-
Notifications
You must be signed in to change notification settings - Fork 790
Expand file tree
/
Copy pathOverview.bs
More file actions
2237 lines (1851 loc) · 92 KB
/
Overview.bs
File metadata and controls
2237 lines (1851 loc) · 92 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
1
103C3
22
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 Values and Units Module Level 3
Group: CSSWG
Shortname: css-values
Level: 3
Status: ED
Work Status: Testing
ED: https://drafts.csswg.org/css-values-3/
TR: https://www.w3.org/TR/css-values/
Previous Version: https://www.w3.org/TR/2015/CR-css-values-3-20150611/
Previous Version: https://www.w3.org/TR/2013/CR-css3-values-20130730/
Previous Version: https://www.w3.org/TR/2013/CR-css3-values-20130404/
Previous Version: https://www.w3.org/TR/2012/CR-css3-values-20120828/
Previous Version: https://www.w3.org/TR/2012/WD-css3-values-20120308/
Previous Version: https://www.w3.org/TR/2011/WD-css3-values-20110906/
Previous Version: https://www.w3.org/TR/2006/WD-css3-values-20060919
Previous Version: https://www.w3.org/TR/2005/WD-css3-values-20050726
Previous Version: https://www.w3.org/TR/2001/WD-css3-values-20010713/
Editor: Tab Atkins, Google, http://xanthir.com/contact/, w3cid 42199
Editor: fantasai, http://fantasai.inkedblade.net/contact, w3cid 35400
Former Editor: Håkon Wium Lie, Opera Software, howcome@opera.com
Abstract: This CSS module describes the common values and units that CSS properties accept and the syntax used for describing them in CSS property definitions.
At Risk: ''attr()''
Ignored Terms: <spacing-limit>, containing block
Ignored Vars: Cn+1, n
Inline Github Issues: yes
</pre>
<pre class='link-defaults'>
spec: css-backgrounds-3; type: type; text: <position>
spec: css21; type: property; text: border-collapse
spec: css-color-4; type: value; text: currentcolor
spec: css-color-4; type: value; text: color
spec: css-cascade-4; type: at-rule; text: @import
</pre>
<style>
code, small { white-space: nowrap }
pre.value { font: inherit; white-space: pre-wrap; margin: 0; padding: 0; }
#propvalues td { text-align: right; }
#propvalues td + td { text-align: left; }
dt + dt::before { content: ", "; }
dt { display: inline; }
td > small { display: block; }
</style>
<h2 id="intro">
Introduction</h2>
The value definition field of each CSS property can contain keywords,
data types (which appear between ''<'' and ''>''), and information on how
they can be combined.
Generic data types (<<length>> being the most widely used)
that can be used by many properties are described in this specification,
while more specific data types (e.g., <<spacing-limit>>)
are described in the corresponding modules.
<h3 id="placement">
Module Interactions</h3>
This module replaces and extends the data type definitions in [[!CSS21]]
sections
<a href="https://www.w3.org/TR/CSS21/about.html#value-defs">1.4.2.1</a>,
<a href="https://www.w3.org/TR/CSS21/syndata.html#values">4.3</a>,
and <a href="https://www.w3.org/TR/CSS21/aural.html#aural-intro">A.2</a>.
<!--
██████ ██ ██ ██ ██ ████████ ███ ██ ██
██ ██ ██ ██ ███ ██ ██ ██ ██ ██ ██
██ ████ ████ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██ ██ ██ ██ ██ ███
██ ██ ██ ████ ██ █████████ ██ ██
██ ██ ██ ██ ███ ██ ██ ██ ██ ██
██████ ██ ██ ██ ██ ██ ██ ██ ██
-->
<h2 id="value-defs">
Value Definition Syntax</h2>
The syntax described here is used to define the set of valid values
for CSS properties. A property value can have one or more components.
<h3 id="component-types">
Component value types</h3>
Component value types are designated in several ways:
<ol>
<li>
<a href="#keywords">keyword</a> values (such as <css>auto</css>, ''disc'', etc.),
which appear literally, without quotes (e.g. <code>auto</code>)
<li>
basic data types, which appear between ''<'' and ''>''
(e.g., <<length>>, <<percentage>>, etc.).
<li>
types that have the same range of values as a property bearing the same name
(e.g., <<'border-width'>>, <<'background-attachment'>>, etc.).
In this case, the type name is the property name (complete with quotes) between the brackets.
Such a type does <em>not</em> include <a href="#common-keywords">CSS-wide keywords</a> such as ''inherit''.
<li>
non-terminals that do not share the same name as a property.
In this case, the non-terminal name appears between ''<'' and ''>'',
as in <<spacing-limit>>.
Notice the distinction between <<border-width>> and <<'border-width'>>:
the latter is defined as the value of the 'border-width' property,
the former requires an explicit expansion elsewhere.
The definition of a non-terminal is typically located near its first appearance in the specification.
</ol>
Some property value definitions also include the slash (/),
the comma (,),
and/or parentheses as literals.
These represent their corresponding tokens.
Other non-keyword literal characters that may appear in a component value,
such as “+”,
must be written enclosed in single quotes.
<strong><dfn lt="," id='comb-comma' export grammar>Commas</dfn> specified in the grammar are implicitly omissible</strong> in some circumstances,
when used to separate optional terms in the grammar.
Within a top-level list in a property or other CSS value,
or a function's argument list,
a comma specified in the grammar must be omitted if:
<ul>
<li>
all items preceding the comma have been omitted
<li>
all items following the comma have been omitted
<li>
multiple commas would be adjacent (ignoring <a href="https://www.w3.org/TR/css-syntax/#whitespace">white space</a>/comments),
due to the items between the commas being omitted.
</ul>
<div class='example'>
For example, if a function can accept three arguments in order,
but all of them are optional,
the grammar can be written like:
<pre class='prod'>
example( first? , second? , third? )
</pre>
Given this grammar,
writing ''example(first, second, third)'' is valid,
as is ''example(first, second)'' or ''example(first, third)'' or ''example(second)''.
However, ''example(first, , third)'' is invalid, as one of those commas are no longer separating two options;
similarly, ''example(,second)'' and ''example(first,)'' are invalid.
''example(first second)'' is also invalid,
as commas are still required to actually separate the options.
If commas were not implicitly omittable,
the grammar would have to be much more complicated
to properly express the ways that the arguments can be omitted,
greatly obscuring the simplicity of the feature.
</div>
All CSS properties also accept the <a href="#common-keywords">CSS-wide keyword values</a>
as the sole component of their property value.
For readability these are not listed explicitly in the property value syntax def
7890
initions.
For example, the full value definition of 'border-color'
is <code><color>{1,4} | inherit | initial | unset</code>
(even though it is listed as <code><color>{1,4}</code>).
Note: This implies that, in general,
combining these keywords with other component values in the same declaration
results in an invalid declaration.
For example,
''background: url(corner.png) no-repeat, inherit;'' is invalid.
<h3 id="component-combinators">
Component value combinators</h3>
Component values can be arranged into property values as follows:
<ul export dfn-type="grammar">
<li>Juxtaposing components means that
all of them must occur, in the given order.
<li>A double ampersand (<dfn id='comb-all'>&&</dfn>) separates two or more components,
all of which must occur, in any order.
<li>A double bar (<dfn id='comb-any'>||</dfn>) separates two or more options:
one or more of them must occur, in any order.
<li>A bar (<dfn id='comb-one'>|</dfn>) separates two or more alternatives:
exactly one of them must occur.
<li>Brackets ([ ]) are for grouping.
</ul>
Juxtaposition is stronger than the double ampersand, the double
ampersand is stronger than the double bar, and the double bar
is stronger than the bar. Thus, the following lines are equivalent:
<pre>
a b | c || d && e f
[ a b ] | [ c || [ d && [ e f ]]]
</pre>
For reorderable combinators (||, &&),
ordering of the grammar does not matter:
components in the same grouping may be interleaved in any order.
Thus, the following lines are equivalent:
<pre>
a || b || c
b || a || c
</pre>
<h3 id="component-multipliers">
Component value multipliers</h3>
Every type, keyword, or bracketed group may be followed by one of
the following modifiers:
<ul export dfn-type="grammar">
<li>An asterisk (<dfn id='mult-zero-plus'>*</dfn>) indicates that the preceding type, word, or
group occurs zero or more times.
<li>A plus (<dfn id='mult-one-plus'>+</dfn>) indicates that the preceding type, word, or group
occurs one or more times.
<li>A question mark (<dfn id='mult-opt'>?</dfn>) indicates that the preceding type, word, or
group is optional (occurs zero or one times).
<li>A single number in curly braces (<dfn id='mult-num'>{<var>A</var>}</dfn>)
indicates that the preceding type, word, or group occurs <var>A</var> times.
<li>A comma-separated pair of numbers in curly braces (<dfn id='mult-num-range'>{<var>A</var>,<var>B</var>}</dfn>)
indicates that the preceding type, word, or group occurs at least
<var>A</var> and at most <var>B</var> times.
The <var>B</var> may be omitted ({<var>A</var>,})
to indicate that there must be at least <var>A</var> repetitions,
with no upper bound on the number of repetitions.
<li>A hash mark (<dfn id='mult-comma'>#</dfn>) indicates that the preceding type, word, or
group occurs one or more times, separated by comma tokens
(which may optionally be surrounded by <a href="https://www.w3.org/TR/css-syntax/#whitespace">white space</a> and/or comments).
It may optionally be followed by the curly brace forms, above,
to indicate precisely how many times the repetition occurs,
like ''<length>#{1,4}''.
<li>An exclamation point (<dfn id='mult-req'>!</dfn>) after a group indicates that the group is required
and must produce at least one value;
even if the grammar of the items within the group would otherwise allow the entire contents to be omitted,
at least one component value must not be omitted.
</ul>
For repeated component values (indicated by ''*'', ''+'', or ''#''),
UAs must support at least 20 repetitions of the component.
If a property value contains more than the supported number of repetitions,
the declaration must be ignored as if it were invalid.
<h3 id='combinator-multiplier-patterns'>
Combinator and Multiplier Patterns</h3>
There are a small set of common ways to combine multiple independent <a>component values</a> in particular numbers and orders.
In particular, it's common to want to express that,
from a set of component value,
the author must select zero or more, one or more, or all of them,
and in either the order specified in the grammar or in any order.
All of these can be easily expressed using simple patterns of <a href="#component-combinators">combinators</a> and <a href="#component-multipliers">multipliers</a>:
<table class='data'>
<thead>
<tr>
<th>
<th>in order
<th>any order
<tbody>
<tr>
<th>zero or more
<td><pre>A? B? C?</pre>
<td><pre> A? || B? || C?</pre>
<tr>
<th>one or more
<td><pre> [ A? B? C? ]!</pre>
<td><pre>A || B || C</pre>
<tr>
<th>all
<td><pre>A B C </pre>
<td><pre>A && B && C</pre>
</table>
Note that all of the "any order" possibilities are expressed using combinators,
while the "in order" possibilities are all variants on juxtaposition.
<h3 id="component-whitespace">
Component values and white space</h3>
Unless otherwise specified,
<a href="https://www.w3.org/TR/css-syntax/#whitespace">white space</a> and/or comments may appear before, after, and/or between
components combined using the above
<a href="#component-combinators">combinators</a> and
<a href="#component-multipliers">multipliers</a>.
Note: In many cases, spaces will in fact be <em>required</em> between components
in order to distinguish them from each other.
For example, the value ''1em2em'' would be parsed as a single <<dimension-token>>
with the number ''1'' and the identifier ''em2em'',
which is an invalid unit.
In this case, a space would be required before the ''2''
to get this parsed as the two lengths ''1em'' and ''2em''.
<h3 id="value-examples">
Property value examples</h3>
Below are some examples of properties with their corresponding value
definition fields
<div class=example>
<table class="data" id="propvalues">
<thead>
<tr><th>Property
<th>Value definition field
<th>Example value
</thead>
<tbody>
<tr><td>'orphans'
<td><integer>
<td>''3''
<tr><td>'text-align'
<td>left | right | center | justify
<td>''text-align/center''
<tr><td>'padding-top'
<td><length> | <percentage>
<td>''5%''
<tr><td>'outline-color'
<td><color> | invert
<td>''#fefefe''
<tr><td>'text-decoration'
<td>none | underline || overline || line-through || blink
<td>''overline underline''
<tr><td><a property>font-family</a>
<td>[ <family-name> | <generic-family> ]#
<td>''"Gill Sans", Futura, sans-serif''
<tr><td>'border-width'
<td>[ <length> | thick | medium | thin ]{1,4}
<td>''2px medium 4px''
<tr><td>'text-shadow'
<td>[ inset? && [ <length>{2,4} && <color>? ] ]# | none
<td>''3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0 0 4px inset''
</tbody>
</table>
</div>
<h2 id="textual-values">
Textual Data Types</h2>
<dfn export lt="identifier|ident">Identifiers</dfn>, generically denoted by <dfn><ident></dfn>,
consist of a sequence of characters conforming to the <<ident-token>> grammar. [[!CSS3SYN]]
Identifiers cannot be quoted;
otherwise they would be interpreted as strings.
<!--
██ ██ ████████ ██ ██ ██ ██ ███████ ████████ ████████ ██████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ████ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
█████ ██████ ██ ██ ██ ██ ██ ██ ████████ ██ ██ ██████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ████████ ██ ███ ███ ███████ ██ ██ ████████ ██████
-->
<h3 id="keywords">
Pre-defined Keywords</h3>
In the value definition fields,
keywords with a pre-defined meaning appear literally.
Keywords are CSS <a>identifiers</a>
and are interpreted <a lt="ASCII case-insensitive">ASCII case-insensitively</a>
(i.e., [a-z] and \[A-Z] are equivalent).
<div class="example">
For example, here is the value definition for the 'border-collapse'
property:
<pre>Value: collapse | separate</pre>
And here is an example of its use:
<pre>table { border-collapse: separate }</pre>
</div>
<h4 id="common-keywords">
CSS-wide keywords: ''initial'', ''inherit'' and ''unset''</h4>
As defined <a href="#component-types">above</a>,
all properties accept the <dfn export>CSS-wide keywords</dfn>,
which represent value computations common to all CSS properties.
The ''initial'' keyword represents the value specified as the property's initial value.
The ''inherit'' keyword represents the computed value of the property on the element's parent.
The ''unset'' keyword acts as either ''inherit'' or ''initial'',
depending on whether the property is inherited or not.
All of these keywords are normatively defined in the Cascade module. [[!CSS3CASCADE]]
Other CSS specifications can define additional CSS-wide keywords.
<!-- Make it easier to add CSS-wide keywords by defining a grammar production. -->
<h3 id='custom-idents'>
Author-defined Identifiers: the <<custom-ident>> type</h3>
Some properties accept arbitrary author-defined identifiers as a component value.
This generic data type is denoted by <dfn id="identifier-value"><custom-ident></dfn>,
and represents any valid CSS <a>identifier</a>
that would not be misinterpreted as a pre-defined keyword in that property's value definition.
Such identifiers are fully case-sensitive,
even in the ASCII range
(e.g. ''example'' and ''EXAMPLE'' are two different, unrelated user-defined identifiers).
The <a>CSS-wide keywords</a> are not valid <<custom-ident>>s.
The ''default'' keyword is reserved
and is also not a valid <<custom-ident>>.
Specifications using <<custom-ident>> must specify clearly
what other keywords are excluded from <<custom-ident>>, if any--
for example by saying that any pre-defined keywords in that property's value definition are excluded.
Excluded keywords are excluded in all <a lt="ASCII case-insensitive">ASCII case permutations</a>.
When parsing positionally-ambiguous keywords in a property value,
a <<custom-ident>> production can only claim the keyword if no other unfulfilled production can claim it.
<div class="example">
For example, the shorthand declaration ''animation: ease-in ease-out''
is equivalent to the longhand declarations
''animation-timing-function: ease-in; animation-name: ease-out;''.
''ease-in'' is claimed by the <<timing-function>> production belonging to 'animation-timing-function',
leaving ''ease-out'' to be claimed by the <<custom-ident>> production belonging to 'animation-name'.
</div>
Note: When designing grammars with <<custom-ident>>,
the <<custom-ident>> should always be "positionally unambiguous",
so that it's impossible to conflict with any keyword values in the property.
<!--
██████ ████████ ████████ ████ ██ ██ ██████
██ ██ ██ ██ ██ ██ ███ ██ ██ ██
██ ██ ██ ██ ██ ████ ██ ██
██████ ██ ████████ ██ ██ ██ ██ ██ ████
██ ██ ██ ██ ██ ██ ████ ██ ██
██ ██ ██ ██ ██ ██ ██ ███ ██ ██
██████ ██ ██ ██ ████ ██ ██ ██████
-->
<h3 id="strings">
Quoted Strings: the <<string>> type</h3>
<dfn export lt="string">Strings</dfn> are denoted by <dfn id="string-value"><string></dfn>
and consist of a sequence of characters delimited by double quotes or
single quotes. They correspond to the <<string-token>> production
in the <a href="https://www.w3.org/TR/css-syntax/">CSS Syntax Module</a> [[!CSS3SYN]].
<div class=example>
Double quotes cannot occur inside double quotes, unless
<a href="https://www.w3.org/TR/CSS21/syndata.html#escaped-characters">escaped</a>
(as <code>"\""</code> or as <code>"\22"</code>).
Analogously for single quotes (<code>'\''</code> or <code>'\27'</code>).
<pre>
content: "this is a 'string'.";
content: "this is a \"string\".";
content: 'this is a "string".';
content: 'this is a \'string\'.'
</pre>
</div>
It is possible to break strings over several lines, for aesthetic or
other reasons, but in such a case the newline itself has to be escaped
with a backslash (\). The newline is subsequently removed from the
string. For instance, the following two selectors are exactly the
same:
<div class="example">
<p style="display:none">Example(s):
<pre>
a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}
</pre>
</div>
Since a string cannot directly represent a newline, to include a
newline in a string, use the escape "\A". (Hexadecimal A is the line
feed character in Unicode (U+000A), but represents the generic notion
of "newline" in CSS.)
<!--
██ ██ ████████ ██
██ ██ ██ ██ ██
██ ██ ██ ██ ██
██ ██ ████████ ██
██ ██ ██ ██ ██
██ ██ ██ ██ ██
███████ ██ ██ ████████
-->
<h3 id="urls">
Resource Locators: the <<url>> type</h3>
The <dfn>url()</dfn> <a>functional notation</a>,
denoted by <<url>>,
represents a <dfn>URL</dfn>,
which is a pointer to a resource.
The typical syntax of a <<url>> is:
<pre class="prod"><dfn id="url-value"><url></dfn> = url( <<string>> <<url-modifier>>* )</pre>
<div class="example">
Below is an example of a URL being used as a background image:
<pre>body { background: url("http://www.example.com/pinkish.gif") }</pre>
</div>
A <<url>> may alternately be written without quotation marks around the URL itself,
in which case it is <a lt="consume a url token" spec=css-syntax-3>specially-parsed</a>
as a <<url-token>> [[!CSS3SYN]].
<div class="example">
For example, the following declarations are identical:
<pre>
background: url("http://www.example.com/pinkish.gif");
background: url(http://www.example.com/pinkish.gif);
</pre>
</div>
Note: This unquoted syntax is cannot accept a <<url-modifier>> argument
and has extra escaping requirements:
parentheses, <a href="https://www.w3.org/TR/css-syntax/#whitespace">whitespace</a> characters,
single quotes (') and double quotes (") appearing in a URL
must be escaped with a backslash,
e.g. ''url(open\(parens)'', ''url(close\)parens)''.
(In quoted <<string>> ''url()''s,
only newlines and the character used to quote the string need to be escaped.)
Depending on the type of URL,
it might also be possible to write these characters as URL-escapes
(e.g. ''url(open%28parens)'' or ''url(close%29parens)'')
as described in [[URL]].
Some CSS contexts (such as ''@import'') also allow a <<url>>
to be represented by a bare <<string>>, without the ''url()'' wrapper.
In such cases the string behaves identically to a ''url()'' function containing that string.
<div class="example">
For example, the following statements are identical:
<pre>
@import url("base-theme.css");
@import "base-theme.css";
</pre>
</div>
<h4 id="relative-urls">
Relative URLs</h4>
In order to create modular style sheets that are not dependent on
the absolute location of a resource, authors should use relative URLs.
Relative URLs (as defined in [[!URL]]) are resolved to full URLs
using a base URL. RFC 3986, section 3, defines the normative
algorithm for this process.
For CSS style sheets, the base URL is that of the style sheet itself,
not that of the styled source document.
Style sheets embedded within a document have
the base URL associated with their container.
When a <<url>> appears in the computed value of a property,
it is resolved to an absolute URL,
as described in the preceding paragraph.
The computed value of a URL that the UA cannot resolve to an absolute URL is the specified value.
<div class="example">
For example, suppose the following rule:
<pre>body { background: url("tile.png") }</pre>
is located in a style sheet designated by the URL:
<pre>http://www.example.org/style/basic.css</pre>
The background of the source document's <code><body></code>
will be tiled with whatever image is described by the resource designated by the URL:
<pre>http://www.example.org/style/tile.png</pre>
The same image will be used regardless of the URL of the source document containing the <code><body></code>.
</div>
<h5 id='local-urls'>
Fragment URLs</h5>
To work around some common eccentriticites in browser URL handling,
CSS has special behavior for fragment-only urls.
If a ''url()''’s value starts with a U+0023 NUMBER SIGN (<code>#</code>) character,
parse it as per normal for URLs,
but additionally set the <dfn export for="url()">local url flag</dfn> of the ''url()''.
When matching a ''url()'' with the <a>local url flag</a> set,
ignore everything but the URL's fragment,
and resolve that fragment against the current document that relative URLs are resolved against.
This reference must always be treated as same-document
(rather than cross-document).
When <a href="https://www.w3.org/TR/cssom-1/#serializing-css-values">serializing</a>
a ''url()'' with the <a>local url flag</a> set,
it must serialize as just the fragment.
<details class=note>
<summary>What “browser eccentricities”?</summary>
Theoretically, browsers should re-resolve any relative URLs,
including fragment-only URLs,
whenever the document's base URL changes
(such as through mutation of the <{base}> element,
or calling {{History/pushState()}}).
In many cases they don't, however,
and so without special handling,
fragment-only URLs will suddenly become cross-document references
(pointing at the previous base URL)
and break in many of the places they're used.
Since fragment-only URLs express a clear semantic
of wanting to refer to the current document
regardless of what its current URL is,
this hack preserves the expected behavior at least in these cases.
</details>
<h4 id="url-empty">
Empty URLs</h4>
If the value of the ''url()'' is the empty string
(like ''url("")'' or ''url()''),
the url must resolve to an invalid resource
(similar to what the url ''about:invalid'' does).
Note: This matches the behavior of empty urls for embedded resources elsewhere in the web platform,
and avoids excess traffic re-requesting the stylesheet or host document
due to editting mistakes leaving the ''url()'' value empty,
which are almost certain to be invalid resources for whatever the ''url()'' shows up in.
Linking on the web platform <em>does</em> allow empty urls,
so if/when CSS gains some functionality to control hyperlinks,
this restriction can be relaxed in those contexts.
<h4 id='url-modifiers'>
URL Modifiers</h4>
The ''url()'' function supports specifying additional <dfn><url-modifier></dfn>s,
which change the meaning or the interpretation of the URL somehow.
A <<url-modifier>> is either an <<ident>> or a <a>functional notation</a>.
This specification does not define any <<url-modifier>>s,
but other specs may do so.
Note: A <<url>> that is either unquoted or not wrapped in ''url()'' notation
cannot accept any <<url-modifier>>s.
<h2 id="numeric-types">
Numeric Data Types</h2>
Properties may restrict numeric values to some range.
If the value is outside the allowed range,
the declaration is invalid and must be <a href="https://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
CSS theoretically supports infinite precision and infinite ranges for all value types;
however in reality implementations have finite capacity.
UAs should support reasonably useful ranges and precisions.
<!--
The recommended minimum ranges and precision,
and the required rounding and clamping rules,
are given in <a href="#required-ranges">Appendix A</a>.
-->
<!--
████ ██ ██ ████████ ████████ ██████ ████████ ████████
██ ███ ██ ██ ██ ██ ██ ██ ██ ██
██ ████ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██████ ██ ████ ██████ ████████
██ ██ ████ ██ ██ ██ ██ ██ ██ ██
██ ██ ███ ██ ██ ██ ██ ██ ██ ██
████ ██ ██ ██ ████████ ██████ ████████ ██ ██
-->
<h3 id="integers">
Integers: the <<integer>> type</h3>
Integer values are denoted by <dfn id="integer-value"><integer></dfn>.
When written literally,
an <dfn export>integer</dfn> is one or more decimal digits ''0'' through ''9''
and corresponds to a subset of the <<number-token>> production
in the CSS Syntax Module [[!CSS3SYN]].
The first digit of an integer may be immediately preceded by <css>-</css> or <css>+</css>
to indicate the integer's sign.
<!--
██ ██ ██ ██ ██ ██ ████████ ████████ ████████
███ ██ ██ ██ ███ ███ ██ ██ ██ ██ ██
████ ██ ██ ██ ████ ████ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ███ ██ ████████ ██████ ████████
██ ████ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ███████ ██ ██ ████████ ████████ ██ ██
-->
<h3 id="numbers">
Real Numbers: the <<number>> type</h3>
Number values are denoted by <dfn id="number-value"><number></dfn>,
and represent real numbers, possibly with a fractional component.
When written literally,
a <dfn export>number</dfn> is either an <a>integer</a>,
or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits
and optionally an exponent composed of "e" or "E" and an integer.
It corresponds to the <<number-token>> production
in the <a href="https://www.w3.org/TR/css-syntax/">CSS Syntax Module</a> [[!CSS3SYN]].
As with integers, the first character of a number may be immediately preceded by ''-'' or ''+''
to indicate the number's sign.
<!--
█████ ██
██ ██ ██
█████ ██
██
██ █████
██ ██ ██
██ █████
-->
<h3 id="percentages">
Percentages: the <<percentage>> type</h3>
Percentage values are denoted by <dfn id="percentage-value"><percentage></dfn>,
and indicates a value that is some fraction of another reference value.
When written literally,
a <dfn export>percentage</dfn> consists of a <a>number</a>
immediately followed by a percent sign ''%''.
It corresponds to the <<percentage-token>> production
in the <a href="https://www.w3.org/TR/css-syntax/">CSS Syntax Module</a> [[!CSS3SYN]].
Percentage values are always relative to another quantity,
for example a length.
Each property that allows percentages also defines the quantity to which the percentage refers.
This quantity can be a value of another property for the same element,
the value of a property for an ancestor element,
a measurement of the formatting context
(e.g., the width of a <a>containing block</a>),
or something else.
In cases where a <<percentage>> can represent the same quantity
as a <a>dimension</a> or <a>number</a> in the same <a>component value</a> position,
and can therefore be combined with them in a ''calc()'' expression,
the following convenience notations may be used in the property grammar:
: <dfn><length-percentage></dfn>
:: Equivalent to <code class=prod>[ <<length>> | <<percentage>> ]</code>,
where the <<percentage>> will resolve to a <<length>>.
: <dfn><frequency-percentage></dfn>
:: Equivalent to <code class=prod>[ <<frequency>> | <<percentage>> ]</code>,
where the <<percentage>> will resolve to a <<frequency>>.
: <dfn><angle-percentage></dfn>
:: Equivalent to <code class=prod>[ <<angle>> | <<percentage>> ]</code>,
where the <<percentage>> will resolve to an <<angle>>.
: <dfn><time-percentage></dfn>
:: Equivalent to <code class=prod>[ <<time>> | <<percentage>> ]</code>,
where the <<percentage>> will resolve to a <<time>>.
: <dfn><number-percentage></dfn>
:: Equivalent to <code class=prod>[ <<number>> | <<percentage>> ]</code>,
where the <<percentage>> will resolve to a <<number>>.
<div class="example">
For example, the 'width' property can accept a <<length>> or a <<percentage>>,
both representing a measure of distance.
This means that ''width: calc(500px + 50%);'' is allowed--
both values are converted to absolute lengths and added.
If the containing block is ''1000px'' wide,
then ''width: 50%;'' is equivalent to ''width: 500px'',
and ''width: calc(50% + 500px)'' thus ends up equivalent to ''width: calc(500px + 500px)'' or ''width: 1000px''.
On the other hand, the second and third arguments of the ''hsl()'' function
can only be expressed as <<percentage>>s.
Although ''calc()'' productions are allowed in their place,
they can only combine percentages with themselves,
as in ''calc(10% + 20%)''.
</div>
Note: Specifications should never alternate <<percentage>> in place of a dimension
in a grammar unless they are <a>compatible</a>.
<h3 id='dimensions'>
Numbers with Units: <a>dimensions</a></h3>
The general term "dimension" refers to a number with a unit attached to it,
and are denoted by <dfn><dimension></dfn>.
When written literally,
a <dfn>dimension</dfn> is a <a>number</a>
immediately followed by a unit identifier,
which is an <a>identifier</a>.
It corresponds to the <<dimension-token>> production
in the <a href="https://www.w3.org/TR/css-syntax/">CSS Syntax Module</a> [[!CSS3SYN]].
Like keywords, unit identifiers are <a>ASCII case-insensitive</a>.
CSS uses <<dimension>>s to specify
distances (<<length>>),
durations (<<time>>),
frequencies (<<frequency>>),
resolutions (<<resolution>>),
and other quantities.
<h3 id="compat">
Compatible Units</h3>
When <a href="https://www.w3.org/TR/cssom-1/#serializing-css-values">serializing</a> <a>computed values</a> [[!CSSOM]],
<dfn export local-lt=compatible>compatible units</dfn>
(those related by a static multiplicative factor,
like the 96:1 factor between ''px'' and ''in'',
or the the computed 'font-size' factor between ''em'' and ''px'')
are converted into a single <dfn export local-lt=canonical>canonical unit</dfn>.
Each group of compatible units defines which among them is the <a>canonical unit</a>
that will be used for serialization.
When serializing <a href="https://www.w3.org/TR/cssom-1/#resolved-values">resolved values</a>
that are <a>used values</a>,
all value types (percentages, numbers, keywords, etc.)
that represent lengths are considered <a>compatible</a> with lengths.
Likewise any future API that returns <a>used values</a>
must consider any values represent distances/durations/frequencies/etc.
as <a>compatible</a> with the relevant class of <a>dimensions</a>,
and canonicalize accordingly.
<!--
██ ████████ ██ ██ ██████ ████████ ██ ██
██ ██ ███ ██ ██ ██ ██ ██ ██
██ ██ ████ ██ ██ ██ ██ ██
██ ██████ ██ ██ ██ ██ ████ ██ █████████
██ ██ ██ ████ ██ ██ ██ ██ ██
██ ██ ██ ███ ██ ██ ██ ██ ██
████████ ████████ ██ ██ ██████ ██ ██ ██
-->
<h2 id="lengths">
Distance Units: the <<length>> type</h2>
Lengths refer to distance measurements
and are denoted by <dfn id="length-value"><length></dfn> in the property definitions.
A length is a <a>dimension</a>.
For zero lengths the unit identifier is optional
(i.e. can be syntactically represented as the <<number>> ''0'').
However, if a ''0'' could be parsed as either a <<number>> or a <<length>> in a property
(such as 'line-height'),
it must parse as a <<number>>.
Properties may restrict the length value to some range.
If the value is outside the allowed range,
the declaration is invalid and must be <a href="https://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
While some properties allow negative length values,
this may complicate the formatting and there may be implementation-specific limits.
If a negative length value is allowed but cannot be supported,
it must be converted to the nearest value that can be supported.
In cases where the <a lt="used value">used</a> length cannot be supported,
user agents must approximate it in the <a lt="actual value">actual</a> value.
There are two types of length units: <a lt="relative length">relative</a> and <a lt="absolute length">absolute</a>.
<h3 id="relative-lengths">
Relative lengths</h3>
<dfn lt="relative length">Relative length units</dfn> specify a length relative to another length.
Style sheets that use relative units can more easily scale from one output environment to another.
The relative units are:
<table class="data">
<caption>Informative Summary of Relative Units</caption>
<thead>
<tr><th>unit<th>relative to
</thead>
<tbody>
<tr><td>''em''
<td>font size of the element
<tr><td>''ex''
<td>x-height of the element's font
<tr><td>''ch''
<td>width of the "0" (ZERO, U+0030) glyph in the element's font
<tr><td>''rem''
<td>font size of the root element
<tr><td>''vw''
<td>1% of viewport's width
<tr><td>''vh''
<td>1% of viewport's height
<tr><td>''vmin''
<td>1% of viewport's smaller dimension
<tr><td>''vmax''
<td>1% of viewport's larger dimension
</tbody>
</table>
Child elements do not inherit the relative values as specified for their parent;
they inherit the <a>computed values</a>.
<!--
████████ ██ ██ ██ ████████ ████████ ██████
██ ███ ███ ██ ██ ██ ██ ██
██ ████ ████ ██ ██ ██ ██
██████ ██ ███ ██ ██ ██████ ██ ██
██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██
████████ ██ ██ ██ ████████ ██ ██████
-->
<h4 id="font-relative-lengths">
Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
The <dfn export id="font-relative-length">font-relative lengths</dfn> refer to the font metrics of the element on which they are used,
or, in the case of ''rem'', the metrics of the root element).
<dl export dfn-type=value dfn-for="<length>">
<dt><dfn id="em" lt="em">em unit</dfn>
<dd>
Equal to the computed value of the 'font-size' property of the element on which it is used.
<div class="example">
The rule:
<pre>h1 { line-height: 1.2em }</pre>
means that the line height of <code>h1</code> elements
will be 20% greater than the font size of <code>h1</code> element.
On the other hand:
<pre>h1 { font-size: 1.2em }</pre>
means that the font size of <code>h1</code> elements
will be 20% greater than the computed font size inherited by <code>h1</code> elements.
</div>
<dt><dfn id="ex" lt="ex">ex unit</dfn>
<dd>
Equal to the used x-height of the <a href="https://www.w3.org/TR/css3-fonts/#first-available-font">first available font</a> [[!CSS3-FONTS]].
The x-height is so called because it is often equal to the height of the lowercase "x".
However, an ''ex'' is defined even for fonts that do not contain an "x".
The x-height of a font can be found in different ways. Some fonts
contain reliable metrics for the x-height. If reliable font metrics
are not available, UAs may determine the x-height from the height
of a lowercase glyph. One possible heuristic is to look at how far
the glyph for the lowercase "o" extends below the baseline, and
subtract that value from the top of its bounding box. In the cases
where it is impossible or impractical to determine the x-height,
a value of 0.5em must be assumed.
<dt><dfn id="ch" lt="ch">ch unit</dfn>
<dd>
Equal to the used <a>advance measure</a> of the "0" (ZERO, U+0030) glyph
found in the font used to render it.
(The <dfn dfn>advance measure</dfn> of a glyph is its advance width or height,
whichever is in the inline axis of the element.)
Note: The advance measure of a glyph depends on writing-mode and text-orientation
as well as font settings, text-transform, and any other properties that affect glyph selection or orientation.
In the cases where it is impossible or impractical to determine the measure of the “0” glyph,
it must be assumed to be 0.5em wide by 1em tall.
Thus, the ''ch'' unit falls back to ''0.5em'' in the general case,
and to ''1em'' when it would be typeset upright
(i.e. 'writing-mode' is ''vertical-rl'' or ''vertical-lr''
and 'text-orientation' is ''text-orientation/upright'').
<dt><dfn id="rem" lt="rem">rem unit</dfn>
<dd>
Equal to the computed value of 'font-size' on the root element.
If used in the 'font-size' property of the root element,
or in a document with no root element,
''1rem'' is equal to the initial value of the 'font-size' property.
</dl>
When used in the value of the 'font-size' property on the element they refer to,
these units refer to the computed font metrics of the parent element
(or the computed font metrics corresponding to the initial values of the 'font' property,
if the element has no parent).
When used outside the context of an element
(such as in <a>media queries</a>),
these units refer to the computed font metrics corresponding to the initial values of the 'font' property.
<!--
██ ██ ██ ██ ██ ████████ ████████ ██████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██
███ ███ ███ ██ ████████ ██ ██████
-->