-
Notifications
You must be signed in to change notification settings - Fork 791
Expand file tree
/
Copy pathOverview.bs
More file actions
1682 lines (1382 loc) · 63.7 KB
/
Overview.bs
File metadata and controls
1682 lines (1382 loc) · 63.7 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 Multi-column Layout Module Level 1
Group: CSSWG
Shortname: css-multicol
Level: 1
Status: ED
Work Status: Testing
WPT Path Prefix: /css/css-multicol/
ED: https://drafts.csswg.org/css-multicol/
TR: https://www.w3.org/TR/css-multicol-1/
Previous Version: https://www.w3.org/TR/2019/WD-css-multicol-1-20191015/
Previous Version: https://www.w3.org/TR/2018/WD-css-multicol-1-20180528/
Previous Version: https://www.w3.org/TR/2017/WD-css-multicol-1-20171005/
Previous Version: https://www.w3.org/TR/2011/CR-css3-multicol-20110412/
Previous Version: https://www.w3.org/TR/2009/CR-css3-multicol-20091217/
Previous Version: https://www.w3.org/TR/2009/WD-css3-multicol-20090630/
Previous Version: https://www.w3.org/TR/2007/WD-css3-multicol-20070606/
Previous Version: https://www.w3.org/TR/2005/WD-css3-multicol-20051215/
Previous Version: https://www.w3.org/TR/2001/WD-css3-multicol-20010118/
Previous Version: http://www.w3.org/1999/06/WD-css3-multicol-19990623
Editor: Håkon Wium Lie, Opera Software, howcome@opera.com, w3cid 9796
Editor: Florian Rivoal, On behalf of Bloomberg, https://florian.rivoal.net, w3cid 43241
Editor: Rachel Andrew, Invited Expert, https://rachelandrew.co.uk, w3cid 81117
Issue Tracking: Disposition of Comments https://drafts.csswg.org/css-multicol-1/issues
Abstract: This specification describes multi-column layouts in CSS, a style sheet language for the web. Using functionality described in the specification, content can be flowed into multiple columns with a gap and a rule between them.
Link Defaults: css-color (property) color, css2 (property) max-height, css-backgrounds-3 (value) hidden
Include Can I Use Panels: yes
</pre>
<pre class="link-defaults">
spec:css-sizing-3; type:property; text:min-height
spec:css-break-4; type:dfn; text:fragment
spec:css-align-3; type:property; text:column-gap
spec:css-align-3; type:value; for:column-gap; text:normal
spec:css-display-3; type:property; text:display
</pre>
<style type="text/css">
.cols { width: 500px; height: 200px; background: #fff; position: relative; border: solid 5px blue; margin: 0.5em 2em 1em 0; font: bold 14px/19px Arial, sans-serif }
.cols p { padding: 3px; margin: 0 }
.col { position: absolute; left: 0px; top: 0; z-index: 6; width: 170px }
.gap { position: absolute; background: green; width: 5px; bottom: 0px; top: 0px; border: 10px solid yellow; border-top-width: 0; border-bottom-width: 0; }
.rep { position: absolute; top: 45px; background: black; height: 110px; width: 100px; color: white; z-index: 4 }
table.breaks { border-collapse: collapse; margin: 1em 0 }
table.breaks td, table.breaks th { border: thin solid black; padding: 0.1em 0.2em }
div.example:before { width: 9em }
</style>
<h2 id="introduction">
Introduction</h2>
(This section is not normative.)
This module describes <dfn export>multi-column layout</dfn> in CSS.
By using functionality described in this document,
style sheets can declare that the content of an element
is to be laid out in multiple columns.
Other layout methods in CSS,
when applied to a parent element,
change the display properties of the direct children.
For example if a three column grid layout is created,
the direct children of the [=grid container=] become [=grid items=]
and are placed into the column tracks,
one element per cell with additional rows created as needed.
The child elements of a [=multi-column container=] however continue in normal flow,
that flow is arranged into a number of columns.
These columns have a flexible inline size,
and therefore respond to available space
by changing the size or number of columns displayed.
Multi-column layouts are easy to describe in CSS.
Here is a simple example:
<div class=example>
<pre highlight="css">body { column-width: 12em }</pre>
In this example, the <code class=html>body</code> element is
set to have columns at least ''12em'' wide. The exact number of
columns will depend on the available space.
</div>
The number of columns can also be set explicitly in the style sheet:
<div class=example>
<pre highlight="css">body { column-count: 2 }</pre>
In this case, the number of columns is fixed
and the column widths will vary depending on the available width.
</div>
The shorthand 'columns' property can be used to set either,
or both, properties in one declaration.
<div class=example>
In these examples, the number of columns, the width of columns, and
both the number and width are set, respectively:
<pre highlight="css">
body { columns: 2 }
body { columns: 12em }
body { columns: 2 12em }
</pre>
</div>
Another group of properties introduced in this module describe
gaps and rules between columns.
<div class=example>
<pre highlight="css">
body {
column-gap: 1em;
column-rule: thin solid black;
}
</pre>
The first declaration in the example above sets the gap
between two adjacent columns to be 1em.
Column gaps are similar to padding areas.
In the middle of the gap there will be a rule
which is described by the 'column-rule' property.
</div>
The values of the 'column-rule' property are similar to those of the CSS 'border' properties.
Like 'border', 'column-rule' is a shorthand property.
<div class=example>
In this example, the shorthand 'column-rule' declaration from the
above example has been expanded:
<pre highlight="css">
body {
column-gap: 1em;
column-rule-width: thin;
column-rule-style: solid;
column-rule-color: black;
}
</pre>
</div>
The 'column-fill' and 'column-span' properties
give style sheets a wider range of visual expressions in multi-column layouts.
<div class=example>
In this example, columns are set to be balanced,
i.e., to have approximately the same length.
Also, <code>h2</code> elements are set to span across all columns.
<pre highlight="css">
div { column-fill: balance }
h2 { column-span: all }
</pre>
</div>
This specification introduces ten new properties,
all of which are used in the examples above.
If all column properties have their initial value,
the layout of an element will be identical
to a multi-column layout with only one column.
<div class="example">
<a>Column gaps</a> (diagonal hatching) and <a>column rules</a>
are shown in this sample rendition of a multi-column container
with padding (cross hatching).
The hatched areas are present for illustrational purposes only.
In actual implementations these areas will be determined by the background,
the second image shows a rendering of a [=multi-column container=] with column-rules.
<figure>
<img alt="a diagram showing the various parts of multi-column layout" src="images/initial-example.svg">
<img alt="key to the conventions used to display invisible parts of diagram" src="images/invisible-elements.svg">
<figcaption>A multi-column layout with the non-visible column-span and padding inside the multicol container highlighted.</figcaption>
</figure>
<figure>
<img alt="a diagram showing the various parts of multi-column layout" src="images/initial-example-b.svg">
<figcaption>The same layout as in the first image, as it would be displayed by an implementation.</figcaption>
</figure>
</div>
<h3 id="values">
Value Definitions</h3>
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS21]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.
<h2 id="the-multi-column-model">
The Multi-Column Model</h2>
An element whose 'column-width' or 'column-count' property is not ''column-width/auto''
establishes a <dfn lt="multi-column container|multicol container" oldids="multi-column-element" export>multi-column container</dfn>
(or <i>multicol container</i> for short),
and therefore acts as a container for [=multi-column layout=].
<wpt>
multicol-width-004.html
multicol-count-computed-004.xht
</wpt>
In the traditional CSS box model,
the content of an element is
flowed into the content box of the corresponding element.
Multi-column layout introduces a [=fragmentation context=]
formed of <a lt="anonymous box">anonymous</a> [=fragmentation containers=]
called <dfn export lt="column box" local-lt=column>column boxes</dfn>
(or <i>columns</i> for short).
These [=column boxes=] establish
an independent [=block formatting context=]
into which the multi-column container's content flows,
and form the [=containing block=] for its non-positioned children.
<div class=example>
In this example, the width of the image is set with these rules:
<pre highlight="css">
img {
display: block;
width: 100%;
}
</pre>
Given that the column box creates a new [=block formatting context=],
the 'width' is calculated relative to the column box.
Therefore the image will not overflow the column box:
<figure>
<img src="images/image-inside-column.svg" alt="an image contained inside a column box">
<figcaption>The image is constrained by the column box that it is displayed in.</figcaption>
</figure>
</div>
<div class="example">
Given that the column box creates a new [=block formatting context=], a top margin set on the first child element of a multicol container will not collapse with the margins of the multicol container.
<figure>
<img src="images/margins-do-not-collapse.svg" alt="The first paragraph has a 'margin-top' of ''1em'', which appears before the text.">
<figcaption>The margin above the first paragraph has not collapsed, leaving a ''1em'' margin above the first line in the multicol container.</figcaption>
</figure>
</div>
<wpt>
multicol-margin-001.xht
multicol-margin-002.xht
multicol-margin-child-001.xht
multicol-nested-margin-001.xht
multicol-nested-margin-002.xht
multicol-nested-margin-003.xht
multicol-nested-margin-004.xht
multicol-nested-margin-005.xht
</wpt>
Floats that appear inside multi-column layouts are positioned with
regard to the [=column box=] where the float appears.
<div class="example">
In this example, this CSS fragment describes the presentation of the image:
<pre highlight="css">
img {
display: block;
float: right;
}
</pre>
In the HTML, the image appears after the sentence ending, "the leg of a chicken".
<figure>
<img src="images/image-floated-in-column.svg" alt="an image floated and contained inside a column box">
<figcaption>The image is floated inside the column box it appears in.</figcaption>
</figure>
<
8096
/div> </div>
Content overflowing a [=column box=] in the [=block axis=]
[=fragments=] and continues in the next [=column box=].
Note: Column boxes, which are [=anonymous boxes=],
do not become the [=containing block=]
for [=absolutely positioned boxes=].
The 'position' property, which establishes a containing block for such boxes,
applies to the [=multicol container=], it being the [=principal box=].
<wpt>
multicol-containing-001.xht
multicol-containing-002.xht
</wpt>
<div class="example">
In this example, the [=multi-column container=] has ''position: relative''
thus becoming the containing block.
The image is a direct child of the [=multi-column container=]
and has ''position: absolute''.
It takes positioning from the [=multi-column container=]
and not from the [=column box=].
<pre highlight="css">
.container {
position: relative;
column-count: 3;
}
img {
position: absolute;
top: 20px;
left: 40px;
}
</pre>
<figure>
<img src="images/column-not-containing-block.svg" alt="The absolutely positioned image is positioned by reference to the [=multi-column container=] not the [=column box=].">
<figcaption>The figure demonstrates that the absolutely positioned image is positioned by reference to the multicol container and not the column box.</figcaption>
</figure>
</div>
Out-of-flow descendants of a [=multi-column container=] do affect column balancing,
and the block-size of the [=multi-column container=].
The column boxes are ordered
in the [=inline base direction=] of the multicol container
and arranged into <dfn lt="multi-column line | multi-col line | multicol line">multicol lines</dfn>.
The <dfn export>column width</dfn> is the length of the column box in the inline direction.
The <dfn export>column height</dfn> is the length of the column box in the block direction.
All column boxes in a line have the same column width,
and all column boxes in a line have the same column height.
Within each [=multicol line=] in the multi-column container,
adjacent column boxes are separated by a <dfn noexport>column gap</dfn>,
which may contain a <dfn noexport>column rule</dfn>.
All column gaps in the same multi-column container are equal.
All column rules in the same multi-column container are also equal, if they appear;
column rules only appear between columns that both have content.
In the simplest case a multicol container will contain only one line
of columns, and the height of each column will be equivalent to the
used height of the multi-column container's content box.
However, [=fragmentation=] or [=spanners=]
can split the content of the [=multi-column container=]
into multiple [=multicol lines=].
If the multi-column container is paginated, the height of each column is
constrained by the page and the content continues in a new line of
column boxes on the next page; a column box never splits across pages.
The same effect occurs when a <i>spanning element</i> divides the
multi-column container: the columns before the spanning element are
balanced and shortened to fit their content. Content after the
spanning element then flows into a new, subsequent line of column boxes.
<div class="example">
<figure>
<img alt="a diagram showing a spanning element causing the shortened columns above the element with text continuing in new columns below" src="images/simple-span-example.svg">
<figcaption>A demonstration of how the spanning element divides the multicol container.</figcaption>
</figure>
</div>
A [=multi-column container=] therefore is a regular [=block container=]
that establishes a new [=independent formatting context=]
whose contents consist of a series of
[=multicol lines=] and multicol spanners.
Each [=multi-column line=] acts as a [=block-level box=]
that establishes a <dfn>multi-column formatting context</dfn>
for its [=column boxes=];
and each [=spanner=] acts as a [=block-level box=]
that establishes an [=independent formatting context=]
with its type depending on its 'display' value as usual.
Nested multi-column containers are allowed,
but there may be implementation-specific limits.
<wpt>
multicol-nested-002.xht
multicol-nested-005.xht
</wpt>
Note: It is not possible to set properties/values on column boxes.
For example, the background of a certain column box cannot be set
and a column box has no concept of padding, margin or borders.
Future specifications may add additional functionality.
For example, columns of different widths and different backgrounds may be supported.
Note: Multicol containers with column heights larger than the viewport may pose accessibility issues.
See <a href="#a11y">Accessibility Considerations</a> for more details.
<h2 id="the-number-and-width-of-columns">
The Number and Width of Columns</h2>
Finding the number and width of columns is fundamental when laying out multi-column content.
These properties are used to set the number and width of columns:
<ul>
<li>'column-count'</li>
<li>'column-width'</li>
</ul>
A third property, 'columns',
is a shorthand property which sets both 'column-width' and 'column-count'.
Other factors, such as explicit column breaks, content, and height constraints,
may influence the actual number and width of columns.
<h3 id='cw'>The Inline Size of Columns: the 'column-width' property</h3>
<pre class=propdef>
Name: column-width
Value: auto | <<length [0,∞]>>
Initial: auto
Applies to: <a>block containers</a> except <a>table wrapper boxes</a>
Inherited: no
Percentages: N/A
Computed value: the keyword ''auto'' or an absolute length
Animation type: by computed value type
</pre
F882
>
This property describes the width of columns in multicol containers.
<dl dfn-type=value dfn-for=column-width>
<dt><dfn>auto</dfn></dt>
<dd>
means that the column width will be determined by other properties
(e.g., 'column-count', if it has a non-auto value).
<dt><dfn><<length [0,∞]>></dfn>
<dd>
describes the optimal column width.
The actual column width may be wider (to fill the available space),
or narrower (only if the available space is smaller than the specified column width).
Negative values are not allowed.
Used values will be clamped to a minimum of ''1px''.
</dd>
</dl>
<wpt>
multicol-basic-003.html
multicol-basic-008.xht
multicol-reduce-000.xht
multicol-width-001.xht
multicol-width-002.xht
multicol-width-003.xht
multicol-width-ch-001.xht
multicol-width-negative-001.xht
multicol-width-invalid-001.xht
multicol-width-large-001.xht
multicol-width-large-002.xht
multicol-inherit-003.xht
multicol-list-item-001.xht
parsing/column-width-computed.html
parsing/column-width-invalid.html
parsing/column-width-valid.html
</wpt>
<div class="example">
For example, consider this style sheet:
<pre highlight="css">
div {
width: 100px;
column-width: 45px;
column-gap: 0;
column-rule: none;
}
</pre>
There is room for two ''45px'' wide columns inside the ''100px'' wide element.
In order to fill the available space
the actual column width will be increased to ''50px''.
</div>
<div class="example">
Also, consider this style sheet:
<pre highlight="css">
div {
width: 40px;
column-width: 45px;
column-gap: 0;
column-rule: none;
}
</pre>
The available space is smaller than the specified column width
and the actual column width will therefore be decreased.
</div>
To ensure that 'column-width' can be used with vertical text,
column width means the length of the line boxes inside the columns.
Note: The reason for making 'column-width' somewhat flexible
is to achieve scalable designs that can fit many screen sizes.
To set an exact column width,
the column gap and the width of the multicol container (assuming horizontal text)
must also be specified.
<h3 id='cc'>The Number of Columns: the 'column-count' property</h3>
<pre class=propdef>
Name: column-count
Value: auto | <<integer [1,∞]>>
Initial: auto
Applies to: <a>block containers</a> except <a>table wrapper boxes</a>
Inherited: no
Percentages: N/A
Computed value: specified value
Animatable: by computed value
</pre>
This property describes the number of columns of a [=multicol container=].
<dl dfn-type=value dfn-for=column-count>
<dt><dfn>auto</dfn>
<dd>
means that the number of columns will be determined by other properties
(e.g., 'column-width', if it has a non-auto value).
<dt><dfn><<integer [1,∞]>></dfn>
<dd>
describes the optimal number of columns into which the content of the element will be flowed.
Values must be greater than 0.
If both 'column-width' and 'column-count' have non-auto values,
the integer value describes the maximum number of columns.
</dd>
</dl>
<wpt>
multicol-count-001.xht
multicol-count-002.xht
multicol-basic-002.html
multicol-basic-006.xht
multicol-width-count-001.xht
multicol-width-count-002.xht
multicol-columns-toolong-001.xht
multicol-count-negative-001.xht
multicol-count-negative-002.xht
multicol-count-non-integer-001.xht
multicol-count-non-integer-002.xht
multicol-count-non-integer-003.xht
multicol-inherit-001.xht
multicol-inherit-002.xht
parsing/column-count-invalid.html
parsing/column-count-valid.html
parsing/column-count-computed.html
multicol-table-cell-001.xht
multicol-table-cell-height-001.xht
multicol-table-cell-height-002.xht
multicol-table-cell-vertical-align-001.xht
</wpt>
<div class="example">
Example:
<pre highlight="css">body { column-count: 3 }</pre>
</div>
<h3 id="columns">The 'column-width' and 'column-count' Shorthand: The 'columns' Property</h3>
<pre class="propdef shorthand">
Name: columns
Value: <<'column-width'>> || <<'column-count'>>
</pre>
This is a shorthand property for setting 'column-width' and 'column-count'.
Omitted values are set to their initial values.
<div class="example">
Here are some valid declarations using the 'columns' property:
<pre highlight="css">
columns: 12em; /* column-width: 12em; column-count: auto */
columns: auto 12em; /* column-width: 12em; column-count: auto */
columns: 2; /* column-width: auto; column-count: 2 */
columns: 2 auto; /* column-width: auto; column-count: 2 */
columns: auto; /* column-width: auto; column-count: auto */
columns: auto auto; /* column-width: auto; column-count: auto */
</pre>
</div>
<wpt>
multicol-columns-001.xht
multicol-columns-002.xht
multicol-columns-003.xht
multicol-columns-004.xht
multicol-columns-005.xht
multicol-columns-006.xht
multicol-columns-007.xht
multicol-columns-invalid-001.xht
multicol-columns-invalid-002.xht
multicol-basic-001.html
multicol-basic-003.html
multicol-basic-005.xht
multicol-basic-007.xht
parsing/columns-invalid.html
parsing/columns-valid.html
</wpt>
<h3 id="pseudo-algorithm">The Pseudo-algorithm</h3>
The pseudo-algorithm below determines the used values for
'column-count' (N) and 'column-width' (W). There is one other variable
in the pseudo-algorithm: U is the used width of the multi-column container.
Note: The used width U of the multi-column container can depend on the element's contents,
in which case it also depends on the computed values of the 'column-count' and 'column-width' properties.
This specification does not define how U is calculated.
Another module (probably the Basic Box Model [[CSS3BOX]]
or the Intrinsic & Extrinsic Sizing Module [[CSS3-SIZING]]) is expected to define this.
<!--
Two assumptions are being made by the pseudo-algorithm:
<ul>
<li>that the block direction is unconstrained
<li>that no column breaks are added through style sheets
</ul>
-->
The <code>floor(X)</code> function returns the largest integer Y ≤ X.
<pre>
(01) if ((column-width = auto) and (column-count = auto)) then
(02) exit; /* not a multicol container */
(03) if column-width = auto then
(04) N := column-count
(05) else if column-count = auto then
(06) N := max(1,
(07) floor((U + column-gap)/(column-width + column-gap)))
(08) else
(09) N := min(column-count, max(1,
(10) floor((U + column-gap)/(column-width + column-gap))))
</pre>
And:
<pre>
(11) W := max(0, ((U + column-gap)/N - column-gap))
</pre>
For the purpose of finding the number of auto-repeated columns,
the UA must floor the column size to a UA-specified value to avoid division by zero.
It is suggested that this floor be 1px or less.
In fragmented contexts such as in [=paged media=],
user agents may perform this calculation on a per-fragment basis.
The used value for 'column-count' is calculated without regard for explicit column breaks or constrained column heights,
while the actual value takes these into consideration.
<wpt>
column-count-used-001.html
</wpt>
<div class="example">
In this example, the actual column-count is higher than the used column-count
due to explicit column breaks:
<pre highlight="css">
div {
width: 40em;
columns: 20em;
column-gap: 0;
}
p {
break-after: column;
}
</pre>
<pre highlight="html">
<div>
<p>one
<p>two
<p>three
</div>
</pre>
<figure>
<img src="images/column-count-higher-than-used-count.svg" alt="Two columns drawn inside the container, one outside">
<figcaption>The computed column-count is auto, the used column-count is 2 and the actual column-count is 3.</figcaption>
</figure>
</div>
<div class=example>
The actual column-count may be lower than the used column-count.
Consider this example:
<pre highlight="css">
div {
width: 80em;
height: 10em;
columns: 20em;
column-gap: 0;
column-fill: auto;
}
</pre>
<pre highlight="html">
<div>foo</div>
</pre>
The computed column-count is auto,
the used column-count is 4,
and the actual column-count is 1.
</div>
<h3 id="stacking-context">
Stacking Context</h3>
All column boxes in a multi-column container are in the same stacking context
and the drawing order of their contents is as specified in CSS 2.1.
Column boxes do not establish new stacking contexts.
<wpt>
multicol-rule-stacking-001.xht
</wpt>
<h2 id="column-gaps-and-rules">Column Gaps and Rules</h2>
Column gaps and rules are placed between columns in the same [=multicol container=].
The length of the column gaps and column rules is equal to the column height.
Column gaps take up space.
That is, column gaps will push apart content in adjacent columns
(within the same [=multicol container=]).
<wpt>
multicol-height-001.xht
multicol-nested-column-rule-001.xht
multicol-rule-nested-balancing-001.html
</wpt>
A [=column rule=] is drawn in the middle of the [=column gap=]
with the endpoints at opposing content edges of the [=multicol container=].
Column rules do not take up space.
That is, the presence or thickness of a [=column rule=] will not alter the placement of anything else.
If a [=column rule=] is wider than its gap,
the adjacent column boxes will overlap the rule,
and the rule may possibly extend outside the box of the [=multicol container=].
Column rules are painted just above the border of the [=multicol container=].
For scrollable multicol containers,
note that while the border and background of the [=multicol container=] obviously aren't scrolled,
the rules need to scroll along with the columns.
Column rules are only drawn between two columns that both have content.
<wpt>
multicol-rule-001.xht
multicol-rule-003.xht
multicol-rule-004.xht
multicol-count-computed-003.xht
multicol-count-computed-005.xht
multicol-rule-fraction-002.xht
multicol-rule-large-001.xht
</wpt>
<h3 id='cg'>Gutters Between Columns: the 'column-gap' property</h3>
The 'column-gap' property is defined in [[!CSS3-ALIGN]].
In a [=multi-column formatting context=]
the used value of ''normal'' for the 'column-gap' property is ''1em''.
This ensures columns are readable when the initial values are used.
If there is a column rule between columns,
it will appear in the middle of the gap.
<wpt>
multicol-gap-animation-001.html
multicol-gap-animation-002.html
multicol-gap-animation-003.html
multicol-gap-fraction-001.xht
multicol-gap-fraction-002.html
multicol-gap-large-001.xht
multicol-gap-large-002.xht
multicol-gap-negative-001.xht
multicol-gap-000.xht
multicol-gap-002.xht
multicol-gap-percentage-001.html
multicol-gap-001.xht
multicol-gap-003.xht
</wpt>
<h3 id='crc'>The Color of Column Rules: the
'column-rule-color' property</h3>
<pre class=propdef>
Name: column-rule-color
Value: <<color>>
Initial: currentcolor
Applies to: multicol containers
Inherited: no
Percentages: N/A
Computed value: computed color
Animation type: by computed v
9E70
alue type
</pre>
<dl>
<dt><dfn value for=column-rule-color><<color>></dfn></dt>
<dd>
Specifies the color of the [=column rule=].
</dd>
</dl>
<wpt>
multicol-rule-color-001.xht
multicol-rule-color-inherit-001.xht
multicol-rule-color-inherit-002.xht
parsing/column-rule-color-computed.html
parsing/column-rule-color-valid.html
parsing/column-rule-color-invalid.html
</wpt>
<h3 id='crs'>The Style Of Column Rules: the 'column-rule-style' property</h3>
<pre class=propdef>
Name: column-rule-style
Value: <<line-style>>
Initial: none
Applies to: multicol containers
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
The 'column-rule-style' property sets the style of the rule between columns of an element.
The <<line-style>> values are interpreted as in the <a href="https://www.w3.org/TR/CSS2/tables.html#collapsing-borders">collapsing border model</a>.
<wpt>
parsing/column-rule-style-computed.html
parsing/column-rule-style-valid.html
parsing/column-rule-style-invalid.html
</wpt>
The ''border-style/none'' value forces the computed value of 'column-rule-width' to be ''0''.
<h3 id='crw'>The Width Of Column Rules: the 'column-rule-width' property</h3>
<pre class=propdef>
Name: column-rule-width
Value: <<line-width>>
Initial: medium
Applies to: multicol containers
Inherited: no
Percentages: N/A
Computed value: absolute length; ''0'' if the column rule style is ''border-style/none'' or ''hidden''
Animation type: by computed value type
</pre>
This property sets the width of the rule between columns.
Negative values are not allowed.
<wpt>
multicol-rule-fraction-001.xht
multicol-rule-px-001.xht
multicol-rule-percent-001.xht
parsing/column-rule-width-computed.html
parsing/column-rule-width-invalid.html
parsing/column-rule-width-valid.html
</wpt>
<h3 id="cr">Column Rule Shorthand: the 'column-rule' property</h3>
<pre class="propdef shorthand">
Name: column-rule
Value: <<'column-rule-width'>> || <<'column-rule-style'>> || <<'column-rule-color'>>
</pre>
This property is a shorthand for setting
'column-rule-width', 'column-rule-style', and 'column-rule-color'
at the same place in the style sheet.
Omitted values are set to their initial values.
<wpt>
multicol-shorthand-001.xht
multicol-rule-shorthand-001.xht
multicol-rule-shorthand-2.xht
multicol-rule-000.xht
multicol-rule-dashed-000.xht
multicol-rule-dotted-000.xht
multicol-rule-double-000.xht
multicol-rule-outset-000.xht
multicol-rule-none-000.xht
multicol-rule-hidden-000.xht
multicol-rule-inset-000.xht
multicol-rule-groove-000.xht
multicol-rule-ridge-000.xht
multicol-rule-solid-000.xht
parsing/column-rule-computed.html
parsing/column-rule-invalid.html
parsing/column-rule-valid.html
</wpt>
<div class=example>
In this example, the [=column rule=] and the [=column gap=] have the same width.
Therefore, they will occupy exactly the same space.
<pre highlight="css">
body {
column-gap: 35px;
column-rule-width: 35px;
column-rule-style: solid;
column-rule-color: black;
}
</pre>
<figure>
<img src="images/rule-same-width-as-gap.svg" alt="The rule completely covers any gap.">
<figcaption>The column rule and column gap occupy the same space.</figcaption>
</figure>
<wpt>
multicol-rule-samelength-001.xht
</wpt>
</div>
<h2 id="column-breaks">
Column Breaks</h2>
When content is laid out in multiple columns,
the user agent must determine where column breaks are placed.
The problem of breaking content into columns is similar to breaking content into pages,
which is described in CSS 2.1, section 13.3.3 [[!CSS21]].
Three new properties are introduced to allow column breaks to be described in the same properties as page breaks:
'break-before', 'break-after', and 'break-inside'.
<wpt>
multicol-break-000.xht
multicol-break-001.xht
</wpt>
<h3 id="break-before-break-after-break-inside">Controlling Fragmentation: the 'break-before', 'break-after', 'break-inside' properties</h3>
'break-before', 'break-after', and 'break-inside'
are defined in [[!CSS3-BREAK]].
<h2 id="spanning-columns">
Spanning Columns</h2>
The 'column-span' property makes it possible for an element to span across several columns.
<h3 id="column-span">Spanning An Element Across Columns: the 'column-span' property</h3>
<pre class="propdef">
Name: column-span
Value: none | all
Initial: none
Applies to: in-flow block-level elements
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
<wpt>
parsing/column-span-invalid.html
</wpt>
This property describes how many columns an element spans across. Values are:
<dl dfn-type=value dfn-for=column-span>
<dt><dfn>none</dfn></dt>
<dd>
The element does not span multiple columns.
<wpt>
multicol-span-none-001.xht
</wpt>
</dd>
<dt><dfn>all</dfn></dt>
<dd>
The element forces a column break and is taken [=out of flow=]
to span across all columns of the nearest multicol ancestor
in the same [=block formatting context=].
Content in the normal flow that appears before the element
is automatically balanced across all columns
in the immediately preceding [=multi-column line=] before the element appears,
and any subsequent content flows into a new [=multi-column line=] after the element.
The element establishes an [=independent formatting context=].
Note: Whether the element establishes a new <a>formatting context</a>
does not depend on whether the element is a descendant of a multicol or not.
When 'column-span' is ''column-span/all'', it always does.
This helps with robustness of designs to later revisions that remove the multicol,