csswg-drafts/selectors-3/Overview.src.html at 9d268ddc73297c528ccaec8f9b03dad2d8f4bdf2 · 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
10A25
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>
  <meta charset=utf8>
  <title>Selectors Level 3</title>
  <link href="../default.css" type="text/css" rel="stylesheet">
  <link href="https://www.w3.org/StyleSheets/TR/W3C-WD.css" type="text/css" rel="stylesheet">
 </head>
 <body>
  <div class="head">
   <!--logo-->

   <h1 id=title>Selectors Level 3</h1>

   <h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>

   <dl>

    <dt>This version:

    <dd>
    <!-- <a href="[VERSION]">
             https://www.w3.org/TR/[YEAR]/REC-[SHORTNAME]-[CDATE]</a> -->
    <a href="https://drafts.csswg.org/selectors3">
             https://drafts.csswg.org/selectors3</a>

    <dt>Latest version:

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

    <dt>Previous version:

    <dd><a href="https://www.w3.org/TR/2009/PR-css3-selectors-20091215/">
                 https://www.w3.org/TR/2009/PR-css3-selectors-20091215/</a>

    <dt>Feedback:
      <dd><a href="mailto:www-style@w3.org?subject=%5Bselectors3%5D%20feedback">www-style@w3.org</a>
          with subject line &ldquo;<kbd>[selectors3] <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
          (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/">archives</a>)

    <dt><a name=editors-list></a>Editors:

    <dd class="vcard"><a lang="tr" class="url fn" href="http://www.tantek.com/">Tantek &Ccedil;elik</a> (Invited Expert)

    <dd class="vcard"><a class="url fn" href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (Invited Expert)

    <dd class="vcard"><span class="fn">Daniel Glazman</span> (Disruptive Innovations SARL)</dd>

    <dd class="vcard"><a href="mailto:ian@hixie.ch" class="url fn">Ian Hickson</a> (<span
    class="company"><a href="http://www.google.com/">Google</a></span>)

    <dd class="vcard"><span class="fn">Peter Linss</span> (former editor, <span class="company"><a
    href="http://www.netscape.com/">Netscape/AOL</a></span>)

    <dd class="vcard"><span class="fn">John Williams</span> (former editor, <span class="company"><a
    href="http://www.quark.com/">Quark, Inc.</a></span>)

   </dl>

   <!--copyright-->

   <hr title="Separator for header">

  </div>

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

  <p><em>Selectors</em> are patterns that match against elements in a
  tree, and as such form one of several technologies that can be used
  to select nodes in an XML document. Selectors have been optimized for
  use with HTML and XML, and are designed to be usable in
  performance-critical code.</p>

  <p><acronym title="Cascading Style Sheets">CSS</acronym> (Cascading
  Style Sheets) is a language for describing the rendering of <acronym
  title="Hypertext Markup Language">HTML</acronym> and <acronym
  title="Extensible Markup Language">XML</acronym> documents on
  screen, on paper, in speech, etc. CSS uses Selectors for binding
  style properties to elements in the document.</p>

  <p>This document describes the selectors that already exist in <abbr
  title="CSS level 1">CSS1</abbr> [[CSS1]] and <abbr title="CSS level
  2">CSS2</abbr> [[!CSS21]], and further introduces new selectors for
  <abbr title="CSS level 3">CSS3</abbr> and other languages that may
  need them.</p>

  <p>Selectors define the following function:</p>

  <pre>expression &#x2217; element &rarr; boolean</pre>

  <p>That is, given an element and a selector, this specification
  defines whether that element matches the selector.</p>

  <p>These expressions can also be used, for instance, to select a set
  of elements, or a single element from a set of elements, by
  evaluating the expression across all the elements in a
  subtree. <acronym title="Simple Tree Transformation
  Sheets">STTS</acronym> (Simple Tree Transformation Sheets), a
  language for transforming XML trees, uses this mechanism. [[STTS3]]</p>

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

  <!--status-->

  <p>A separate <a
  href="/Style/CSS/Test/CSS3/Selectors/20091025/reports/CR-ImpReport.html">
  implementation report</a> contains a test suite and shows several
  implementations of the specification.

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

  <h2 id=context>Introduction</h2>

  <p>Selectors Level 1 and Selectors Level 2 are defined as the subsets of selector
    functionality defined in the <a href="https://www.w3.org/TR/REC-CSS1">CSS1</a>
    and <a href="https://www.w3.org/TR/CSS21/">CSS2.1</a> specifications,
    respectively.</p>

  <h3 id=dependencies>Dependencies</h3>

  <p>Some features of this specification are specific to CSS, or have
  particular limitations or rules specific to CSS. In this
  specification, these have been described in terms of CSS2.1. [[!CSS21]]</p>

  <h3 id=terminology>Terminology</h3>

  <p>All of the text of this specification is normative except
  examples, notes, and sections explicitly marked as
  non-normative.</p>

  <p>Additional terminology is defined in the
  <a href="https://www.w3.org/TR/CSS21/conform.html#defs">Definitions</a>
  section of [[!CSS21]].
  Examples of document source code and fragments are given in XML
  [[XML10]] or HTML [[HTML401]]
  syntax.

  <h3 id=changesFromCSS2>Changes from CSS2</h3>

  <p><em>This section is non-normative.</em></p>

  <p>The main differences between the selectors in CSS2 and those in
  Selectors are:

  <ul>

   <li>the list of basic definitions (selector, group of selectors,
   simple selector, etc.) has been changed; in particular, what was
   referred to in CSS2 as a simple selector is now called a sequence
   of simple selectors, and the term "simple selector" is now used for
   the components of this sequence</li>

   <li>an optional namespace component is now allowed in element type
   selectors, the universal selector and attribute selectors</li>

   <li>a <a href="#general-sibling-combinators">new combinator</a> has been introduced</li>

   <li>new simple selectors including substring matching attribute
   selectors, and new pseudo-classes</li>

   <li>new pseudo-elements, and introduction of the "::" convention
   for pseudo-elements</li>

   <li>the grammar has been rewritten</li>

   <li>profiles to be added to specifications integrating Selectors
   and defining the set of selectors which is actually supported by
   each specification</li>

   <li>Selectors are now a CSS3 Module and an independent
   specification; other specifications can now refer to this document
   independently of CSS</li>

   <li>the specification now has its own test suite</li>

  </ul>

<h2 id=selectors>Selectors</h2>

<p><em>This section is non-normative, as it merely summarizes the
following sections.</em></p>

<p>A Selector represents a structure. This structure can be used as a
condition (e.g. in a CSS rule) that determines which elements a
selector matches in the document tree, or as a flat description of the
HTML or XML fragment corresponding to that structure.</p>

<p>Selectors may range from simple element names to rich contextual
representations.</p>

<p>The following table summarizes the Selector syntax:</p>

<table class="selectorsReview">
  <thead>
  <tr>
    <th class="pattern">Pattern</th>
    <th class="meaning">Represents</th>
    <th class="described">Description</th>
    <th class="origin">Level</th></tr>
  <tbody>
  <tr>
    <td class="pattern">*</td>
    <td class="meaning">any element</td>
    <td class="described"><a
      href="#universal-selector">Universal
      selector</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E</td>
    <td class="meaning">an element of type E</td>
    <td class="described"><a
      href="#type-selectors">Type selector</a></td>
    <td class="origin">1</td></tr>
  <tr>
    <td class="pattern">E[foo]</td>
    <td class="meaning">an E element with a "foo" attribute</td>
    <td class="described"><a
      href="#attribute-selectors">Attribute
      selectors</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E[foo="bar"]</td>
    <td class="meaning">an E element whose "foo" attribute value is exactly
      equal to "bar"</td>
    <td class="described"><a
      href="#attribute-selectors">Attribute
      selectors</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E[foo~="bar"]</td>
    <td class="meaning">an E element whose "foo" attribute value is a list of
      whitespace-separated values, one of which is exactly equal to "bar"</td>
    <td class="described"><a
      href="#attribute-selectors">Attribute
      selectors</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E[foo^="bar"]</td>
    <td class="meaning">an E element whose "foo" attribute value begins exactly
      with the string "bar"</td>
    <td class="described"><a
      href="#attribute-selectors">Attribute
      selectors</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E[foo$="bar"]</td>
    <td class="meaning">an E element whose "foo" attribute value ends exactly
      with the string "bar"</td>
    <td class="described"><a
      href="#attribute-selectors">Attribute
      selectors</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E[foo*="bar"]</td>
    <td class="meaning">an E element whose "foo" attribute value contains the
      substring "bar"</td>
    <td class="described"><a
      href="#attribute-selectors">Attribute
      selectors</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E[foo|="en"]</td>
    <td class="meaning">an E element whose "foo" attribute has a hyphen-separated
      list of values beginning (from the left) with "en"</td>
    <td class="described"><a
      href="#attribute-selectors">Attribute
      selectors</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E:root</td>
    <td class="meaning">an E element, root of the document</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:nth-child(n)</td>
    <td class="meaning">an E element, the n-th child of its parent</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:nth-last-child(n)</td>
    <td class="meaning">an E element, the n-th child of its parent, counting
      from the last one</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:nth-of-type(n)</td>
    <td class="meaning">an E element, the n-th sibling of its type</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:nth-last-of-type(n)</td>
    <td class="meaning">an E element, the n-th sibling of its type, counting
      from the last one</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:first-child</td>
    <td class="meaning">an E element, first child of its parent</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E:last-child</td>
    <td class="meaning">an E element, last child of its parent</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:first-of-type</td>
    <td class="meaning">an E element, first sibling of its type</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:last-of-type</td>
    <td class="meaning">an E element, last sibling of its type</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:only-child</td>
    <td class="meaning">an E element, only child of its parent</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:only-of-type</td>
    <td class="meaning">an E element, only sibling of its type</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:empty</td>
    <td class="meaning">an E element that has no children (including text
    nodes)</td>
    <td class="described"><a
      href="#structural-pseudos">Structural
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:link<br>E:visited</td>
    <td class="meaning">an E element being the source anchor of a hyperlink of
      which the target is not yet visited (:link) or already visited
    (:visited)</td>
    <td class="described"><a
      href="#link">The link
      pseudo-classes</a></td>
    <td class="origin">1</td></tr>
  <tr>
    <td class="pattern">E:active<br>E:hover<br>E:focus</td>
    <td class="meaning">an E element during certain user actions</td>
    <td class="described"><a
      href="#useraction-pseudos">The user
      action pseudo-classes</a></td>
    <td class="origin">1 and 2</td></tr>
  <tr>
    <td class="pattern">E:target</td>
    <td class="meaning">an E element being the target of the referring URI</td>
    <td class="described"><a
      href="#target-pseudo">The target
      pseudo-class</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:lang(fr)</td>
    <td class="meaning">an element of type E in language "fr" (the document
      language specifies how language is determined)</td>
    <td class="described"><a
      href="#lang-pseudo">The :lang()
      pseudo-class</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E:enabled<br>E:disabled</td>
    <td class="meaning">a user interface element E which is enabled or
    disabled</td>
    <td class="described"><a
      href="#UIstates">The UI element states
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E:checked<!--<br>E:indeterminate--></td>
    <td class="meaning">a user interface element E which is checked<!-- or in an
      indeterminate state--> (for instance a radio-button or checkbox)</td>
    <td class="described"><a
      href="#UIstates">The UI element states
      pseudo-classes</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E::first-line</td>
    <td class="meaning">the first formatted line of an E element</td>
    <td class="described"><a
      href="#first-line">The ::first-line
      pseudo-element</a></td>
    <td class="origin">1</td></tr>
  <tr>
    <td class="pattern">E::first-letter</td>
    <td class="meaning">the first formatted letter of an E element</td>
    <td class="described"><a
      href="#first-letter">The ::first-letter
      pseudo-element</a></td>
    <td class="origin">1</td></tr>
  <tr>
    <td class="pattern">E::before</td>
    <td class="meaning">generated content before an E element</td>
    <td class="described"><a
      href="#gen-content">The ::before
      pseudo-element</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E::after</td>
    <td class="meaning">generated content after an E element</td>
    <td class="described"><a
      href="#gen-content">The ::after
      pseudo-element</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E.warning</td>
    <td class="meaning">an E element whose class is
"warning" (the document language specifies how class is determined).</td>
    <td class="described"><a
      href="#class-html">Class
    selectors</a></td>
    <td class="origin">1</td></tr>
  <tr>
    <td class="pattern">E#myid</td>
    <td class="meaning">an E element with ID equal to "m
898F
yid".</td>
    <td class="described"><a
      href="#id-selectors">ID
    selectors</a></td>
    <td class="origin">1</td></tr>
  <tr>
    <td class="pattern">E:not(s)</td>
    <td class="meaning">an E element that does not match simple selector s</td>
    <td class="described"><a
      href="#negation">Negation
      pseudo-class</a></td>
    <td class="origin">3</td></tr>
  <tr>
    <td class="pattern">E F</td>
    <td class="meaning">an F element descendant of an E element</td>
    <td class="described"><a
      href="#descendant-combinators">Descendant
      combinator</a></td>
    <td class="origin">1</td></tr>
  <tr>
    <td class="pattern">E &gt; F</td>
    <td class="meaning">an F element child of an E element</td>
    <td class="described"><a
      href="#child-combinators">Child
      combinator</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E + F</td>
    <td class="meaning">an F element immediately preceded by an E element</td>
    <td class="described"><a
      href="#adjacent-sibling-combinators">Next-sibling combinator</a></td>
    <td class="origin">2</td></tr>
  <tr>
    <td class="pattern">E ~ F</td>
    <td class="meaning">an F element preceded by an E element</td>
    <td class="described"><a
      href="#general-sibling-combinators">Following-sibling combinator</a></td>
    <td class="origin">3</td></tr></tbody></table>

<h2 id=casesens>Case sensitivity</h2>

<p>All Selectors syntax is case-insensitive within the ASCII range
(i.e. [a-z]  and [A-Z] are equivalent), except for parts that are
not under the control of Selectors. The case sensitivity of document
language element names, attribute names, and attribute values in
selectors depends on the document language. For example, in HTML,
element names are case-insensitive, but in XML, they are
case-sensitive. Case sensitivity of namespace prefixes is defined in
[[!CSS3NAMESPACE]].</p>

<h2 id=selector-syntax>Selector syntax</h2>

<p>A <dfn>selector</dfn> is a chain of one
or more <a href="#sequence">sequences of simple selectors</a>
separated by <a href="#combinators">combinators</a>.  One <a
href="#pseudo-elements">pseudo-element</a> may be appended to the last
sequence of simple selectors in a selector.</p>

<p>A <dfn><a name=sequence>sequence of simple selectors</a></dfn>
is a chain of <a href="#simple-selectors-dfn">simple selectors</a>
that are not separated by a <a href="#combinators">combinator</a>. It
always begins with a <a href="#type-selectors">type selector</a> or a
<a href="#universal-selector">universal selector</a>. No other type
selector or universal selector is allowed in the sequence.</p>

<p>A <dfn><a name=simple-selectors-dfn></a><a
href="#simple-selectors">simple selector</a></dfn> is either a <a
href="#type-selectors">type selector</a>, <a
href="#universal-selector">universal selector</a>, <a
href="#attribute-selectors">attribute selector</a>, <a
href="#class-html">class selector</a>, <a
href="#id-selectors">ID selector</a>, or <a
href="#pseudo-classes">pseudo-class</a>.</p>

<p><dfn>Combinators</dfn> are: whitespace, &quot;greater-than
sign&quot; (U+003E, <code>&gt;</code>), &quot;plus sign&quot; (U+002B,
<code>+</code>) and &quot;tilde&quot; (U+007E, <code>~</code>).  White
space may appear between a combinator and the simple selectors around
it. <a name=whitespace></a>Only the characters "space" (U+0020), "tab"
(U+0009), "line feed" (U+000A), "carriage return" (U+000D), and "form
feed" (U+000C) can occur in whitespace. Other space-like characters,
such as "em-space" (U+2003) and "ideographic space" (U+3000), are
never part of whitespace.</p>

<p>The elements of a document tree that are represented by a selector
are the <dfn><a name=subject></a>subjects of the selector</dfn>. A
selector consisting of a single sequence of simple selectors
represents any element satisfying its requirements. Prepending another
sequence of simple selectors and a combinator to a sequence imposes
additional matching constraints, so the subjects of a selector are
always a subset of the elements represented by the last sequence of
simple selectors.</p>

<p>An empty selector, containing no sequence of simple selectors and
no pseudo-element, is an <a href="#Conformance">invalid
selector</a>.</p>

<p>Characters in Selectors can be escaped with a backslash according
to the same <a href="https://www.w3.org/TR/CSS21/syndata.html#characters">escaping
rules</a> as CSS. [[!CSS21]].</p>

<p id="nsdecl">Certain selectors support namespace prefixes. The
mechanism by which namespace prefixes are <dfn>declared</dfn> should
be specified by the language that uses Selectors. If the language does
not specify a namespace prefix declaration mechanism, then no prefixes
are declared. In CSS, namespace prefixes are declared with the
<a href="https://www.w3.org/TR/css3-namespace/#declaration"><code>@namespace</code></a>
rule. [[!CSS3NAMESPACE]]</p>


<h2 id=grouping>Groups of selectors</h2>

<p>A comma-separated list of selectors represents the union of all
elements selected by each of the individual selectors in the list.
(A comma is U+002C.) For example, in CSS when several selectors share
the same declarations, they may be grouped into a comma-separated
list. White space may appear before and/or after the comma.</p>

<div class="example">
<p>CSS example:</p>
<p>In this example, we condense three rules with identical
declarations into one. Thus,</p>
<pre>h1 { font-family: sans-serif }
h2 { font-family: sans-serif }
h3 { font-family: sans-serif }</pre>
<p>is equivalent to:</p>
<pre>h1, h2, h3 { font-family: sans-serif }</pre>

<p><strong>Warning</strong>: the equivalence is true in this example
because all the selectors are valid selectors. If just one of these
selectors were invalid, the entire group of selectors would be
invalid. This would invalidate the rule for all three heading
elements, whereas in the former case only one of the three individual
heading rules would be invalidated.</p>

<div class="example">
<p>Invalid CSS example:</p>
<pre>h1 { font-family: sans-serif }
h2..foo { font-family: sans-serif }
h3 { font-family: sans-serif }</pre>
<p>is not equivalent to:</p>
<pre>h1, h2..foo, h3 { font-family: sans-serif }</pre>
<p>because the above selector (<code>h1, h2..foo, h3</code>)
is entirely invalid and the entire style rule is dropped. (When
the selectors are not grouped, only the rule for <code>h2..foo</code>
is dropped.)</p>
</div>

<h2 id=simple-selectors>Simple selectors</h2>

<h3 id=type-selectors>Type selector</h3>

<p>A <dfn>type selector</dfn> is the name of a document language
element type written using the syntax of
<a href="https://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified
names</a> [[!CSS3NAMESPACE]]. A type selector represents
an instance of the element type in the document tree. </p>

<div class="example">
 <p>Example:</p>
 <p>The following selector represents an <code>h1</code> element in the document tree:</p>
 <pre>h1</pre>
</div>


<h4 id=typenmsp>Type selectors and namespaces</h4>

<p>Type selectors allow an optional namespace component: a namespace
prefix that has been previously <a href="#nsdecl">declared</a> may be
prepended to the element name separated by the namespace separator
&quot;vertical bar&quot; (U+007C, <code>|</code>). (See, e.g., [[XML-NAMES]] for the use of namespaces in
XML.)</p>

<p>The namespace component may be left empty (no prefix before the
namespace separator) to indicate that the selector is only to
represent elements with no namespace.</p>

<p>An asterisk may be used for the namespace prefix, indicating that
the selector represents elements in any namespace (including elements
with no namespace).</p>

<p>Element type selectors that have no namespace component (no
namespace separator) represent elements without regard to the
element's namespace (equivalent to "<code>*|</code>") unless a default
namespace has been <a href="#nsdecl">declared</a>  for namespaced selectors (e.g. in CSS, in
the style sheet). If a default namespace has been declared, such
selectors will represent only elements in the default namespace.</p>

<p>A type selector containing a namespace prefix that has not been
previously <a href="#nsdecl">declared</a> for namespaced selectors is
an <a href="#Conformance">invalid</a> selector.</p>

<p>In a namespace-aware client, the name part of element type
selectors (the part after the namespace separator, if it is present)
will only match against the <a
href="https://www.w3.org/TR/REC-xml-names/#NT-LocalPart">local part</a>
of the element's <a
href="https://www.w3.org/TR/REC-xml-names/#ns-qualnames">qualified
name</a>.</p>

<p>In summary:</p>

<dl>
  <dt><code>ns|E</code></dt>
  <dd>elements with name E in namespace ns</dd>
  <dt><code>*|E</code></dt>
  <dd>elements with name E in any namespace, including those without a
  namespace</dd>
  <dt><code>|E</code></dt>
  <dd>elements with name E without a namespace</dd>
  <dt><code>E</code></dt>
  <dd>if no default namespace has been <a href="#nsdecl">declared</a>  for selectors, this is
  equivalent to *|E.  Otherwise it is equivalent to ns|E where ns is
  the default namespace.</dd>
</dl>

<div class="example">
 <p>CSS examples:</p>

 <pre>@namespace foo url(http://www.example.com);
 foo|h1 { color: blue }  /* first rule */
 foo|* { color: yellow } /* second rule */
 |h1 { color: red }      /* ...*/
 *|h1 { color: green }
 h1 { color: green }</pre>

 <p>The first rule (not counting the <code>@namespace</code> at-rule)
 will match only <code>h1</code> elements in the
 "http://www.example.com" namespace.</p>

 <p>The second rule will match all elements in the
 "http://www.example.com" namespace.</p>

 <p>The third rule will match only <code>h1</code> elements with
 no namespace.</p>

 <p>The fourth rule will match <code>h1</code> elements in any
 namespace (including those without any namespace).</p>

 <p>The last rule is equivalent to the fourth rule because no default
 namespace has been defined.</p>

</div>

<h3 id=universal-selector>Universal selector </h3>

<p>The <dfn>universal selector</dfn>, written as a
<a href="https://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified
name</a> [[!CSS3NAMESPACE]] with an asterisk (<code>*</code>
U+002A) as the local name, represents the qualified name of any element
type. It represents any single element in the document tree in any
namespace (including those without a namespace) if no default
namespace has been specified for selectors. If a default namespace has
been specified, see <a href="#univnmsp">Universal selector and
Namespaces</a> below.</p>

<p>If a universal selector represented by <code>*</code> (i.e. without a
namespace prefix) is not the only component of a <a href="#sequence">sequence
of simple selectors</a> selectors or is immediately followed by a
<a href="#pseudo-elements">pseudo-element</a>, then the <code>*</code> may
be omitted and the universal selector's presence implied.</p>

<div class="example">
 <p>Examples:</p>
 <ul>
  <li><code>*[hreflang|=en]</code> and <code>[hreflang|=en]</code> are equivalent,</li>
  <li><code>*.warning</code> and <code>.warning</code> are equivalent,</li>
  <li><code>*#myid</code> and <code>#myid</code> are equivalent.</li>
 </ul>
</div>

<p class="note"><strong>Note:</strong> it is recommended that the
<code>*</code> not be omitted, because it decreases the potential
confusion between, for example,
<code style="white-space: nowrap">div :first-child</code>
and <code style="white-space: nowrap">div:first-child</code>.
Here, <code style="white-space: nowrap">div *:first-child</code>
is more readable.</p>

<h4 id=univnmsp>Universal selector and namespaces</h4>

<p>The universal selector allows an optional namespace component. It
is used as follows:</p>

<dl>
 <dt><code>ns|*</code></dt>
 <dd>all elements in namespace ns</dd>
 <dt><code>*|*</code></dt>
 <dd>all elements</dd>
 <dt><code>|*</code></dt>
 <dd>all elements without a namespace</dd>
 <dt><code>*</code></dt>
 <dd>if no default namespace has been specified, this is equivalent to *|*.
 Otherwise it is equivalent to ns|* where ns is the default namespace.</dd>
</dl>

<p>A universal selector containing a namespace prefix that has not
been previously <a href="#nsdecl">declared</a> is an <a
href="#Conformance">invalid</a> selector.</p>


<h3 id=attribute-selectors>Attribute selectors</h3>

<p>Selectors allow the representation of an element's attributes. When
a selector is used as an expression to match against an element,
attribute selectors must be considered to match an element if that
element has an attribute that matches the attribute represented by the
attribute selector.</p>

<h4 id=attribute-representation>Attribute presence and value selectors</h4>

<p>CSS2 introduced four attribute selectors:</p>

<dl>
  <dt><code>[att]</code>
  <dd>Represents an element with the <code>att</code> attribute, whatever the value of
  the attribute.</dd>
  <dt><code>[att=val]</code></dt>
  <dd>Represents an element with the <code>att</code> attribute whose value is exactly
  "val".</dd>
  <dt><code>[att~=val]</code></dt>
  <dd>Represents an element with the <code>att</code> attribute whose value is a <a
  href="#whitespace">whitespace</a>-separated list of words, one of
  which is exactly "val". If "val" contains whitespace, it will never
  represent anything (since the words are <em>separated</em> by
  spaces). Also if "val" is the empty string, it will never represent
  anything.</dd>
  <dt><code>[att|=val]</code>
  <dd>Represents an element with the <code>att</code> attribute, its
  value either being exactly "val" or beginning with "val" immediately
  followed by "-" (U+002D).  This is primarily intended to allow
  language subcode matches (e.g., the <code>hreflang</code> attribute
  on the <code>a</code> element in HTML) as described in BCP 47 ([[BCP47]]) or its successor.  For
  <code>lang</code> (or <code>xml:lang</code>) language subcode
  matching, please see <a href="#lang-pseudo">the <code>:lang</code>
  pseudo-class</a>.</dd>
</dl>

<p>Attribute values must be CSS <a href="https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">identifiers</a>
or <a href="https://www.w3.org/TR/CSS21/syndata.html#strings">strings</a>. [[!CSS21]]
The case-sensitivity of attribute names and values in selectors depends on
the document language.</p>

<div class="example">

  <p>Examples:</p>

  <p>The following attribute selector represents an <code>h1</code>
  element that carries the <code>title</code> attribute, whatever its
  value:</p>

  <pre>h1[title]</pre>

  <p>In the following example, the selector represents a
  <code>span</code> element whose <code>class</code> attribute has
  exactly the value "example":</p>

  <pre>span[class="example"]</pre>

  <p>Multiple attribute selectors can be used to represent several
  attributes of an element, or several conditions on the same
  attribute. Here, the selector represents a <code>span</code> element
  whose <code>hello</code> attribute has exactly the value "Cleveland"
  and whose <code>goodbye</code> attribute has exactly the value
  "Columbus":</p>

  <pre>span[hello="Cleveland"][goodbye="Columbus"]</pre>

  <p>The following CSS rules illustrate the differences between
  "=" and "~=".  The first selector would match, for example, an
  <code>a</code> element with the value "copyright copyleft
  copyeditor" on a <code>rel</code> attribute. The second selector
  would only match an <code>a</code> element with an <code>href</code>
  attribute having the exact value "http://www.w3.org/".</p>

  <pre>a[rel~="copyright"] { ... }
a[href="http://www.w3.org/"] { ... }</pre>

  <p>The following selector represents an <code>a</code> element
  whose <code>hreflang</code> attribute is exactly "fr".</p>

  <pre>a[hreflang=fr]</pre>

  <p>The following selector represents an <code>a</code> element for
  which the value of the <code>hreflang</code> attribute begins with
  "en", including "en", "en-US", and "en-scouse":</p>

  <pre>a[hreflang|="en"]</pre>

  <p>The following selectors represent a <code>DIALOGUE</code> element
  whenever it has one of two different values for an attribute
  <code>character</code>:</p>

  <pre>DIALOGUE[character=romeo]
DIALOGUE[character=juliet]</pre>

</div>

<h4 id=attribute-substrings>Substring matching attribute
selectors</h4>

<p>Three additional attribute selectors are provided for matching
substrings in the value of an attribute:</p>

<dl>
  <dt><code>[att^=val]</code></dt>
  <dd>Represents an element with the <code>att</code> attribute whose value begins
  with the prefix "val". If "val" is the empty string then the selector does not
  represent anything.</dd>
  <dt><code>[att$=val]</code>
  <dd>Represents an element with the <code>att</code> attribute whose value ends with
  the suffix "val". If "val" is the empty string then the selector does not represent
  anything.</dd>
  <dt><code>[att*=val]</code>
  <dd>Represents an element with the <code>att</code> attribute whose value contains
  at least one instance of the substring "val". If "val" is the empty string then
  the selector does not represent anything.</dd>
</dl>

<p>Attribute values must be CSS <a href="https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">identifiers</a>
or <a href="https://www.w3.org/TR/CSS21/syndata.html#strings">strings</a>. [[!CSS21]]
The case-sensitivity of attribute names in selectors depends on the
document language.</p>

<div class="example">
 <p>Examples:</p>
 <p>The following selector represents an HTML <code>object</code>, referencing an
 image:</p>
 <pre>object[type^="image/"]</pre>
 <p>The following selector represents an HTML anchor <code>a</code> with an
 <code>href</code> attribute whose value ends with ".html".</p>
 <pre>a[href$=".html"]</pre>
 <p>The following selector represents an HTML paragraph with a <code>title</code>
 attribute whose value contains the substring "hello"</p>
 <pre>p[title*="hello"]</pre>
</div>

<h4 id=attrnmsp>Attribute selectors and namespaces</h4>

<p>The attribute name in an attribute selector is given as a
<a href="https://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified
name</a>: a namespace prefix that has been previously <a href="#nsdecl">declared</a>
may be prepended to the attribute name separated by the namespace
separator &quot;vertical bar&quot; (<code>|</code>). In keeping with
the Namespaces in the XML recommendation, default namespaces do not
apply to attributes, therefore attribute selectors without a namespace
component apply only to attributes that have no namespace (equivalent
to "<code>|attr</code>"). An asterisk may be used for
the namespace prefix indicating that the selector is to match all
attribute names without regard to the attribute's namespace.

<p>An attribute selector with an attribute name containing a namespace
prefix that has not been previously <a href="#nsdecl">declared</a> is
an <a href="#Conformance">invalid</a> selector.</p>

<div class="example">
  <p>CSS examples:</p>
  <pre>@namespace foo "http://www.example.com";
[foo|att=val] { color: blue }
[*|att] { color: yellow }
[|att] { color: green }
[att] { color: green }</pre>

  <p>The first rule will match only elements with the attribute
  <code>att</code> in the "http://www.example.com" namespace with the
  value "val".</p>

  <p>The second rule will match only elements with the attribute
  <code>att</code> regardless of the namespace of the attribute
  (including no namespace).</p>

  <p>The last two rules are equivalent and will match only elements
  with the attribute <code>att</code> where the attribute is not
  in a namespace.</p>

</div>

<h4 id=def-values>Default attribute values in DTDs</h4>

<p>Attribute selectors represent attribute values in the document tree.
How that document tree is constructed is outside the scope of Selectors.
In some document formats default attribute values can be defined in a DTD or
elsewhere, but these can only be selected by attribute selectors if they
appear in the document tree. Selectors should be designed so that they
work whether or not the default values are included in the document tree.</p>

<p>For example, a XML UA may, but is <em>not</em> required to read an "external
subset" of the DTD but <em>is</em> required to look for default
attribute values in the document's "internal subset." (See, e.g., [[XML10]] for definitions of these subsets.) Depending
on the UA, a default attribute value defined in the external subset of the
DTD might or might not appear in the document tree.</p>

<p>A UA that recognizes an XML namespace may, but is not required to use its
knowledge of that namespace to treat default attribute values as if
they were present in the document. (For example, an XHTML UA is not
required to use its built-in knowledge of the XHTML DTD. See, e.g., [[XML-NAMES]] for details on namespaces in XML
1.0.)</p>

<p class="note"><strong>Note:</strong> Typically, implementations
choose to ignore external subsets. This corresponds to the behaviour
of non-validating processors as defined by the XML specification.</p>

<div class="example">
<p>Example:</p>

<p>Consider an element <code>EXAMPLE</code> with an attribute <code>radix</code>
that has a default value of <code>"decimal"</code>. The DTD fragment might be</p>

<pre class="dtd-example">&lt;!ATTLIST EXAMPLE radix (decimal,octal) "decimal"></pre>

<p>If the style sheet contains the rules</p>

<pre>EXAMPLE[radix=decimal] { /*... default property settings ...*/ }
EXAMPLE[radix=octal]   { /*... other settings...*/ }</pre>

<p>the first rule might not match elements whose <code>radix</code> attribute is
set by default, i.e. not set explicitly. To catch all cases, the
attribute selector for the default value must be dropped:</p>

<pre>EXAMPLE                { /*... default property settings ...*/ }
EXAMPLE[radix=octal]   { /*... other settings...*/ }</pre>

<p>Here, because the selector <code>EXAMPLE[radix=octal]</code> is
more specific than the type selector alone, the style declarations in
the second rule will override those in the first for elements that
have a <code>radix</code> attribute value of <code>"octal"</code>. Care has to be taken that
all property declarations that are to apply only to the default case
are overridden in the non-default cases' style rules.</p>

</div>

<h3 id=class-html>Class selectors</h3>

<p>Working with HTML, authors may use the "period" notation (also
known as "full stop", U+002E, <code>.</code>) as an alternative to the
<code>~=</code> notation when representing the <code>class</code>
attribute. Thus, for HTML, <code>div.value</code> and
<code>div[class~=value]</code> have the same meaning. The attribute
value must immediately follow the full stop
(<code>.</code>).</p>

<p>UAs may apply selectors using the period (.) notation in XML
documents if the UA has namespace-specific knowledge that allows it to
determine which attribute is the "class" attribute for the
respective namespace. One such example of namespace-specific knowledge
is the prose in the specification for a particular namespace (e.g. SVG
1.1 [[SVG11]] describes the <a
href="https://www.w3.org/TR/SVG11/styling.html#ClassAttribute">SVG
<code>class</code> attribute</a> and how a UA should interpret it, and
similarly MathML [[MATHML3]] describes the <a
href="https://www.w3.org/TR/MathML3/chapter2.html#fund.globatt">MathML
<code>class</code> attribute</a>.)</p>

<div class="example">
 <p>CSS examples:</p>