forked from w3c/csswg-drafts
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathOverview.src.html
More file actions
1027 lines (945 loc) · 38.8 KB
/
Overview.src.html
File metadata and controls
1027 lines (945 loc) · 38.8 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
<!DOCTYPE html public '-//W3C//DTD HTML 4.01//EN' 'http://www.w3.org/TR/html4/strict.dtd'>
<html lang="en">
<head profile="http://www.w3.org/2006/03/hcard">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>CSS Fragmentation Controls Module Level 3</title>
<link rel="stylesheet" type="text/css" href="../default.css" />
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css" />
</head>
<body>
<div class="head">
<!--logo-->
<h1 id="css-fragmentation-module">CSS Fragmentation Controls Module Level 3</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
<dt>This version:</dt>
<dd><a href="[VERSION]">http://www.w3.org/csswg/css3-break/</a></dd>
<dt>Latest version:</dt>
<dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/csswg/[SHORTNAME]/</a></dd>
<dt>Editor's Draft:</dt>
<dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a></dd>
<dt>Previous version:</dt>
<dd>None</dd>
<dt>Editors:</dt>
<dd class="vcard"><span class="fn">Rossen Atanassov</span>, <span class="org"><a class="fn org url" href="http://www.microsoft.com/">Microsoft Corporation</a></span>, <span class="email">ratan@microsoft.com</span></dd>
<dd class="vcard"><span class="fn"><a class="fn" href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a></span>, <span class="org"><a class="fn org url" href="http://www.mozilla.com/">Mozilla Corporation</a></span></dd>
<dt>Issues List:</dt>
<dd><a href="http://wiki.csswg.org/spec/[SHORTNAME]">http://wiki.csswg.org/spec/[SHORTNAME]</a></dd>
<dt>Discussion:</dt>
<dd><a href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a> with subject line "<code>[css3-break] …message topic…</code>"</dd>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id="abstract">
Abstract</h2>
<p>
This module describes the fragmentation model that partitions a flow into pages.
It builds on the Page model module and introduces and defines the fragmentation
model. It adds functionality for pagination, breaking variable fragment size and
orientation, widows and orphans.
</p>
<h2 class="no-num no-toc" id="status">
Status of this document</h2>
<!--status-->
<!--<p>The following features are at risk: …</p>-->
<h2 class="no-num no-toc" id="contents">
Table of contents</h2>
<!--toc-->
<h2 id="intro">
Introduction</h2>
<p><em>This section is not normative.</em></p>
<p>
In paged media (e.g., paper, transparencies, photo album pages, pages
displayed on computer screens as printed output simulations), as
opposed to <a href="/TR/CSS2/media.html#continuous-media-group">
continuous media</a>, the content of the document is split into one or
more discrete display surfaces. In order to avoid awkward breaks
(such as halfway through a line of text), the layout engine must be
able to shift around content that would fall across the page break.
This process is called <dfn>pagination</dfn>.
</p>
<p>
In CSS, in addition to paged media, certain layout features such as
<a href="http://www.w3.org/TR/css3-regions/">regions</a> [[CSS3-REGIONS]]
and <a href="http://www.w3.org/TR/css3-multicol/">multi-column layout</a>
[[CSS3COL]] create a similarly fragmented environment.
The generic term for breaking content across containers is
<dfn>fragmentation</dfn>.
This module explains how content breaks across <i>fragmentation containers</i>
such as pages and columns and how such breaks can be
<a href="#breaking-controls">controlled by the author</a>.
</p>
<h3 id="placement">
Module Interactions</h3>
<p>
This module replaces and extends the pagination controls defined in
[[CSS21]] <a href="http://www.w3.org/TR/CSS21/page.html#page-breaks">section 13.3</a>
and in [[CSS3PAGE]].
</p>
<!-- End section "Modules Interactions" -->
<h3 id="values">
Values</h3>
<p>
This specification follows the <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">
CSS property definition conventions</a> from [[!CSS21]]. Value types not defined
in this specification are defined in CSS Level 2 Revision 1 [[!CSS21]]. Other CSS
modules may expand the definitions of these value types: for example [[CSS3COLOR]],
when combined with this module, expands the definition of the <color> value
type as used in this specification.</p>
<p>
In addition to the property-specific values listed in their definitions, all properties
defined in this specification also accept the <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">
inherit</a> keyword as their property value. For readability it has not been
repeated explicitly.
</p>
<h2 id="fragmentation-model">
Fragmentation Model and Terminology</h2>
<dl>
<dt><dfn>fragmented flow</dfn></dt>
<dd>
A content flow that is being laid out in a <i>fragmentation context</i>,
such as a <a href="http://www.w3.org/TR/css3-multicol/">multi-column element</a>,
a chain of <a href="http://www.w3.org/TR/css3-regions">CSS regions</a>,
or a <a href="http://www.w3.org/TR/css3-page/">paged media display</a>.
</dd>
<dt><dfn>fragmenter</dfn></dt>
<dd>
A box—such as a page box, column box, or region—that contains
a portion (or all) of a <i>fragmented flow</i>.
Fragmenters can be pre-defined, or generated as needed.
When breakable content overflows a fragmenter in the block dimension,
instead of overflowing it breaks into the next container in its
<i>fragmentation context</i>.
</dd>
<dt><dfn>fragmentation context</dfn></dt>
<dd>
An ordered series of <i>fragmenters</i>.
A given fragmentation context can only have one block flow direction
across all its fragmenters.
</dd>
<dt><dfn>fragmentation</dfn></dt>
<dd>
The process of splitting a content flow across the <i>fragmenters</i>
that form a <i>fragmentation context</i>.
</dd>
<dt><dfn>box fragment</dfn> or <dfn>fragment</dfn></dt>
<dd>
The portion of a box that belongs to exactly one <i>fragmenter</i>.
A box in continuous flow always consists of only one fragment.
A box in a fragmented flow consists of one or more fragments.
</dl>
<p>
Each <dfn>fragmentation break</dfn> (hereafter, <dfn>break</dfn>) ends
layout in the current <i>fragmenter</i> and causes the remaining content
of the fragmented flow to be laid out in the next <i>fragmenter</i>, in
some cases causing a new fragmenter to be generated to hold the deferred
content.
</p>
<p>
Breaking a fragmenter F effectively splits the fragmenter into two
fragmenters (F<sub>1</sub> and F<sub>2</sub>). The only difference
is that the type of break between the two pieces F<sub>1</sub> and
F<sub>2</sub> is the <a href="#break-types">type of break</a> created
by the fragmentation context that split F, not the type of break
normally created by F's own fragmentation context.
<p class="note">
Breaking inline content into lines is another form of fragmentation,
and similarly creates box fragments when it breaks
<a href="http://www.w3.org/TR/CSS21/visuren.html#inline-boxes">inline boxes</a>
across <a href="http://www.w3.org/TR/CSS21/visuren.html#line-box">line boxes</a>.
However, inline breaking is not covered here; see [[CSS3TEXT]] and
the 'box-decoration-break' property in [[CSS3BACKGROUND]].
</p>
<h2 id="breaking-controls">
Breaking Controls</h2>
<p>
The following sections explain how breaks are controlled in a fragmented
flow. Five properties indicate where the user agent may or should break
the content flow. In the case of pagination, the author can also specify
on which page (<a href="http://www.w3.org/TR/css3-page/#left-right-first">left or right</a>)
the subsequent content should resume.
</p>
<h3 id="explicit-break-properties">
Breaking before, after or inside elements: the 'break-before', 'break-after', 'break-inside'
properties.</h3>
<table class="propdef" summary="property definition">
<tr>
<td><em>Name:</em>
<td><dfn>break-before</dfn>
<tr>
<td><em>Value:</em>
<td>auto | always | left | right | page | column | region | avoid | avoid-page | avoid-column | avoid-region
<tr>
<td><em>Initial:</em>
<td>auto
<tr>
<td><em>Applies to:</em>
<td>block-level elements, table row groups, table rows (but see prose)
<tr>
<td><em>Inherited:</em>
<td>no
<tr>
<td><em>Percentages:</em>
<td>N/A
<tr>
<td><em>Media:</em>
<td>paged
<tr>
<td><em>Computed value:</em>
<td>specified value
</table>
<table class="propdef" summary="property definition">
<tr>
<td><em>Name:</em>
<td><dfn>break-after</dfn>
<tr>
<td><em>Value:</em>
<td>auto | always | left | right | page | column | region | avoid | avoid-page | avoid-column | avoid-region
<tr>
<td><em>Initial:</em>
<td>auto
<tr>
<td><em>Applies to:</em>
<td>block-level elements, table row groups, table rows (but see prose)
<tr>
<td><em>Inherited:</em>
<td>no
<tr>
<td><em>Percentages:</em>
<td>N/A
<tr>
<td><em>Media:</em>
<td>paged
<tr>
<td><em>Computed value:</em>
<td>specified value
</table>
<table class="propdef" summary="property definition">
<tr>
<td><em>Name:</em>
<td><dfn>break-inside</dfn>
<tr>
<td><em>Value:</em>
<td>auto | avoid | avoid-page | avoid-column | avoid-region
<tr>
<td><em>Initial:</em>
<td>auto
<tr>
<td><em>Applies to:</em>
<td>block-level elements, block containers, table row groups, table rows (but see prose)
<tr>
<td><em>Inherited:</em>
<td>no
<tr>
<td><em>Percentages:</em>
<td>N/A
<tr>
<td><em>Media:</em>
<td>paged
<tr>
<td><em>Computed value:</em>
<td>specified value
</table>
<p>
These properties describe page/column/region break behavior before/after/inside the generated
box. These values have the following meaning:
</p>
<dl>
<dt>
auto
</dt>
<dd>
Neither force nor forbid a break before/after/inside the principle box.
</dd>
<dt>
always
</dt>
<dd>
Always force a break before/after the principle box.
</dd>
<dt>
avoid
</dt>
<dd>
Avoid a break before/after/inside the principle box.
</dd>
<dt>
left
</dt>
<dd>
Force one or two page breaks before/after the principle box so that the next page is formatted as a left page.
</dd>
<dt>
right
</dt>
<dd>
Force one or two page breaks before/after the principle box so that the next page is formatted as a right page.
</dd>
<dt>
page
</dt>
<dd>
Always force a page break before/after the principle box.
</dd>
<dt>
column
</dt>
<dd>
Always force a column break before/after the principle box.
</dd>
<dt>
region
</dt>
<dd>
Always force a region break before/after the principle box.
</dd>
<dt>
avoid-page
</dt>
<dd>
Avoid a page break before/after/inside the principle box.
</dd>
<dt>
avoid-column
</dt>
<dd>
Avoid a column break before/after/inside the principle box.
</dd>
<dt>
avoid-region
</dt>
<dd>
Avoid a region break before/after/inside the principle box.
</dd>
</dl>
<p>
A potential page/column/region break location is typically under the influence of
the parent element's 'break-inside' property, the 'break-after' property of the
preceding element, and the 'break-before' property of the following element. When
these properties have values other than ''auto'', the <dfn>forced break values</dfn> (''always'',
''left'', ''right'', ''page'', ''column'' and ''region'') take precedence over the
<dfn>avoid break values</dfn> (''avoid'', ''avoid-page'', ''avoid-column'' and ''avoid-region''). See the section
on <a href="#breaking-rules">rules for breaking</a> for the exact rules on how these
properties affect forced breaks.
</p>
<p>
User Agents must apply these properties to block-level boxes and to table rows,
table row groups, and—in the case of 'break-inside'—block containers
in the normal flow of the root element. User agents should also apply these properties
to floated boxes whose containing block is in the normal flow of the root element.
User agents may also apply these properties to other boxes.
</p>
<h3 id="breaks-inside">
Breaks inside elements: 'orphans', 'widows'</h3>
<table class="propdef" summary="property definition">
<tr>
<th>Name:
<td><dfn id="orphans">orphans</dfn>
<tr>
<th>Value:
<td><integer>
<tr>
<th>Initial:
<td>2
<tr>
<th>Applies to:
<td>block containers
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
</table>
<table class="propdef" summary="property definition">
<tr>
<th>Name:
<td><dfn id="widows">widows</dfn>
<tr>
<th>Value:
<td><integer>
<tr>
<th>Initial:
<td>2
<tr>
<th>Applies to:
<td>block containers
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
</table>
<p>
The 'orphans' property specifies the minimum number of line boxes in a block container
that must be left at the end of a <i>fragment</i>, unless this is the last <i>fragment</i>
of the box. The 'widows' property specifies the minimum number of line boxes of
a block container that must be le
9F7E
ft at the start of a <i>fragment</i>, unless this
is the last <i>fragment</i> of the box. Examples of how they are used to control
fragmentation breaks are given below.
</p>
<p>
Only positive integers are allowed as values of 'orphans' and 'widows'. Negative
values and zero are invalid must cause the declaration to be
<a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
</p>
<p>
If a block contains fewer lines than the value of 'widows' or 'orphans', the rule
simply becomes that all lines in the block must be kept together.
</p>
<h2 id="breaking-rules">
Rules for Breaking</h2>
<p>
A fragmented flow may be broken across fragmenters at a number of
<a href="#possible-breaks">possible break points</a>. In the case of
<a href="#forced-breaks">forced breaks</a>, the UA is required to
break the flow at that point. In the case of
<a href="#unforced-breaks">unforced breaks</a> the UA has to choose
among the possible breaks that are allowed.
</p>
<h3 id="possible-breaks">
Possible Break Points</h3>
<p>
Fragmentation breaks boxes in the block flow dimension.
Breaks may occur at the following places:
</p>
<ol>
<li id="btw-blocks">
Between sibling block-level boxes (or rows in a table).
</li>
<li id="btw-lines">
Between line boxes inside a block container box (if the lines are
perpendicular to the fragmenter's block flow direction); or between
column boxes in a multi-column element or table (if the columns are
perpendicular to the fragmenter's block flow direciton).
</li>
<li id="end-block">
Between the content edge of a block box and the outer edges of its
child content (margin edges of block-level children or line box
edges for inline-level children) <em>if</em> there is a (non-zero)
gap between them.
</li>
</ol>
<p>
Since breaks are usually only possible between siblings (1), not between
a child and its parent, a 'break-before' value on a first-child box is
propagated to its parent. Likewise a 'break-after' value on a last-child
box is propagated to its parent.
</p>
<p>
When paginating, if there are no possible break points below the top
of the page, and not all the content fits, the UA may break anywhere
in order to avoid losing content off the edge of the page.
</p>
<p>
The UA is not required to fragment the scrollable contents of elements
with 'overflow' set to ''auto'' or ''scroll'' and may instead graphically
slice their contents as necessary to fragment the element. In such cases
it must treat the element as having ''break-inside: avoid''.
</p>
<h3 id="break-types">
Types of Breaks</h3>
<p>
There are different types of breaks in CSS, defined based on the type of
fragmenters they span:
</p>
<dl>
<dt><dfn>page break</dfn></dt>
<dd>
A break between two <a href="http://www.w3.org/TR/css3-page/#page-box">page boxes</a>.
[[!CSS3PAGE]]
</dd>
<dt><dfn>spread break</dfn></dt>
<dd>
A break between two page boxes that are not associated with
<a href="http://www.w3.org/TR/css3-page/#facing-pages">facing pages</a>.
A spread break is always also a page break.
[[!CSS3PAGE]]
</dd>
<dt><dfn>column break</dfn></dt>
<dd>
A break between two <a href="http://www.w3.org/TR/css3-multicol/#column-box">column boxes</a>.
Note that if the column boxes are on different pages, then the break is
also a <i>page break</i>.
[[!CSS3COL]]
</dd>
<dt><dfn>region break</dfn></dt>
<dd>
A break between two <a href="http://www.w3.org/TR/css3-regions/#regions">regions</a>.
Note that if the region boxes are on different pages, then the break is
also a <i>page break</i>.
[[!CSS3-REGIONS]]
</dd>
</dl>
<p class="note">
A fifth type of break is the <dfn>line break</dfn>, which is a break between
two <a href="http://www.w3.org/TR/CSS21/visuren.html#line-box">line boxes</a>.
These are not covered in this specification; see [[!CSS21]].
</p>
<h3 id="forced-breaks">
Forced breaks</h3>
<p>
A forced break then occurs at <a href="#btw-blocks">(1)</a> if, among the
'break-after' properties specified on or propagated to the earlier
sibling box and the 'break-before' properties specified on or propagated
to the later sibling box there is at least one with a <i>forced break
value</i>.
</p>
<p>
When multiple <i>forced break values</i> apply, they combine such all types
of break are honored and no content-empty page boxes are generated, except
for at most one content-empty page as may be required by the ''right'' or
''left'' values to position ensuing content on a right- or left-facing page.
When ''left'' and ''right'' are both combined, the value specified on the
latest element in document order wins.
</p>
<p class="note">
A page break must also occur at <a href="#brk-btw-blocks">(1)</a>
if the last line box above this margin and the first one below it do not have the
same value for 'page'. See [[!CSS3PAGE]]
</p>
<h3 id="unforced-breaks">
Unforced Breaks</h3>
<p>
While <a href="#breaking-controls">breaking controls</a> can force breaks,
they can also discourage them. The following rules control whether breaking
at a <a href="#posible-breaks">possible break point</a> is allowed:
</p>
<dl>
<dt>Rule A</dt>
<dd>
A fragmented flow may break at <a href="#btw-blocks">(1)</a> only if
all the 'break-after' and 'break-before' values applicable to this
break point allow it, which is when at least one of them forces a
break or when all of them are ''auto''.
</dd>
<dt>Rule B</dt>
<dd>
However, if all of them are ''auto'' and a common ancestor of all
the elements has a 'break-inside' value of ''avoid'', then breaking
here is not allowed.
</dd>
<dt>Rule C</dt>
<dd>
Breaking at <a href="#btw-lines">(2)</a> is allowed only if the number
of line boxes between the break and the start of the enclosing block
box is the value of 'orphans' or more, and the number of line boxes
between the break and the end of the box is the value of 'widows' or more.
</dd>
<dt>Rule D</dt>
<dd>
Additionally, breaking at <a href="#btw-blocks">(2)</a> or
<a href="#end-block">(3)</a> is allowed only if the 'break-inside'
property of all ancestors is ''auto''.
</dd>
</dl>
<p>
If the above doesn't provide enough break points to keep content from
overflowing the page boxes, then rules A, B and D are dropped in order
to find additional breakpoints.
In this case the UA may use the ''avoid''s that are in effect at those
points to weigh the appropriateness of the new breakpoints; however,
this specification does not suggest a precise algorithm.
</p>
<p>
If that still does not lead to sufficient break points, rule C is
dropped as well, to find still more break points.
</p>
<h3 id="best-pg-brk">
Optimizing Unforced Breaks</h3>
<p>While CSS3 requires that a fragmented flow must break at allowed
break points in order to avoid overflowing the fragmenters in its
fragmentation context, it does not define <em>which</em> of a set of
possible <a href="#unforced-breaks">allowed breaks</a> must be taken.
However, it is recommended that user agents observe the following
guidelines (while recognizing that they are sometimes contradictory):
</p>
<ul>
<li>Break as few times as possible.</li>
<li>Make all fragmenters that don't end with a forced break appear
to be equally filled with content.</li>
<li>Avoid breaking inside a replaced element.</li>
</ul>
<div class="example">
<p>
Suppose, for example, that the style sheet contains ''orphans : 4'',
''widows : 2'', and there are 20 lines (line boxes) available
at the bottom of the current page, and the next block in normal flow
is considered for placement:
</p>
<ul>
<li>
If the block contains 20 line boxes or fewer, it should be placed
on the current page.
</li>
<li>
If the block contains 21 or 22 line boxes, the second fragment of
the paragraph must not violate the 'widows' constraint, and so
the second fragment must contain at least two line boxes;
likewise the first fragment must contain at least four line boxes.
</li>
<li>
If the block contains 23 line boxes or more, the first fragment should
contain 20 lines and the second fragment the remaining lines. But if
any fragment of the block is placed on the current page, that fragment
must contain at least four line boxes and the second fragment at least
two line boxes.
</li>
</ul>
<p>
Now suppose that 'orphans' is ''10'', 'widows' is ''20'', and there
are 8 lines available at the bottom of the current page:
</p>
<ul>
<li>
If the block contains 8 lines or fewer, it should be placed
on the current page.
</li>
<li>
If the block contains 9 lines or more, it must NOT be split
(that would violate the 'orphans' constraint), so it must
move as a block to the next page.
</li>
</ul>
</div>
<h3 id="breaking-boxes">
Box Model for Breaking</h3>
<p>
When a flow is fragmented into varying-size fragmenters, the following
rules are observed for adapting layout:
</p>
<p class="note">
The sizing terminology used in this section is defined in Appendix D of
[[CSS3-WRITING-MODES]].
</p>
<ul>
<li>
Intrinsic sizes are calculated and maintained across the entire element.
Where an initial containing block size is needed, assume an initial
containing block using the page's size of the first fragment's page.
</li>
<li>
Layout is performed per-fragmenter, with each fragmenter continuing
progress from the breakpoint on the previous, but recalculating sizes
and positions using its own size as if the entire element were fragmented
across fragmenters of this size. Progress is measured in percentages
(not absolute lengths) of used/available space and in amount of
used/remaining content.
However, when laying out scrollable elements and certain replaced
elements the UA may instead maintain a consistent measure and resolved
extent across fragmenters.
</li>
<li><strong class="issue">Option A:</strong>
Fragments of boxes that began on a previous fragmenter must start at
the top of the new fragmenter.
If this results in multiple floats side-by-side that would otherwise
be staggered (if they were not continuations) in order to fit, the
floats' widths are reduced (on that fragmenter only) in proportion to
their original widths until they fit. However they are not reduced
past their min-content width.
</li>
<li><strong class="issue">Option B:</strong>
Fragments of boxes that began on a previous fragmenter must obey
placement rules with the additional constraint that fragments must
not be positioned above the before edge of the fragmenter and must
otherwise be placed as high as possible while not violating other
constraints. If this results in a float's continuation fragment
shifting away from the before edge of the fragmenter, then
''box-decoration-break: clone'', if specified, wraps the fragment
with the box's margin in addition to its padding and border.
</li>
</ul>
<p>Below are listed some implications of these rules:</p>
<ul>
<li>
Boxes (including tables) fullfilling layout constraints at their
<i>fill-available</i> or percentage-based size may change measure
across pages.
</li>
<li>
Boxes (including tables) fulfilling layout constraints at their
<i>min-content</i>, <i>max-content</i>, or absolute-length size
will maintain their measure across pages.
</li>
<li><strong class="issue">Option A:</strong>
Opposite-side side-by-side floats might overlap if, e.g. both begin
on a wide page, but their min-content measures taken together are
too wide to fit on a later, narrower page. (Auto-sized floats will
not overlap if only their max-content measures are too wide, since
the shrink-wrap algorithm will give them narrower measures due to the
narrower available measure.)
</li>
<li><strong class="issue">Option A:</strong>
Same-side side-by-side floats might overflow their containing block
if, e.g. both begin on a wide page, but their min-content measures
taken together are too wide to fit on a later, narrower page.
</li>
<li><strong class="issue">Option B:</strong>
Float continuation fragments may be placed below the top of the
page if, e.g. multiple floats continue onto a new page that is
narrower than the previous page and their (recalculated) widths
together are wider than their containing block.
</li>
<li><strong class="issue">Option B:</strong>
A block-level continuation fragment may be placed below the top of
the page if, e.g. it establishes a block formatting context root
that is placed beside a float and both continue onto a narrower page
that is too narrow to hold both of them side-by-side.
</li>
<li><strong class="issue">Option B:</strong>
Content adjacent to a preceding float on one page may wind up above
the float on the next page if, e.g. that float is pushed down because
it no longer fits side-by-side with an earlier float that also
continues to this narrower page.
</li>
</ul>
<div class="example">
<p>Here is an example that shows the use of percentage-based progress:
Suppose we have an absolutely-positioned element that is positioned
''top: calc(150% + 30px)'' and has ''height: calc(100% - 10px)''. If
it is placed into a paginated context with a first page of 400px,
a second page of 200px, and a third page of 600px, its layout progresses
as follows:
<ul>
<li>First, the top position is resolved against the first page's size.
This results in 630px. Since the first page only has 400px, layout
moves to the second page, recording progress of 400/630 = 63.49% with
36.51% left to go.
<li>Now on the second page, the top position is again resolved, this
time against the second page size. This results in 330px. The remining
36.51% of progress thus resolves to 120.5px, placing the top edge of
the element 120.5px down the second page.
<li>Now the height is resolved against the second page; it resolves
to 190px. Since there are only 79.5px left on the page, layout moves
to the third page, recording progress of 79.5/190 = 41.84%, with 48.16%
left to go.
<li>On the third page, the height resolves to 590px. The remaining
48.16% of progress thus resolves to 246.9px, which fits on this page
and completes the element.
</ul>
</div>
<p>
When an unforced break occurs between block-level boxes, any adjoining
margins are set to zero. When a forced break occurs there, any margins
before the break are truncated, but margins after the break are preserved.
</p>
<p>
When a break splits a box, the 'box-decoration-break' property controls
whether the box's margins, borders, and padding wrap the fragment effect
where the split occurs. If they do not (i.e. ''slice'' is specified),
the box's background and side margins, border, and padding extend from
the effective break point to the end of the fragmenter (and across the
ensuing blank fragmenter if one is generated due to ''left'' or ''right''
breaking). <span class="issue">Does this use up height?</span>
</p>
<h2 id="conformance">
Conformance</h2>
<h3 id="conventions">
Document Conventions</h3>
<p>
Conformance requirements are expressed with a combination of descriptive assertions
and RFC 2119 terminology. The key words “must”, “must NOT”, “REQUIRED”, “SHALL”,
“SHALL NOT”, “should”, “should NOT”, “RECOMMENDED”, “may”, and “OPTIONAL” in the
normative parts of this document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase letters in
this specification.
</p>
<p>
All of the text of this specification is normative except sections explicitly marked
as non-normative, examples, and notes. [[!RFC2119]]
</p>
<p>
Examples in this specification are introduced with the words “for example” or are
set apart from the normative text with <code>class="example"</code>, like this:
</p>
<div class="example">
<p>This is an example of an informative example.</p>
</div>
<p>
Informative notes begin with the word “Note” and are set apart from the normative
text with <code>class="note"</code>, like this:
</p>
<p class="note">Note, this is an informative note.</p>
<h3 id="conformance-classes">Conformance Classes</h3>
<p>Conformance to CSS Fragmentation is defined for three conformance classes:</p>
<dl>
<dt><dfn title="style sheet!!as conformance class">style sheet</dfn></dt>
<dd>
A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS style sheet</a>.</dd>
<dt><dfn>renderer</dfn></dt>
<dd>
A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a> that interprets
the semantics of a style sheet and renders documents that use them.</dd>
<dt><dfn id="authoring-tool">authoring tool</dfn></dt>
<dd>
A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a> that writes
a style sheet.</dd>
</dl>
<p>
A style sheet is conformant to CSS Fragmentation if all of its declarations
that use properties defined in this module have values that are valid according
to the generic CSS grammar and the individual grammars of each property as given
in this module.
</p>
<p>
A renderer is conformant to CSS Fragmentation if, in addition to interpreting
the style sheet as defined by the appropriate specifications, it supports all the
features defined by CSS Fragmentation by parsing them correctly and rendering
the document accordingly. However, the inability of a UA to correctly render a document
due to limitations of the device does not make the UA non-conformant. (For example,
a UA is not required to render color on a monochrome monitor.)
</p>
<p>
An authoring tool is conformant to CSS Fragmentation if it writes style
sheets that are syntactically correct according to the generic CSS grammar and the
individual grammars of each feature in this module, and meet all other conformance
requirements of style sheets as described in this module.
</p>
<h3 id="partial">
Partial Implementations</h3>
<p>
So that authors can exploit the forward-compatible parsing rules to assign fallback
values, CSS renderers <strong>must</strong> treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">
ignore as appropriate</a>) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of support. In
particular, user agents <strong>must not</strong> selectively ignore unsupported
component values and honor supported values in a single multi-value property declaration:
if any value is considered invalid (as unsupported values must be), CSS requires
that the entire declaration be ignored.
</p>
<h3 id="experimental">
Experimental Implementations</h3>
<p>
To avoid clashes with future CSS features, the CSS2.1 specification reserves a <a
href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed syntax</a>
for proprietary and experimental extensions to CSS.</p>
<p>
Prior to a specification reaching the Candidate Recommendation stage in the W3C
process, all implementations of a CSS feature are considered experimental. The CSS
Working Group recommends that implementations use a vendor-prefixed syntax for such
features, including those in W3C Working Drafts. This avoids incompatibilities with
future changes in the draft.
</p>
<h3 id="testing">
Non-Experimental Implementations</h3>
<p>
Once a specification reaches the Candidate Recommendation stage, non-experimental
implementations are possible, and implementors should release an unprefixed implementation
of any CR-level feature they can demonstrate to be correctly implemented according
to spec.
</p>
<p>
To establish and maintain the interoperability of CSS across implementations, the
CSS Working Group requests that non-experimental CSS renderers submit an implementation
report (and, if necessary, the testcases used for that implementation report) to
the W3C before releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS Working Group.
</p>
<p>
Further information on submitting testcases and implementation reports can be found
from on the CSS Working Group's website at <a href="http://www.w3.org/Style/CSS/Test/">
http://www.w3.org/Style/CSS/Test/</a>. Questions should be directed to the <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">
public-css-testsuite@w3.org</a> mailing list.
</p>
<h3 id="cr-exit-criteria">
CR Exit Criteria</h3>
<p>
For this specification to be advanced to Proposed Recommendation, there must be
at least two independent, interoperable implementations of each feature. Each feature
may be implemented by a different set of products, there is no requirement that
all features be implemented by a single product. For the purposes of this criterion,
we define the following terms:
</p>
<dl>
<dt>independent </dt>
<dd>
each implementation must be developed by a different party and cannot share, reuse,
or derive from code used by another qualifying implementation. Sections of code
that have no bearing on the implementation of this specification are exempt from
this requirement.</dd>
<dt>interoperable</dt>
<dd>
passing the respective test case(s) in the official CSS test suite, or, if the implementation
is not a Web browser, an equivalent test. Every relevant test in the test suite
should have an equivalent test created if such a user agent (UA) is to be used to
claim interoperability. In addition if such a UA is to be used to claim interoperability,
then there must one or more additional UAs which can also pass those equivalent
tests in the same way for the purpose of interoperability. The equivalent tests
must be made publicly available for the purposes of peer review.
</dd>
<dt>implementation</dt>
<dd>
a user agent which:
<ol class="inline">
<li>implements the specification.</li>
<li>is available to the general public. The implementation may be a shipping product
or other publicly available version (i.e., beta version, preview release, or “nightly
build”). Non-shipping product releases must have implemented the feature(s) for
a period of at least one month in order to demonstrate stability.</li>
<li>is not experimental (i.e., a version specifically designed to pass the test suite
and is not intended for normal usage going forward).</li>
</ol>
</dd>
</dl>
<p>
The specification will remain Candidate Recommendation for at least six months.
</p>
<h2 class="no-num" id="acknowledgments">
Acknowledgments</h2>
<p>
The editors would like to extend special thanks to
Michael Day,
and the former [[CSS3PAGE]] editors
Jim Bigelow (HP),
Melinda Grant (HP),
Håkon Wium Lie (Opera),
and
Jacob Refstrup (HP)
for their contributions to this specification.
</p>
<h2 class="no-num" id="references">References</h2>
<h3 class="no-num" id="normative-references">Normative references</h3>
<!--normative-->
<h3 class="no-num" id="other-references">Other references</h3>
<!--informative-->
<h2 class="no-num" id="index">Index</h2>