csswg-drafts/css-conditional-3/Overview.bs at 31d3710ab09573c9ebb2e9851aa45a73f94861d3 · 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
383384
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
<pre class="metadata">
Title: CSS Conditional Rules Module Level 3
Group: csswg
Shortname: css-conditional
Level: 3
Status: ED
Work Status: Testing
ED: https://drafts.csswg.org/css3-conditional/
TR: https://www.w3.org/TR/css3-conditional/
Previous Version: https://www.w3.org/TR/2013/CR-css3-conditional-20130404/
Test Suite: http://test.csswg.org/suites/css3-conditional/nightly-unstable/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Link Defaults: css-color-3 (property) color
Abstract: 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&nbsp;2 [[!CSS21]], which builds on CSS level&nbsp;1
  [[CSS1]].  The main extensions compared to level&nbsp;2 are allowing nesting of
  certain at-rules inside '@media', and the addition of the '@supports' rule for
  conditional processing.
At Risk: The inclusion of @font-face rules and @keyframes 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.
At Risk: 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.
At Risk: The @supports 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.
</pre>
  <style>.css::before, .css::after, .property::before, .property::after { content: none !important;}</style>

<!--
  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 id="introduction">Introduction</h2>

<h3 id="context">Background</h3>

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

  <p>[[!CSS21]] defines one type of conditional group rule, the
  '@media' rule, and allows only style rules (not other @-rules)
  inside of it.  The '@media' rule provides the ability to
  have media-specific style sheets, which is also provided by style
  sheet linking features such as '@import' and
  <code class="html">&lt;link&gt;</code>.  The restrictions on the contents of
  '@media' 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>

  <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>

  <p>This specification also defines an additional type of conditional
  group rule, '@supports', to
  address author and user requirements.</p>

  <p>The '@supports' 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.</p>

<h3 id="placement">Module Interactions</h3>

  <p>This module replaces and extends the '@media' rule
  feature defined in [[!CSS21]] section 7.2.1 and
  incorporates the modifications previously made non-normatively by
  [[!MEDIAQ]] section 1.</p>

  <p>Its current definition depends on @-rules defined in [[!CSS3-FONTS]]
  and [[!CSS3-ANIMATIONS]], 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.</p>

  <p>In order to allow these new @-rules in CSS style sheets, this
  specification modifies the <code>stylesheet</code> production in the <a
  href="https://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
  [[!CSS21]] by replacing the <code>media</code> production defined in
  [[!CSS21]] with the <code>media</code> production defined in this one,
  and additionally inserting <code>| supports_rule</code>
  alongside <code>ruleset | media | page</code>.</p>

<h2 id="processing">Processing of conditional group rules</h2>

<p>This specification defines some CSS @-rules, called <dfn export lt="conditional group rule">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.</p>

<div class="example">
<p>For example, this rule:</p>
<pre>
@media print {
  /* hide navigation controls when printing */
  #navigation { display: none }
}
</pre>
<p>causes a particular CSS rule
(making elements with ID &ldquo;navigation&rdquo; be display:none)
apply only when the style sheet is used for a print medium.
</div>


992E
<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>

<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.</p>

<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 ''#navigation { display: none }'' applies
whenever this style sheet is applied to print media, and the rule
''.note { float: none }'' 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 [[CSS3-TRANSITIONS]] and
[[!CSS3-ANIMATIONS]]).</p>

<h2 id="contents-of">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>group rule body</dfn>,
which is a block (pair of braces) containing a sequence of rules.</p>

<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 '@charset', '@import',
and '@namespace' rules) are not allowed inside of conditional group
rules.  Conditional group rules can be nested.</p>

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

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

<pre>
<dfn>nested_statement</dfn>
  : ruleset | <a>media</a> | page | font_face_rule | keyframes_rule |
    <a>supports_rule</a>
  ;

<dfn>group_rule_body</dfn>
  : '{' S* <a>nested_statement</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 [[!CSS3-FONTS]], <code>keyframes_rule</code> defined in
[[!CSS3-ANIMATIONS]], and <code>media</code> and <code>supports_rule</code>
defined in this specification.</p>

<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 <code>nested_statement</code> production to keep the grammar
accurate.</p>

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

<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="https://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section
4.2 (Rules for handling parsing errors)</a>, <a
href="https://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
(At-rules)</a>, and <a
href="https://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
(Rule sets, declaration blocks, and selectors)</a> of [[!CSS21]].</p>

<h2 id="use">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>

<p>Any rules that are not allowed after a style rule (e.g., ''@charset'',
''@import'', or ''@namespace'' 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.</p>

<h2 id="at-media">Media-specific style sheets:  the '@media' 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
'@media' followed by a (possibly empty) media query list (as
defined in [[!MEDIAQ]]), followed by a group rule body.  The condition
of the rule is the result of the media query.</p>

<div class="example">
<p>This '@media' rule:</p>
<pre>
@media screen and (min-width: 35em),
       print and (min-width: 40em) {
  #section_navigation { float: left; width: 10em; }
}
</pre>
<p>has the condition
''screen and (min-width: 35em), print and (min-width: 40em)'',
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
''#section_navigation { float: left; width: 10em; }''
is applied.</p>
</div>

<p>In terms of the grammar, this specification extends the
<code>media</code> production in the
<a href="https://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) into:
<pre>
<dfn>media</dfn>
  : MEDIA_SYM S* media_query_list <a>group_rule_body</a>
  ;
<p>where the <code>group_rule_body</code> production is defined in this
specification, the <code>media_query_list</code> production is defined
in [[!MEDIAQ]], and the others are defined in the <a
href="https://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G).

<h2 id="at-supports">Feature queries: the '@supports' 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>

<p>The syntax of the condition in the '@supports' rule is
slightly more complicated than for the other conditional group rules
(though has some similarities to media queries) since:</p>
<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>
  <li>conjunction (and) is needed so that multiple required features can
  be tested</li>
  <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</li>
</ul>

<p>Therefore, the syntax of the '@supports' rule allows
testing for property:value pairs, and arbitrary conjunctions (and),
disjunctions (or), and negations (not) of them.</p>

<p>This extends the lexical scanner in the
<a href="https://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) by adding:
<pre>
@{S}{U}{P}{P}{O}{R}{T}{S} {return <dfn>SUPPORTS_SYM</dfn>;}
{O}{R}                    {return <dfn>OR</dfn>;}
</pre>

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

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

<dfn>supports_condition</dfn>
  : <a>supports_negation</a> | <a>supports_conjunction</a> | <a>supports_disjunction</a> |
    <a>supports_condition_in_parens</a>
  ;

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

<dfn>supports_negation</dfn>
  : <a href="https://www.w3.org/TR/css3-mediaqueries/#syntax">NOT</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a>supports_condition_in_parens</a>
  ;

<dfn>supports_conjunction</dfn>
  : <a>supports_condition_in_parens</a> ( <a href="https://www.w3.org/TR/css3-mediaqueries/#syntax">AND</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a>supports_condition_in_parens</a> )+
  ;

<dfn>supports_disjunction</dfn>
  : <a>supports_condition_in_parens</a> ( <a>OR</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a>supports_condition_in_parens</a> )+
  ;

<dfn>supports_declaration_condition</dfn>
  : '(' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>* <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">declaration</a> ')' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>*
  ;

<dfn>general_enclosed</dfn>
  : ( <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">FUNCTION</a> | '(' ) ( <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">any</a> | <a href="https://www.w3.org/TR/CSS21/syndata.html#tokenization">unused</a> )* ')' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner">S</a>*
  ;
</pre>

<p>The <dfn data-dfn-type="type" data-export>&lt;supports-condition></dfn> production is defined as matching the <code>supports_condition</code> grammar, defined above.</p>

<p>
  Implementations <strong>must</strong> parse ''@supports'' 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>

<p>
  The above grammar is purposely very loose for forwards-compatibility reasons,
  since the <code>general_enclosed</code> production
  allows for substantial future extensibility.
  Any ''@supports'' rule that does not parse according to the grammar above
  (that is, a rule that does not match this loose grammar
  which includes the <code>general_enclosed</code> 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:</p>
<dl>
<dt>supports_condition</dt>
<dd>
  The result is the result of the single child term.
</dd>

<dt>supports_condition_in_parens</dt>
<dd>
  The result is the result of the single <code>supports_condition</code>
  or <code>supports_declaration_condition</code> child term.
</dd>

<dt>supports_negation</dt>
<dd>
  The result is the <em>negation</em> of the result of the
  <code>supports_condition_in_parens</code> child term.
</dd>

<dt>supports_conjunction</dt>
<dd>
  The result is true if the result of <em>all</em> of the
  <code>supports_condition_in_parens</code> child terms is true;
  otherwise it is false.
</dd>

<dt>supports_disjunction</dt>
<dd>
  The result is true if the result of <em>any</em> of the
  <code>supports_condition_in_parens</code> child terms is true;
  otherwise it is false.
</dd>

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

<dt>general_enclosed</dt>
<dd>
  The result is always false.
  Additionally, style sheets <strong>must not</strong>
  write ''@supports'' 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>
</dd>
</dl>

<p>The condition of the '@supports' rule is the result of the
<code>supports_condition</code> term that is a child of the
<code>supports_rule</code> term.</p>

<div class="example">
<p>For example, the following rule</p>
<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 '@supports' rule only when
''display: flex'' is supported.</p>
</div>

<div class="example">
<p>The following example shows an additional '@supports' rule that can
be used to provide an alternative for when ''display: flex'' is not
supported:</p>
<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 'width' declarations may be harmful to the
flex-based layout, so it is important that they be present only in
the non-flex styles.</p>
</div>

<div class="example">
<p>The following example checks for support for the 'box-shadow'
property, including checking for support for vendor-prefixed versions of
it.  When the support is present, it specifies both 'box-shadow' (with
the prefixed versions) and 'border' in a way what would cause the box to
become invisible were 'box-shadow' not supported.</p>
<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 ''and'' and ''or'', the syntax requires
that both ''and'' and ''or'' 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 ''and'', ''or'',
and ''not'' operators to be mixed without a layer of parentheses.</p>

<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:</p>
<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>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:</p>
<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.</p>

<div class="example">
<p>For example, authors may write:</p>
<pre>
@supports ((display: flex)) {
  // ...
}
</pre>
</div>

<p>A trailing ''!important'' 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">Definition of support</h3>

<p>For forward-compatibility,
<a href="https://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
(Declarations and properties)</a> of [[!CSS21]]
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>

<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>

<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 'color' property
even though declarations of the 'color' 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>

<p>These rules (and the equivalence between them) allow
authors to use fallback (either in the [[CSS1]] sense of declarations
that are overridden by later declarations or with the new capabilities
provided by the ''@supports'' 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 ''@supports'' 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]]&#39;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]]&#39;s <code>use</code> element), then the
address of the container document is used.</p>

<p class="note">Note:  In [[HTML5]], this is the
<a href="http://dev.w3.org/html5/spec/dom.html#documents">document's address</a>
of a document in a
<a href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing context</a>.</p>

<div class="issue">What form of normalization is done on URLs and domains
before matching?  In particular, this specification needs to describe:
<ul>
<li>what form is used for the <a href="#url-of-doc">URL of the document
being styled</a> (and what has been normalized in that form)</li>
<li>what normalization (if any) happens to the argument of each of the match
functions before the comparison that they describe and</li>
<li>whether the
comparison algorithm used is string comparison or some other URL
comparison algorithm.</li></ul></div>

<p>The '@document' rule's condition is written as a
comma-separated list of <dfn>URL matching functions</dfn>, and the
condition evaluates to true whenever any one of those functions
evaluates to true.  The following URL matching functions are
permitted:</p>

<dl>
  <dt><dfn id="url-exact" title="url()|URL matching functions::exact">&lt;url&gt;</dfn></dt>

  <dd>
    <p>The 'url()' function is the <dfn>exact url matching
    function</dfn>.  It evaluates to true whenever the <a
    href="#url-of-doc">URL of the document being styled</a> is exactly
    the URL given.</p>

    <p class="Note">The 'url()' function, since it is a core syntax
    element in CSS, is allowed (subject to different character
    limitations and thus escaping requirements) to contain an unquoted
    value (in addition to the string values that are allowed as
    arguments for all four functions).</p>

    <div class="example">
      <p>For example, this rule:</p>
      <pre>
      @document url("http://www.w3.org/Style/CSS/") {
        #summary { background: yellow; color: black}
      }
      </pre>
      <p>styles the <code class="html">summary</code> element on the page
      <code>http://www.w3.org/Style/CSS/</code>, but not on any other
      pages.</p>
    </div>
  </dd>

  <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The 'url-prefix()' function is the <dfn>url prefix
    matching function</dfn>.  It evaluates to true whenever the
    <a href="#url-of-doc">URL of the document being styled</a>
    has the argument to the function as an
    initial substring (which is true when the two strings are equal).
    When the argument is the empty string, it evaluates to true for all
    documents.</p>
    <div class="example">
      <p>For example, this rule:</p>
      <pre>
      @document url-prefix("http://www.w3.org/Style/CSS/") {
        #summary { background: yellow; color: black}
      }
      </pre>
      <p>styles the <code class="html">summary</code> element on the page
      <code>http://www.w3.org/Style/CSS/</code> and on the page
      <code>http://www.w3.org/Style/CSS/Test</code>, but it does not
      affect the page <code>http://www.w3.org/</code> or the page
      <code>http://www.example.com/Style/CSS/</code>.</p>
    </div>
  </dd>

  <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The 'domain()' function is the <dfn>domain
    matching function</dfn>.  It evaluates to true whenever
    the <a href="#url-of-doc">URL of the document being styled</a>
    has a host subcomponent (as defined in [[!URI]])
    and that host subcomponent is exactly the argument to the
    'domain()' function or a final substring of the host
    component is a period (U+002E) immediately followed by the argument
    to the 'domain()' function.</p>
    <div class="example">
      <p>For example, this rule:</p>
      <pre>
      @document domain("w3.org") {
        body { font-size: 16px ! important }
      }
      </pre>
      <p>changes the font size of the body element for pages such as
      <code>http://www.w3.org/Style/CSS/</code> and
      <code>http://w3.org/Style/CSS/</code> and
      <code>http://lists.w3.org/Archives/Public/www-style/</code>
      but it does not affect the page
      <code>http://www.example.com/Style/CSS/</code>.</p>
    </div>
  </dd>

  <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(&lt;string&gt;)</dfn></dt>

<
1898
/div>
  <dd>
    <p>The contents of the &lt;string&gt; argument <strong>must</strong>
    match the JavaScript <code>Pattern</code> production
    ([[!ECMA-262-5.1]], section 15.10.1).  However,
    failing to do so is not a CSS syntax error and does not trigger any
    error handling for CSS syntax errors.</p>

    <p>The ''regexp()'' function evaluates to true whenever the string
    argument compiled as a JavaScript regular expression with the
    <code>global</code>, <code>ignoreCase</code> and
    <code>multiline</code> flags <em>disabled</em>
    (see [[!ECMA-262-5.1]], sections 15.10.7.2 through 15.10.7.4)
    compiles successfully and the resulting regular expression matches
    the entirety of the
    <a href="#url-of-doc">URL of the document being styled</a>.</p>

    <p class="note">Note that regular expression must match the entire
    URL, not just a part of it.</p>

    <p class="note">Note that this definition intentionally matches the
    behavior of the <a
    href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code class="html">pattern</code>
    attribute</a> on the <code class="html">input</code> element
    in [[HTML5]].</p>

    <div class="example">
      <p>For example, this rule:</p>
      <pre>
      @document regexp("https://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
        body { font-size: 20px ! important }
      }
      </pre>
      <p>changes the font size of the body element for pages such as
      <code>https://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
      <p class="note">Note that the backslashes in the regular
      expression require CSS escaping as ''\\''.</p>
    </div>
  </dd>

</dl>

<p>Implementations <strong>must</strong> treat any unknown URL matching
functions as a syntax error, and thus ignore the '@document' rule.
<span class="issue">Should we instead have more complicated error
handling rules to make forward-compatibility work differently, or is
this rule the best solution for such future expansion anyway?</span></p>

<div class="issue">This syntax doesn't offer any ability to do negations,
which has been requested in <a
href="https://bugzilla.mozilla.org/show_bug.cgi?id=349813">Mozilla bug
349813</a>.  Use cases that people have wanted negations for
include:
<ul>
  <li>User style sheets that want a particular rule in general, but know
  that that rule does more harm than good on specific sites.</li>
  <li>Authors who have a rule that they want to apply to most of their
  pages, but wish to make a few exceptions for.</li>
</ul>
</div>

<p>This extends the lexical scanner in the
<a href="https://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) by adding:
<pre>@{D}{O}{C}{U}{M}{E}{N}{T}	{return DOCUMENT_SYM;}</pre>
<p>and the grammar by adding</p>
<pre>
<dfn>document_rule</dfn>
  : DOCUMENT_SYM S+ <a>url_match_fn</a> ( "," S* <a>url_match_fn</a> )* <a>group_rule_body</a>
  ;

<dfn>url_match_fn</dfn>
  : (URI | FUNCTION S* STRING S* ')' ) S*
  ;
</pre>
-->


<h2 id="apis">APIs</h2>

<h3 id='extentions-to-cssrule-interface'>
Extensions to the <code>CSSRule</code> interface</h3>

<p>The <code>CSSRule</code> interface is extended as follows:

<pre class='idl'>
partial interface CSSRule {
    const unsigned short SUPPORTS_RULE = 12;
    <!--
    const unsigned short DOCUMENT_RULE = 13;
    -->
};
</pre>


<h3 id='the-cssgroupingrule-interface'>
The <code>CSSGroupingRule</code> interface</h3>

<p>The {{CSSGroupingRule}} interface represents an at-rule that contains other rules nested inside itself.

<pre class='idl'>
[Exposed=Window]
interface CSSGroupingRule : CSSRule {
    readonly attribute <a href="https://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a> cssRules;
    unsigned long insertRule (CSSOMString rule, unsigned long index);
    void deleteRule (unsigned long index);
};
</pre>

<dl class='idl-attributes'>
  <dt><code>cssRules</code> of type <code><a href="https://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSRuleList">CSSRuleList</a></code>, readonly
  <dd>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code>
    object for the list of CSS rules nested inside the grouping rule.
</dl>

<dl class='idl-methods'>
  <dt><code>insertRule(CSSOMString rule, unsigned long index)</code>, returns
    <code>unsigned long</code>
  <dd>
    The <code>insertRule</code> operation must
    insert a CSS rule <var>rule</var>
    into the CSS rule list returned by <code>cssRules</code>,
    such that the inserted rule will be at position <var>index</var>,
    and any rules previously at <var>index</var> or higher
    will increase their index by one.
    It must throw INDEX_SIZE_ERR
    if index is greater than <code>cssRules.length</code>.
    It must throw SYNTAX_ERR
    if <var>rule</var> has a syntax error and is unparseable;
    this does not include syntax errors handled by error handling rules
    for constructs inside of the rule,
    but this does include cases where the string given
    does not parse into a single CSS rule (such as when the string is empty)
    or where there is anything other than whitespace or comments
    after that single CSS rule.
    It must throw HIERARCHY_REQUEST_ERR
    if the rule cannot be inserted at the location specified,
    for example, if an ''@import'' rule is inserted inside a group rule.

    <p>The return value is the <var>index</var> parameter.

  <dt><code>deleteRule (unsigned long index)</code>, return <code>void</code>
  <dd>
    The <code>deleteRule</code> operation must
    remove a CSS rule from
    the CSS rule list returned by <code>cssRules</code> at <var>index</var>.
    It must throw INDEX_SIZE_ERR
    if index is greater than or equal to <code>cssRules.length</code>.
</dl>


<h3 id="the-cssconditionrule-interface">
The <code>CSSConditionRule</code> interface</h3>

<p>The {{CSSConditionRule}} interface represents
all the &ldquo;conditional&rdquo; at-rules,
  which consist of a condition and a statement block.

<pre class='idl' export>
[Exposed=Window]
interface CSSConditionRule : CSSGroupingRule {
    attribute CSSOMString conditionText;
};
</pre>

<dl class='idl-attributes'>

  <dt><code>conditionText</code> of type <code>CSSOMString</code>
  <dd>
    <p>The <code>conditionText</code> attribute represents
    the condition of the rule.
    Since what this condition does
    varies between the derived interfaces of <code>CSSConditionRule</code>,
    those derived interfaces
    may specify different behavior for this attribute
    (see, for example, <code>CSSMediaRule</code> below).
    In the absence of such rule-specific behavior,
    the following rules apply:</p>

    <p>The <code>conditionText</code> attribute, on getting, must return
    the result of serializing the associated condition.

    <p>On setting the <code>conditionText</code> attribute these steps
      must be run:

    <ol>
      <li>Trim the given value of white space.
      <li>If the given value matches the grammar of the
        appropriate condition production for the given rule,
        replace the associated CSS condition with the given value.
      <li>Otherwise, do nothing.
    </ol>
</dl>


<h3 id="the-cssmediarule-interface">
The <code>CSSMediaRule</code> interface</h3>

<p>The {{CSSMediaRule}} interface represents a ''@media'' at-rule:

<pre class='idl'>
[Exposed=Window]
interface CSSMediaRule : CSSConditionRule {
    [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
};
</pre>

<dl class='idl-attributes'>
  <dt><code>media</code> of type {{MediaList}}, readonly
  <dd>The <code>media</code> attribute must return a {{MediaList}} object
    for the list of media queries specified with the ''@media'' at-rule.

  <dt><code>conditionText</code> of type <code>CSSOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
  <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
    on getting, must return the value of <code>media.mediaText</code> on the rule.

    <p>Setting the <code>conditionText</code> attribute
      must set the <code>media.mediaText</code> attribute on the rule.
</dl>


<h3 id="the-csssupportsrule-interface">
The <code>CSSSupportsRule</code> interface</h3>

<p>The {{CSSSupportsRule}} interface represents a ''@supports'' rule.</p>

<pre class='idl'>
[Exposed=Window]
interface CSSSupportsRule : CSSConditionRule {
};
</pre>

<dl class='idl-attributes'>
  <dt><code>conditionText</code> of type <code>CSSOMString</code> (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
  <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
    on getting, must return the condition that was specified,
    without any logical simplifications,
    so that the returned condition will evaluate to the same result
    as the specified condition
    in any conformant implementation of this specification
    (including implementations that implement future extensions
    allowed by the <a>general_enclosed</a> exensibility mechanism in this specification).
    In other words,
    token stream simplifications are allowed
    (such as reducing whitespace to a single space
    or omitting it in cases where it is known to be optional),
    but logical simplifications (such as removal of unneeded parentheses,
    or simplification based on evaluating results) are not allowed.

</dl>

<!--
<h3 id="the-cssdocumentrule-interface">
The <code>CSSDocumentRule</code> interface</h3>

<p>The {{CSSDocumentRule}} interface represents a ''@document'' rule.</p>

<pre class='idl'>
[Exposed=Window]
interface CSSDocumentRule : CSSConditionRule {
};
</pre>
-->


<h3 id='the-css-interface'>
The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>