-
Notifications
You must be signed in to change notification settings - Fork 791
Expand file tree
/
Copy pathselector.src
More file actions
1431 lines (1168 loc) · 50.9 KB
/
selector.src
File metadata and controls
1431 lines (1168 loc) · 50.9 KB
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
10BC0
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE HTML PUBLIC '-//W3C//DTD HTML 4.01//EN'>
<html lang="en">
<!-- $Id: selector.src,v 2.118 2010-09-29 15:53:39 bbos Exp $ -->
<head>
<title>Selectors</title>
</head>
<body>
<h1>Selectors</h1>
<h2><a name="pattern-matching">Pattern matching</a></h2>
<p>In CSS, pattern matching rules determine which style rules apply to
elements in the <a href="conform.html#doctree">document
tree</a>. These patterns, called <span class="index-inst"
title="selector">selectors,</span> may range from simple element names
to rich contextual patterns. If all conditions in the pattern are true
for a certain element, the selector <span class="index-def"
title="match|selector::match"><dfn>matches</dfn></span> the element.
<P>The case-sensitivity of document language element names in
selectors depends on the document language. For example, in HTML,
element names are case-insensitive, but in XML they are
case-sensitive.
<P>The following table summarizes CSS 2.1 selector syntax:</P>
<table border=1>
<TR><TH>Pattern<TH>Meaning<TH>Described in section</TR>
<TR><TD>*<TD>Matches any element.<TD><a href="#universal-selector">Universal
selector</a></TR>
<TR><TD>E<TD>Matches any E element (i.e., an element of type E).<TD><a href="#type-selectors">Type
selectors</a></TR>
<TR><TD>E F<TD>Matches any F element that is a descendant of
an E element.<TD><a href="#descendant-selectors">Descendant
selectors</a></TR>
<TR><TD>E > F<TD>Matches any F element that is a child of
an element E.<TD><a href="#child-selectors">Child selectors</a></TR>
<TR><TD>E:first-child<TD>Matches element E when E is the first
child of its parent.
<TD><a href="#first-child">The :first-child pseudo-class</a></TR>
<TR><TD>E:link<br>E:visited <TD>Matches element E if E is the source
anchor of a hyperlink of which the target is not yet visited (:link)
or already visited (:visited).
<TD><a href="#link-pseudo-classes">The link pseudo-classes</a></TR>
<TR><TD>E:active<br>E:hover<br>E:focus <TD>Matches E during certain
user actions.
<TD><a href="#dynamic-pseudo-classes">The dynamic pseudo-classes</a>
<TR><TD>E:lang(c) <TD>Matches element of type E if it is in (human) language c
(the document language specifies how language is determined).
<TD><a href="#lang">The :lang() pseudo-class</a>
<TR><TD>E + F<TD>Matches any F element immediately preceded by
a sibling element E.<TD><a href="#adjacent-selectors">Adjacent selectors</a>
</TR>
<TR><TD>E[foo]<TD>Matches any E element with the
"foo" attribute set (whatever the value).
<TD><a href="#attribute-selectors">Attribute selectors</a>
</TR>
<TR><TD>E[foo="warning"]<TD>Matches any E element whose
"foo" attribute value is exactly equal to "warning".
<TD><a href="#attribute-selectors">Attribute selectors</a>
</TR>
<TR><TD>E[foo~="warning"]<TD>Matches any E element whose
"foo" attribute value is a list of space-separated values, one of
which is exactly equal to "warning".
<TD><a href="#attribute-selectors">Attribute selectors</a>
</TR>
<TR><TD>E[lang|="en"]<TD>Matches any E element whose
"lang" attribute has a hyphen-separated list of values
beginning (from the left) with "en".
<TD><a href="#attribute-selectors">Attribute selectors</a>
</TR>
<TR><TD>DIV.warning<TD><em>Language specific.</em> (In HTML, the same
as DIV[class~="warning"].)
<TD><a href="#class-html">Class selectors</a></TR>
<TR><TD>E#myid<TD>Matches any E element with ID
equal to "myid".<TD><a href="#id-selectors">ID selectors</a></TR>
</table>
<h2><a name="selector-syntax">Selector syntax</a></h2>
<P>A <span class="index-def" title="simple selector"><a
name="simple-selector"><dfn>simple selector</dfn></a></span> is either
a <a href="#type-selectors">type selector</a> or <a
href="#universal-selector">universal selector</a> followed immediately
by zero or more <a href="#attribute-selectors">attribute
selectors</a>, <a href="#id-selectors">ID selectors</a>, or <a
href="#pseudo-classes">pseudo-classes</a>, in any order. The simple
selector matches if all of its components match.
<p class="note">Note: the terminology used here in CSS 2.1 is
different from what is used in CSS3. For example, a "simple selector"
refers to a smaller part of a selector in CSS3 than in CSS 2.1.
See the CSS3 Selectors module [[-CSS3SEL]].
<P>A <span class="index-def"
title="selector"><dfn>selector</dfn></span> is a chain of one or more
simple selectors separated by combinators. <span class="index-def"
title="combinator"><a
name="combinator"><dfn>Combinators</dfn></a></span> are: white space,
">", and "+". White space may appear between a combinator and the
simple selectors around it.
<P>The elements of the document tree that match a selector are called
<span class="index-def" title="subject (of selector)|selector::subject
of"><a name="subject"><dfn>subjects</dfn></a></span> of the selector.
A selector consisting of a single simple selector matches any element
satisfying its requirements. Prepending a simple selector and
combinator to a chain imposes additional matching constraints, so the
subjects of a selector are always a subset of the elements matching
the last simple selector.
<P>One <a href="#pseudo-elements">pseudo-element</a> may be appended
to the last simple selector in a chain, in which case the style
information applies to a subpart of each subject.
<h3><a name="grouping">Grouping</a></h3>
<p>When several selectors share the same declarations, they may be
grouped into a comma-separated list.</p>
<div class="example">
<P>In this example, we condense three rules with identical declarations
into one. Thus,
<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>
</div>
<P>CSS offers other "shorthand" mechanisms as well, including
<span class="index-def" title="multiple declarations">
<a href="syndata.html#declaration">multiple declarations</a></span>
and <span class="index-inst" title="shorthand property"><a
href="about.html#shorthand">shorthand properties</a></span>.
<h2><a name="universal-selector">Universal selector</a></h2>
<P>The <span class="index-def" title="universal selector">universal
selector</span>, written "*", matches the name of any element
type. It matches any single element in the <a
href="conform.html#doctree">document tree.</a>
<P>If the universal selector is not the only component of a <a
href="#simple-selector">simple selector</a>, the "*" may be
omitted. For example:</P>
<ul>
<li><code>*[lang=fr] </code> and <code>[lang=fr]</code> are equivalent.
<li><code>*.warning</code> and <code>.warning</code> are equivalent.
<li><code>*#myid</code> and <code>#myid</code> are equivalent.
</ul>
<h2><a name="type-selectors">Type selectors</a></h2>
<p>A <span class="index-def" title="type selector"><em>type
selector</em></span> matches the name of a document language element
type. A type selector matches every instance of the element type in
the document tree.
<div class="example">
<P>The following rule matches all H1 elements in the
document tree:</p>
<pre>
h1 { font-family: sans-serif }
</pre>
</div>
<h2><a name="descendant-selectors">Descendant selectors</a></h2>
<p>At times, authors may want selectors to match an element that is
the descendant of another element in the document tree (e.g., "Match
those EM elements that are contained by an H1 element"). <span
class="index-def" title="descendant-selectors">Descendant
selectors</span> express such a relationship in a pattern. A
descendant selector is made up of two or more selectors separated by
<a href="syndata.html#whitespace">white space</a>. A descendant
selector of the form "<code>A B</code>" matches when an element
<code>B</code> is an arbitrary descendant of some <a
href="conform.html#doctree">ancestor</a> element <code>A</code>.
<div class="example"><p>
For example, consider the following rules:</p>
<pre>
h1 { color: red }
em { color: red }
</pre>
<p>Although the intention of these rules is to add emphasis to text by
changing its color, the effect will be lost in a case such as:</p>
<pre class="html-example">
<H1>This headline is <EM>very</EM> important</H1>
</pre>
<p>We address this case by supplementing the previous rules with a
rule that sets the text color to blue whenever an EM occurs anywhere
within an H1:</p>
<pre>
h1 { color: red }
em { color: red }
h1 em { color: blue }
</pre>
<p>The third rule will match the EM in the following fragment:</p>
<pre class="html-example">
<H1>This <SPAN class="myclass">headline
is <EM>very</EM> important</SPAN></H1>
</pre>
</div>
<div class="example"><P>
The following selector:</p>
<PRE>
div * p
</PRE>
<P>matches a P element that is a grandchild or later descendant of a
DIV element. Note the white space on either side of the "*" is not part
of the universal selector; the white space is a combinator
indicating
that the DIV must be the ancestor of some element, and that that
element must be an ancestor of the P.
</div>
<div class="example"><p>
The selector in the following rule, which combines
descendant and <a href="#attribute-selectors">attribute selectors</a>,
matches any element that (1) has the "href" attribute set and
(2) is inside a P that is itself inside a DIV:</p>
<pre>
div p *[href]
</pre>
</div>
<h2><a name="child-selectors">Child selectors</a></h2>
<p>A <span class="index-def" title="child selector"><em>child
selector</em></span> matches when an element is the <a
href="conform.html#doctree">child</a> of some element. A child
selector is made up of two or more selectors separated by ">".
<div class="example"><p>
The following rule sets the style of all P elements that
are children of BODY:</p>
<pre>
body > P { line-height: 1.3 }
</pre>
</div>
<div class="example"><P>
The following example combines descendant selectors and child selectors:</p>
<pre>
div ol>li p
</pre>
<P>It matches a P element that is a descendant of an LI; the LI element
must be the child of an OL element; the OL element must be a
descendant of a DIV. Notice that the optional white space around
the ">" combinator has been left out.
</div>
<P>For information on selecting the first child of an element, please
see the section on the <a href="#first-child">:first-child</a>
pseudo-class below.
<h2><a name="adjacent-selectors">Adjacent sibling selectors</a></h2>
<p>Adjacent sibling selectors have the following syntax: E1 + E2,
where E2 is the subject of the selector. The selector matches if E1
and E2 share the same parent in the document tree and E1 immediately
precedes E2,
ignoring non-element nodes (such as text nodes and comments).
<div class="example"><p>
Thus, the following rule states that when a P element immediately
follows a MATH element, it should not be indented:</p>
<pre>
math + p { text-indent: 0 }
</pre>
<p>The next example reduces the vertical space separating
an H1 and an H2 that immediately follows it:</p>
<pre>
h1 + h2 { margin-top: -5mm }
</pre>
</div>
<div class="example"><p>
The following rule is similar to the one in the previous example,
except that it adds a class selector. Thus, special formatting only
occurs when H1 has <samp>class="opener"</samp>:</p>
<pre>
h1.opener + h2 { margin-top: -5mm }
</pre>
</div>
<h2><a name="attribute-selectors">Attribute selectors</a></h2>
<p>CSS 2.1 allows authors to specify rules that match elements
which have certain attributes defined
in the source document.
<h3><a name="matching-attrs">Matching attributes and attribute values</a></h3>
<P>Attribute selectors may match in four ways:</p>
<dl>
<dt><code>[att]</code>
<dd>Match when the element sets the "att" attribute, whatever
the value of the attribute.
<dt><span class="index-def" title="exact
matching|="><code>[att=val]</code></span>
<dd>Match when the element's "att" attribute value is exactly "val".
<dt><span class="index-def" title="space-separated
matching|~="><code>[att~=val]</code></span>
<dd>Represents an element with the <code>att</code> attribute whose
value is a white space-separated list of words, one of which is exactly
"val". If "val" contains white space, it will never represent anything
(since the words are <em>separated</em> by spaces).
If "val" is the empty string, it will never represent anything either.
<dt><span class="index-def" title="hyphen-separated
matching||="><code>[att|=val]</code></span>
<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 <span
class="index-inst" title="language code">language subcode</span>
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">the <code>:lang</code> pseudo-class</a>.
</dl>
<p>Attribute values must be identifiers or strings. The
case-sensitivity of attribute names and values in selectors depends on
the document language.
<div class="example"><p>
For example, the following attribute selector
matches all H1 elements that specify the "title" attribute,
whatever its value:</p>
<pre>
h1[title] { color: blue; }
</pre>
</div>
<div class="example"><p>
In the following example, the selector matches all SPAN elements whose
"class" attribute has exactly the value "example":</p>
<pre>
span[class=example] { color: blue; }
</pre>
</div>
<P>Multiple attribute selectors can be used to refer to several
attributes of an element, or even several times to the same attribute.
<div class="example"><p>
Here, the selector matches all SPAN elements whose
"hello" attribute has exactly the value "Cleveland" and whose
"goodbye" attribute has exactly the value "Columbus":</P>
<pre>
span[hello="Cleveland"][goodbye="Columbus"] { color: blue; }
</pre>
</div>
<div class="example"><p>
The following selectors illustrate the differences between "=" and "~=".
The first selector will match, for example, the value
"copyright copyleft copyeditor" for the "rel" attribute. The second
selector will only match when the "href" attribute has the value
"http://www.w3.org/".
</p>
<pre>
a[rel~="copyright"]
a[href="http://www.w3.org/"]
</pre>
</div>
<div class="example"><P>
The following rule hides all elements for which the value of the
"lang" attribute is "fr" (i.e., the language is French).
<PRE>
*[lang=fr] { display : none }
</PRE>
</div>
<div class="example"><P>
The following rule will match for values of the "lang" attribute
that begin with "en", including "en", "en-US", and "en-cockney":</p>
<PRE>
*[lang|="en"] { color : red }
</PRE>
</div>
<div class="example"><P>
Similarly, the following aural style sheet rules allow a script
to be read aloud in different voices for each role:</p>
<pre class="example">
DIALOGUE[character=romeo]
{ voice-family: "Laurence Olivier", charles, male }
DIALOGUE[character=juliet]
{ voice-family: "Vivien Leigh", victoria, female }
</pre>
</div>
<H3><a name="default-attrs">Default attribute values in DTDs</a></H3>
<P>Matching takes place on attribute values in the document tree.
Default attribute values may be defined in a <span class="index-inst"
title="DTD">DTD</span> or elsewhere, but cannot always be selected by
attribute selectors. Style sheets should be designed so that they work
even if the default values are not included in the document tree.
<p>More precisely, a 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 [[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>A UA that recognizes an XML namespace [[-XMLNAMESPACES]] 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. (E.g., an
XHTML UA is not required to use its built-in knowledge of the XHTML
DTD.)
<p class="note">Note that, typically, implementations choose to ignore
external subsets.
<div class="example">
<P style="display:none">Example:</P>
<P>For example, consider an element EXAMPLE with an attribute "notation"
that has a default value of "decimal". The DTD fragment might be
<pre class="dtd-example">
<!ATTLIST EXAMPLE notation (decimal,octal) "decimal">
</pre>
<p>If the style sheet contains the rules
<pre class="example">
EXAMPLE[notation=decimal] { /*... default property settings ...*/ }
EXAMPLE[notation=octal] { /*... other settings...*/ }
</pre>
<p>the first rule might not match elements whose "notation" 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 class="example">
EXAMPLE { /*... default property settings ...*/ }
EXAMPLE[notation=octal] { /*... other settings...*/ }
</pre>
<p>Here, because the selector <code>EXAMPLE[notation=octal]</code> is
more <a href="cascade.html#specificity">specific</a> than the type
selector alone, the style declarations in the second rule will override
those in the first for elements that have a "notation" attribute value
of "octal". 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.
</div>
<h3><a name="class-html">Class selectors</a></h3>
<p>Working with HTML, authors may use the period (<code>.</code>)
notation 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
"period" (<code>.</code>). 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="http://www.w3.org/TR/2003/REC-SVG11-20030114/styling.html#ClassAttribute">SVG
"class" attribute</a> and how a UA should interpret it, and
similarly MathML 2.0 [[-MATH20]] describes the <a
href="http://www.w3.org/TR/MathML2/chapter2.html#fund.globatt">MathML
"class" attribute</a>.)
<div class="example"><p>
For example, we can assign style information to all elements with
<samp>class~="pastoral"</samp> as follows:</p>
<pre>
*.pastoral { color: green } /* all elements with class~=pastoral */
</pre>
or just
<pre>
.pastoral { color: green } /* all elements with class~=pastoral */
</pre>
<p>The following assigns style only to H1 elements with
<samp>class~="pastoral"</samp>:</p>
<pre>
H1.pastoral { color: green } /* H1 elements with class~=pastoral */
</pre>
<p>Given these rules, the first H1 instance below would not have green
text, while the second would:</p>
<pre>
<H1>Not green</H1>
<H1 class="pastoral">Very green</H1>
</pre>
</div>
<p>To match a subset of "class" values, each value must be preceded
by a ".".</P>
<div class="example"><P>
For example, the following rule matches any P element whose "class" attribute
has been assigned a list of space-separated values that includes "pastoral"
and "marine":</p>
<pre>
p.marine.pastoral { color: green }
</pre>
<p>This rule matches when <samp>class="pastoral blue aqua
marine"</samp> but does not match for <samp>class="pastoral
blue"</samp>.
</div>
<div class="note"><p> <em><strong>Note.</strong> CSS gives so much
power to the "class" attribute, that authors could conceivably design
their own "document language" based on elements with almost no
associated presentation (such as DIV and SPAN in HTML) and assigning
style information through the "class" attribute. Authors should avoid
this practice since the structural elements of a document language
often have recognized and accepted meanings and author-defined classes may
not.</em>
</div>
<div class="note"><p><em><strong>Note:</strong> If an element has
multiple class attributes, their values must be concatenated with
spaces between the values before searching for the class. As of this
time the working group is not aware of any manner in which this
situation can be reached, however, so this behavior is explicitly
non-normative in this specification.</em>
</div>
<h2><a name="id-selectors">ID selectors</a></h2>
<P>Document languages may contain attributes that are declared to be
of type ID. What makes attributes of type ID special is that no two
such attributes can have the same value; whatever the document
language, an ID attribute can be used to uniquely identify its
element. In HTML all ID attributes are named "id"; XML
applications may name ID attributes differently, but the
same restriction applies.
<p>The ID attribute of a document language allows authors to assign an
identifier to one element instance in the document tree. CSS ID
selectors match an element instance based on its identifier. A CSS
ID selector contains a "#" immediately followed by the ID
value, which must be an identifier.</p>
<p class=note>Note that CSS does not specify how a UA knows the ID
attribute of an element. The UA may, e.g., read a document's DTD, have
the information hard-coded or ask the user.
<div class="example"><p>
The following ID selector matches the H1 element whose ID
attribute has the value "chapter1":</p>
<PRE>
h1#chapter1 { text-align: center }
</PRE>
</div>
<div class="html-example"><p>
In the following example, the style rule matches
the element that has the ID value "z98y".
The rule will thus match for the P element:</p>
<pre>
<HEAD>
<TITLE>Match P</TITLE>
<STYLE type="text/css">
*#z98y { letter-spacing: 0.3em }
</STYLE>
</HEAD>
<BODY>
<P id=z98y>Wide text</P>
</BODY>
</pre>
<p>In the next example, however, the style rule will only match an H1
element that has an ID value of "z98y". The rule will not match the
P element in this example:</p>
<pre>
<HEAD>
<TITLE>Match H1 only</TITLE>
<STYLE type="text/css">
H1#z98y { letter-spacing: 0.5em }
</STYLE>
</HEAD>
<BODY>
<P id=z98y>Wide text</P>
</BODY>
</pre>
</div>
<P>ID selectors have a higher specificity than attribute selectors.
For example, in HTML, the selector <samp>#p123</samp> is more specific
than <samp>[id=p123]</samp> in terms of the <a
href="cascade.html">cascade</a>.
<div class="note">
<p><em><strong>Note.</strong> In XML 1.0 [[-XML10]], the information about which
attribute contains an element's IDs is contained in a DTD. When
parsing XML, UAs do not always read the DTD, and thus may not know
what the ID of an element is. If a style sheet designer knows or
suspects that this will be the case, he should use normal attribute
selectors instead: <code>[name=p371]</code> instead of
<code>#p371</code>. However, the cascading order of normal attribute
selectors is different from ID selectors. It may be necessary to add
an "!important" priority to the declarations: <code>[name=p371]
{color: red ! important}</code>.</em>
</div>
<p>If an element has multiple ID attributes, all of them must be
treated as IDs for that element for the purposes of the ID
selector. Such a situation could be reached using mixtures of xml:id
[[-XMLID]], DOM3 Core [[-DOM-LEVEL-3-CORE]], XML DTDs [[-XML10]] and
namespace-specific knowledge.
<h2><a name="pseudo-elements">Pseudo-elements</a> and <a
name="pseudo-classes">pseudo-classes</a></h2>
<p>In CSS 2.1, style is normally attached to an element based on its
position in the <a href="conform.html#doctree">document tree</a>. This
simple model is sufficient for many cases, but some common publishing
scenarios may not be possible due to the structure of the <a
href="conform.html#doctree">document tree</a>. For instance, in HTML
4 (see [[HTML4]]), no element refers to the first line of a
paragraph, and therefore no simple CSS selector may refer to it.</p>
<p>CSS introduces the concepts of <span class="index-def"
title="pseudo-elements"><dfn>pseudo-elements</dfn></span> and <span
class="index-def"
title="pseudo-classes"><dfn>pseudo-classes</dfn></span> to permit
formatting based on information that lies outside the document
tree. </p>
<ul>
<li>Pseudo-elements 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. CSS pseudo-elements allow style sheet
designers to refer to this otherwise inaccessible
information. Pseudo-elements may also provide style sheet designers a
way to assign style to content that does not exist in the source
document (e.g., the <a
href="generate.html#before-after-content">:before and :after</a>
pseudo-elements give access to generated content).
<li>Pseudo-classes classify elements on characteristics o
3FAC
ther than
their name, attributes or content; in principle characteristics that
cannot be deduced from the document tree. Pseudo-classes may be
dynamic, in the sense that an element may acquire or lose a
pseudo-class while a user interacts with the document. The exceptions
are <a href="#first-child">':first-child'</a>, which <em>can</em> be
deduced from the document tree, and
<a href="#lang">':lang()'</a>, which can be
deduced from the document tree in some cases.
</ul>
<P>Neither pseudo-elements nor pseudo-classes appear in the document
source or document tree.
<P>Pseudo-classes are allowed anywhere in selectors while
pseudo-elements may only be appended after the last simple selector of
the selector.
</p>
<p>Pseudo-element and pseudo-class names are case-insensitive.</p>
<P>Some pseudo-classes are mutually exclusive, while others can be
applied simultaneously to the same element. In case of conflicting
rules, the normal <a href="cascade.html#cascading-order">cascading
order</A> determines the outcome.
<h2><a name="pseudo-class-selectors">Pseudo-classes</a></h2>
<h3><a name="first-child">:first-child</a> pseudo-class</h3>
<P>The <span class="index-def"
title="first-child|:first-child">:first-child</span> pseudo-class
matches an element that is the first child element of some other element.
<div class="example"><P>
In the following example, the selector matches any P element
that is the first child of a DIV element. The rule
suppresses indentation for the first paragraph of a DIV:</p>
<PRE>
div > p:first-child { text-indent: 0 }
</PRE>
This selector would match the P inside the DIV of the
following fragment:
<pre class="html-example">
<P> The last P before the note.
<DIV class="note">
<P> The first P inside the note.
</DIV>
</pre>
but would not match the second P in the following
fragment:
<pre class="html-example">
<P> The last P before the note.
<DIV class="note">
<H2>Note</H2>
<P> The first P inside the note.
</DIV>
</pre>
</div>
<div class="example">
<p>The following rule sets the font weight to 'bold' for any EM
element that is some descendant of a P element that is a first
child:</p>
<pre>
p:first-child em { font-weight : bold }
</pre>
</div>
<P>Note that since <a href="visuren.html#anonymous">anonymous</a>
boxes are not part of the document tree, they are not counted when
calculating the first child.</p>
<div class="html-example"><P>
For example, the EM in:</p>
<PRE>
<P>abc <EM>default</EM>
</PRE>
is the first child of the P.
</div>
<P>The following two selectors are equivalent:</p>
<PRE>
* > a:first-child /* A is first child of any element */
a:first-child /* Same */
</PRE>
<h3><a name="link-pseudo-classes">The link pseudo-classes</a>: <span
class="index-def" title="pseudo-classes:::link|:link|link
(pseudo-class)">:link</span> and <span class="index-def"
title="pseudo-classes:::visited|:visited|visited
(pseudo-class)">:visited</span></h3>
<p>User agents commonly display unvisited links differently from
previously visited ones. CSS provides the pseudo-classes ':link' and
':visited' to distinguish them:</p>
<ul>
<li> The :link pseudo-class applies for links that have
not yet been visited.
<li> The :visited pseudo-class applies once the link has been
visited by the user.
</ul>
<p>UAs may return a visited link to the (unvisited) ':link' state at
some point.
<P>The two states are mutually exclusive.
<p>The document language determines which elements are hyperlink
source anchors. For example, in HTML4, the link pseudo-classes
apply to A elements with an "href" attribute. Thus, the following
two CSS 2.1 declarations have similar effect:</p>
<pre>
a:link { color: red }
:link { color: red }
</pre>
<div class="example"><p>
If the following link:</p>
<pre class="html-example">
<A class="external" href="http://out.side/">external link</A>
</pre>
has been visited, this rule:
<pre>
a.external:visited { color: blue }
</pre>
will cause it to be blue.
</div>
<p class=note>Note. It is possible for style sheet authors to abuse the
:link and :visited pseudo-classes to determine which sites a user has
visited without the user's consent.
<p>UAs may therefore treat all links as unvisited links, or implement
other measures to preserve the user's privacy while rendering visited
and unvisited links differently. See [[-P3P]] for more information
about handling privacy.
<h3><a name="dynamic-pseudo-classes">The dynamic pseudo-classes:</a>
<span class="index-def" title="pseudo-classes:::hover|:hover|hover
(pseudo-class)">:hover</span>, <span class="index-def"
title="pseudo-classes:::active|:active|active
(pseudo-class)">:active</span>, and <span class="index-def"
title="pseudo-classes:::focus|:focus|focus
(pseudo-class)">:focus</span></h3>
<p>Interactive user agents sometimes change the rendering in response
to user actions. CSS provides three pseudo-classes for common cases:</p>
<ul>
<li> The :hover pseudo-class applies while the user designates an
element (with some pointing device), but does not activate it. For
example, a visual user agent could apply this pseudo-class when the
cursor (mouse pointer) hovers over a box generated by the element.
User agents not supporting
<a href="media.html#interactive-media-group">interactive media</a>
do not have to support this pseudo-class.
Some conforming user agents supporting
<a href="media.html#interactive-media-group">interactive media</a>
may not be able to support this pseudo-class (e.g., a pen device).
<li> The :active pseudo-class applies while an element is being
activated by the user. For example, between the times the user presses
the mouse button and releases it.
<li> The :focus pseudo-class applies while an element has the
focus (accepts keyboard events or other forms of text input).
</ul>
<p>An element may match several pseudo-classes at the same time.
<p>CSS does not define which elements may be in the above states, or
how the states are entered and left. Scripting may change whether
elements react to user events or not, and different devices and UAs
may have different ways of pointing to, or activating elements.
<p>CSS 2.1 does not define if the parent of an element that is
':active' or ':hover' is also in that state.
<p>User agents are not required to reflow a currently displayed
document due to pseudo-class transitions. For instance, a style sheet
may specify that the <span
class="propinst-font-size">'font-size'</span> of an :active link
should be larger than that of an inactive link, but since this may
cause letters to change position when the reader selects the link, a
UA may ignore the corresponding style rule.</p>
<div class="example"><p>
<pre>
a:link { color: red } /* unvisited links */
a:visited { color: blue } /* visited links */
a:hover { color: yellow } /* user hovers */
a:active { color: lime } /* active links */
</pre>
<p>Note that the A:hover must be placed after the A:link and A:visited
rules, since otherwise the cascading rules will hide the <span
class="propinst-color">'color'</span> property of the A:hover
rule. Similarly, because A:active is placed after A:hover, the active
color (lime) will apply when the user both activates and hovers over
the A element.
</div>
<div class="example">
<p>An example of combining dynamic pseudo-classes:
<pre>
a:focus { background: yellow }
a:focus:hover { background: white }
</pre>
<P>The last selector matches A elements that are in pseudo-class
:focus and in pseudo-class :hover.
</div>
<!--In order to support the :focus pseudo-class, user agents are
expected to be able to reformat a document dynamically. [this
conflicts with the statement above, HWL] -->
<P>For information about the presentation of focus outlines, please
consult the section on <a href="ui.html#dynamic-outlines">dynamic
focus outlines</a>.
<div class="note"><P>
<em><strong>Note.</strong>
In CSS1, the ':active' pseudo-class was mutually
exclusive with ':link' and ':visited'. That is no longer the case. An
element can be both ':visited' and ':active' (or ':link' and
':active') and the normal cascading rules determine which style
declarations
apply.
</em>
</div>
<div class="note"><P>
<em><strong>Note.</strong>
Also note that in CSS1, the ':active' pseudo-class only applied to
links.</em>
</div>
<h3><a name="lang">The language pseudo-class:</a> <span
class="index-def" title="pseudo-classes:::lang|:lang|lang
(pseudo-class)">:lang</span></h3>
<p>If the document language specifies how the <span class="index-inst"
title="language (human)">human language</span> of an element is
determined, it is possible to write selectors in CSS that match an
element based on its language. For example, in HTML [[HTML4]], the
language is determined by a combination of the "lang" attribute, the
META element, and possibly by information from the protocol (such as
HTTP headers). XML uses an attribute called xml:lang, and there may be
other document language-specific methods for determining the language.
<p>The pseudo-class ':lang(C)' matches if the element is in language
C. Whether there is a match is based solely on the identifier C
being either equal to, or a hyphen-separated substring of, the
element's language value, in the same way as if performed by the <a
href="#attribute-selectors">'|='</a> operator. The matching of C
against the element's language value is performed case-insensitively
for characters within the ASCII range.
The identifier C does not have to be a valid language name.</p>
<p>C must not be empty.
<p class=note>Note: It is recommended that documents and
protocols indicate language using codes from BCP 47 [[-BCP47]] or
its successor, and by means of "xml:lang" attributes in the case of
XML-based documents [[XML10]]. See <a
href="http://www.w3.org/International/questions/qa-lang-2or3">
"FAQ: Two-letter or three-letter language codes."</a></p>
<div class="example">
<p>The following rules set the quotation marks for an HTML document
that is either in Canadian French or German:</p>
<pre>
html:lang(fr-ca) { quotes: '« ' ' »' }
html:lang(de) { quotes: '»' '«' '\2039' '\203A' }
:lang(fr) > Q { quotes: '« ' ' »' }
:lang(de) > Q { quotes: '»' '«' '\2039' '\203A' }
</pre>
<p>The second pair of rules actually set the <span
class="propinst-quotes">'quotes'</span> property on Q elements