csswg-drafts/css-variables/Overview.html at 4c2c2dd65b6a2ec5fb6f92f07c13300ff7a7de30 · w3c/csswg-drafts · GitHub

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

<html lang=en>
 <head><meta content="text/html; charset=utf-8" http-equiv=Content-Type>

  <title>CSS Variables Module Level 1</title>
  <link href="http://dev.w3.org/csswg/default.css" rel=stylesheet
  type="text/css">
  <link href="http://www.w3.org/StyleSheets/TR/W3C-WD.css" rel=stylesheet
  type="text/css">

  <style>
		pre.idl { border:solid thin; background:#eee; color:#000; padding:0.5em }
		pre.idl :link, pre.idl :visited { color:inherit; background:transparent }
	</style>

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

   <h1>CSS Variables Module Level 1</h1>
   <!--<h2 class="no-num no-toc">W3C Working Draft 22 March 2012</h2>-->
   <h2 class="no-num no-toc" id=w3c-working-draft-15-march-2012>W3C Working
    Draft 15 March 2012</h2>

   <dl>
    <dt>This version:

    <dd><a
     href="http://www.w3.org/TR/2012/WD-css-variables-20120315/">http://www.w3.org/TR/2012/WD-css-variables-20120315/</a>


    <dt>Latest version:

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


    <dt>Editor's draft:

    <dd><a
     href="http://dev.w3.org/csswg/css-variables/">http://dev.w3.org/csswg/css-variables/</a>


    <dt>Editors:

    <dd class=vcard><span class=fn>Luke Macpherson</span>, <span
     class=org>Google, Inc.</span>, <span
     class=email>macpherson@google.com</span>

    <dd class=vcard> <a class=url href="http://xanthir.com/contact/"> <span
     class=fn>Tab Atkins Jr.</span>, </a> <span class=org>Google, Inc.</span>


    <dd class=vcard> <span class=fn>Daniel Glazman</span>, <span
     class=org>Disruptive Innovations</span>, <span
     class=email>daniel.glazman@disruptive-innovations.com</span>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2012 <a href="http://www.w3.org/"><abbr
    title="World Wide Web Consortium">W3C</abbr></a><sup>&reg;</sup> (<a
    href="http://www.csail.mit.edu/"><abbr
    title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
    href="http://www.ercim.eu/"><abbr
    title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
    <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
    <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
    and <a
    href="http://www.w3.org/Consortium/Legal/copyright-documents">document
    use</a> rules apply.</p>
   <!--end-copyright-->
   <hr title="Separator for header">
  </div>

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

  <p>CSS is a language for describing the rendering of structured documents
   (such as HTML and XML) on screen, on paper, in speech, etc. This module
   contains the features of CSS level&nbsp;3 relating to variables. It
   includes and extends the functionality of CSS level&nbsp;2 <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds
   on CSS level&nbsp;1 <a href="#CSS1"
   rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>. The main extensions compared to
   level&nbsp;2 are the introduction of the variable as a new primitive value
   type that is accepted by all properties.

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

  <p><em>This section describes the status of this document at the time of
   its publication. Other documents may supersede this document. A list of
   current W3C publications and the latest revision of this technical report
   can be found in the <a href="http://www.w3.org/TR/">W3C technical reports
   index at http://www.w3.org/TR/.</a></em>

  <p>Publication as a Working Draft does not imply endorsement by the W3C
   Membership. This is a draft document and may be updated, replaced or
   obsoleted by other documents at any time. It is inappropriate to cite this
   document as other than work in progress.

  <p>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a href="mailto:www-style@w3.org">www-style@w3.org</a> (see
   <a href="http://www.w3.org/Mail/Request">instructions</a>) is preferred
   for discussion of this specification. When sending e-mail, please put the
   text &#8220;css-variables&#8221; in the subject, preferably like this:
   &#8220;[<!---->css-variables<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

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

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

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

  <ul class=toc>
   <li><a href="#intro"><span class=secno>1. </span> Introduction</a>
    <ul class=toc>
     <li><a href="#placement"><span class=secno>1.1. </span> Module
      Interactions</a>

     <li><a href="#values"><span class=secno>1.2. </span> Values</a>
    </ul>

   <li><a href="#defining-variables"><span class=secno>2. </span> Defining
    Variables With Variable Properties</a>

   <li><a href="#using-variables"><span class=secno>3. </span> Using
    Variables: the var() function</a>
    <ul class=toc>
     <li><a href="#using-invalid-variables"><span class=secno>3.1. </span>
      Using Invalid Variables</a>

   <li><a href="#cssom"><span class=secno>4. </span> CSSOM</a>
    <ul class=toc>
     <li><a href="#cssom-cssvariable"><span class=secno>4.1. </span>
      Interface CSSVariableComponentValue</a>

     <li><a href="#cssstyledeclaration-interface"><span class=secno>4.2.
      </span> Additions to the <code>CSSStyleDeclaration</code> Interface</a>

    </ul>

   <li><a href="#conformance"><span class=secno>5. </span> Conformance</a>
    <ul class=toc>
     <li><a href="#conventions"><span class=secno>5.1. </span> Document
      Conventions</a>

     <li><a href="#conformance-classes"><span class=secno>5.2. </span>
      Conformance Classes</a>

     <li><a href="#partial"><span class=secno>5.3. </span> Partial
      Implementations</a>

     <li><a href="#experimental"><span class=secno>5.4. </span> Experimental
      Implementations</a>

     <li><a href="#testing"><span class=secno>5.5. </span>Non-Experimental
      Implementations</a>

     <li><a href="#cr-exit-criteria"><span class=secno>5.6. </span> CR Exit
      Criteria</a>
    </ul>

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

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

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

   <li class=no-num><a href="#index">Index</a>

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

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

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

  <p>Large documents or applications (and even small ones) can contain quite
   a bit of CSS. Many of the values in the CSS file will be duplicate data;
   for example, a site may establish a color scheme and reuse three or four
   colors throughout the site. Altering this data can be difficult and
   error-prone, since it's scattered throughout the CSS file (and possibly
   across multiple files), and may not be amenable to Find-and-Replace.

  <p>This module introduces a family of custom user-defined properties known
   collectively as <a href="#variable-property"><i>variable
   properties</i></a>, which allow an author to assign arbitrary values to a
   property with an author-chosen name, and then use those values in other
   properties elsewhere in the document. This makes it easier to read large
   files, as seemingly-arbitrary values now have informative names, and makes
   editting such files much easier and less error-prone, as one only has to
   change the value once, at the variable definition site, and the change
   will propagate to all uses of that variable automatically.

  <h3 id=placement><span class=secno>1.1. </span> Module Interactions</h3>

  <p>This module defines a new type of primitive value, the <a
   href="#type-variable"><i>variable</i></a>, which is accepted by all
   properties.

  <h3 id=values><span class=secno>1.2. </span> Values</h3>

  <p>This specification follows the <a
   href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
   definition conventions</a> from <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
   this specification are defined in CSS Level 2 Revision 1 <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
   modules may expand the definitions of these value types: for example <a
   href="#CSS3COLOR" rel=biblioentry>[CSS3COLOR]<!--{{CSS3COLOR}}--></a>,
   when combined with this module, expands the definition of the
   &lt;color&gt; value type as used in this specification.

  <h2 id=defining-variables><span class=secno>2. </span> Defining Variables
   With Variable Properties</h2>

  <p>This specification defines an open-ended set of properties called <a
   href="#variable-property"><i>variable properties</i></a>, which are used
   to define <a href="#type-variable"><i>variables</i></a>.

  <table class=propdef>
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=variable-property
      title="variable property|variable properties">var-*</dfn>

    <tr>
     <th>Values:

     <td>see prose

    <tr>
     <th>Initial:

     <td>invalid (a keyword, but see prose)

    <tr>
     <th>Applies To:

     <td>all elements

    <tr>
     <th>Inherited:

     <td>yes

    <tr>
     <th>Computed Value:

     <td>specified value with variables substituted (but see prose for
      "invalid variables")

    <tr>
     <th>Media:

     <td>all
  </table>

  <p><em>Any</em> property name starting with the prefix "var-" is a <a
   href="#variable-property"><i>variable property</i></a>. Variable
   properties are defined to be valid but meaningless as they are meant
   solely for allowing authors to pass custom data around their page, similar
   to the <a
   href="http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#embedding-custom-non-visible-var-with-the-var-*-attributes">custom
   data attributes</a> in HTML. Other specifications and user agents must not
   assign a particular meaning to <a href="#variable-property"><i>variable
   properties</i></a> or attach a specific effect to them beyond the bare
   minimum that comes from them being valid properties.

  <p class=issue>As defined here, the syntax for variable usage is different
   from the syntax for variable definition (i.e. var-foo for definition,
   var(foo) for usage). Some have suggested that the syntaxes should should
   match, using functional syntax in both cases. Others have suggested using
   a prefixed symbol instead of functional syntax (e.g. $foo) for both the
   property and usage.

  <p>The valid possible values of a variable property are almost completely
   unrestricted. A variable property can contain anything that is valid
   according to the <code>value</code> production in the <a
   href="http://www.w3.org/TR/CSS2/syndata.html#tokenization">CSS Core
   Grammar</a>. The values do not have to correspond to any existing CSS
   values, as they are not evaluated except to replace variables occuring
   within them until they are actually referenced in a normal property with a
   variable.

  <p>This specification reserves the use of all function tokens starting with
   the prefix "var" within variable properties. Authors must not use any such
   functions except as defined in this specification or future updates. If a
   variable property contains such a function, it must match the grammar
   defined in this specification or future updates; the use of such a
   function that does not follow the grammar, or that utilizes such a
   function that is not yet defined, makes the variable property invalid and
   it must be ignored.

  <div class=example>
   <p>For example, even though this spec doesn't define a &lsquo;<code
    class=css>var-inherit()</code>&rsquo; function, that name is reserved by
    default and cannot be used. Similarly, the &lsquo;<code
    class=css>var()</code>&rsquo; function <em>is</em> defined by this spec,
    so any use of it must match the definition in the spec. The following
    property declarations are thus invalid:</p>

   <pre>
:root {
  var-foo: var-inherit(bar);   /* Use of reserved, but undefined, function. */
  var-foo: var(bar, baz, qux); /* Invalid use of var() function. */
}</pre>
  </div>

  <p>For each <a href="#variable-property"><i>variable property</i></a>,
   there is an associated variable with the same name save for the "var-"
   prefix. For example, a variable property named "var-foo" is associated
   with the variable named "foo". See the next chapter for details on how to
   use variables.

  <div class=example>
   <p>This style rule:</p>

   <pre>
:root {
  var-header-color: #06c;
}</pre>

   <p>declares a <a href="#variable-property"><i>variable property</i></a>
    named "var-header-color" on the root element, and assigns to it the value
    "#06c". This property is then inherited to the elements in the rest of
    the document. Its value can be referenced via the "header-color"
    variable:

   <pre>h1 { background-color: var(header-color); }</pre>

   <p>The preceding rule is equivalent to writing &lsquo;<code
    class=css>background-color: #06c;</code>&rsquo;, except that the variable
    name makes the origin of the color clearer, and if &lsquo;<code
    class=css>var(header-color)</code>&rsquo; is used on other elements in
    the document, all of the uses can be updated at once by changing the
    &lsquo;<code class=property>var-header-color</code>&rsquo; property on
    the root element.</p>
  </div>

  <p>Variable properties are ordinary properties, so they can be declared on
   any element, are resolved with the normal inheritance and cascade rules,
   can be made conditional with &lsquo;<code class=css>@media</code>&rsquo;
   and other conditional rules, can be used in HTML's <code>style</code>
   attribute, can be read or set using the CSSOM, etc..

  <div class=example>
   <p>If a <a href="#variable-property"><i>variable property</i></a> is
    declared multiple times, the standard cascade rules help resolve it.
    Variables always draw from the computed value of the associated variable
    property on the same element:</p>

   <pre>
:root { var-color: blue; }
div { var-color: green; }
#alert { var-color: red; }
* { color: var(color); }

&lt;p>I inherited blue from the root element!&lt;/p>
&lt;div>I got green set directly on me!&lt;/div>
&lt;div id='alert'>
  While I got red set directly on me!
  &lt;p>I'm red too, because of inheritance!&lt;/p>
&lt;/div></pre>
  </div>

  <p><a href="#variable-property"><i>Variable properties</i></a> may use
   variables in their own values to build up composite variables. This can
   create cyclic dependencies where two or more <a
   href="#variable-property"><i>variable properties</i></a> each attempt to
   use the variable that the other defines; doing so makes all the <a
   href="#variable-property"><i>variable properties</i></a> involved in the
   cycle compute to their initial value (which is a guaranteed-invalid
   value).

  <div class=example>
   <p>This example shows a variable property safely using a variable:</p>

   <pre>
:root {
	var-main-color: #c06;
	var-accent-background: linear-gradient(to top, var(main-color), white);
}</pre>

   <p>The &lsquo;<code class=property>var-accent-background</code>&rsquo;
    property will automatically update when the &lsquo;<code
    class=property>var-main-color</code>&rsquo; property is changed.</p>
  </div>

  <div class="example invalid-example">
   <p>On the other hand, this example shows an invalid instance of variables
    depending on each other:</p>

   <pre>
:root {
	var-one: calc(var(two) + 20px);
	var-two: calc(var(one) - 20px);
}</pre>

   <p>Both &lsquo;<code class=property>var-one</code>&rsquo; and &lsquo;<code
    class=property>var-two</code>&rsquo; now define <a
    href="#invalid-variable"><i>invalid variables</i></a> rather than
    lengths.</p>
  </div>

  <p>It is important to note that <a href="#variable-property"><i>variable
   properties</i></a> resolve any <a
   href="#type-variable"><i>variables</i></a> in their values at
   computed-value time, which occurs <em>before</em> the value is inherited.
   In general, cyclic dependencies occur only when multiple variable
   properties on the same element refer to each other; variable properties
   defined on elements higher in the element tree can never cause a cyclic
   reference with properties defined on elements lower in the element tree.

  <div class=example>
   <p>For example, given the following structure, these variable properties
    are <strong>not</strong> cyclic, and all define valid variables:</p>

   <pre>
&lt;one>&lt;two>&lt;three />&lt;/two>&lt;/one>
one   { var-foo: 10px; }
two   { var-bar: calc(var(foo) + 10px); }
three { var-foo: calc(var(bar) + 10px); }</pre>

   <p>The &lt;one> element defines a value for &lsquo;<code
    class=property>var-foo</code>&rsquo;. The &lt;two> element inherits this
    value, and additionally assigns a value to &lsquo;<code
    class=property>var-bar</code>&rsquo; using the &lsquo;<code
    class=css>foo</code>&rsquo; variable. Finally, the &lt;three> element
    inherits the &lsquo;<code class=property>var-bar</code>&rsquo; value
    <em>after</em> variable substitution (in other words, it sees the value
    &lsquo;<code class=css>calc(10px + 10px)</code>&rsquo;), and then
    redefines &lsquo;<code class=property>var-foo</code>&rsquo; in terms of
    that value. Since the value it inherited for &lsquo;<code
    class=property>var-bar</code>&rsquo; no longer contains a reference to
    the &lsquo;<code class=property>var-foo</code>&rsquo; property defined on
    &lt;one>, defining &lsquo;<code class=property>var-foo</code>&rsquo;
    using the &lsquo;<code class=css>var(bar)</code>&rsquo; variable is not
    cyclic, and actually defines a value that will eventually (when
    referenced as a variable in a normal property) resolve to &lsquo;<code
    class=css>30px</code>&rsquo;.</p>
  </div>

  <p>The initial value of a <a href="#variable-property"><i>variable
   property</i></a> is a special invalid value which makes the associated
   variable an <a href="#invalid-variable"><i>invalid variable</i></a>. This
   is represented by the keyword &lsquo;<code
   class=css>invalid</code>&rsquo;, but that keyword has no special meaning
   in itself, and is valid if set explicitly in a variable property.

  <div class=example>
   <pre>
&lt;div>
  &lt;p>Sample text&lt;/p>
&lt;/div>
&lt;style>
p { var-foo: invalid; }
div,p { font-family: var(foo); }
&lt;/style></pre>

   <p>In this example, the "foo" variable is an <a
    href="#invalid-variable"><i>invalid variable</i></a> at the time the DIV
    element references it, because the &lsquo;<code
    class=property>var-foo</code>&rsquo; property still has its initial
    value. This causes the DIV's &lsquo;<code
    class=property>font-family</code>&rsquo; property to compute to the
    initial value for &lsquo;<code class=property>font-family</code>&rsquo;.</p>

   <p>On the other hand, the P element defines an explicit value for the
    &lsquo;<code class=property>var-foo</code>&rsquo; property. Its
    &lsquo;<code class=property>font-family</code>&rsquo; property thus
    references a font named "invalid".</p>
  </div>

  <h2 id=using-variables><span class=secno>3. </span> Using Variables: the
   var() function</h2>

  <p>A <dfn id=variable>variable</dfn> allows the value of a <a
   href="#variable-property"><i>variable property</i></a> on an element to be
   substituted into another property on the element. Variables are used with
   the &lsquo;<code class=css>var()</code>&rsquo; function, taking the name
   of a variable and returning the value of the associated variable property.
   The syntax of the &lsquo;<code class=css>var()</code>&rsquo; function is:

  <pre><dfn id=type-variable
   title="variable|variables">&lt;variable></dfn> = var( IDENT )</pre>

  <p>The &lsquo;<code class=css>var()</code>&rsquo; function references the
   <a href="#variable-property"><i>variable property</i></a> with the name
   obtained by prepending "var-" to the function's argument. <span
   class=note>Note that all possible <a href="#variable-property"><i>variable
   properties</i></a> exist; if one with the given name has not yet been
   defined in the document, it simply takes its initial value.</span> When a
   property resolves its computed value, a &lsquo;<code
   class=css>var()</code>&rsquo; function is replaced by the value of the
   referenced <a href="#variable-property"><i>variable property</i></a>.

  <p>A variable can be used anywhere a value is expected in CSS. Variables
   can not be used as property names, selectors, or anything else besides
   property values - doing so either produces an invalid value or, in some
   situations like the attribute value of an attribute selector, a valid
   value that nonetheless has no relation to the variable of that name.

  <p>A variable is substituted for its value in the property value at
   computed-value time. If a declaration, once all variables are substituted
   in, is invalid, the declaration is <a
   href="#invalid-at-computed-value-time"><i>invalid at computed-value
   time</i></a>.

  <div class=example>
   <p>For example, the following usage is fine from a syntax standpoint, but
    results in nonsense when the variable is substituted in:</p>

   <pre>
:root { var-looks-valid: 20px; }
p { background-color: var(looks-valid); }</pre>

   <p>Since &lsquo;<code class=css>20px</code>&rsquo; is an invalid value for
    &lsquo;<code class=property>background-color</code>&rsquo;, this instance
    of the property computes to &lsquo;<code
    class=property>transparent</code>&rsquo; (the initial value for
    &lsquo;<code class=property>background-color</code>&rsquo;) instead.</p>
  </div>

  <h3 id=using-invalid-variables><span class=secno>3.1. </span> Using Invalid
   Variables</h3>

  <p>When a variable property has its initial value, a &lsquo;<code
   class=css>var()</code>&rsquo; function referencing that property
   represents an <dfn id=invalid-variable
   title="invalid variable|invalid variables">invalid variable</dfn>. Using
   an <a href="#invalid-variable"><i>invalid variable</i></a> in a property
   value makes the declaration <a
   href="#invalid-at-computed-value-time"><i>invalid at computed-value
   time</i></a>.

  <p>A declaration that is <dfn id=invalid-at-computed-value-time>invalid at
   computed-value time</dfn> results from either us
DB4E
ing an <a
   href="#invalid-variable"><i>invalid variable</i></a> in a property value,
   or using a valid variable that produces an invalid declaration when it is
   substituted in. When this happens, the declaration must compute to the
   property's initial value.

  <div class=example>
   <p>For example, in the following code:</p>

   <pre>
:root { var-not-a-color: 20px; }
p { background-color: red; }
p { background-color: var(not-a-color); }</pre>

   <p>the &lt;p> elements will have transparent backgrounds (the initial
    value for &lsquo;<code class=property>background-color</code>&rsquo;),
    rather than red backgrounds. The same would happen if the variable itself
    was invalid.</p>

   <p>Note the difference between this and what happens if the author had
    just written &lsquo;<code class=css>background-color: 20px</code>&rsquo;
    directly in their stylesheet - that would be a normal syntax error, which
    would cause the rule to be discarded, so the &lsquo;<code
    class=css>background-color: red</code>&rsquo; rule would be used instead.</p>
  </div>

  <p class=note>The <a href="#invalid-at-computed-value-time"><i>invalid at
   computed-value time</i></a> concept exists because variables can't "fail
   early" like other syntax errors can, so by the time the user agent
   realizes a property value is invalid, it's already thrown away the other
   cascaded values. I think &lsquo;<code class=css>attr()</code>&rsquo; needs
   to rely on it as well, as its behavior is almost identical to variables.

  <h2 id=cssom><span class=secno>4. </span> CSSOM</h2>

  <p><a href="#variable-property"><i>Variable properties</i></a> are ordinary
   properties, and can be read or modified using all of the existing CSSOM
   APIs for reading or modifying properties.

  <p>The specification extends the IDL definitions in the CSSOM spec to
   represent the use of the &lsquo;<code class=css>var()</code>&rsquo;
   function.

  <h3 id=cssom-cssvariable><span class=secno>4.1. </span> Interface
   CSSVariableComponentValue</h3>

  <p>The CSSVariableComponentValue interface represents a use of the
   &lsquo;<code class=css>var()</code>&rsquo; function.

  <dl>
   <dt>IDL Definition

   <dd>
    <pre class=idl>
[NoInterfaceObject] interface CSSVariableComponentValue {
           attribute DOMString variableName;
  readonly attribute any variableValue;
}</pre>

   <dt>Attributes

   <dd>
    <dl>
     <dt><code>variableName</code> of type <code>DOMString</code>

     <dd>This attribute represents the argument to the function. Changing
      this attribute changes the <a href="#variable-property"><i>variable
      property</i></a> being referred to.

     <dt><code>variableValue</code> of type <code>any</code>, readonly

     <dd>This attribute represents the value of the <a
      href="#variable-property"><i>variable property</i></a> the function is
      referencing.
    </dl>
  </dl>

  <h3 id=cssstyledeclaration-interface><span class=secno>4.2. </span>
   Additions to the <code>CSSStyleDeclaration</code> Interface</h3>

  <p class=issue>The set of variable properties is open-ended, so it's not
   clear how best to represent this. Ideally, the CSSOM would expose the
   current set of properties with a non-initial value and allow setting of
   arbitrary properties. The most natural way seems to be to first, set up a
   getter behavior on the interface somehow that deals with variable
   properties, and second, set up a <code>vars</code> map that exposes the
   variable properties that aren't set to their initial value.

  <h2 id=conformance><span class=secno>5. </span> Conformance</h2>

  <h3 id=conventions><span class=secno>5.1. </span> Document Conventions</h3>

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

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

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

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

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

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

  <h3 id=conformance-classes><span class=secno>5.2. </span> Conformance
   Classes</h3>

  <p>Conformance to CSS Variables Module is defined for three conformance
   classes:

  <dl>
   <dt><dfn id=style-sheet title="style sheet!!as conformance class">style
    sheet</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
    style sheet</a>.

   <dt><dfn id=renderer>renderer</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
    that interprets the semantics of a style sheet and renders documents that
    use them.

   <dt><dfn id=authoring-tool>authoring tool</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
    that writes a style sheet.
  </dl>

  <p>A style sheet is conformant to CSS Variables Module if all of its
   declarations that use properties defined in this module have values that
   are valid according to the generic CSS grammar and the individual grammars
   of each property as given in this module.

  <p>A renderer is conformant to CSS Variables Module if, in addition to
   interpreting the style sheet as defined by the appropriate specifications,
   it supports all the features defined by CSS Variables Module by parsing
   them correctly and rendering the document accordingly. However, the
   inability of a UA to correctly render a document due to limitations of the
   device does not make the UA non-conformant. (For example, a UA is not
   required to render color on a monochrome monitor.)

  <p>An authoring tool is conformant to CSS Variables Module if it writes
   style sheets that are syntactically correct according to the generic CSS
   grammar and the individual grammars of each feature in this module, and
   meet all other conformance requirements of style sheets as described in
   this module.

  <h3 id=partial><span class=secno>5.3. </span> Partial Implementations</h3>

  <p>So that authors can exploit the forward-compatible parsing rules to
   assign fallback values, CSS renderers <strong>must</strong> treat as
   invalid (and <a
   href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore as
   appropriate</a>) any at-rules, properties, property values, keywords, and
   other syntactic constructs for which they have no usable level of support.
   In particular, user agents <strong>must not</strong> selectively ignore
   unsupported component values and honor supported values in a single
   multi-value property declaration: if any value is considered invalid (as
   unsupported values must be), CSS requires that the entire declaration be
   ignored.

  <h3 id=experimental><span class=secno>5.4. </span> Experimental
   Implementations</h3>

  <p>To avoid clashes with future CSS features, the CSS2.1 specification
   reserves a <a
   href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
   syntax</a> for proprietary and experimental extensions to CSS.

  <p>Prior to a specification reaching the Candidate Recommendation stage in
   the W3C process, all implementations of a CSS feature are considered
   experimental. The CSS Working Group recommends that implementations use a
   vendor-prefixed syntax for such features, including those in W3C Working
   Drafts. This avoids incompatibilities with future changes in the draft.

  <h3 id=testing><span class=secno>5.5. </span>Non-Experimental
   Implementations</h3>

  <p>Once a specification reaches the Candidate Recommendation stage,
   non-experimental implementations are possible, and implementors should
   release an unprefixed implementation of any CR-level feature they can
   demonstrate to be correctly implemented according to spec.

  <p>To establish and maintain the interoperability of CSS across
   implementations, the CSS Working Group requests that non-experimental CSS
   renderers submit an implementation report (and, if necessary, the
   testcases used for that implementation report) to the W3C before releasing
   an unprefixed implementation of any CSS features. Testcases submitted to
   W3C are subject to review and correction by the CSS Working Group.

  <p>Further information on submitting testcases and implementation reports
   can be found from on the CSS Working Group's website at <a
   href
3072
="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
   Questions should be directed to the <a
   href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
   mailing list.

  <h3 id=cr-exit-criteria><span class=secno>5.6. </span> CR Exit Criteria</h3>

  <p> For this specification to be advanced to Proposed Recommendation, there
   must be at least two independent, interoperable implementations of each
   feature. Each feature may be implemented by a different set of products,
   there is no requirement that all features be implemented by a single
   product. For the purposes of this criterion, we define the following
   terms:

  <dl>
   <dt>independent

   <dd>each implementation must be developed by a different party and cannot
    share, reuse, or derive from code used by another qualifying
    implementation. Sections of code that have no bearing on the
    implementation of this specification are exempt from this requirement.

   <dt>interoperable

   <dd>passing the respective test case(s) in the official CSS test suite,
    or, if the implementation is not a Web browser, an equivalent test. Every
    relevant test in the test suite should have an equivalent test created if
    such a user agent (UA) is to be used to claim interoperability. In
    addition if such a UA is to be used to claim interoperability, then there
    must one or more additional UAs which can also pass those equivalent
    tests in the same way for the purpose of interoperability. The equivalent
    tests must be made publicly available for the purposes of peer review.

   <dt>implementation

   <dd>a user agent which:
    <ol class=inline>
     <li>implements the specification.

     <li>is available to the general public. The implementation may be a
      shipping product or other publicly available version (i.e., beta
      version, preview release, or “nightly build”). Non-shipping product
      releases must have implemented the feature(s) for a period of at least
      one month in order to demonstrate stability.

     <li>is not experimental (i.e., a version specifically designed to pass
      the test suite and is not intended for normal usage going forward).
    </ol>
  </dl>

  <p>The specification will remain Candidate Recommendation for at least six
   months.

  <h2 class=no-num id=acknowledgments>Acknowledgments</h2>

  <p>Thanks to Daniel Glazman and Dave Hyatt for writing the original
   Variables draft in 2008. Thanks to many WG members for keeping the idea of
   variables alive through the years. Thanks to Roland Steiner and Shane
   Stephens for invaluable feedback and implementation experience.

  <h2 class=no-num id=references>References</h2>

  <h3 class=no-num id=normative-references>Normative references</h3>
  <!--begin-normative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS21>[CSS21]

   <dd>Bert Bos; et al. <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607"><cite>Cascading Style
    Sheets Level 2 Revision 1 (CSS&#160;2.1) Specification.</cite></a> 7 June
    2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607">http://www.w3.org/TR/2011/REC-CSS2-20110607</a>
    </dd>
   <!---->

   <dt id=RFC2119>[RFC2119]

   <dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key
    words for use in RFCs to Indicate Requirement Levels.</cite></a> Internet
    RFC 2119. URL: <a
    href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
    </dd>
   <!---->
  </dl>
  <!--end-normative-->

  <h3 class=no-num id=other-references>Other references</h3>
  <!--begin-informative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS1>[CSS1]

   <dd>H&#229;kon Wium Lie; Bert Bos. <a
    href="http://www.w3.org/TR/2008/REC-CSS1-20080411"><cite>Cascading Style
    Sheets (CSS1) Level 1 Specification.</cite></a> 11 April 2008. W3C
    Recommendation. URL: <a
    href="http://www.w3.org/TR/2008/REC-CSS1-20080411">http://www.w3.org/TR/2008/REC-CSS1-20080411</a>
    </dd>
   <!---->

   <dt id=CSS3COLOR>[CSS3COLOR]

   <dd>Tantek &#199;elik; Chris Lilley; L. David Baron. <a
    href="http://www.w3.org/TR/2011/REC-css3-color-20110607"><cite>CSS Color
    Module Level 3.</cite></a> 7 June 2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-css3-color-20110607">http://www.w3.org/TR/2011/REC-css3-color-20110607</a>
    </dd>
   <!---->
  </dl>
  <!--end-informative-->

  <h2 class=no-num id=index>Index</h2>
  <!--begin-index-->

  <ul class=indexlist>
   <li>authoring tool, <a href="#authoring-tool"
    title="authoring tool"><strong>5.2.</strong></a>

   <li>invalid at computed-value time, <a
    href="#invalid-at-computed-value-time"
    title="invalid at computed-value time"><strong>3.1.</strong></a>

   <li>invalid variable, <a href="#invalid-variable"
    title="invalid variable"><strong>3.1.</strong></a>

   <li>invalid variables, <a href="#invalid-variable"
    title="invalid variables"><strong>3.1.</strong></a>

   <li>renderer, <a href="#renderer" title=renderer><strong>5.2.</strong></a>


   <li>style sheet
    <ul>
     <li>as conformance class, <a href="#style-sheet"
      title="style sheet, as conformance class"><strong>5.2.</strong></a>
    </ul>

   <li>variable, <a href="#type-variable"
    title=variable><strong>3.</strong></a>, <a href="#variable"
    title=variable><strong>3.</strong></a>

   <li>variable properties, <a href="#variable-property"
    title="variable properties"><strong>2.</strong></a>

   <li>variable property, <a href="#variable-property"
    title="variable property"><strong>2.</strong></a>

   <li>variables, <a href="#type-variable"
    title=variables><strong>3.</strong></a>
  </ul>
  <!--end-index-->

  <h2 class=no-num id=property-index>Property index</h2>
  <!--begin-properties-->

  <table class=proptable>
   <thead>
    <tr>
     <th>Property

     <th>Values

     <th>Initial

     <th>Applies&nbsp;to

     <th>Inh.

     <th>Percentages

     <th>Media

   <tbody>
    <tr>
     <th><span class=property>var-*</span>

     <td>see prose

     <td>invalid (a keyword, but see prose)

     <td>all elements

     <td>yes

     <td>specified value with variables substituted (but see prose for
      "invalid variables")

     <td>all
  </table>
  <!--end-properties-->
</html>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-declaration:"~/SGML/HTML4.decl"
sgml-default-doctype-name:"html"
sgml-minimize-attributes:t
sgml-nofill-elements:("pre" "style" "br")
sgml-live-element-indicator:t
sgml-omittag:nil
sgml-shorttag:nil
sgml-namecase-general:t
sgml-general-insert-case:lower
sgml-always-quote-attributes:t
sgml-indent-step:nil
sgml-indent-data:t
sgml-parent-document:nil
sgml-exposed-tags:nil
sgml-local-catalogs:nil
sgml-local-ecat-files:nil
End:
-->