-
Notifications
You must be signed in to change notification settings - Fork 790
Expand file tree
/
Copy pathOverview.bs
More file actions
674 lines (554 loc) · 25.3 KB
/
Overview.bs
File metadata and controls
674 lines (554 loc) · 25.3 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
<h1>CSS Custom Properties for Cascading Variables Module Level 1</h1>
<pre class='metadata'>
!Date: 2014-05-06
Status: ED
Shortname: css-variables
Level: 1
Group: csswg
TR: http://www.w3.org/TR/css-variables-1/
ED: http://dev.w3.org/csswg/css-variables/
Previous Version: http://www.w3.org/TR/2013/WD-css-variables-1-20130620/
Previous Version: http://www.w3.org/TR/2013/WD-css-variables-20130312/
Previous Version: http://www.w3.org/TR/2012/WD-css-variables-20120410/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact
Abstract: This module introduces cascading variables as a new primitive value type that is accepted by all CSS properties, and custom properties for defining them.
Ignored Terms: --foo, case-insensitive, getpropertyvalue()
Link Defaults: css-color-3 (property) color, cssom-1 (interface) cssstyledeclaration
</pre>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
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.
This module introduces a family of custom author-defined properties known collectively as <a>custom properties</a>,
which allow an author to assign arbitrary values to a property with an author-chosen name,
and the ''var()'' function,
which allow an author to 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 editing such files much easier and less error-prone,
as one only has to change the value once,
in the <a>custom property</a>,
and the change will propagate to all uses of that variable automatically.
<h2 id='defining-variables'>
Defining Custom Properties: the '--*' family of properties</h2>
This specification defines an open-ended set of properties called <a>custom properties</a>,
which, among other things, are used to define the <a title="substitute a var()">substitution value</a> of ''var()'' functions.
<pre class='propdef'>
Name: --*
Value: <<any-value>>
Initial: (nothing, see prose)
Applies to: all elements
Inherited: yes
Computed value: specified value with variables substituted (but see prose for "invalid variables")
Media: all
Animatable: no
</pre>
A <dfn export>custom property</dfn> is any property
whose name starts with two dashes (U+002D HYPHEN-MINUS),
like ''--foo''.
The <dfn><custom-property-name></dfn> production corresponds to this:
it's defined as any valid <a>identifier</a> that starts with two dashes.
<a>Custom properties</a> are solely for use by authors and users;
CSS will never give them a meaning beyond what is presented here.
<div class='example'>
Custom properties define variables,
referenced with the ''var()'' notation,
which can be used for many purposes.
For example, a page that consistently uses a small set of colors in its design
can store the colors in custom properties
and use them with variables:
<pre>
:root {
--main-color: #06c;
--accent-color: #006;
}
/* The rest of the CSS file */
#foo h1 {
color: var(--main-color);
}
</pre>
The naming provides a mnemonic for the colors,
prevents difficult-to-spot typos in the color codes,
and if the theme colors are ever changed,
focuses the change on one simple spot
(the custom property value)
rather than requiring many edits across all stylesheets in the webpage.
</div>
Unlike other CSS properties,
custom property names are <a>case-sensitive</a>.
<div class='example'>
While both ''--foo'' and ''--FOO'' are valid,
they are distinct properties -
using ''var(--foo)'' will refer to the first one,
while using ''var(--FOO)'' will refer to the second.
</div>
Custom properties are <strong>not</strong> reset by the 'all' property.
<span class='note'>We may define a property in the future that resets all variables.</span>
The <a spec=css-values>CSS-wide keywords</a> can be used in custom properties,
with the same meaning as in any another property.
Note: That is, they're interpreted at cascaded-value time as normal,
and are not preserved as the custom property's value,
and thus are not substituted in by the corresponding variable.
Note: While this module focuses on the use of <a>custom properties</a> with the ''var()'' function to create “variables”,
they can also be used as actual custom properties,
parsed by and acted on by script.
It's expected that the CSS Extensions spec [[CSS-EXTENSIONS]]
will expand on these use-cases and make them easier to do.
<h3 id='syntax'>
Custom Property Value Syntax</h3>
The allowed syntax for <a>custom properties</a> is extremely permissive.
The <dfn><any-value></dfn> production matches <em>any</em> sequence of one or more tokens,
so long as the sequence does not contain
<<bad-string-token>>,
<<bad-url-token>>,
unmatched <<)-token>>, <<]-token>>, or <<}-token>>,
or top-level <<semicolon-token>> tokens or <<delim-token>> tokens with a value of "!".
Note: This definition,
along with the general CSS syntax rules,
implies that a custom property value never includes an unmatched quote or bracket,
and so cannot have any effect on larger syntax constructs,
like the enclosing style rule,
when reserialized.
Note: Custom properties can contain a trailing ''!important'',
but this is automatically removed from the property's value by the CSS parser,
and makes the custom property "important" in the CSS cascade.
In other words, the prohibition on top-level "!" characters
does not prevent ''!important'' from being used,
as the ''!important'' is removed before syntax checking happens.
Note: While <<any-value>> must represent at least one token,
that one token may be whitespace.
This implies that ''--foo: ;'' is valid,
and the corresponding ''var(--foo)'' call would have a single space as its substitution value,
but ''--foo:;'' is invalid.
<div class='example'>
For example, the following is a valid custom property:
<pre>
--foo: if(x > 5) this.width = 10;
</pre>
While this value is obviously useless as a <em>variable</em>,
as it would be invalid in any normal property,
it might be read and acted on by JavaScript.
</div>
The values of custom properties,
and the values of ''var()'' functions substituted into custom properties,
are <em>case-sensitive</em>,
and must be preserved in their original author-given casing.
(Many CSS values are case-insensitive,
which user agents can take advantage of by "canonicalizing" them into a single casing,
but that isn't allowed for custom properties.)
The initial value of a <a>custom property</a> is an empty value;
that is, nothing at all.
This initial value has a special interaction with the ''var()'' notation,
which is explained in the section defining ''var()''.
Custom 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 ''@media'' and other conditional rules,
can be used in HTML's <code>style</code> attribute,
can be read or set using the CSSOM, etc.
Notably, they can even be transitioned or animated,
but since the UA has no way to interpret their contents,
they always use the "flips at 50%" behavior
that is used for any other pair of values that can't be intelligently interpolated.
However, any <a>custom property</a> used in a ''@keyframes'' rule
becomes <dfn>animation-tainted</dfn>,
which affects how it is treated when referred to via the ''var()'' function in an animation property.
<div class='example'>
This style rule:
<pre>
:root {
--header-color: #06c;
}
</pre>
declares a <a>custom property</a> named "--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 with the ''var()'' function:
<pre>
h1 { background-color: var(--header-color); }
</pre>
The preceding rule is equivalent to writing ''background-color: #06c;'',
except that the variable name makes the origin of the color clearer,
and if ''var(--header-color)'' is used on other elements in the document,
all of the uses can be updated at once
by changing the '--header-color' property on the root element.
</div>
<div class='example'>
If a <a>custom property</a> is declared multiple times,
the standard cascade rules help resolve it.
Variables always draw from the computed value of the associated custom property on the same element:
<pre>
:root { --color: blue; }
div { --color: green; }
#alert { --color: red; }
* { color: var(--color); }
<p>I inherited blue from the root element!</p>
<div>I got green set directly on me!</div>
<div id='alert'>
While I got red set directly on me!
<p>I'm red too, because of inheritance!</p
77F4
>
</div>
</pre>
</div>
<h3 id='cycles'>
Resolving Dependency Cycles</h3>
<a>Custom properties</a> are left almost entirely unevaluated,
except that they allow and evaluate the ''var()'' function in their value.
This can create cyclic dependencies
where a custom property uses a ''var()'' referring to itself,
or two or more <a>custom properties</a> each attempt to refer to each other.
For each element,
create an undirected dependency graph,
containing nodes for each <a>custom property</a>.
If the value of a <a>custom property</a> <var>prop</var>
contains a ''var()'' function referring to the property <var>var</var>
(including in the fallback argument of ''var()''),
add an edge between <var>prop</var> and the <var>var</var>.
<span class='note'>Edges are possible from a custom property to itself.</span>
If there is a cycle in the dependency graph,
all the <a>custom properties</a> in the cycle
must compute to their initial value
(which is a guaranteed-invalid value).
<div class='example'>
This example shows a custom property safely using a variable:
<pre>
:root {
--main-color: #c06;
--accent-background: linear-gradient(to top, var(--main-color), white);
}
</pre>
The '--accent-background' property
(along with any other properties that use ''var(--main-color)'')
will automatically update when the '--main-color' property is changed.
</div>
<div class='example invalid-example'>
On the other hand,
this example shows an invalid instance of variables depending on each other:
<pre>
:root {
--one: calc(var(--two) + 20px);
--two: calc(var(--one) - 20px);
}
</pre>
Both '--one' and '--two' now compute to their initial value,
rather than lengths.
</div>
It is important to note that
<a>custom properties</a> resolve any ''var()'' functions in their values at computed-value time,
which occurs <em>before</em> the value is inherited.
In general,
cyclic dependencies occur only when multiple custom properties on the same element refer to each other;
custom 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'>
For example,
given the following structure,
these custom properties are <strong>not</strong> cyclic,
and all define valid variables:
<pre>
<one><two><three /></two></one>
one { --foo: 10px; }
two { --bar: calc(var(--foo) + 10px); }
three { --foo: calc(var(--bar) + 10px); }
</pre>
The <one> element defines a value for '--foo'.
The <two> element inherits this value,
and additionally assigns a value to '--bar' using the ''foo'' variable.
Finally,
the <three> element inherits the '--bar' value
<em>after</em> variable substitution
(in other words, it sees the value ''calc(10px + 10px)''),
and then redefines '--foo' in terms of that value.
Since the value it inherited for '--bar' no longer contains a reference to the '--foo' property defined on <one>,
defining '--foo' using the ''var(--bar)'' variable is not cyclic,
and actually defines a value that will eventually
(when referenced as a variable in a normal property)
resolve to ''30px''.
</div>
<!--
UUUUUUUU UUUUUUUU iiii
U::::::U U::::::U i::::i
U::::::U U::::::U iiii
UU:::::U U:::::UU
U:::::U U:::::U ssssssssss iiiiiiinnnn nnnnnnnn ggggggggg ggggg
U:::::D D:::::U ss::::::::::s i:::::in:::nn::::::::nn g:::::::::ggg::::g
U:::::D D:::::Uss:::::::::::::s i::::in::::::::::::::nn g:::::::::::::::::g
U:::::D D:::::Us::::::ssss:::::s i::::inn:::::::::::::::ng::::::ggggg::::::gg
U:::::D D:::::U s:::::s ssssss i::::i n:::::nnnn:::::ng:::::g g:::::g
U:::::D D:::::U s::::::s i::::i n::::n n::::ng:::::g g:::::g
U:::::D D:::::U s::::::s i::::i n::::n n::::ng:::::g g:::::g
U::::::U U::::::Ussssss s:::::s i::::i n::::n n::::ng::::::g g:::::g
U:::::::UUU:::::::Us:::::ssss::::::si::::::i n::::n n::::ng:::::::ggggg:::::g
UU:::::::::::::UU s::::::::::::::s i::::::i n::::n n::::n g::::::::::::::::g
UU:::::::::UU s:::::::::::ss i::::::i n::::n n::::n gg::::::::::::::g
UUUUUUUUU sssssssssss iiiiiiii nnnnnn nnnnnn gggggggg::::::g
g:::::g
gggggg g:::::g
g:::::gg gg:::::g
93D4
g::::::ggg:::::::g
gg:::::::::::::g
ggg::::::ggg
gggggg
-->
<h2 id='using-variables'>
Using Cascading Variables: the ''var()'' notation</h2>
The value of a <a>custom property</a> can be substituted into the value of another property
with the ''var()'' function.
The syntax of ''var()'' is:
<pre class='prod'>
<dfn>var()</dfn> = var( <<custom-property-name>> [, <<any-value>> ]? )
</pre>
The ''var()'' function can be used in place of any part of a value in any property on an element.
The ''var()'' function can not be used as
property names,
selectors,
or anything else besides property values.
(Doing so usually produces invalid syntax,
or else a value whose meaning has no connection to the variable.)
The first argument to the function is the name of the <a>custom property</a> to be substituted.
The second argument to the function, if provided,
is a fallback value,
which is used as the substitution value when the referenced <a>custom property</a> is invalid.
Note: The syntax of the fallback, like that of <a>custom properties</a>, allows commas.
For example, ''var(--foo, red, blue)'' defines a fallback of ''red, blue'';
that is, anything between the first comma and the end of the function is considered a fallback value.
If a property contains one or more ''var()'' functions,
and those functions are syntactically valid,
the entire property's grammar must be assumed to be valid at parse time.
It is only syntax-checked at computed-value time,
after ''var()'' functions have been <a title="substitute a var()">substituted</a>.
To <dfn export>substitute a var()</dfn> in a property's value:
<ol>
<li>
If the <a>custom property</a> named by the first argument to the ''var()'' function is <a>animation-tainted</a>,
and the ''var()'' function is being used in the 'animation' property or one of its longhands,
treat the <a>custom property</a> as having its initial value for the rest of this algorithm.
<li>
If the value of the <a>custom property</a> named by the first argument to the ''var()'' function
is anything but the initial value,
replace the ''var()'' function by the value of the corresponding <a>custom property</a>.
<li>
Otherwise, if the ''var()'' function has a fallback value as its second argument,
replace the ''var()'' function by the fallback value.
If there are any ''var()'' references in the fallback,
<a title="substitute a var()">substitute</a> them as well.
<li>
Otherwise,
the property containing the ''var()'' function is <a>invalid at computed-value time</a>.
Note: Other things can also make a property <a>invalid at computed-value time</a>.
</ol>
<div class='example'>
The fallback value allows for some types of defensive coding.
For example,
an author may create a component
intended to be included in a larger application,
and use variables to style it
so that it's easy for the author of the larger application
to theme the component to match the rest of the app.
Without fallback,
the app author must supply a value for every variable that your component uses.
With fallback, the component author can supply defaults,
so the app author only needs to supply values for the variables they wish to override.
<pre>
/* In the component's style: */
.component .header {
color: var(--header-color, blue);
}
.component .text {
color: var(--text-color, black);
}
/* In the larger application's style: */
.component {
--text-color: #080;
/* header-color isn't set,
and so remains blue,
the fallback value */
}
</pre>
</div>
<div class='example'>
For example, the following code incorrectly attempts to use a variable as a property name:
<pre>
.foo {
--side: margin-top;
var(--side): 20px;
}
</pre>
This is <em>not</em> equivalent to setting ''margin-top: 20px;''.
Instead, the second declaration is simply thrown away as a syntax error
for having an invalid property name.
Similarly, you can't build up a single token where part of it is provided by a variable:
<pre>
.foo {
--gap: 20;
margin-top: var(--gap)px;
}
</pre>
Again, this is <em>not</em> equivalent to setting ''margin-top: 20px;'' (a length).
Instead, it's equivalent to ''margin-top: 20 px;'' (a number followed by an ident),
which is simply an invalid value for the 'margin-top' property.
Note, though, that ''calc()'' can be used to validly achieve the same thing, like so:
<pre>
.foo {
--gap: 20;
margin-top: calc(var(--gap) * 1px);
}
</pre>
</div>
''var()'' functions are <a title="substitute a var()">substituted</a> at computed-value time.
If a declaration,
once all ''var()'' functions are substituted in,
is invalid,
the declaration is <a>invalid at computed-value time</a>.
<div class='example'>
For example,
the following usage is fine from a syntax standpoint,
but results in nonsense when the variable is substituted in:
<pre>
:root { --looks-valid: 20px; }
p { background-color: var(--looks-valid); }
</pre>
Since ''20px'' is an invalid value for 'background-color',
this instance of the property computes to ''transparent''
(the initial value for 'background-color')
instead.
If the property was one that's inherited by default,
such as 'color',
it would compute to the inherited value
rather than the initial value.
</div>
<h3 id='invalid-variables'>
Invalid Variables</h3>
When a <a>custom property</a> has its initial value,
''var()'' functions cannot use it for substitution.
Attempting to do so
makes the declaration <a>invalid at computed-value time</a>,
unless a valid fallback is specified.
A declaration can be <dfn>invalid at computed-value time</dfn>
if it contains a ''var()'' that references a <a>custom property</a> with its initial value,
as explained above,
or if it uses a valid <a>custom property</a>,
but the property value,
after substituting its ''var()'' functions,
is invalid.
When this happens,
the computed value of the property is either
the property's inherited value
or its initial value
depending on whether the property is inherited or not, respectively,
as if the property's value had been specified as the ''unset'' keyword.
<div class='example'>
For example, in the following code:
<pre>
:root { --not-a-color: 20px; }
p { background-color: red; }
p { background-color: var(--not-a-color); }
</pre>
the <p> elements will have transparent backgrounds
(the initial value for 'background-color'),
rather than red backgrounds.
The same would happen if the <a>custom property</a> itself was unset,
or contained an invalid ''var()'' function.
Note the difference between this
and what happens if the author had just written ''background-color: 20px'' directly in their stylesheet -
that would be a normal syntax error,
which would cause the rule to be discarded,
so the ''background-color: red'' rule would be used instead.
</div>
Note: The <a>invalid at computed-value time</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.
<h3 id=variables-in-shorthands>
Variables in Shorthand Properties</h3>
The use of ''var()'' functions in <a>shorthand properties</a> presents some unique difficulties.
Ordinarily, the value of a shorthand property is separated out into its component <a>longhand properties</a> at parse time,
and then the longhands themselves participate in the <a>cascade</a>,
with the shorthand more-or-less discarded.
If a ''var()'' functions is used in a shorthand, however,
one can't tell what values are meant to go where;
it may in fact be impossible to separate it out at parse time,
as a single ''var()'' function may substitute in the value of several longhands at once.
To get around this,
implementations must fill in longhands with a special, unobservable-to-authors <dfn export>pending-substitution value</dfn>
that indicates the shorthand contains a variable,
and thus the longhand's value is pending variable substitution.
This value must then be cascaded as normal,
and at computed-value time,
after ''var()'' functions are finally substituted in,
the shorthand must be parsed and the longhands must be given their appropriate values at that point.
<a>Pending-substitution values</a> must be serialized as the empty string,
if an API allows them to be observed.
Similarly,
while [[CSSOM]] defines that shorthand properties are serialized
by appropriately concatenating the values of their corresponding longhands,
shorthands that are specified with explicit ''var()'' functions
must serialize to the original, ''var()''-containing value.
For other shorthands,
if any of the longhand subproperties for that shorthand have <a>pending-substitution values</a>
then the serialized value of the shorthand must be the empty string.
<!--
AAA PPPPPPPPPPPPPPPPP IIIIIIIIII
A:::A P::::::::::::::::P I::::::::I
A:::::A P::::::PPPPPP:::::P I::::::::I
A:::::::A PP:::::P P:::::PII::::::II
A:::::::::A P::::P P:::::P I::::I
A:::::A:::::A P::::P P:::::P I::::I
A:::::A A:::::A P::::PPPPPP:::::P I::::I
A:::::A A:::::A P:::::::::::::PP I::::I
A:::::A A:::::A P::::PPPPPPPPP I::::I
A:::::AAAAAAAAA:::::A P::::P I::::I
A:::::::::::::::::::::A P::::P I::::I
A:::::AAAAAAAAAAAAA:::::A P::::P I::::I
A:::::A A:::::A PP::::::PP II::::::II
A:::::A A:::::A P::::::::P I::::::::I
A:::::A A:::::A P::::::::P I::::::::I
AAAAAAA AAAAAAAPPPPPPPPPP IIIIIIIIII
-->
<h2 id='apis'>
APIs</h2>
All <a>custom property</a> <a>declarations</a> have the <a>case-insensitive</a> flag set.
Note: Custom properties do not appear on a CSSStyleDeclaration object in camel-cased form,
because their names may have both upper and lower case letters
which indicate distinct custom properties.
The sort of text transformation that automatic camel-casing performs is incompatible with this.
They can still be accessed by their proper name via <a method>getPropertyValue()</a>/etc.
<h3 id='serializing-custom-props'>
Serializing Custom Properties</h3>
Custom property names must be serialized with the casing as provided by the author.
<p class='note'>
Ordinarily, property names are restricted to the ASCII range and are case-insensitive,
so implementations typically serialize the name lowercased.
<h2 id='changes'>
Changes since the June 20 2013 Last Call Working Draft</h2>
<ul>
<li>
The syntax of custom properties was changed from ''var-foo'' to ''--foo''.
<li>
The ''var()'' function takes the full custom property name as its first argument,
rather than the name minus a prefix.
<li>
The CSSVariablesMap interface was removed,
in favor of a better custom-property API to be defined in the future.
</ul>
<h2 id="acks">
Acknowledgments</h2>
<p>
Many thanks to several people in the CSS Working Group for keeping the dream of variables alive over the years,
particularly Daniel Glazman and David Hyatt.
Thanks to multiple people on the mailing list for helping contribute to the development of this incarnation of variables,
particularly
Brian Kardell,
David Baron,
François Remy,
Roland Steiner,
and Shane Stephens.