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
3179 lines (2690 loc) · 126 KB
/
Copy pathOverview.bs
File metadata and controls
3179 lines (2690 loc) · 126 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 Counter Styles Module Level 3
Shortname: css-counter-styles
Level: 3
Status: ED
Work Status: Testing
Group: csswg
ED: https://drafts.csswg.org/css-counter-styles/
TR: https://www.w3.org/TR/css-counter-styles-3/
Previous Version: https://www.w3.org/TR/2017/CR-css-counter-styles-3-20171214/
Previous Version: https://www.w3.org/TR/2015/CR-css-counter-styles-3-20150611/
Previous Version: https://www.w3.org/TR/2014/WD-css-counter-styles-3-20140826/
Previous Version: https://www.w3.org/TR/2013/WD-css-counter-styles-3-20130718/
Previous Version: https://www.w3.org/TR/2013/WD-css-counter-styles-3-20130221/
Previous Version: https://www.w3.org/TR/2012/WD-css-counter-styles-3-20121009/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
Abstract: This module introduces the ''@counter-style'' rule, which allows authors to define their own custom counter styles for use with CSS list-marker and generated-content counters [[CSS-LISTS-3]]. It also predefines a set of common counter styles, including the ones present in CSS2 and CSS2.1.
Link Defaults: css-text-3 (dfn) grapheme cluster, css-pseudo-4 (selector) ::marker, dom-ls (dfn) ASCII case-insensitive
Deadline: 2015-05-03
At Risk: the <<image>> value in <<symbol>>
</pre>
<style>
pre.stylesheet { white-space: pre-wrap; }
code.inline { display: inline-block; }
</style>
<pre class=link-defaults>
spec:css-text-3; type:dfn; text: grapheme cluster
spec:css-pseudo-4; type:selector; text: ::marker
spec:dom; type:dfn; text: ASCII case-insensitive
spec:css-values-3; type:dfn; text:identifier
</pre>
<h2 id='intro'>
Introduction</h2>
CSS 1 defined a handful of useful counter styles based on the styles that HTML traditionally allowed on ordered and unordered lists.
While this was expanded slightly by CSS2.1,
it doesn't address the needs of worldwide typography.
This module introduces the ''@counter-style'' rule which allows CSS to address this in an open-ended manner,
by allowing the author to define their own counter styles.
These styles can then be used in the 'list-style-type' property
or in the ''counter()'' and ''counters()'' functions.
It also defines some additional predefined counter styles,
particularly ones which are common but complicated to represent with ''@counter-style''.
<h2 id='counter-styles'>
Counter Styles</h2>
A <dfn export>counter style</dfn> defines how to convert a counter value into a string.
Counter styles are composed of:
<ul>
<li>
a name,
to identify the style
<li>
an algorithm,
which transforms integer counter values into a basic string representation
<li>
a negative sign,
which is prepended or appended to the representation of a negative counter value.
<li>
a prefix,
to prepend to the representation
<li>
a suffix
to append to the representation
<li>
a range,
which limits the values that a counter style handles
<li>
a spoken form,
which describes how to read out the counter style in a speech synthesizer
<li>
and a fallback style,
to render the representation with when the counter value is outside the counter style's range
or the counter style otherwise can't render the counter value
</ul>
When asked to <dfn export lt='generate a counter|generate a counter representation'>generate a counter representation</dfn>
using a particular counter style for a particular counter value,
follow these steps:
<ol>
<li>
If the counter style is unknown,
exit this algorithm and instead <a>generate a counter representation</a>
using the ''decimal'' style and the same counter value.
<li>
If the counter value is outside the '@counter-style/range' of the counter style,
exit this algorithm and instead <a>generate a counter representation</a>
using the counter style's fallback style and the same counter value.
<li>
Using the counter value and the counter algorithm for the counter style,
generate an <dfn>initial representation for the counter value</dfn>.
If the counter value is negative
and the counter style <a>uses a negative sign</a>,
instead generate an initial representation using the absolute value of the counter value.
<li>
Prepend symbols to the representation as specified in the '@counter-style/pad' descriptor.
<li>
If the counter value is negative
and the counter style <a>uses a negative sign</a>,
wrap the representation in the counter style's negative sign
as specified in the '@counter-style/negative' descriptor.
<li>
Return the representation.
</ol>
Note: '@counter-style/prefix' and '@counter-style/suffix' don't play a part in this algorithm.
This is intentional;
the prefix and suffix aren't part of the string returned by the counter() or counters() functions.
Instead, the prefix and suffix are added by the algorithm that constructs the value of the 'content' property
for the ''::marker'' pseudo-element.
This also implies that the prefix and suffix always come from the specified counter-style,
even if the actual representation is constructed by a fallback style.
Some values of '@counter-style/system' (''symbolic'', ''additive'')
and some descriptors ('@counter-style/pad')
can generate representations with size linear to an author-supplied number.
This can potentially be abused to generate excessively large representations
and consume undue amounts of the user's memory or even hang their browser.
User agents must support representations at least 60 Unicode codepoints long,
but they may choose to instead use the fallback style for representations that would be longer than 60 codepoints.
<!--
███████ ██████ ███████ ██ ██ ██ ██ ████████ ████████ ████████ ██████ ████████ ██ ██ ██ ████████
██ ██ ██ ██ ██ ██ ██ ██ ███ ██
10BC8
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ███ ██ ██ ██ ██ ██ ██ ████ ██ ██ ██ ██ ██ ██ ██ ████ ██ ██
██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██████ ████████ ███████ ██████ ██ ██ ██ ██████
██ █████ ██ ██ ██ ██ ██ ██ ████ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
███████ ██████ ███████ ███████ ██ ██ ██ ████████ ██ ██ ██████ ██ ██ ████████ ████████
-->
<h2 id='the-counter-style-rule'>
Defining Custom Counter Styles: the ''@counter-style'' rule</h2>
The <dfn at-rule>@counter-style</dfn> rule allows authors to define a custom <a>counter style</a>.
The components of a <a>counter style</a> are specified by descriptors in the ''@counter-style'' rule.
The algorithm is specified implicitly by a combination of the ''@counter-style/system'', ''@counter-style/symbols'', and ''@counter-style/additive-symbols'' properties.
The general form of an ''@counter-style'' rule is:
<pre class=prod>
@counter-style <<counter-style-name>> { <<declaration-list>> }
</pre>
<dfn><counter-style-name></dfn> is a <<custom-ident>>
that is not an <a>ASCII case-insensitive</a> match for ''list-style-type/none''.
When used here, to define a counter style,
it also cannot be any of the [=non-overridable counter-style names=]
(in other uses that merely reference a counter style,
such as the ''extend'' system,
these are allowed).
The <<counter-style-name>> is a [=tree-scoped name=].
The <dfn>non-overridable counter-style names</dfn> are
the keywords ''decimal'', ''disc'', ''square'', ''circle'', ''disclosure-open'', and ''disclosure-closed''.
Note: Note that <<custom-ident>> also automatically excludes the <a spec=css-values>CSS-wide keywords</a>.
In addition, some names, like ''list-style-position/inside'',
are valid as counter style names,
but conflict with the existing values of properties like 'list-style',
and so won't be usable there.
Counter style names are case-sensitive.
However, the names defined in this specification are ASCII lowercased on parse
wherever they are used as counter styles, e.g.
in the 'list-style' set of properties,
in the ''@counter-style'' rule,
and in the ''counter()'' functions.
Each ''@counter-style'' rule specifies a value for every counter-style descriptor,
either implicitly or explicitly.
Those not given explicit value in the rule take the initial value listed with each descriptor in this specification.
These descriptors apply solely within the context of the ''@counter-style'' rule in which they are defined,
and do not apply to document language elements.
There is no notion of which elements the descriptors apply to or whether the values are inherited by child elements.
When a given descriptor occurs multiple times in a given ''@counter-style'' rule,
only the last-specified valid value is used;
all prior values for that descriptor must be ignored.
Defining a ''@counter-style'' makes it available to the entire document in which it is included.
If multiple ''@counter-style'' rules are defined with the same name,
only one wins,
according to standard cascade rules.
''@counter-style'' rules cascade "atomically":
if one replaces another of the same name,
it replaces it <em>entirely</em>,
rather than just replacing the specific descriptors it specifies.
Note: Note that even the predefined counter styles can be overridden;
the UA stylesheet occurs before any other stylesheets,
so the predefined ones always lose in the cascade.
This at-rule conforms with the forward-compatible parsing requirement of CSS;
conformant parsers that don't understand these rules will ignore them without error.
Any descriptors that are not recognized or implemented by a given user agent,
or whose value does not match the grammars given here or in a future version of this specification,
must be ignored in their entirety;
they do not make the ''@counter-style'' rule invalid.
<!--
██████ ██ ██ ██████ ████████ ████████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ███ ███
██ ████ ██ ██ ██ ████ ████
██████ ██ ██████ ██ ██████ ██ ███ ██
██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██████ ██ ████████ ██ ██
-->
<h3 id='counter-style-system'>
Counter algorithms: the '@counter-style/system' descriptor</h3>
<pre class='descdef'>
Name: system
For: @counter-style
Value: cyclic | numeric | alphabetic | symbolic | additive | <span class=nobr>[fixed <<integer>>?]</span> | <span class=nobr>[ extends <<counter-style-name>> ]</span>
Initial: symbolic
</pre>
The '@counter-style/system' descriptor specifies which algorithm will be used to construct
the counter's representation based on the counter value. For example,
''cyclic'' counter styles just cycle through their symbols repeatedly,
while ''system/numeric'' counter styles interpret their symbols as digits and
build their representation accordingly.
The systems are defined in the following subsections.
Each '@counter-style/system' value is associated with either the '@counter-style/symbols' or '@counter-style/additive-symbols' descriptors,
and has a minimum length that the appropriate descriptor must have;
each entry below defines what this is.
If a ''@counter-style'' rule fails to meet this requirement,
it does not define a [=counter style=].
(The rule is still syntactically valid, but has no effect.)
<!--
██████ ██ ██ ██████ ██ ████ ██████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ████ ██ ██ ██ ██
██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██████ ████████ ████ ██████
-->
<h4 id="cyclic-system">
Cycling Symbols: the ''cyclic'' system</h4>
The <dfn value for="@counter-style/system">cyclic</dfn> counter system cycles repeatedly through its provided symbols,
looping back to the beginning when it reaches the end of the list.
It can be used for simple bullets
(just provide a single <a>counter symbol</a>),
or for cycling through multiple symbols.
The first <a>counter symbol</a> is used as the representation of the value 1,
the second <a>counter symbol</a> (if it exists) is used as the representation of the value 2, etc.
If the system is ''cyclic'',
the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>,
otherwise the rule does not define a counter style (but is still a valid rule).
This system is defined over all counter values.
<div class=example>
A "triangle bullet" counter style can be defined as:
<pre>
@counter-style <dfn noexport>triangle</dfn> {
system: cyclic;
symbols: ‣;
suffix: " ";
}
</pre>
It will then produce lists that look like:
<pre>
‣ One
‣ Two
‣ Three
</pre>
</div>
If there are <var>N</var> <a>counter symbols</a>
and a representation is being constructed for the integer <var>value</var>,
the representation is the <a>counter symbol</a>
at index ( (<var>value</var>-1) mod <var>N</var>)
of the list of <a>counter symbols</a> (0-indexed).
<!--
████████ ████ ██ ██ ████████ ████████
██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██
██████ ██ ███ ██████ ██ ██
██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██
██ ████ ██ ██ ████████ ████████
-->
<h4 id="fixed-system">
Exhaustible Symbols: the ''system/fixed'' system</h4>
The <dfn value for="@counter-style/system">fixed</dfn> counter system runs through its list of counter symbols once,
then falls back.
It is useful for representing counter styles that only have a finite number of representations.
For example, Unicode defines several limited-length runs of special characters meant for lists,
such as circled digits.
If the system is ''system/fixed'',
the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>,
otherwise the rule does not define a counter style (but is still a valid rule).
This system is defined over counter values in a finite range,
starting with the <a>first symbol value</a> and having a length equal to the length of the list of <a>counter symbols</a>.
When this system is specified,
it may optionally have an integer provided after it,
which sets the <dfn export>first symbol value</dfn>.
If it is omitted, the <a>first symbol value</a> is 1.
<div class=example>
A "box-corner" counter style can be defined as:
<pre>
@counter-style <dfn noexport>box-corner</dfn> {
system: fixed;
symbols: ◰ ◳ ◲ ◱;
suffix: ': ';
}
</pre>
It will then produce lists that look like:
<pre>
◰: One
◳: Two
◲: Three
◱: Four
5: Five
6: Six
</pre>
</div>
The first <a>counter symbol</a> is the representation for the <a>first symbol value</a>,
and subsequent counter values are represented by subsequent <a>counter symbols</a>.
Once the list of <a>counter symbols</a> is exhausted,
further values cannot be represented by this counter style,
and must instead be represented by the fallback counter style.
<!--
██████ ██ ██ ██ ██ ████████ ███████ ██ ████ ██████
██ ██ ██ ██ ███ ███ ██ ██ ██ ██ ██ ██ ██ ██
██ ████ ████ ████ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ███ ██ ████████ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██ ████████ ███████ ████████ ████ ██████
-->
<h4 id="symbolic-system">
Repeating Symbols: the ''symbolic'' system</h4>
The <dfn value for="system">symbolic</dfn> counter system cycles repeatedly through its provided symbols,
doubling, tripling, etc. the symbols on each successive pass through the list.
For example, if the original symbols were "*" and "†",
then on the second pass they would instead be "**" and "††",
while on the third they would be "***"and "†††", etc.
It can be used for footnote-style markers,
and is also sometimes used for alphabetic-style lists for a slightly different presentation than what the ''system/alphabetic'' system presents.
If the system is ''symbolic'',
the '@counter-style/symbols' descriptor must contain at least one <a>counter symbol</a>,
otherwise the rule does not define a counter style (but is still a valid rule).
This system is defined only over strictly positive counter values.
<div class=example>
An "footnote" counter style can be defined as:
<pre>
@counter-style <dfn noexport>footnote</dfn> {
system: symbolic;
symbols: '*' ⁑ † ‡;
suffix: " ";
}
</pre>
It will then produce lists that look like:
<pre>
* One
⁑ Two
† Three
‡ Four
** Five
⁑⁑ Six
</pre>
</div>
<div class=example>
Some style guides mandate a list numbering that looks similar to ''upper-alpha'',
but repeats differently after the first 26 values,
instead going "AA", "BB", "CC", etc.
This can be achieved with the symbolic system:
<pre>
@counter-style <dfn noexport>upper-alpha-legal</dfn> {
system: symbolic;
symbols: A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z;
}
</pre>
This style is identical to ''upper-alpha'' through the first 27 values,
but they diverge after that, with ''upper-alpha'' going "AB", "AC", "AD", etc.
Starting at the 53rd value, ''upper-alpha'' goes "BA", "BB", "BC", etc.,
while this style jumps into triple digits with "AAA", "BBB", "CCC", etc.
</div>
To construct the representation, run the following algorithm:
Let <var>N</var> be the length of the list of <a>counter symbols</a>,
<var>value</var> initially be the counter value,
<var>S</var> initially be the empty string,
and <var>symbol(n)</var> be the nth <a>counter symbol</a> in the list of <a>counter symbols</a> (0-indexed).
<ol>
<li>Let the <var>chosen symbol</var> be <code class='inline'>symbol( (<var>value</var> - 1) mod <var>N</var>)</code>.
<li>Let the <var>representation length</var> be <code class='inline'>ceil( <var>value</var> / <var>N</var> )</code>.
<li>Append the <var>chosen symbol</var> to <var>S</var> a number of times equal to the <var>representation length</var>.
</ol>
Finally, return <var>S</var>.
<!--
███ ██ ████████ ██ ██ ███ ████████ ████████ ████████ ████ ██████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ████████ █████████ ██ ██ ████████ ██████ ██ ██ ██
█████████ ██ ██ ██ ██ █████████ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ████████ ██ ██ ██ ██ ██ ████████ ████████ ██ ████ ██████
-->
<h4 id="alphabetic-system">
Bijective Numerals: the ''alphabetic'' system</h4>
The <dfn value for="@counter-style/system">alphabetic</dfn> counter system interprets the list of <a>counter symbols</a> as digits to an <em>alphabetic</em> numbering system,
similar to the default ''lower-alpha'' counter style,
which wraps from "a", "b", "c", to "aa", "ab", "ac".
Alphabetic numbering systems do not contain a digit representing 0;
so the first value when a new digit is added is composed solely of the first digit.
Alphabetic numbering systems are commonly used for lists,
and also appear in many spreadsheet programs to number columns.
The first <a>counter symbol</a> in the list is interpreted as the digit 1,
the second as the digit 2,
and so on.
If the system is ''system/alphabetic'',
the '@counter-style/symbols' descriptor must contain at least two <a>counter symbols</a>,
otherwise the rule does not define a counter style (but is still a valid rule).
This system is defined only over strictly positive counter values.
<div class=example>
A counter style using go stones can be defined as:
<pre>
@counter-style <dfn noexport>go</dfn> {
system: alphabetic;
symbols: url(white.svg) url(black.svg);
suffix: " ";
}
</pre>
It will then produce lists that look like:
<div class='alphabetic-example'>
<span><img src=images/white.svg width=16 height=16 alt=""></span> One<br>
<span><img src=images/black.svg width=16 height=16 alt=""></span> Two<br>
<span><img src=images/white.svg width=16 height=16 alt=""><img src=images/white.svg width=16 height=16 alt=""></span> Three<br>
<span><img src=images/white.svg width=16 height=16 alt=""><img src=images/black.svg width=16 height=16 alt=""></span> Four<br>
<span><img src=images/black.svg width=16 height=16 alt=""><img src=images/white.svg width=16 height=16 alt=""></span> Five<br>
<span><img src=images/black.svg width=16 height=16 alt=""><img src=images/black.svg width=16 height=16 alt=""></span> Six<br>
<span><img src=images/white.svg width=16 height=16 alt=""><img src=images/white.svg width=16 height=16 alt=""><img src=images/white.svg width=16 height=16 alt=""></span> Seven
</div>
Note: This example requires support for SVG images to display correctly.
</div>
If there are <var>N</var> <a>counter symbols</a>,
the representation is a base <var>N</var> alphabetic number using the <a>counter symbols</a> as digits.
To construct the representation, run the following algorithm:
Let <var>N</var> be the length of the list of <a>counter symbols</a>,
<var>value</var> initially be the counter value,
<var>S</var> initially be the empty string,
and <var>symbol(n)</var> be the nth <a>counter symbol</a> in the list of <a>counter symbols</a> (0-indexed).
While <var>value</var> is not equal to 0:
<ol>
<li>Set <var>value</var> to <code><var>value</var> - 1</code>.
<li>Prepend <var>symbol( <var>value</var> mod <var>N</var> )</var>
to <var>S</var>.
<li>Set <var>value</var> to <code>floor( <var>value</var> / <var>N</var> )</code>.
</ol>
Finally, return <var>S</var>.
<!--
██ ██ ██ ██ ██ ██ ████████ ████████ ████ ██████
███ ██ ██ ██ ███ ███ ██ ██ ██ ██ ██ ██
████ ██ ██ ██ ████ ████ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ███ ██ ██████ ████████ ██ ██
██ ████ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ███████ ██ ██ ████████ ██ ██ ████ ██████
-->
<h4 id="numeric-system">
Positional Numerals: the ''numeric'' system</h4>
The <dfn value for="@counter-style/system">numeric</dfn> counter system interprets the list of <a>counter symbols</a>
as digits to a "place-value" numbering system,
similar to the default ''decimal'' counter style.
The first <a>counter symbol</a> in the list is interpreted as the digit 0,
the second as the digit 1,
and so on.
If the system is ''system/numeric'',
the '@counter-style/symbols' descriptor must contain at least two <a>counter symbols</a>,
otherwise the rule does not define a counter style (but is still a valid rule).
This system is defined over all counter values.
<div class=example>
A "trinary" counter style can be defined as:
<pre>
@counter-style <dfn noexport>trinary</dfn> {
system: numeric;
symbols: '0' '1' '2';
}
</pre>
It will then produce lists that look like:
<pre>
1. One
2. Two
10. Three
11. Four
12. Five
20. Six
</pre>
</div>
If there are <var>N</var> <a>counter symbols</a>,
the representation is a base <var>N</var> number using the <a>counter symbols</a> as digits.
To construct the representation, run the following algorithm:
Let <var>N</var> be the length of the list of <a>counter symbols</a>,
<var>value</var> initially be the counter value,
<var>S</var> initially be the empty string,
and <var>symbol(n)</var> be the nth <a>counter symbol</a> in the list of <a>counter symbols</a> (0-indexed).
<ol>
<li>If <var>value</var> is 0, append <code>symbol(0)</code> to
<var>S</var> and return <var>S</var>.
<li>While <var>value</var> is not equal to 0:
<ol>
<li>Prepend <var>symbol( <var>value</var> mod <var>N</var> )</var>
to <var>S</var>.
<li>Set <var>value</var> to <code>floor( <var>value</var> / <var>N</var> )</code>.
</ol>
<li>Return <var>S</var>.
</ol>
<!--
███ ████████ ████████ ████ ████████ ████ ██ ██ ████████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██████
█████████ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ████████ ████████ ████ ██ ████ ███ ████████
-->
<h4 id="additive-system">
Accumulating Numerals: the ''additive'' system</h4>
The <dfn value for="@counter-style/system">additive</dfn> counter system is used to represent "sign-value" numbering systems,
which, rather than reusing digits in different positions to change their value,
define additional digits with much larger values,
so that the value of the number can be obtained by adding all the digits together.
This is used in Roman numerals
and other numbering systems around the world.
If the system is ''additive'',
the '@counter-style/additive-symbols' descriptor must contain at least one <a>additive tuple</a>,
otherwise the rule does not define a counter style (but is still a valid rule).
This system is nominally defined over all counter values
(see algorithm, below, for exact details).
<div class=example>
A "dice" counter style can be defined as:
<pre>
@counter-style <dfn noexport>dice</dfn> {
system: additive;
additive-symbols: 6 ⚅, 5 ⚄, 4 ⚃, 3 ⚂, 2 ⚁, 1 ⚀;
suffix: " ";
}
</pre>
It will then produce lists that look like:
<pre>
⚀ One
⚁ Two
⚂ Three
...
⚅⚄ Eleven
⚅⚅ Twelve
⚅⚅⚀ Thirteen
</pre>
</div>
<div algorithm="additive representation">
To construct the representation:
1. Let |value| initially be the counter value,
|S| initially be the empty string,
and |symbol list| initially be the list of <a>additive tuples</a>.
2. If |value| is zero:
1. If |symbol list| contains a tuple with a weight of zero,
append that tuple's <a>counter symbol</a> to |S| and return |S|.
2. Otherwise, the given counter value cannot be represented by this counter style,
and must instead be represented by the fallback counter style.
3. For each |tuple| in |symbol list|:
1. Let |symbol| and |weight| be |tuple|'s [=counter symbol=] and weight, respectively.
2. If |weight| is zero, or |weight| is greater than |value|,
[=iteration/continue=].
3. Let |reps| be <code>floor( |value| / |weight| )</code>.
4. Append |symbol| to |S| |reps| times.
5. Decrement |value| by <code>|weight| * |reps|</code>.
6. If |value| is zero, return |S|.
4. Assertion: |value| is still non-zero.
The given counter value cannot be represented by this counter style,
and must instead be represented by the fallback counter style.
</div>
Note: All of the predefined additive ''@counter-style'' rules in this specification
produce representations for every value in their range,
but it's possible to produce values for ''@counter-style/additive-symbols'' that will fail to find a representation
with the algorithm defined above,
even though theoretically a representation could be found.
For example, if a ''@counter-style'' was defined with ''additive-symbols: 3 "a", 2 "b";'',
the algorithm defined above will fail to find a representation for a counter value of 4,
even though theoretically a "bb" representation would work.
While unfortunate, this is required to maintain the property that the algorithm runs in linear time
relative to the size of the counter value.
<!--
████████ ██ ██ ████████ ████████ ██ ██ ████████ ██████
██ ██ ██ ██ ██ ███ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ████ ██ ██ ██ ██
██████ ███ ██ ██████ ██ ██ ██ ██ ██ ██████
██ ██ ██ ██ ██ ██ ████ ██ ██ ██
██ ██ ██ ██ ██ ██ ███ ██ ██ ██ ██
████████ ██ ██ ██ ████████ ██ ██ ████████ ██████
-->
<h4 id="extends-system">
Building from Existing Counter Styles: the ''extends'' system
<span id='override-system'></span></h4>
The <dfn value for="@counter-style/system">extends</dfn> system allows an author to use the algorithm of another counter style,
but alter other aspects,
such as the negative sign or the suffix.
If a counter style uses the ''extends'' system,
any unspecified descriptors must be taken from the extended counter style specified,
rather than taking their initial values.
If a ''@counter-style'' uses the ''extends'' system,
it must not contain a '@counter-style/symbols' or '@counter-style/additive-symbols' descriptor,
otherwise the rule does not define a counter style (but is still a valid rule).
If the specified <<counter-style-name>> is an [=ASCII case-insensitive=] match
for ''disc'', ''circle'', ''square'', ''disclosure-open'', or ''disclosure-closed''
(any of the predefined symbolic counter styles),
using ''extend'' extends from the “standard” definition of the rules
provided in the normative stylesheet
(rather than the exception allowing them to be drawn in a different, user-agent-specific fashion.)
If the specified counter style name isn't the name of any defined counter style,
it must be treated as if it was extending the ''decimal'' counter style.
If one or more ''@counter-style'' rules form a cycle with their ''extends'' values,
all of the counter styles participating in the cycle
must be treated as if they were extending the ''decimal'' counter style instead.
<div class='example'>
For example, if you wanted a counter style that was identical to decimal,
but used a parenthesis rather than a period after it, like:
<pre>
1) first item
2) second item
3) third item
</pre>
Rather than writing up an entirely new counter style,
this can be done by just extending ''decimal'':
<pre>
@counter-style decimal-paren {
system: extends decimal;
suffix: ") ";
}
</pre>
</div>
<!--
██ ██ ████████ ██████ ███ ████████ ████ ██ ██ ████████
███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
████ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██████ ██ ████ ██ ██ ██ ██ ██ ██ ██████
██ ████ ██ ██ ██ █████████ ██ ██ ██ ██ ██
██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ████████ ██████ ██ ██ ██ ████ ███ ████████
-->
<h3 id='counter-style-negative'>
Formatting negative values: the '@counter-style/negative' descriptor</h3>
<pre class='descdef'>
Name: negative
For: @counter-style
Value: <<symbol>> <<symbol>>?
Initial: "-"
</pre>
The '@counter-style/negative' descriptor defines how to alter the representation when
the counter value is negative.
The first <<symbol>> in the value is prepended to the representation when the
counter value is negative. The second <<symbol>>, if specified, is appended
to the representation when the counter value is negative.
<div class='example'>
For example, specifying ''negative: "(" ")";'' will make negative
values be wrapped in parentheses, which is sometimes used in financial
contexts, like "(2) (1) 0 1 2 3...".
</div>
Not all '@counter-style/system' values use a negative sign.
In particular, a counter style <dfn export lt="use a negative sign|uses a negative sign">uses a negative sign</dfn>
if its '@counter-style/system' value is
''symbolic'',
''system/alphabetic'',
''system/numeric'',
''additive'',
or ''extends'' if the extended counter style itself <a>uses a negative sign</a>.
If a counter style does not <a>use a negative sign</a>,
it ignores the negative sign when <a lt="generate a counter representation">generating a counter representation</a>.
<!--
████████ ████████ ████████ ████████ ████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
████████ ████████ ██████ ██████ ██ ███
██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ████████ ██ ████ ██ ██
-->
<h3 id='counter-style-prefix'>
Symbols before the marker: the '@counter-style/prefix' descriptor</h3>
<pre class='descdef'>
Name: prefix
For: @counter-style
Value: <<symbol>>
Initial: ""
</pre>
The '@counter-style/prefix' descriptor specifies a <<symbol>>
that is prepended to the marker representation.
Prefixes come before any negative sign.
Note: Prefixes are only added by the algorithm for constructing
the default contents of the ''::marker'' pseudo-element; the prefix is not
added automatically when the ''counter()'' or ''counters()'' functions are used.
<!--
██████ ██ ██ ████████ ████████ ████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██████ ██████ ██ ███
██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ███████ ██ ██ ████ ██ ██
-->
<h3 id='counter-style-suffix'>
Symbols after the marker: the '@counter-style/suffix' descriptor</h3>
<pre class='descdef'>
Name: suffix
For: @counter-style
Value: <<symbol>>
Initial: ". "
</pre>
The '@counter-style/suffix' descriptor specifies a <<symbol>>
that is appended to the marker representation.
Suffixes are added to the representation after negative signs.
Note: Suffixes are only added by the algorithm for constructing
the default contents of the ''::marker'' pseudo-element; the suffix is not
added automatically when the counter() or counters() functions are used.
<!--
████████ ███ ██ ██ ██████ ████████
██ ██ ██ ██ ███ ██ ██ ██ ██
██ ██ ██ ██ ████ ██ ██ ██
████████ ██ ██ ██ ██ ██ ██ ████ ██████
██ ██ █████████ ██ ████ ██ ██ ██
██ ██ ██ ██ ██ ███ ██ ██ ██
██ ██ ██ ██ ██ ██ ██████ ████████
-->
<h3 id='counter-style-range'>
Limiting the counter scope: the '@counter-style/range' descriptor</h3>
<pre class='descdef'>
Name: range
For: @counter-style
Value: [ [ <<integer>> | infinite ]{2} ]# | auto
Initial: auto
</pre>
The '@counter-style/range' descriptor defines the ranges over which the counter style is defined.
If a counter style is used to represent a counter value outside of its ranges,
the counter style instead drops down to its fallback counter style.
<dl dfn-for="@counter-style/range">
<dt><dfn value>auto</dfn>
<dd>
The range depends on the counter system:
<ul>
<li>
For ''cyclic'', ''system/numeric'', and ''system/fixed'' systems,
the range
A7C6
is negative infinity to positive infinity.
<li>
For ''system/alphabetic'' and ''symbolic'' systems,
the range is 1 to positive infinity.
<li>
For ''additive'' systems,
the range is 0 to positive infinity.
<li>
For ''extends'' systems,
the range is whatever ''range/auto'' would produce for the extended system;
if extending a complex predefined style (<a section href="#complex-predefined-counters"></a>),
the range is the style's defined range.
</ul>
<dt>[ [ <<integer>> | infinite ]{2} ]#
<dd>
This defines a comma-separated list of ranges.
For each individual range,
the first value is the lower bound
and the second value is the upper bound.
This range is inclusive - it contains both the lower and upper bound numbers.
If ''range/infinite'' is used as the first value in a range,
it represents negative infinity;
if used as the second value,
it represents positive infinity.
The range of the counter style is the union of all the ranges defined in the list.
If the lower bound of any range is higher than the upper bound,
the entire descriptor is invalid and must be ignored.
</dl>
Implementations must support ranges with a lower bound of at least -2<sup>15</sup>
and an upper bound of at least 2<sup>15</sup>-1
(the range of a signed 2-byte int).
They may support higher ranges.
If any specified bound is outside of the implementation's supported bounds,
it must be treated as the closest bound that the implementation does support.
<!--
████████ ███ ████████
██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██
████████ ██ ██ ██ ██
██ █████████ ██ ██
██ ██ ██ ██ ██
██ ██ ██ ████████
-->
<h3 id='counter-style-pad'>
Zero-Padding and Constant-Width Representations: the '@counter-style/pad' descriptor</h3>
<pre class='descdef'>
Name: pad
For: @counter-style
Value: <<integer [0,∞]>> && <<symbol>>
Initial: 0 ""
</pre>
The '@counter-style/pad' descriptor allows an author to specify a "fixed-width" counter style,
where representations shorter than the pad value are padded with a particular <<symbol>>.
Representations larger than the specified pad value are constructed as normal.
<dl>
<dt><<integer [0,∞]>> && <<symbol>>
<dd>
The <<integer>> specifies a minimum length that all counter representations must reach.
Let <var>difference</var> be the provided <<integer>>
minus the number of <a>grapheme clusters</a> in the <a>initial representation for the counter value</a>.
<span class='note'>(Note that, per the algorithm to <a>generate a counter representation</a>,
this occurs before adding prefixes/suffixes/negatives.)</span>
If the counter value is negative
and the counter style <a>uses a negative sign</a>,
further reduce <var>difference</var>
by the number of <a>grapheme clusters</a> in the counter style’s '@counter-style/negative' descriptor’s <<symbol>>(s).
If <var>difference</var> is greater than zero,
prepend <var>difference</var> copies of the specified <<symbol>> to the representation.
Negative <<integer>> values are not allowed.
</dl>
<div class='example'>
The most common example of "fixed-width" numbering is zero-padded decimal numbering.
If an author knows that the numbers used will be less than a thousand, for example,
it can be zero-padded with a simple ''pad: 3 "0";'' descriptor,
ensuring that all of the representations are 3 digits wide.
This will cause, for example,
1 to be represented as "001",
20 to be represented as "020",
300 to be represented as "300",
4000 to be represented as "4000",
and -5 to be represented as "-05".
</div>
Note: The '@counter-style/pad' descriptor counts the number of <a>grapheme clusters</a> in the representation,
but pads it with <<symbol>>s.
If the specified '@counter-style/pad' <<symbol>> is multi-character,
this will likely not have the desired effect.
Unfortunately, there's no way to use the number of <a>grapheme clusters</a> in the '@counter-style/pad' <<symbol>>
without violating useful constraints.
It is recommended that authors only specify <<symbol>>s of a single <a>grapheme cluster</a>
in the '@counter-style/pad' descriptor.
<!--
████████ ███ ██ ██ ████████ ███ ██████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██ ██ ████████ ██ ██ ██ █████