors)</a> of <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.

  <h2 id=use><span class=secno>4. </span>Placement of conditional group rules</h2>

  <p>Conditional group rules are allowed at the top-level of a style sheet,
   and inside other conditional group rules. CSS processors
   <strong>must</strong> process such rules as <a
   href="#processing">described above</a>.

  <p>Any rules that are not allowed after a style rule (e.g., ‘<code
   class=css>@charset</code>’, ‘<code class=css>@import</code>’, or
   ‘<code class=css>@namespace</code>’ rules) are also not allowed after
   a conditional group rule. Therefore, style sheets <strong>must
   not</strong> place such rules after a conditional group rules, and CSS
   processors <strong>must</strong> ignore such rules.

  <h2 id=at-media><span class=secno>5. </span>Media-specific style sheets:
   the ‘<a href="#at-ruledef-media"><code class=css>@media</code></a>’
   rule</h2>

  <p>The <dfn data-dfn-type=at-rule id=at-ruledef-media>@media</dfn> rule is
   a conditional group rule whose condition is a media query. It consists of
   the at-keyword ‘<a href="#at-ruledef-media"><code
   class=css>@media</code></a>’ followed by a (possibly empty) media query
   list (as defined in <a href="#ref-MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>), followed by a group rule
   body. The condition of the rule is the result of the media query.

  <div class=example>
   <p>This ‘<a href="#at-ruledef-media"><code
    class=css>@media</code></a>’ rule:

   <pre>@media screen and (min-width: 35em),
       print and (min-width: 40em) {
  #section_navigation { float: left; width: 10em; }
}</pre>

   <p>has the condition ‘<code class=css>screen and (min-width: 35em),
    print and (min-width: 40em)</code>’, which is true for screen displays
    whose viewport is at least 35 times the initial font size and for print
    displays whose viewport is at least 40 times the initial font size. When
    either of these is true, the condition of the rule is true, and the rule
    ‘<code class=css>#section_navigation { float: left; width: 10em;
    }</code>’ is applied.
  </div>

  <p>In terms of the grammar, this specification extends the <a
   href="#media"><code>media</code></a> production in the <a
   href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
   href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix
   G) into:

  <pre><dfn id=media>media</dfn>
  : MEDIA_SYM S* media_query_list <a
   href="#group_rule_body"><i>group_rule_body</i></a>
  ;</pre>

  <p>where the <a href="#group_rule_body"><code>group_rule_body</code></a>
   production is defined in this specification, the
   <code>media_query_list</code> production is defined in <a
   href="#ref-MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>, and the
   others are defined in the <a
   href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
   href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix
   G).

  <h2 id=at-supports><span class=secno>6. </span>Feature queries: the ‘<a
   href="#at-ruledef-supports"><code class=css>@supports</code></a>’ rule</h2>

  <p>The <dfn data-dfn-type=at-rule id=at-ruledef-supports>@supports</dfn>
   rule is a conditional group rule whose condition tests whether the user
   agent supports CSS property:value pairs. Authors can use it to write style
   sheets that use new features when available but degrade gracefully when
   those features are not supported. CSS has existing mechanisms for graceful
   degradation, such as ignoring unsupported properties or values, but these
   are not always sufficient when large groups of styles need to be tied to
   the support for certain features, as is the case for use of new layout
   system features.

  <p>The syntax of the condition in the ‘<a
   href="#at-ruledef-supports"><code class=css>@supports</code></a>’ rule
   is slightly more complicated than for the other conditional group rules
   (though has some similarities to media queries) since:

  <ul>
   <li>negation is needed so that the new-feature styles and the fallback
    styles can be separated (within the forward-compatible grammar's rules
    for the syntax of @-rules), and not required to override each other

   <li>conjunction (and) is needed so that multiple required features can be
    tested

   <li>disjunction (or) is needed when there are multiple alternative
    features for a set of styles, particularly when some of those
    alternatives are vendor-prefixed properties or values
  </ul>

  <p>Therefore, the syntax of the ‘<a href="#at-ruledef-supports"><code
   class=css>@supports</code></a>’ rule allows testing for property:value
   pairs, and arbitrary conjunctions (and), disjunctions (or), and negations
   (not) of them.

  <p>This extends the lexical scanner in the <a
   href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
   href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix
   G) by adding:

  <pre>
@{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn
   id="supports_sym">SUPPORTS_SYM</dfn>;}
{O}{R}                    {return <dfn
   id=or>OR</dfn>;}
</pre>

  <p>This then extends the grammar in the <a
   href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>,
   using the lexical scanner there, with the additions of <code><a
   href="http://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a></code> and
   <code><a
   href="http://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a></code>
   tokens defined in the Media Queries specification <a href="#ref-MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a> and the <a
   href="#or"><code>OR</code></a> and <a
   href="#supports_sym"><code>SUPPORTS_SYM</code></a> tokens defined above,
   and with <code><a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a></code>,
   <code><a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a></code>,
   and <code><a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a></code>
   productions and the <code><a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a></code>
   token taken from the core syntax of CSS defined in <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1
   (Tokenization)</a> of <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, by adding:

  <pre><dfn id="supports_rule">supports_rule</dfn>
  : <a
   href="#supports_sym"><i>SUPPORTS_SYM</i></a> <a
   href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <a
   href="#supports_condition"><i>supports_condition</i></a> <a
   href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <a
   href="#group_rule_body"><i>group_rule_body</i></a>
  ;

<dfn
   id="supports_condition">supports_condition</dfn>
  : <a
   href="#supports_negation"><i>supports_negation</i></a> | <a
   href="#supports_conjunction"><i>supports_conjunction</i></a> | <a
   href="#supports_disjunction"><i>supports_disjunction</i></a> |
    <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a>
  ;

<dfn
   id="supports_condition_in_parens">supports_condition_in_parens</dfn>
  : ( '(' <a
   href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* <a
   href="#supports_condition"><i>supports_condition</i></a> <a
   href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>* ')' ) | <a
   href="#supports_declaration_condition"><i>supports_declaration_condition</i></a> |
    <a
   href="#general_enclosed"><i>general_enclosed</i></a>
  ;

<dfn
   id="supports_negation">supports_negation</dfn>
  : <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>+ <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a>
  ;

<dfn
   id="supports_conjunction">supports_conjunction</dfn>
  : <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a> ( <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>+ <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a> )+
  ;

<dfn
   id="supports_disjunction">supports_disjunction</dfn>
  : <a
   href="#supports_condition_in_parens"><i>supports_condition_in_pare
570E
ns</i></a> ( <a
   href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <a
   href="#or"><i>OR</i></a> <a
   href="http://www.w3.org/TR/CSS21/grammar.html#scanner"><i>S</i></a>+ <a
   href="#supports_condition_in_parens"><i>supports_condition_in_parens</i></a> )+
  ;

<dfn
   id="supports_declaration_condition">supports_declaration_condition</dfn>
  : '(' <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> ')'
  ;

<dfn
   id="general_enclosed">general_enclosed</dfn>
  : ( <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> )* ')'
  ;
</pre>

  <p> Implementations <strong>must</strong> parse ‘<a
   href="#at-ruledef-supports"><code class=css>@supports</code></a>’ rules
   based on the above grammar, and when interpreting the above grammar,
   <strong>must</strong> match the production before an <code>|</code>
   operator in preference to the one after it.

  <p> The above grammar is purposely very loose for forwards-compatibility
   reasons, since the <a
   href="#general_enclosed"><code>general_enclosed</code></a> production
   allows for substantial future extensibility. Any ‘<a
   href="#at-ruledef-supports"><code class=css>@supports</code></a>’ rule
   that does not parse according to the grammar above (that is, a rule that
   does not match this loose grammar which includes the <a
   href="#general_enclosed"><code>general_enclosed</code></a> production) is
   invalid. Style sheets <strong>must not</strong> use such a rule and
   processors <strong>must</strong> ignore such a rule (including all of its
   contents).

  <p>Each of these grammar terms is associated with a boolean result, as
   follows:

  <dl>
   <dt>supports_condition

   <dd> The result is the result of the single child term.

   <dt>supports_condition_in_parens

   <dd> The result is the result of the single <a
    href="#supports_condition"><code>supports_condition</code></a> or <a
    href="#supports_declaration_condition"><code>supports_declaration_condition</code></a>
    child term.

   <dt>supports_negation

   <dd> The result is the <em>negation</em> of the result of the <a
    href="#supports_condition_in_parens"><code>supports_condition_in_parens</code></a>
    child term.

   <dt>supports_conjunction

   <dd> The result is true if the result of <em>all</em> of the <a
    href="#supports_condition_in_parens"><code>supports_condition_in_parens</code></a>
    child terms is true; otherwise it is false.

   <dt>supports_disjunction

   <dd> The result is true if the result of <em>any</em> of the <a
    href="#supports_condition_in_parens"><code>supports_condition_in_parens</code></a>
    child terms is true; otherwise it is false.

   <dt>supports_declaration_condition

   <dd> The result is whether the CSS processor <a
    href="#support-definition">supports</a> the declaration within the
    parentheses.

   <dt>general_enclosed

   <dd> The result is always false. Additionally, style sheets <strong>must
    not</strong> write ‘<a href="#at-ruledef-supports"><code
    class=css>@supports</code></a>’ rules that match this grammar
    production. (In other words, this production exists only for future
    extensibility, and is not part of the description of a valid style sheet
    in this level of the specification.) <span class=note>Note that future
    levels may define functions or other parenthesized expressions that can
    evaluate to true.</span>
  </dl>

  <p>The condition of the ‘<a href="#at-ruledef-supports"><code
   class=css>@supports</code></a>’ rule is the result of the <a
   href="#supports_condition"><code>supports_condition</code></a> term that
   is a child of the <a href="#supports_rule"><code>supports_rule</code></a>
   term.

  <div class=example>
   <p>For example, the following rule

   <pre>@supports ( display: flex ) {
  body, #navigation, #content { display: flex; }
  #navigation { background: blue; color: white; }
  #article { background: white; color: black; }
}</pre>

   <p>applies the rules inside the ‘<a href="#at-ruledef-supports"><code
    class=css>@supports</code></a>’ rule only when ‘<code
    class=css>display: flex</code>’ is supported.
  </div>

  <div class=example>
   <p>The following example shows an additional ‘<a
    href="#at-ruledef-supports"><code class=css>@supports</code></a>’ rule
    that can be used to provide an alternative for when ‘<code
    class=css>display: flex</code>’ is not supported:

   <pre>@supports not ( display: flex ) {
  body { width: 100%; height: 100%; background: white; color: black; }
  #navigation { width: 25%; }
  #article { width: 75%; }
}</pre>

   <p>Note that the ‘<code class=property>width</code>’ declarations may
    be harmful to the flex-based layout, so it is important that they be
    present only in the non-flex styles.
  </div>

  <div class=example>
   <p>The following example checks for support for the ‘<code
    class=property>box-shadow</code>’ property, including checking for
    support for vendor-prefixed versions of it. When the support is present,
    it specifies both ‘<code class=property>box-shadow</code>’ (with the
    prefixed versions) and ‘<code class=property>border</code>’ in a way
    what would cause the box to become invisible were ‘<code
    class=property>box-shadow</code>’ not supported.

   <pre>.noticebox {
  border: 1px solid black;
  padding: 1px;
}
@supports ( box-shadow: 0 0 2px black inset ) or
          ( -moz-box-shadow: 0 0 2px black inset ) or
          ( -webkit-box-shadow: 0 0 2px black inset ) or
          ( -o-box-shadow: 0 0 2px black inset ) {
  .noticebox {
    -moz-box-shadow: 0 0 2px black inset;
    -webkit-box-shadow: 0 0 2px black inset;
    -o-box-shadow: 0 0 2px black inset;
    box-shadow: 0 0 2px black inset; /* unprefixed last */
    /* override the rule above the @supports rule */
    border: none;
    padding: 2px;
  }
}</pre>
  </div>

  <p>To avoid confusion between ‘<code class=css>and</code>’ and ‘<a
   href="#or"><code class=css>or</code></a>’, the syntax requires that both
   ‘<code class=css>and</code>’ and ‘<a href="#or"><code
   class=css>or</code></a>’ be specified explicitly (rather than, say,
   using commas or spaces for one of them). Likewise, to avoid confusion
   caused by precedence rules, the syntax does not allow ‘<code
   class=css>and</code>’, ‘<a href="#or"><code
   class=css>or</code></a>’, and ‘<code class=css>not</code>’ operators
   to be mixed without a layer of parentheses.

  <div class=example>
   <p>For example, the following rule is not valid:

   <pre class=illegal>@supports (transition-property: color) or
          (animation-name: foo) and
          (transform: rotate(10deg)) {
  // ...
}</pre>

   <p>Instead, authors must write one of the following:

   <pre>@supports ((transition-property: color) or
           (animation-name: foo)) and
          (transform: rotate(10deg)) {
  // ...
}</pre>

   <pre>@supports (transition-property: color) or
          ((animation-name: foo) and
           (transform: rotate(10deg))) {
  // ...
}</pre>
  </div>

  <p>Furthermore, whitespace is required after a ‘<code
   class=css>not</code>’ and on both sides of an ‘<code
   class=css>and</code>’ or ‘<a href="#or"><code
   class=css>or</code></a>’.

  <p>The declaration being tested must always occur within parentheses, when
   it is the only thing in the expression.

  <p>

  <div class=example>
   <p>For example, the following rule is not valid:

   <pre class=illegal>@supports display: flex {
  // ...
}</pre>

   <p>Instead, authors must write:

   <pre>@supports (display: flex) {
  // ...
}</pre>
  </div>

  <p>The syntax allows extra parentheses when they are not needed. This
   flexibility is sometimes useful for authors (for example, when commenting
   out parts of an expression) and may also be useful for authoring tools.

  <div class=example>
   <p>For example, authors may write:

   <pre>@supports ((display: flex)) {
  // ...
}</pre>
  </div>

  <p>A trailing ‘<code class=css>!important</code>’ on a declaration
   being tested is allowed, though it won't change the validity of the
   declaration.

  <div class=example>
   <p>For example, the following rule is valid:

   <pre>@supports (display: flex !important) {
  // ...
}</pre>
  </div>

  <h3 id=support-definition><span class=secno>6.1. </span>Definition of
   support</h3>

  <p>For forward-compatibility, <a
   href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
   (Declarations and properties)</a> of <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> defines rules for handling
   invalid properties and values. CSS processors that do not implement or
   partially implement a specification <strong>must</strong> treat any part
   of a value that they do not implement, or do not have a usable level of
   support for, as invalid according to this rule for handling invalid
   properties and values, and therefore <strong>must</strong> discard the
   declaration as a parse error.

  <p>A CSS processor is considered to <dfn id=dfn-support>support</dfn> a
   declaration (consisting of a property and value) if it accepts that
   declaration (rather than discarding it as a parse error). If a processor
   does not implement, with a usable level of support, the value given, then
   it <strong>must not</strong> accept the declaration or claim support for
   it.

  <p class=note>Note that properties or values whose support is effectively
   disabled by user preferences are still considered as supported by this
   definition. For example, if a user has enabled a high-contrast mode that
   causes colors to be overridden, the CSS processor is still considered to
   support the ‘<code class=property>color</code>’ property even though
   declarations of the ‘<code class=property>color</code>’ property may
   have no effect. On the other hand, a developer-facing preference whose
   purpose is to enable or disable support for an experimental CSS feature
   does affect this definition of support.

  <p>These rules (and the equivalence between them) allow authors to use
   fallback (either in the <a href="#ref-CSS1"
   rel=biblioentry>[CSS1]<!--{{CSS1}}--></a> sense of declarations that are
   overridden by later declarations or with the new capabilities provided by
   the ‘<a href="#at-ruledef-supports"><code
   class=css>@supports</code></a>’ rule in this specification) that works
   correctly for the features implemented. This applies especially to
   compound values; implementations must implement all parts of the value in
   order to consider the declaration supported, either inside a style rule or
   in the declaration condition of an ‘<a href="#at-ruledef-supports"><code
   class=css>@supports</code></a>’ rule.</p>
  <!--
<h2 id="at-document">Document queries: the '@document' rule</h2>

<p>The <dfn>'@document' rule</dfn> is a conditional group
rule whose condition depends on the
<a href="#url-of-doc">URL of the document being styled</a>.
This allows style sheets, particularly user style sheets, to have styles
that only apply to a set of pages rather than to all pages using the
style sheet.</p>

<p class="issue">Given that this @-rule is intended primarily for user
style sheets, what should this specification say about its use in author
style sheets?  Should it be forbidden?  Should use instead be
discouraged?  Or should this specification remain neutral on the
topic, since there are valid uses in author style sheets?</p>

<p id="url-of-doc">The <dfn>URL of the document being styled</dfn> is
the URI at which the document is located, excluding any fragment
identifiers.  (This means, for example, that HTTP redirects have been
followed.)  If the styles are being applied inside a complete document
embedded into the presentation of another (e.g., [[HTML5]]'s <code
class="html">iframe</code>, <code class="html">object</code>, or <code
class="html">img</code> elements), the relevant URI is that of the
frame, not of its container.  However, if content from other documents
is mixed in via mechanisms that mix content from one document into
another (e.g., [[SVG11]]'s <code>use</code> element), then the
address of the container document is used.</p>

<p class="note">Note:  In [[HTML5]], this is the