csswg-drafts/selectors/Overview.src.html at 53d3ce4d92d11a07540ef29a910f7033ca404f9a · 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
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
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en">
 <head>
  <meta charset=utf8>
  <title>Selectors Level 4</title>
  <link href="../default.css" type="text/css" rel="stylesheet">
  <style type="text/css">
    .tprofile td, th { vertical-align: baseline; padding: 0 0.5em; }
    .tprofile th { text-align: right; }
  </style>
  <link href="http://www.w3.org/StyleSheets/TR/W3C-WD.css" type="text/css" rel="stylesheet">
 </head>
 <body>
  <div class="head">
   <!--logo-->

   <h1 id="title">Selectors Level 4</h1>

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

   <dl>
    <dt>This version:
    <dd><a href="[VERSION]">
                 http://www.w3.org/TR/[YEAR]/ED-[SHORTNAME]-[CDATE]/</a>

    <dt>Editor's draft:
    <dd><a href="http://dev.w3.org/csswg/selectors4/">
                 http://dev.w3.org/csswg/selectors4/</a>

    <dt>Latest version of Selectors Level 4:
    <dd><a href="http://www.w3.org/TR/selectors4/">
                 http://www.w3.org/TR/selectors4/</a>

    <dt>Latest Selectors specification:
    <dd><a href="http://www.w3.org/TR/selectors/">
                 http://www.w3.org/TR/selectors/</a>

    <dt>Previous version:
    <dd><a href="http://www.w3.org/TR/2011/WD-selectors4-20110929/">
                 http://www.w3.org/TR/2011/WD-selectors4-20110929/</a>

  <dt>Feedback:
    <dd><a href="mailto:www-style@w3.org?subject=%5Bselectors4%5D%20feedback">www-style@w3.org</a>
        with subject line &ldquo;<kbd>[selectors4] <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
        (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/">archives</a>)

    <dt>Editors:
    <dd class="vcard"><a class="url fn" href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (Mozilla)
    <dd class='vcard'>
      <a class='url fn' href="http://www.xanthir.com/contact/">Tab Atkins Jr.</a> (<span class='org'>Google</span>)

    <dt>Previous Editors:
    <dd class="vcard"><a lang="tr" class="url fn" href="http://www.tantek.com/">Tantek &Ccedil;elik</a> (Microsoft)
    <dd class="vcard"><span class="fn">Daniel Glazman</span> (Disruptive Innovations SARL)
    <dd class="vcard"><span class="fn">Ian Hickson</span> (Opera Softare ASA)
    <dd class="vcard"><span class="fn">Peter Linss</span> (Netscape/AOL)
    <dd class="vcard"><span class="fn">John Williams</span> (Quark, Inc.)
   </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 a document. Selectors have been optimized for
  use with HTML and XML, and are designed to be usable in
  performance-critical code. They are a core component of
  <abbr title="Cascading Style Sheets">CSS</abbr> (Cascading
  Style Sheets), which uses Selectors to bind style properties to
  elements in the document.

  <p>Selectors Level 4 describes the selectors that already exist in
  [[!SELECT]], and further introduces new selectors for CSS and
  other languages that may need them.

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

  <!--status-->

  <p>This module is a somewhat-unstable Working Draft.
  <strong>If you are looking for a stable Selectors specification, use
  <a href="http://www.w3.org/TR/css3-selectors/">Selectors 3</a>.</strong>
  Read the <a href="http://www.w3.org/TR/CSS/">CSS Snapshot</a> for
  an overview of the CSS development process.
  See the <a href="#overview">Selectors Overview</a> for a summary of
  additions to level 3.

  <p>The following features are at-risk and may be dropped during the CR period
  if there is not sufficient implementer interest:
  the reference combinator,
  the column combinator,
  the '':drop()'' pseudo-class.

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

<h2 id="context">
Introduction</h2>

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

  <p>
    A selector is a boolean predicate
    that takes an element in a tree structure
    and tests whether the element matches the selector or not.

  <p>
    These expressions may be used for many things:

  <ul>
    <li>
      directly on an element to test whether it matches some criteria,
      such as in the <code>Element.matches()</code> function defined in [[SELECTORS-API2]]

    <li>
      applied to an entire tree of elements
      to filter it into a set of elements that match the criteria,
      such as in the <code>document.findAll()</code> function defined in [[SELECTORS-API2]]
      or the selector of a CSS style rule.

    <li>
      used "in reverse" to generate markup that would match a given selector,
      such as in <a href="http://haml.info/">HAML</a> or <a href="http://en.wikipedia.org/wiki/Zen_Coding">Zen Coding</a>
  </ul>


  <p>
    Selectors Levels 1, 2, and 3 are defined as the subsets of selector
    functionality defined in the <a href="http://www.w3.org/TR/REC-CSS1">CSS1</a>,
    <a href="http://www.w3.org/TR/CSS21/">CSS2.1</a>, and
    <a href="http://www.w3.org/TR/css3-selectors/">Selectors Level 3</a>
    specifications, respectively. This module defines Selectors Level 4.

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

  <p>This module replaces the definitions of
    and extends the set of selectors defined for CSS in [[SELECT]] and [[CSS21]].

  <p>
    Pseudo-element selectors,
    which define abstract elements in a rendering tree,
    are not part of this specification:
    their generic syntax is described here,
    but, due to their close integration with the rendering model and irrelevance to other uses such as DOM queries,
    they will be defined in other modules.

<h2 id="overview">
Selectors Overview</h2>

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

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

  <p>The following table summarizes the Selector syntax:

  <table class="data">
    <col class="pattern">
    <col class="meaning">
    <col class="section">
    <col class="level">
  <thead>
    <tr>
      <th>Pattern
      <th>Represents
      <th>Section
      <th>Level
  <tbody>
    <tr>
      <td><code>*</code>
      <td>any element
      <td><a href="#universal-selector">Universal selector</a>
      <td>2
    <tr>
      <td><code>E</code>
      <td>an element of type E
      <td><a href="#type-selectors">Type (tag name) selector</a>
      <td>1
  <tbody>
    <tr>
      <td><code>E:not(<var>s1</var>, <var>s2</var>)</code>
      <td>an E element that does not match either <i>compound selector</i> <var>s1</var>
          or <i>compound selector</i> <var>s2</var>
      <td><a href="#negation">Negation pseudo-class</a>
      <td>3/4
    <tr>
      <td><code>E:matches(<var>s1</var>, <var>s2</var>)</code>
      <td>an E element that matches <i>compound selector</i> <var>s1</var>
          and/or <i>compound selector</i> <var>s2</var>
      <td><a href="#matches">Matches-any pseudo-class</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E.warning</code>
      <td>an E element belonging to the class <code>warning</code>
          (the document language specifies how class is determined).
      <td><a href="#class-html">Class selectors</a>
      <td>1
    <tr>
      <td><code>E#myid</code>
      <td>an E element with ID equal to <code>myid</code>.
      <td><a href="#id-selectors">ID selectors</a>
      <td>1
    <tr>
      <td><code>E[foo]</code>
      <td>an E element with a <code>foo</code> attribute
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
    <tr>
      <td><code>E[foo="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value is
          exactly equal to <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
    <tr>
      <td><code>E[foo="bar" i]</code>
      <td>an E element whose <code>foo</code> attribute value is
          exactly equal to any (ASCII-range) case-permutation of <code>bar</code>
      <td><a href="#attribute-case">Attribute selectors: Case-sensitivity</a>
      <td>4
    <tr>
      <td><code>E[foo~="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value is
          a list of whitespace-separated values, one of which is
          exactly equal to <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
    <tr>
      <td><code>E[foo^="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value
          begins exactly with the string "bar"
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>3
    <tr>
      <td><code>E[foo$="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value
          ends exactly with the string <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>3
    <tr>
      <td><code>E[foo*="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value
          contains the substring <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>3
    <tr>
      <td><code>E[foo|="en"]</code>
      <td>an E element whose <code>foo</code> attribute value is
          a hyphen-separated list of values beginning with <code>en</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
  <tbody>
    <tr>
      <td><code>E:dir(ltr)</code>
      <td>an element of type E in with left-to-right directionality
        (the document language specifies how directionality is determined)
      <td><a href="#dir-pseudo">The :dir() pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:lang(zh, *-hant)</code>
      <td>an element of type E tagged as being either in Chinese
        (any dialect or writing system)
        or othewise written with traditional Chinese characters
      <td><a href="#lang-pseudo">The :lang() pseudo-class</a>
      <td>2/4
  <tbody>
    <tr>
      <td><code>E:any-link</code>
      <td>an E element being the source anchor of a hyperlink
      <td><a href="#any-link-pseudo">The hyperlink pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:link</code>
      <td>an E element being the source anchor of a hyperlink
          of which the target is not yet visited
      <td><a href="#link">The link history pseudo-classes</a>
      <td>1
    <tr>
      <td><code>E:visited</code>
      <td>an E element being the source anchor of a hyperlink
          of which the target is already visited
      <td><a href="#link">The link history pseudo-classes</a>
      <td>1
    <tr>
      <td><code>E:local-link</code>
      <td>an E element being the source anchor of a hyperlink
          of which the target is the current document
      <td><a href="#local-pseudo">The local link pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:local-link(0)</code>
      <td>an E element being the source anchor of a hyperlink
          of which the target is within the current domain
      <td><a href="#local-pseudo">The local link pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:target</code>
      <td>an E element being the target of the referring URL
      <td><a href="#target-pseudo">The target pseudo-class</a>
      <td>3
    <tr>
      <td><code>E:scope</code>
      <td>an E element being a designated reference element
      <td><a href="#scope-pseudo">The scope pseudo-class</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E:current</code>
      <td>an E element that is currently presented in a time-dimensional canvas
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:current(<var>s</var>)</code>
      <td>an E element that is the deepest <code>:current</code> element that
          matches selector <var>s</var>
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:past</code>
      <td>an E element that is in the past in a time-dimensional canvas
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:future</code>
      <td>an E element that is in the future in a time-dimensional canvas
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E:active</code>
      <td>an E element that is in an activated state
      <td><a href="#useraction-pseudos">The user action pseudo-classes</a>
      <td>1
    <tr>
      <td><code>E:hover</code>
      <td>an E element that is under the cursor,
          or that has a descendant under the cursor
      <td><a href="#useraction-pseudos">The user action pseudo-classes</a>
      <td>2
    <tr>
      <td><code>E:focus</code>
      <td>an E element that has user input focus
      <td><a href="#useraction-pseudos">The user action pseudo-classes</a>
      <td>2
    <tr>
      <td><code>E:active-drop</code>
      <td>an E element that will receive the item currently being dragged
      <td><a href="#drag-pseudos">The drag-and-drop pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:valid-drop</code>
      <td>an E element that could receive the item currently being dragged
      <td><a href="#drag-pseudos">The drag-and-drop pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:invalid-drop</code>
      <td>an E element that cannot receive the item currently being dragged, but could receive some other item
      <td><a href="#drag-pseudos">The drag-and-drop pseudo-classes</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E:enabled<br>E:disabled</code>
      <td>a user interface element E that is enabled or disabled, respectively
      <td><a href="#enableddisabled">The :enabled and :disabled pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:read-only</code><br><code>E:read-write</code>
      <td>a user interface element E that
      <td><a href="#rw-pseudos">The mutability pseudo-classes</a>
      <td>3-UI/4
    <tr>
      <td><code>E:placeholder-shown</code>
      <td>an input control currently showing placeholder text
      <td><a href="#rw-pseudos">The placeholder-shown pseudo-class</a>
      <td>3-UI/4
    <tr>
      <td><code>E:default</code>
      <td>a user interface element E that
      <td><a href="#default-pseudo">The default option pseudo-class :default</a>
      <td>3-UI/4
    <tr>
      <td><code>E:checked</code>
      <td>a user interface element E that is checked/selected
         (for instance a radio-button or checkbox)
      <td><a href="#checked">The selected-option pseudo-class</a>
      <td>3
    <tr>
      <td><code>E:indeterminate</code>
      <td>a user interface element E that is in an indeterminate state
          (neither checked nor unchecked)
      <td><a href="#indeterminate">The indeterminate-value pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:valid</code><br><code>E:invalid</code>
      <td>a user-input element E that
      <td><a href="#range-pseudos">The validity pseudo-classes</a>
      <td>3-UI/4
    <tr>
      <td><code>E:in-range</code><br><code>E:out-of-range</code>
      <td>a user-input element E whose value is in-range/out-of-range
      <td><a href="#range-pseudos">The range pseudo-classes</a>
      <td>3-UI/4
    <tr>
      <td><code>E:required</code><br><code>E:optional</code>
      <td>a user-input element E that requires/does not require input
      <td><a href="#opt-pseudos">The optionality pseudo-classes</a>
      <td>3-UI/4
    <tr>
      <td><code>E:required</code><br><code>E:optional</code>
      <td>a user-input element E with incorrect input
      <td><a href="#opt-pseudos">The optionality pseudo-classes</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E:root</code>
      <td>an E element, root of the document
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:empty</code>
      <td>an E element that has no children (not even text nodes)
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:blank</code>
      <td>an E element that has no content except maybe white space
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-child(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-last-child(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th child of its parent,
          counting from the last one
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:first-child</code>
      <td>an E element, first child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>2
    <tr>
      <td><code>E:last-child</code>
      <td>an E element, last child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:only-child</code>
      <td>an E element, only child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-of-type(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-last-of-type(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th sibling of its type,
          counting from the last one
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:first-of-type</code>
      <td>an E element, first sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:last-of-type</code>
      <td>an E element, last sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:only-of-type</code>
      <td>an E element, only sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-match(<var>n</var> of <var title=''>selector</var>)</code>
      <td>an E element, the <var>n</var>-th sibling matching <var title=''>selector</var>
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-last-match(<var>n</var> of <var title=''>selector</var>)</code>
      <td>an E element, the <var>n</var>-th sibling matching <var title=''>selector</var>,
          counting from the last one
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E F</code>
      <td>an F element descendant of an E element
      <td><a href="#descendant-combinators">Descendant combinator</a>
      <td>1
    <tr>
      <td><code>E > F</code>
      <td>an F element child of an E element
      <td><a href="#child-combinators">Child combinator</a>
      <td>2
    <tr>
      <td><code>E + F</code>
      <td>an F element immediately preceded by an E element
      <td><a href="#adjacent-sibling-combinators">Next-sibling combinator</a>
      <td>2
    <tr>
      <td><code>E ~ F</code>
      <td>an F element preceded by an E element
      <td><a href="#general-sibling-combinators">Following-sibling combinator</a>
      <td>3
    <tr>
      <td><code>E /foo/ F</code>
      <td>an F element ID-referenced by an E element's <code>foo</code> attribute
      <td><a href="#idref-combinators">Reference combinator</a>
      <td>4
    <tr>
      <td><code>!E > F</code>
      <td>an E element parent of an F element
      <td><a href="#subject">Determining the subject of a selector</a> +
          <a href="#child-combinators">Child combinator</a>
      <td>4
  <tbody>
    <tr>
      <td><code>F || E</code>
      <td>an E element that represents a cell in a grid/table
          belonging to a column represented by an element F
      <td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-column(<var>n</var>)</code>
      <td>an E element that represents a cell belonging to the
          <var>n</var>th column in a grid/table
      <td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-last-column(<var>n</var>)</code>
      <td>an E element that represents a cell belonging to the
          <var>n</var>th column in a grid/table, counting from the last one
      <td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
      <td>4
  </tbody></table>

  <p class="note">Some Level 4 selectors (noted above as "3-UI") were
    introduced in [[CSS3UI]].

<h3 id="profiles">
<i>Fast</i> vs <i>Complete</i> Selector Profiles</h3>

  <p>
    Selectors are used in many different contexts,
    with wildly varying performance characteristics.
    Some powerful selectors are unfortunately too slow
    to realistically include in the more performance-sensitive contexts.
    To accommodate this, two profiles of the Selectors spec are defined:

  <dl>
    <dt><dfn id='fast-profile'>fast</dfn>
    <dd>
      The <i>fast</i> profile is appropriate for use in any context,
      including dynamic browser CSS selector matching.
      It includes every selector defined in this document,
      except for:

      <ul>
        <li><i>Combinators</i> within <code>:matches()</code>, <code>:not()</code>, <code>:nth-match()</code>, and <code>:nth-last-match()</code>.
          <p class='issue'>We are considering whether this restriction should be lifted.
        <li>The <a href="#idref-combinators">reference combinator</a>
        <li>The <a href="#subject">subject indicator</a>
      </ul>

    <dt><dfn id='complete-profile'>complete</dfn>
    <dd>
      The <i>complete</i> profile is appropriate for contexts which aren't extremely performance sensitive.
      For example, implementations of the Selectors API specification [[SELECTORS-API]] should use the ''complete'' profile.
      It includes all of the selectors defined in this document.
  </dl>

  <p>CSS implementations conformant to Selectors Level 4 must use the ''fast'' profile for CSS selection.

  <p class='issue'>
    The categorization of things into the "fast" or "complete" profiles needs implementor review.
    If some things currently not in the fast profile can reasonably be done in CSS Selectors,
    we should move them.


<h2 id="syntax">
Selector Syntax and Structure</h2>

<h3 id="structure">
Structure and Terminology</h3>

  <p>The term <dfn>selector</dfn> can refer to a <i>simple selector</i>,
    <i>compound selector</i>, <i>complex selector</i>, or <i>selector list</i>.

  <p>A <i>selector list</i> is a comma-separated list of <i>selectors</i>;
    see <a href="#grouping">Selector Lists</a>.

  <p>A <dfn id="complex">complex selector</dfn> is a chain of one
  or more <i>compound selectors</i>
  separated by <i>combinators</i>.

  <p>A <dfn id="compound">compound selector</dfn>
  is a chain of <i>simple selectors</i>
  that are not separated by a <i>combinator</i>. It
  always begins with a <i>type selector</i> or a
  (possibly implied) <i>universal selector</i>.
  No other type selector or universal selector is allowed in the sequence.

  <p>
    A <dfn id="simple">simple selector</dfn> is either a
    <i>type selector</i>,
    <i>universal selector</i>,
    <i>attribute selector</i>,
    <i>class selector</i>,
    <i>ID selector</i>,
    or <i>pseudo-class</i>.

  <p>A <dfn>combinator</dfn> is punctuation that represents a particular
  kind of relationship between the <i>compound selectors</i> on either side.
  Combinators in Selectors level 4 include: whitespace, &quot;greater-than
  sign&quot; (U+003E, <code>></code>), &quot;plus sign&quot; (U+002B,
  <code>+</code>) and &quot;tilde&quot; (U+007E, <code>~</code>).
  <a href="#whitespace">White space</a> may appear between a combinator
  and the simple selectors around it.

  <p>An empty selector, containing no <i>compound selector</i>, is an
  <a href="#invalid">invalid selector</a>.

<h3 id='evaluating-selectors'>
Evaluating a Selector</h3>

  <p>A <i>selector</i> is evaluated against some initial list of elements,
  as determined by the context that the selector is evaluated in:
  the <dfn data-export>selector match list</dfn>.
  The selector is processed from left to right in order,
  with <i>simple selectors</i> filtering the <i>selector match set</i>,
  and <i>combinators</i> changing the <i>selector match set</i> into something new.
  When this process is done, the elements in the <i>selector match set</i>
  are the elements said to match the selector.

  <p>For example, to evaluate the selector "div > i.name" against a document,
  the <i>selector match list</i> is first set to all the elements in the entire document.
  Then, the "div" type selector is evaluated, filtering the <i>selector match list</i>
  to only contain elements with a tagname of "div".
  Then, the ">" child combinator is evaluated, transforming the <i>selector match list</i>
  by replacing each element currently in it with the element's children.
  Then, the "i" type selector is evaluated, filtering the <i>selector match list</i>
  to only contain elements with a tagname of "i".
  Finally, the ".name" class selector is evaluated, filter the <i>selector match list</i>
  to only contain elements with a class of "name".

  <p class='note'>
    Note that many implementations of selectors instead evaluate them right to left,
    as it's more efficient to do so in many cases.
    This, as usual, is completely valid,
    as long as it results in the same elements being returned
    as the spec's algorithm would.

  <p>Each context that a selector can be evaluated in
  must define the <dfn data-export>initial selector match list</dfn>,
  the list of elements that every selector in that context begins with.

  <p>For HTML,
  when evaluating a selector against a document,
  the <i>initial selector match list</i>
  is the root element and all of its descendants,
  ordered by a pre-order traversal.


<h3 id='data-model'>
Data Model</h3>

  <p>Selectors are matched against an <dfn data-export>element tree</dfn>,
  which is an abstract tree structure consisting of elements.

  Each element may have any of:

  <ul>
    <li>A type, which is a string.
    <li>An ID, which is a string.
    <li>Classes, which are strings.
    <li>Attributes, which are pairs of strings consisting of an attribute name and an attribute value.
    <li>A list of child elements.
    <li>Arbitrary additional information,
      which isn't directly accessible,
      but can be used in the matching of pseudo-classes and pseudo-elements.
  </ul>

  <p>In HTML,
  the DOM is used as the <i>element tree</i>.
  Each DOM element is an element in the <i>element tree</i>,
  with the DOM element's tagname as the type,
  and its ID, class list, attributes, and child elements
  as the ID, classes, attributes, and child elements in the <i>element tree</i>.
  The rest of the information in the DOM is also accessible
  as the arbitrary additional information in the <i>element tree</i>.

<h3 id="subject">
Determining the Subject of a Selector</h3>

  <p>The elements of a document tree that are represented by a selector
  are the <dfn title="subject">subjects</dfn> of the selector.

  <p>By default, the subjects of a selector are the elements represented
  by the last <i>compound selector</i> in the selector.
  Thus a selector consisting of a single <i>compound selector</i>
  represents any element satisfying its requirements
  Prepending another <i>compound selector</i> and a combinator
  to a sequence imposes additional matching constraints,
  so the subjects of the selector are always a subset of
  the elements represented by the last <i>compound selector</i>.

  <p>As a feature of the <i>complete</i> Selectors profile,
  the subject of the selector can be explicitly identified
  by prepending an exclamation mark (!)
  to one of the <i>compound selectors</i> in a selector.
  Although the element structure that the selector represents is the same
  with or without the exclamation mark, indicating the subject in this way
  can change which <i>compound selector</i> represents the subject in that structure.

  <p class="issue">
    Should the exclamation mark be prepended or appended to the subject? Or both?
    Or prepend two, to avoid the "! = not" issue?

  <div class="example">
  <p>For example, the following selector represents a list item <code>LI</code>
    unique child of an ordered list <code>OL</code>:
    <pre>OL > LI:only-child</pre>
    <p>However the following one represents an ordered list <code>OL</code>
    having a unique child, that child being a <code>LI</code>:
    <pre>!OL > LI:only-child</pre>
    <p>The tree structures represented by these two selectors are the same,
    but the subjects of the selectors are not.
  </div>

  of the selectors are the elements represented by all <i>compound selectors</i>
  prepended by a subject indicator.</p>

  <div class="example">
    <p>For example, the following selectors represents all paragraphs of class
      <code>warning</code> contained in a div AND their containing <code>div</code> elements:</p>
    <pre>!div > !p.warning</pre>
  </div>

<h3 id="scoping">
Scoped Selectors</h3>

  <p>Some host applications may choose to <dfn title="scope | scoped selector">scope</dfn> selectors
  to a particular subtree or fragment of the document.
  The root of the scoping subtree is called the <dfn>scoping root</dfn>,
  and may be either a true element (the <dfn>scoping element</dfn>)
  or a <dfn title="virtual scoping root">virtual</dfn> one (such as a <code>DocumentFragment</code>).

  <p>There are two methods of scoping selectors:

  <dl>
    <dt><dfn title="scope-contained | scope-contained selector">scope-contained selectors</dfn>
    <dd>
      With this method of scoping, selectors match as if
      the <i>scoping root</i> were the root of the document:
      all <i>compound selectors</i> must be represented within the scope.
      (The <code>:root</code> pseudo-class, however,
      still only matches the actual root of the document.)

    <dt><dfn title="scope-filtered | scope-filtered selector">scope-filtered</dfn> selectors
    <dd>
      With this method of scoping, a selector matches if
      the <i>subject</i> of the selector is within the scope,
      even if other components of the selector are outside the scope.
      (A <i>scoping element</i> is considered to be in scope.)
  </dl>

  <div class='example'>
    <p>
      For example,
      the <code>element.querySelector()</code> function defined in [[SELECTORS-API2]]
      allows the author to define a <i>scope-filtered</i> selector.

    <p>
      On the other hand, the selectors within an [[HTML5]] scoped stylesheet
      define <i>scope-contained</i> selectors.
  </div>

  <p>Unless otherwise specified,
  the <i>scoping root</i> also forms the <i>reference element set</i>
  and matches the <code>:scope</code> pseudo-class.

<h3 id="relative">
Relative Selectors</h3>

  <p>
    Certain contexts may accept <dfn title="relative selector | relative | scope-relative">relative selectors</dfn>,
    which are a shorthand for selectors that represent elements relative to a <i>reference element</i>
    (i.e. an element that matches <code>:scope</code>).
    In a <i>relative selector</i>,
    “<code>:scope</code> ” (the <code>:scope</code> pseudo-class followed by a space)
    is implied at the beginning of each <i>complex selector</i>
    that does not already contain the <code>:scope</code> pseudo-class.
    This allows the selector to begin syntactically with a <i>combinator</i>.
    However, it must be <a href="#absolutize">absolutized</a> before matching.

  <p>
    Relative selectors, once absolutized, can additionally be <i>scope-contained</i>
    (such as within the <code>::distributed()</code> pseudo-class [[SHADOWDOM]])
    or <i>scope-filtered</i>.

<h4 id='absolutizing'>
Absolutizing a Relative Selector</h4>

  <p>
    To <dfn id='absolutize'>absolutize a relative selector</dfn>:

  <p>
    If the <i>reference element set</i> is empty or consists of one or more elements:

  <ol>
    <li>
      If the selector starts with a <i>combinator</i> other than the <i>descendant combinator</i>,
      prepend <code>:scope</code> as the initial <i>compound selector</i>.

    <li>
      Otherwise, if the selector does not contain any instance of the <code>:scope</code> pseudo-class
      (either at the top-level or as an argument to a functional pseudo-class),
      prepend <code>:scope</code> followed by the <i>descendant combinator</i>.

    <li>
      Otherwise, the selector is already absolute.
  </ol>

  <p class='issue'>
    The above algorithm is different from Selectors API: its step 2, which bailed if the reference element set was empty, has been removed.
    This is because handling an explicitly empty list should return nothing;
    if a reference element set was not given at all,
    Selectors API 2 should be amended to say that the selector must be absolute already.
    (Broken example: doing <code>document.find('img', [list-of-links]))</code> finds images that are descendants of links normally,
    but instead finds all images in the document if the list-of-links is empty.)

  <p>
    Otherwise, if the <i>reference element set</i> consists of only a <i>virtual scoping root</i>:

  <ol>
    <li>
      If the selector starts with a <i>child combinator</i>,
      remove the child combinator.
      The selector is now absolute,
      with the additional constraint that the first compound selector in the selector
      only matches elements without a parent.

    <li>
      Otherwise, if the selector starts with any combinator other than the <i>descendant combinator</i>,
      change the selector to '':not(*)''.
      <span class='note'>This is the shortest selector that is valid, but guaranteed to match nothing.</span>

    <li>
      Otherwise, the selector is already absolute.
  </ol>

  <p>
    Otherwise, this is a spec error.  Please report it to the relevant standards body.

  <p>
    To <dfn id='absolutize-list'>absolutize a relative selector list</dfn>,
    absolutize each relative selector in the list.


<h3 id="pseudo-classes">
Pseudo-classes</h3>

  <p>The <dfn>pseudo-class</dfn> concept is introduced to permit selection based on
  information that lies outside of the document tree
  or that can be awkward or impossible to express using the other simple selectors.

  <p>A <i>pseudo-class</i> always consists of a “colon”
  (<code>:</code>) followed by the name of the <i>pseudo-class</i> and, for
  <dfn>functional pseudo-classes</dfn>, by one or more arguments between parentheses
  (similar to CSS functions).
  White space is optionally allowed between the parentheses and the argument,
  but not between the pseudo-class name and the parentheses.
  If arguments are separated by commas, white space is optionally allowed
  before/after each comma.

  <p><i>Pseudo-classes</i> are allowed in all <i>compound selectors</i> contained in a selector.
  <i>Pseudo-classes</i> are allowed anywhere in a <i>compound selector</i>
  after the leading type selector or (possibly omitted) universal selector.
  <i>Pseudo-class</i> names are <a href="#case-sensitive">ASCII case-insensitive</a>.
  Some <i>pseudo-classes</i> are mutually exclusive
  (such that a <i>compound selector</i> containing them, while valid, will never match anything),
  while others can apply simultaneously to the same element.
  <i>Pseudo-classes</i> may be dynamic,
  in the sense that an element can acquire or lose a pseudo-class
  while a user interacts with the document.

  <p>Dynamic <i>pseudo-classes</i> classify elements on characteristics
  other than their name, attributes, or content,
  but rather on characteristics that cannot be deduced from the document tree.
  They do not appear in or modify the document source or document tree.

<h3 id="pseudo-elements">Pseudo-elements</h3>

  <p><dfn id='pseudo-element' title="pseudo-element">Pseudo-elements</dfn> create abstractions about the document tree
  beyond those specified by the document language.
  For instance, document languages do not offer mechanisms to access
  the first letter or first line of an element's content.
  <i>Pseudo-elements</i> allow authors to refer
  to this otherwise inaccessible information.
  <i>Pseudo-elements</i> may also provide authors a way
  to refer to content that does not exist in the source document
  (e.g., the <code>::before</code> and <code>::after</code> <i>pseudo-elements</i>
  give access to generated content in CSS [[CSS21]]).

  <p>A <i>pseudo-element</i> is made of two colons (<code>::</code>)
  followed by the name of the <i>pseudo-element</i>.
  <i>Pseudo-element</i> names are <a href="#case-sensitive">ASCII case-insensitive</a>.

  <p>This <code>::</code> notation was chosen in order to establish
  a discrimination between <i>pseudo-classes</i> (which subclass existing elements)
  and <i>pseudo-elements</i> (which are elements not represented in the document tree).
  However, for compatibility with existing style sheets,
  user agents must also accept the previous one-colon notation
  for <i>pseudo-elements</i> introduced in CSS levels 1 and 2
  (namely, <code>:first-line</code>, <code>:first-letter</code>,
  <code>:before</code> and <code>:after</code>).
  This compatibility notation is not allowed any other <i>pseudo-elements</i>.

  <p class="note">A future version of this specification may allow
  multiple pseudo-elements per selector.

  <p>Syntactically, a <i>pseudo-element</i> immediately follows
  the <i>compound selector</i> representing its <dfn>originating element</dfn>,
  i.e. the element to which it is associated.
  Unless otherwise overridden by the definition of the <i>pseudo-element</i>:
  <ul>
    <li>only one <i>pseudo-element</i> may appear per <i>complex selector</i>
    <li>the <i>pseudo-element</i> must appear after the <i>compound selector</i>
      that represents the <a href="#subject">subjects</a> of the selector
    <li>the <i>pseudo-element</i> may appear only if the subject of the selector
      is the last <i>compound selector</i> in the selector.
  </ul>

  <p>A <i>pseudo-element</i> may be immediately followed
  by any combination of the <a href="#useraction-pseudos">user action pseudo-classes</a>,
  in which case the <i>pseudo-element</i> is represented only when it is in the corresponding state.
  Whether these pseudo-classes can match on the <i>pseudo-element</i>
  depends on the <i>pseudo-class</i> and <i>pseudo-element</i>&rdquo;s definitions:
  unless otherwise-specified, none of these <i>pseudo-classes</i>
  will match on the <i>pseudo-element</i>.

  <div class="example">
    <p>For example, the <code>:hover</code> pseudo-class specifies that it
    can apply to any pseudo-element, i.e. <code>::first-line:hover</code>
    will match when the first line is hovered.
    However, since neither <code>:focus</code> nor <code>::first-line</code>
    define that <code>:focus</code> can apply to <code>::first-line</code>,
    the selector <code>::first-line:focus</code> will never match anything.
  </div>

  <p>The host language defines which pseudo-elements exist and their meaning.
  For CSS, [[CSS21]] defines the ::before, ::after, ::first-line and ::first-letter pseudo-elements.

<h3 id="case-sensitive">
Characters and case sensitivity</h3>

  <p>All Selectors syntax is case-insensitive within the ASCII range
  (i.e. [a-z]  and [A-Z] are equivalent),
  except for the following parts,
  which are not under the control of Selectors:
  the case-sensitivity of
  document language element names,
  attribute names,
  and attribute values
  depends on the document language.
  For example,
  <a href="http://www.whatwg.org/html/selectors.html#case-sensitivity">in HTML, element names are case-insensitive</a>,
  but in XML, they are case-sensitive.
  Case sensitivity of namespace prefixes is defined in [[!CSS3NAMESPACE]].
  Case sensitivity of <i>language ranges</i> is defined in the <code>:lang()</code> section.

  <p><dfn id="whitespace">White space</dfn> in Selectors consists of the
  code points 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 code points, such as EM SPACE (U+2003) and
  IDEOGRAPHIC SPACE (U+3000), are never part of white space.

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