forked from w3c/csswg-drafts
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathOverview.bs
More file actions
1692 lines (1391 loc) · 67.1 KB
/
Overview.bs
File metadata and controls
1692 lines (1391 loc) · 67.1 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
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
<pre class='metadata'>
Title: CSS Cascading and Inheritance Level 5
Shortname: css-cascade
Level: 5
Status: ED
Work Status: Exploring
Group: csswg
ED: https://drafts.csswg.org/css-cascade/
TR: https://www.w3.org/TR/css-cascade-5/
Previous Version: https://www.w3.org/TR/2021/WD-css-cascade-5-20210317/
Previous Version: https://www.w3.org/TR/2021/WD-css-cascade-5-20210119/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Miriam E. Suzanne, Invited Expert, http://miriamsuzanne.com/contact, w3cid 117151
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
Abstract: This CSS module describes how to collate style rules and assign values to all properties on all elements. By way of cascading and inheritance, values are propagated for all properties on all elements.
Abstract:
Abstract: New in this level is <a>cascade layers</a>.
At Risk: the ''revert-layer'' keyword
Ignored Terms: auto, flex items, <supports-condition>
Include Can I Use Panels: yes
Informative Classes: ex
</pre>
<pre class=link-defaults>
spec:css-syntax-3; type:dfn; text:css identifier
spec:dom; type:dfn; text:shadow tree
spec:css-color-4; type:property; text:color
spec:css-values-3; type: value; text:ex
spec:css-conditional-3; type:at-rule; text:@medi
10921
a
spec:mediaqueries-4; type:type; for:@media; text:all
spec:mediaqueries-4; type:type; text:<media-query>
</pre>
<pre class=ignored-specs>
spec:mediaqueries-5
spec:css-values-4
spec:css-fonts-4
</pre>
<h2 id="intro">
Introduction</h2>
CSS defines a finite set of parameters,
called <dfn export for=CSS lt="property">properties</dfn>,
that direct the rendering of a document.
Each [=property=] has a name
(e.g., 'color', 'font-size', or 'border-style'),
a value space
(e.g., <<color>>, <<length-percentage>>, ''[ solid | dashed | dotted | … ]''),
and a defined behavior on the rendering of the document.
Properties values are assigned to various parts of the document
via [=property declarations=],
which assign the property a value
(e.g. ''red'', ''12pt'', ''border-style/dotted'')
for the associated element or box.
One of the fundamental design principles of CSS is <a lt="cascade">cascading</a>,
which allows several style sheets to influence the presentation of a document.
When different declarations try to set a value for the same element/property combination,
the conflicts must somehow be resolved.
The opposite problem arises when no declarations try to set a the value for an element/property combination.
In this case, a value is be found by way of <a>inheritance</a>
or by looking at the property's <a>initial value</a>.
The <a href="#cascade">cascading</a> and <a href="#defaulting">defaulting</a> process takes a set of declarations as input,
and outputs a <a>specified value</a> for each property on each element.
The rules for finding the specified value for all properties on all elements in the document are described in this specification.
The rules for finding the specified values in the page context and its margin boxes are described in [[css-page-3]].
<h3 id="placement">
Module Interactions</h3>
<em>This section is normative.</em>
This module replaces and extends
the rules for assigning property values, cascading, and inheritance defined in [[!CSS2]] chapter 6.
Other CSS modules may expand the definitions of some of the syntax and features defined here.
For example, the Media Queries Level 4 specification,
when combined with this module, expands the definition of
the <<media-query>> value type as used in this specification.
For the purpose of this specification,
<a>text nodes</a> are treated as <a spec=css-display-3>element</a> children of their associated element,
and possess the full set of properties;
since they cannot be targeted by selectors
all of their computed values are assigned by <a href="#defaulting">defaulting</a>.
<!--
███████ ████ ██ ██ ████████ ███████ ████████ ████████
██ ██ ██ ███ ███ ██ ██ ██ ██ ██ ██ ██
██ ███ ██ ██ ████ ████ ██ ██ ██ ██ ██ ██ ██
██ ███ ██ ██ ██ ███ ██ ████████ ██ ██ ████████ ██
██ █████ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██
███████ ████ ██ ██ ██ ███████ ██ ██ ██
-->
<h2 id="at-import">
Importing Style Sheets: the ''@import'' rule</h2>
The <dfn>@import</dfn> rule allows users to import style rules from other style sheets.
If an ''@import'' rule refers to a valid stylesheet,
user agents must treat the contents of the stylesheet as if they were written in place of the ''@import'' rule,
with two exceptions:
* If a feature
(such as the ''@namespace'' rule)
<em>explicitly</em> defines that it only applies to a particular stylesheet,
and not any imported ones,
then it doesn't apply to the imported stylesheet.
* If a feature relies on the relative ordering of two or more constructs in a stylesheet
(such as the requirement that ''@namespace'' rules must not have any other rules other than
''@import'' preceding it),
it only applies between constructs in the same stylesheet.
<p class='example'>
For example, declarations in style rules from imported stylesheets interact with the cascade
as if they were written literally into the stylesheet at the point of the ''@import''.
Any ''@import'' rules must precede all other valid at-rules and style rules in a style sheet
(ignoring ''@charset'' and empty ''@layer'' definitions), or else the ''@import'' rule is invalid.
The syntax of ''@import'' is:
<pre class='prod'>
@import [ <<url>> | <<string>> ]
[ layer | layer(<<layer-ident>>) ]?
[ supports( [ <<supports-condition>> | <<declaration>> ] ) ]?
<<media-query-list>>? ;</pre>
where:
* the <<url>> or <<string>>
gives the URL of the style sheet to be imported.
* the optional ''layer'' keyword or ''layer()'' function
assigns the contents of the style sheet
into its own anonymous [=cascade layer=]
or into the named [=cascade layer=].
* the optional [<<supports-condition>>|<<declaration>>] and <<media-query-list>>
(collectively, the <dfn export>import conditions</dfn>)
state the conditions under which it applies.
<div class="example">
The following <a href="#conditional-import">conditional <css>@import</css> rule</a>
only loads the style sheet when the UA
<a href="https://www.w3.org/TR/css-conditional-3/#support-definition">supports</a> ''display: flex'',
and only applies the style sheet on a <a href="https://www.w3.org/TR/CSS2/media.html#media-types">handheld</a> device
with a <a href="https://www.w3.org/TR/mediaqueries-4/#width">maximum viewport width</a> of 400px.
<pre>@import url("narrow.css") supports(display: flex) handheld and (max-width: 400px);</pre>
</div>
<div class="example">
The following layer imports load the style sheets into
the ''framework component'' layer, and an un-named layer, respectively:
<pre>
@import url("tabs.css") layer(framework component);
@import url("override.css") layer();
</pre>
</div>
If a <<string>> is provided,
it must be interpreted as a <<url>> with the same value.
<div class="example">
The following lines are equivalent in meaning
and illustrate both ''@import'' syntaxes
(one with ''url()'' and one with a bare string):
<pre class='lang-css'>
@import "mystyle.css";
@import url("mystyle.css");
</pre>
</div>
<h3 id=conditional-import>
Conditional ''@import'' Rules</h3>
The <a>import conditions</a> allow the import to be media– or feature-support–dependent.
In the absence of any <a>import conditions</a>, the import is unconditional.
(Specifying ''@media/all'' for the <<media-query-list>> has the same effect.)
If the <a>import conditions</a> do not match,
the rules in the imported stylesheet do not apply,
exactly as if the imported stylesheet were wrapped in ''@media'' and/or ''@supports'' blocks with the given conditions.
<div class=example>
The following rules illustrate how ''@import'' rules can be made media-dependent:
<pre class='lang-css'>
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
@import url("narrow.css") handheld and (max-width: 400px);
</pre>
</div>
User agents may therefore avoid fetching a conditional import
as long as the <a>import conditions</a> do not match.
Additionally, if a <<supports-condition>> blocks the application of the imported style sheet,
the UA <em>must not</em> fetch the style sheet (unless it is loaded through some other link)
and <em>must</em> return null for the import rule's CSSImportRule.styleSheet value
(even if it is loaded through some other link).
<div class="example">
The following rule illustrates how an author can provide fallback rules for legacy user agents
without impacting network performance on newer user agents:
<pre class='lang-css'>
@import url("fallback-layout.css") supports(not (display: flex));
@supports (display: flex) {
...
}
</pre>
</div>
A <dfn><<media-query>></dfn> corresponds to the <code>media_query_list</code> production
and is interpreted as a <a>media query</a>,
and a <dfn><<supports-condition>></dfn> corresponds to a <code>supports_condition</code> production
and is interpreted as an ''@supports'' condition.
If a <dfn><<declaration>></dfn> (a <code>declaration</code> production) is given in place of a <<supports-condition>>,
it must be interpreted as a <code>supports_declaration_condition</code> production
(i.e. the extra set of parentheses is implied)
and treated as a <<supports-condition>>.
<div class="example">
For example, the following two lines are equivalent:
<pre class='lang-css'>
@import "mystyle.css" supports(display: flex);
@import "mystyle.css" supports((display: flex));
</pre>
</div>
The evaluation and full syntax of the <a>import conditions</a>
are defined by the <a href="https://www.w3.org/TR/mediaqueries/">Media Queries</a> [[!MEDIAQ]]
and <a href="https://www.w3.org/TR/css-conditional/">CSS Conditional Rules</a> [[!CSS-CONDITIONAL-3]] specifications.
<h3 id=import-processing>
Processing Stylesheet Imports</h3>
When the same style sheet is imported or linked to a document in multiple places,
user agents must process (or act as though they do) each link
as though the link were to an independent style sheet.
Note: This does not place any requirements on resource fetching,
only how the style sheet is reflected in the CSSOM and used in specs such as this one.
Assuming appropriate caching,
it is perfectly appropriate for a UA to fetch a style sheet only once,
even though it's linked or imported multiple times.
The <a>origin</a> of an imported style sheet is the <a>origin</a> of the style sheet that imported it.
The <a>environment encoding</a> of an imported style sheet is the encoding of the style sheet that imported it. [[css-syntax-3]]
<h3 id='content-type'>
Content-Type of CSS Style Sheets</h3>
The processing of imported style sheets depends on the actual type of the linked resource:
* If the resource does not have <l spec=html>[=Content-Type metadata=]</l>,
the type is treated as <code>text/css</code>.
* If the host document is in [=quirks mode=],
and the host document's origin is [=same origin=]
with the linked resource [=/response's=] [=response/URL's=] origin,
the type is treated as <code>text/css</code>.
* Otherwise, the type is determined from its <l spec=html>[=Content-Type metadata=]</l>.
If the linked resource's type is <code>text/css</code>,
it must be interpreted as a CSS style sheet.
Otherwise, it must be interpreted as a network error.
<!--
██████ ██ ██ ███████ ████████ ████████ ██ ██ ███ ██ ██ ████████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ███ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ████ ██ ██ ██
██████ █████████ ██ ██ ████████ ██ █████████ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ █████████ ██ ████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ███ ██ ██
██████ ██ ██ ███████ ██ ██ ██ ██ ██ ██ ██ ██ ██ ████████
-->
<h2 id='shorthand'>
Shorthand Properties</h2>
Some properties are <dfn export lt="shorthand property | shorthand">shorthand properties</dfn>,
meaning that they allow authors to specify the values of several properties with a single property.
A <a>shorthand property</a> sets all of its <dfn export lt="longhand | longhand property | sub-property">longhand sub-properties</dfn>,
exactly as if expanded in place.
When values are omitted from a <a>shorthand</a> form,
unless otherwise defined,
each “missing” <a>sub-property</a> is assigned its <a>initial value</a>.
<div class='note'>
This means that a <a>shorthand</a> property declaration always sets <em>all</em> of its <a>sub-properties</a>,
even those that are not explicitly set.
Carelessly used, this might result in inadvertently resetting some <a>sub-properties</a>.
Carefully used, a <a>shorthand</a> can guarantee a “blank slate”
by resetting <a>sub-properties</a> inadvertently cascaded from other sources.
For example, writing ''background: green'' rather than ''background-color: green''
ensures that the background color overrides any earlier declarations
that might have set the background to an image with 'background-image'.
</div>
<div class='example'>
For example, the CSS Level 1 'font' property
is a <a>shorthand</a> property for setting <a property>font-style</a>, <a property>font-variant</a>, <a property>font-weight</a>, 'font-size', 'line-height', and <a property>font-family</a> all at once.
The multiple declarations of this example:
<pre class='lang-css'>
h1 {
font-weight: bold;
font-size: 12pt;
line-height: 14pt;
font-family: Helvetica;
font-variant: normal;
font-style: normal;
}
</pre>
can therefore be rewritten as
<pre class='lang-css'>h1 { font: bold 12pt/14pt Helvetica }</pre>
As more 'font' <a>sub-properties</a> are introduced into CSS,
the shorthand declaration resets those to their initial values as well.
</div>
In some cases, a <a>shorthand</a> might have different syntax
or special keywords
that don't directly correspond to values of its <a>sub-properties</a>.
(In such cases, the <a>shorthand</a> will explicitly define the expansion of its values.)
In other cases, a property might be a <dfn export>reset-only sub-property</dfn> of the shorthand:
Like other <a>sub-properties</a>, it is reset to its initial value by the shorthand when unspecified,
but the shorthand might not include syntax to set the <a>sub-property</a>
to any of its other values.
<span class=ex>For example, the 'border' shorthand resets 'border-image'
to its initial value of ''border-image/none'',
but has no syntax to set it to anything else. [[css-backgrounds-3]]</span>
If a <a>shorthand</a> is specified as one of the <a href="https://www.w3.org/TR/css-values/#common-keywords">CSS-wide keywords</a> [[!css-values-3]],
it sets all of its <a>sub-properties</a> to that keyword,
including any that are <a>reset-only sub-properties</a>.
(Note that these keywords cannot be combined with other values in a single declaration, not even in a shorthand.)
Declaring a <a>shorthand</a> property to be ''!important''
is equivalent to declaring all of its <a>sub-properties</a> to be ''!important''.
<h3 id="aliasing">
Aliasing</h3>
Properties sometimes change names after being supported for a while,
such as vendor-prefixed properties being standardized.
The original name still needs to be supported for compatibility reasons,
but the new name is preferred.
To accomplish this, CSS defines two different ways of “aliasing” old syntax to new syntax.
<dl export>
<dt><dfn lt="legacy name alias">legacy name aliases</dfn>
<dd>
When the old property’s value syntax is identical
to that of the new property,
the two names are aliased with an operation on par with case-mapping:
at parse time, the old property is converted into the new property.
This conversion also applies in the CSSOM,
both for string arguments and property accessors:
requests for the old property name
transparently transfer to the new property name instead.
<div class=example highlight=js>
For example, if
<css>old-name</css> is a <a>legacy name alias</a> for <css>new-name</css>,
<code>getComputedStyle(el).oldName</code>
will return the computed style of the <code>newName</code> property,
and <code>el.style.setPropertyValue("old-name", "value")</code>
will set the <css>new-name</css> property to <code>"value"</code>.
</div>
<dt><dfn lt="legacy shorthand">legacy shorthands</dfn>
<dd>
When the old property has a distinct syntax from the new property,
the two names are aliased using the <a>shorthand</a> mechanism.
These shorthands are defined to be <a>legacy shorthands</a>,
and their use is <em>deprecated</em>.
They otherwise behave exactly as regular shorthands,
except that the CSSOM will not use them
when serializing declarations. [[CSSOM]]
<div class=example highlight=js>
For example, the 'page-break-*' properties
are <a>legacy shorthands</a> for the 'break-*' properties
(see [[css-break-3#page-break-properties]]).
Setting ''page-break-before: always'' expands to ''break-before: page'' at parse time,
like other shorthands do.
Similarly, if ''break-before: page'' is set,
calling <code>getComputedStyle(el).pageBreakBefore</code> will return <code>"always"</code>.
However, when serializing a style block
(see [[cssom-1#serializing-css-values]]),
the 'page-break-before' property will never be chosen as the shorthand to serialize to,
regardless of whether it or 'break-before' was specified;
instead, 'break-before' will always be chosen.
</div>
</dl>
<h3 id="all-shorthand" caniuse="css-all">
Resetting All Properties: the 'all' property</h3>
<pre class="propdef shorthand">
Name: all
Value: initial | inherit | unset | revert | revert-layer
</pre>
The 'all' property is a <a>shorthand</a>
that resets <em>all</em> CSS properties
except 'direction' and 'unicode-bidi'.
It only accepts the <a href="https://www.w3.org/TR/css-values/#common-keywords">CSS-wide keywords</a>.
It does not reset <a>custom properties</a> [[css-variables-1]].
Note: The excepted CSS properties 'direction' and 'unicode-bidi'
are actually markup-level features,
and <a href="https://www.w3.org/TR/css-writing-modes-3/#text-direction">should not be set in the author's style sheet</a>.
(They exist as CSS properties only to style document languages not supported by the UA.)
Authors should use the appropriate markup, such as HTML's <code>dir</code> attribute, instead.
[[css-writing-modes-3]]
<div class='example'>
For example, if an author specifies ''all: initial'' on an element,
it will block all inheritance and reset all properties,
as if no rules appeared in the author, user, or user-agent levels of the cascade.
This can be useful for the root element of a "widget" included in a page,
which does not wish to inherit the styles of the outer page.
Note, however, that any "default" style applied to that element
(such as, e.g. ''display: block'' from the UA style sheet on block elements such as <code><div></code>)
will also be blown away.
</div>
<!--
██████ ████████ ███ ██████ ████████ ██████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██ ██ ████ ██████ ██████
██ ██ █████████ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██ ██████ ████████ ██████
-->
<h2 id="value-stages">
Value Processing</h2>
Once a user agent has parsed a document and constructed a document tree,
it must assign,
to every element in the tree,
and correspondingly to every box in the formatting structure,
a value to every property that applies to the target media type.
The final value of a CSS property for a given element or box
is the result of a multi-step calculation:
<ol>
<li>
First, all the <a>declared values</a> applied to an element are collected,
for each property on each element.
There may be zero or many <a>declared values</a> applied to the element.
<li>
Cascading yields the <a>cascaded value</a>.
There is at most one <a>cascaded value</a> per property per element.
<li>
Defaulting yields the <a>specified value</a>.
Every element has exactly one <a>specified value</a> per property.
<li>
Resolving value dependencies yields the <a>computed value</a>.
Every element has exactly one <a>computed value</a> per property.
<li>
Formatting the document yields the <a>used value</a>.
An element only has a <a>used value</a> for a given property
if that property applies to the element.
<li>
Finally, the used value is transformed to the <a>actual value</a>
based on constraints of the display environment.
As with the <a>used value</a>, there may or may not be an <a>actual value</a>
for a given property on an element.
</ol>
Elements that are not [=connected=]
or are not part of the document’s [=flattened element tree=]
do not participate in CSS value processing,
and do not have [=declared=], [=cascaded=], [=specified=], [=computed=], [=used=], or [=actual=] values,
even if they potentially have style declarations assigned to them
(for example, by a <code>style</code> attribute).
<h3 id="declared">
Declared Values</h3>
Each property declaration <a href="#filtering">applied to an element</a>
contributes a <dfn export local-lt="declared">declared value</dfn> for that property
associated with the element.
See <a href="#filtering">Filtering Declarations</a> for details.
These values are then processed by the <a>cascade</a>
to choose a single “winning value”.
<h3 id="cascaded">
Cascaded Values</h3>
The <dfn export local-lt="cascaded">cascaded value</dfn>
represents the result of <a href="#cascade">the cascade</a>:
it is the <a>declared value</a> that wins the cascade
(is sorted first in the <a>output of the cascade</a>).
If the <a>output of the cascade</a> is an empty list,
there is no <a>cascaded value</a>.
<h3 id="specified">
Specified Values</h3>
The <dfn export local-lt="specified">specified value</dfn> is
the value of a given property that the style sheet authors intended for that element.
It is the result of putting the <a>cascaded value</a> through the <a href="#defaulting">defaulting</a> processes,
guaranteeing that a <a>specified value</a> exists for every property on every element.
In many cases, the <a>specified value</a> is the <a>cascaded value</a>.
However, if there is no <a>cascaded value</a> at all,
the <a>specified value</a> is <a href="#defaulting">defaulted</a>.
The [=CSS-wide keywords=] are handled specially
when they are the <a>cascaded value</a> of a property,
setting the <a>specified value</a> as required by that keyword,
see [[#defaulting-keywords]].
<h3 id="computed">
Computed Values</h3>
The <dfn export local-lt="computed">computed value</dfn> is
the result of resolving the <a>specified value</a>
as defined in the “Computed Value” line of the property definition table,
generally absolutizing it in preparation for <a>inheritance</a>.
Note: The <a>computed value</a> is the value that is transferred from parent to child during <a>inheritance</a>.
For historical reasons,
it is not necessarily the value returned by the {{getComputedStyle()}} function,
which sometimes returns <a>used values</a>. [[CSSOM]]
Furthermore, the <a>computed value</a> is an abstract data representation:
their definitions reflect that data representation,
not how that data is serialized.
For example, serialization rules often allow omitting certain values which are implied during parsing;
but those values are nonetheless part of the <a>computed value</a>.
<div class="example">
A <a>specified value</a> can be either absolute (i.e., not relative to another value, as in ''red'' or ''2mm'')
or relative (i.e., relative to another value, as in ''auto'', ''2em'').
Computing a relative value generally absolutizes it:
<ul>
<li>
values with relative units
(''em'', ''ex'', ''vh'', ''vw'')
must be made absolute by multiplying with the appropriate reference size
<li>
certain keywords
(e.g., ''smaller'', ''bolder'')
must be replaced according to their definitions
<li>
percentages on some properties must be multiplied by a reference value
(defined by the property)
<li>
valid relative URLs must be resolved to become absolute.
</ul>
See examples (f), (g) and (h) in the <a href="#stages-examples">table below</a>.
</div>
Note: In general, the <a>computed value</a> resolves the <a>specified value</a>
as far as possible without laying out the document
or performing other expensive or hard-to-parallelize operations,
such as resolving network requests
or retrieving values other than from the element and its parent.
The <a>computed value</a> exists even when the property does not apply.
However, some properties may change how they determine the <a>computed value</a>
based on whether the property [=applies to=] the element.
<h3 id='used'>
Used Values</h3>
The <dfn export local-lt="used">used value</dfn> is
the result of taking the <a>computed value</a>
and completing any remaining calculations to make it the absolute theoretical value
used in the formatting of the document.
<p class='example'>
For example, a declaration of ''width: auto'' can't be resolved into a length without knowing the layout of the element's ancestors,
so the <a>computed value</a> is ''auto'',
while the <a>used value</a> is an absolute length, such as ''100px''. [[CSS2]]
<p class='example'>
As another example, a <code><div></code> might have a computed 'break-before' value of ''auto'',
but acquire a used 'break-before' value of ''break-before/page'' by propagation from its first child. [[css-break-3]]
If a property does not [=apply to=]
this element or box type
then it has no <a>used value</a> for that property.
<p class='example'>
For example, the 'flex' property has no <a>used value</a>
on elements that aren't <a>flex items</a>.
Note: A property defined to apply to “all elements”
applies to all elements and [=display types=],
but not necessarily to all [=pseudo-element=] types,
since pseudo-elements often have their own specific rendering models
or other restrictions.
The ''::before'' and ''::after'' pseudo-elements, however,
are defined to generate boxes almost exactly like normal elements
and are therefore defined accept all properties that apply to “all elements”.
See [[CSS-PSEUDO-4]]
for more information about [=pseudo-elements=].
<h4 id="applies-to">
Applicable Properties</h4>
If a property does not <dfn export for=CSS id="apply">apply to</dfn>
an element or box type--
as noted in its “Applies to” line--
this means it does not directly take effect on that type of box or element.
Note: A property that does not apply
can still have <em>indirect</em> formatting effects
if its computed value affects the computation of other properties
that do apply;
and of course its [=computed value=],
which always exists,
can still inherit to descendants
and take effect on them.
<div class=example>
Even though 'writing-mode' and 'text-orientation' do not apply to table rows
(they do not affect how the table row or its children are laid out),
setting them on such boxes
will still affect the calculation of font relative units such as ''ch'',
and thus possibly any property that takes a <<length>>.
</div>
<div class=example>
Setting 'text-transform' on an HTML <{p}> element
(which is ''display: block'' by default)
will have an effect,
even though 'text-transform' only applies to [=inline boxes=],
because the property inherits
into the paragraph's anonymous [=root inline box=]
and applies to the text it contains.
</div>
Note: A property defined to apply to “all elements”
applies to all elements and [=display types=],
but not necessarily to all [=pseudo-element=] types,
since pseudo-elements often have their own specific rendering models
or other restrictions.
The ''::before'' and ''::after'' pseudo-elements, however,
are defined to generate boxes almost exactly like normal elements
and are therefore defined accept all properties that apply to “all elements”.
See [[CSS-PSEUDO-4]]
for more information about [=pseudo-elements=].
<h3 id="actual">
Actual Values</h3>
A <a>used value</a> is in principle ready to be used,
but a user agent may not be able to make use of the value in a given environment.
<span class=ex>For example, a user agent may only be able to render borders with integer pixel widths
and may therefore have to approximate the <a lt="used value">used</a> width.
Also, the font size of an element may need adjustment based on the availability of fonts
or the value of the 'font-size-adjust' property.</span>
The <dfn export local-lt="actual">actual value</dfn> is
the used value after any such adjustments have been made.
Note: By probing the actual values of elements,
much can be learned about how the document is laid out.
However, not all information is recorded in the actual values.
For example, the actual value of the 'page-break-after' property
does not reflect whether there is a page break or not after the element.
Similarly, the actual value of 'orphans'
does not reflect how many orphan lines there is in a certain element.
See examples (j) and (k) in the <a href="#stages-examples">table below</a>.
<h3 id="stages-examples">
Examples</h3>
<table class="data non-normative">
<caption>Examples of CSS Value Computation</caption>
<thead>
<tr>
<th>
<th>Property
<th>Winning declaration
<th>Cascaded value
<th>Specified value
<th>Computed value
<th>Used value
<th>Actual value
<tbody>
<tr>
<td>(a)
<th>'text-align'
<td><code class="declaration">text-align: left</code>
<td><css>left</css>
<td><css>left</css>
<td><css>left</css>
<td><css>left</css>
<td><css>left</css>
<tr>
<td>(b)
<th>'border-top-width', 'border-right-width', 'border-bottom-width', 'border-left-width'
<td><code class="declaration">border-width: inherit</code>
<td><css>inherit</css>
<td class="say"><css>4.2px</css>
<td><css>4.2px</css>
<td><css>4.2px</css>
<td><css>4px</css>
<tr>
<td>(c)
<th>'width'
<td><small>(none)</small>
<td><small>(none)</small>
<td><css>auto</css> <small>(initial value)</small>
<td><css>auto</css>
<td><css>120px</css>
<td><css>120px</css>
<tr>
<td>(d)
<th>'list-style-position'
<td><code class="declaration">list-style-position: inherit</code>
<td><css>inherit</css>
<td class="say"><css>inside</css>
<td><css>inside</css>
<td><css>inside</css>
<td><css>inside</css>
<tr>
<td>(e)
<th>'list-style-position'
<td><code class="declaration">list-style-position: initial</code>
<td><css>initial</css>
<td><css>outside</css> <small>(initial value)</small>
<td><css>outside</css>
<td><css>outside</css>
<td><css>outside</css>
<tr>
<td>(f)
<th>'font-size'
<td><code class="declaration">font-size: 1.2em</code>
<td><css>1.2em</css>
<td><css>1.2em</css>
<td class="say"><css>14.1px</css>
<td><css>14.1px</css>
<td><css>14px</css>
<tr>
<td>(g)
<th>'width'
<td><code class="declaration">width: 80%</code>
<td><css>80%</css>
<td><css>80%</css>
<td><css>80%</css>
<td><css>354px</css>
<tr>
<td>(h)
<th>'width'
<td><code class="declaration">width: auto</code>
<td><css>auto</css>
<td><css>auto</css>
<td><css>auto</css>
<td class="say"><css>134px</css>
<td><css>134px</css>
<tr>
<td>(i)
<th>'height'
<td><code class="declaration">height: auto</code>
<td><css>auto</css>
<td><css>auto</css>
<td><css>auto</css>
<td class="say"><css>176px</css>
<td><css>176px</css>
<tr>
<td>(j)
<th>'page-break-after'
&l
F3A0
t;td><small>(none)</small>
<td><small>(none)</small>
<td><css>auto</css> <small>(initial value)</small>
<td><css>auto</css>
<td><css>auto</css>
<td><css>auto</css>
<tr>
<td>(k)
<th>'orphans'
<td><code class="declaration">orphans: 3</code>
<td><css>3</css>
<td><css>3</css>
<td><css>3</css>
<td><css>3</css>
<td><css>3</css>
</table>
<h3 id="fragments">
Per-Fragment Value Processing</h3>
Certain CSS features
can interfere with value processing
on a per-fragment basis.
See for example [[css-pseudo-4#first-line-inheritance]]
which alters inheritance for fragments within the ''::first-line'' pseudo-element.
In such cases, where individual fragments are given different [=specified values=],
any values that resolve
based on the [=computed value=] of other properties
(such as ''currentcolor'' or ''em'' units)
are resolved per [=box fragment=].
Subsequent value processing proceeds as normal in each fragment.
APIs that assume a singular value per [=box=] (rather than per [=box fragment=])
must ignore the effects of non-[=tree-abiding=] [=pseudo-elements=].
(For example, ''::first-line'' styles have no effect on the value returned by {{getComputedStyle()}}.)
<div class=example>
For example, given the following markup:
<xmp highlight=html>
<div><span>First line<br />Second line</span></div>
<div><span>First line</span></div>
<div>First line<br><span>Second line</span></div>
<style>
div { color: blue; }
div::first-line { color: yellow; }
span { border: thin solid currentcolor; }
</style>
</xmp>
In each <{div}>, the “First line” text is yellow and the “Second line” text is blue;
the border for each fragment of the <{span}>s that wrap each line matches that color.
However, {{getComputedStyle()}} on all three of the spans
will return <code>"blue"</code> for 'border-color',
because the effects of a ''::first-line'' pseudo-element
are ignored for APIs that aren't fragment-aware.
</div>
<!--
████████ ████ ██ ████████ ████████ ████████ ████ ██ ██ ██████
██ ██ ██ ██ ██ ██ ██ ██ ███ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ████ ██ ██
██████ ██ ██ ██ ██████ ████████ ██ ██ ██ ██ ██ ████
██ ██ ██ ██ ██ ██ ██ ██ ██ ████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ███ ██ ██
██ ████ ████████ ██ ████████ ██ ██ ████ ██ ██ ██████
-->
<h2 id='filtering'>
Filtering</h2>
In order to find the <a>declared values</a>,
implementations must first identify all declarations that apply to each element.
A declaration applies to an element if:
<ul>
<li>
It belongs to a style sheet that currently applies to this document.
<li>
It is not qualified by a conditional rule [[!CSS-CONDITIONAL-3]] with a false condition.
<li>
It belongs to a style rule whose selector matches the element. [[!SELECT]]
(Taking <a href="https://www.w3.org/TR/selectors-4/#scoping">scoping</a> into account, if necessary.)
<li>
It is syntactically valid:
the declaration's property is a known property name,
and the declaration's value matches the syntax for that property.
</ul>
The values of the declarations that apply form,
for each property on each element,
a list of <a>declared values</a>.
The next section,
the <a>cascade</a>,
prioritizes these lists.
<!--
██████ ███ ██████ ██████ ███ ████████ ████████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██████ ██ ██ ██ ██ ██ ██████
██ █████████ ██ ██ █████████ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██████ ██████ ██ ██ ████████ ████████
-->
<h2 id='cascading'>
Cascading</h2>
The <dfn export>cascade</dfn>
takes an unordered list of <a>declared values</a>
for a given property on a given element,
sorts them by their declaration’s precedence as determined below,
and outputs a single <a>cascaded value</a>.
<h3 id="cascade-sort">
Cascade Sorting Order</h3>
The cascade sorts declarations according to the following criteria,
in descending order of priority:
<dl>
<dt id='cascade-origin'>Origin and Importance
<dd>
The <a>origin</a> of a declaration is based on where it comes from
and its <a lt="important">importance</a> is
whether or not it is declared with ''!important''
(see [[#importance|below]]).
The precedence of the various <a>origins</a> is, in descending order:
<ol>
<li>Transition declarations [[!css-transitions-1]]
<li>[=Important=] [=user-agent origin|user agent=] declarations
<li>[=Important=] [=user origin|user=] declarations
<li>[=Important=] [=author origin|author=] declarations
<li>Animation declarations [[!css-animations-1]]
<li>[=Normal=] [=author origin|author=] declarations
<li>[=Normal=] [=user origin|user=] declarations
<li>[=Normal=] [=user-agent origin|user agent=] declarations
</ol>
Declarations from <a>origins</a> earlier in this list win over declarations from later <a>origins</a>.
<dt id='cascade-context'>Context
<dd>
A document language can provide for blending declarations sourced
from different <dfn local-lt="context">encapsulation contexts</dfn>,
such as the nested [=tree contexts=] of [=shadow trees=] in the [[!DOM]].
When comparing two declarations
that are sourced from different [=encapsulation contexts=],
then for [=normal=] rules
the declaration from the outer context wins,
and for [=important=] rules
the declaration from the inner context wins.
For this purpose,
[[DOM]] [=tree contexts=] are considered to be nested
in [=shadow-including tree order=].
Note: This effectively means that
[=normal=] declarations belonging to an [=encapsulation context=]
can set defaults that are easily overridden by the outer context,
while [=important=] declarations belonging to an [=encapsulation context=]
can enforce requirements that cannot be overridden by the outer context.
<!--
<dt id='cascade-scope'>Scope
<dd>
A declaration can be <dfn export>scoped</dfn> to a subtree of the document
so that it only affects its <dfn export>scoping element</dfn> and that element's descendants.
For example, [[HTML]] defines scoped <code><style></code> elements,
whose style sheets are scoped to the element's parent.
If the <a>scoping elements</a> of two declarations
have an ancestor/descendant relationship,
then for [=normal=] rules the declaration whose <a>scoping element</a> is the descendant wins,
and for [=important=] rules the declaration whose <a>scoping element</a> is the ancestor wins.
Note: In other words, for [=normal=] declarations the inner scope's declarations override,
but for ''!important'' rules <em>outer</em> scope's override.
For the purpose of this step,
all unscoped declarations are considered to be <a>scoped</a> to the root element.
[=Normal=] declarations from style attributes
are considered to be <a>scoped</a> to the element with the attribute,
whereas important declarations from style attributes
are considered to be <a>scoped</a> to the root element.
[[!CSSSTYLEATTR]]
Note: This odd handling of ''!important'' style attribute declarations
is to match the behavior defined in CSS Levels 1 and 2,
where style attributes simply have higher specificity than any other author rules. [[CSS2]]
-->