forked from w3c/csswg-drafts
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathOverview.html
More file actions
executable file
·4807 lines (3803 loc) · 186 KB
/
Copy pathOverview.html
File metadata and controls
executable file
·4807 lines (3803 loc) · 186 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://dublincore.org/documents/2008/08/04/dc-html/ http://www.w3.org/2006/03/hcard">
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Positioned Layout Module Level 3</title>
<link href="http://purl.org/dc/terms/" rel=schema.dcterms>
<link href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=dcterms.rights>
<meta content="CSS Positioned Layout Module Level 3" name=dcterms.title>
<meta content=text name=dcterms.type>
<meta content=2014-01-04 name=dcterms.date>
<meta content="Rossen Atanassov" name=dcterms.creator>
<meta content="Arron Eicholz" name=dcterms.creator>
<meta content=W3C name=dcterms.publisher>
<meta content="http://dev.w3.org/csswg/css3-positioning/"
name=dcterms.identifier>
<link href=default.css rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<style type="text/css">
.cb-example-table
{
border: 1px solid;
border-collapse: collapse;
font-size: smaller;
text-align: center;
margin: 0 2em;
}
.cb-example-table td, .cb-example-table th
{
border: 1px solid;
padding: 0 3px;
}
code span, td span
{
white-space: nowrap;
}
.relationship-table
{
border: 1px solid;
border-collapse: collapse;
font-size: smaller;
margin: 0 6em;
}
.relationship-table th
{
text-align: center;
white-space:nowrap;
}
.relationship-table td, .relationship-table th
{
border: 1px solid;
padding: 0 3px;
}
h2 dfn, h3 dfn
{
font: inherit;
}
table.lpr
{
border-collapse: collapse;
}
.lpr td, .lpr th
{
border: 1px solid;
text-align: center;
}
.lpr col
{
background: rgb(221, 238, 255);
border-right: 2px solid;
font-weight: bold;
padding: 0 5px;
}
.lpr div
{
font-weight: bold;
-moz-transform: rotate(-90deg) translatex(-0.5em);
-ms-transform: rotate(-90deg) translatex(-0.5em);
-o-transform: rotate(-90deg) translatex(-0.5em);
-webkit-transform: rotate(-90deg) translatex(-0.5em);
transform: rotate(-90deg) translatex(-0.5em);
width: 1.5em;
}
.lpr th:first-child
{
border: none;
background: white;
}
.lpr tbody td:first-child
{
font-weight: normal;
}
.lpr thead td
{
background: rgb(238, 238, 238);
font-weight: bold;
padding: 0 5px;
vertical-align: bottom;
white-space: nowrap;
}
.lpr thead tr:nth-child(2)
{
border-bottom: 2px solid;
}
</style>
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS Positioned Layout Module Level 3</h1>
<h2 class="no-num no-toc">Editor's Draft 4 January 2014</h2>
<dl>
<dt>This version:
<dd><a
href="http://dev.w3.org/csswg/css3-positioning/">http://dev.w3.org/csswg/css3-positioning/</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-positioning/">http://www.w3.org/TR/css3-positioning/</a>
<dt>Editor's draft:
<dd><a
href="http://dev.w3.org/csswg/css3-positioning/">http://dev.w3.org/csswg/css3-positioning/</a>
<dt>Previous version:
<dd><a
href="http://www.w3.org/TR/2012/WD-css3-positioning-20120207/">http://www.w3.org/TR/2012/WD-css3-positioning-20120207/</a>
<dt>Issues List
<dd><a
href="http://wiki.csswg.org/spec/css3-positioning/">http://wiki.csswg.org/spec/css3-positioning/</a>
<dt>Feedback:
<dd><a
href="mailto:www-style@w3.org?subject=%5Bcss-position%5D%20feedback">www-style@w3.org</a>
with subject line “<kbd>[css-position] <var>… message topic
…</var></kbd>” (<a
href="http://lists.w3.org/Archives/Public/www-style/"
rel=discussion>archives</a>)
<dt>Editors:
<dd class=vcard><span class=fn>Rossen Atanassov</span>, <span
class=org>Microsoft Corporation</span>
<dd class=vcard><span class=fn>Arron Eicholz</span>, <span
class=org>Microsoft Corporation</span>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2014 <a href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><abbr
title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
href="http://www.ercim.eu/"><abbr
title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
<a href="http://www.keio.ac.jp/">Keio</a>, <a
href="http://ev.buaa.edu.cn/">Beihang</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p> CSS is a language for describing the rendering of structured documents
(such as HTML and XML) on screen, on paper, in speech, etc. This module
contains the features of CSS level 3 relating to positioning and stacking
of elements. It includes and extends the functionality of CSS level 2 <a
href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which
builds on CSS level 1 <a href="#ref-CSS1"
rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>. The main extensions compared to
level 2 are the ability to position elements based on CSS Region boxes,
and the ability to specify a different containing blocks for elements.
<p> Other kinds of layout, such as tables, "floating" boxes, ruby
annotations, grid layouts, columns and basic handling of normal "flow"
content, are described in other modules. Also, the layout of text inside
each line is defined elsewhere.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss3-positioning%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“css3-positioning” in the subject, preferably like this:
“[<!---->css3-positioning<!---->] <em>…summary of comment…</em>”
<p>This document was produced by the <a
href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of
the <a href="http://www.w3.org/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
2004 W3C Patent Policy</a>. W3C maintains a <a
href="http://www.w3.org/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
6 of the W3C Patent Policy</a>.</p>
<!--end-status-->
<!--<p>The following features are at risk:</p>-->
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#intro"><span class=secno>1. </span>Introduction</a>
<ul class=toc>
<li><a href="#placement"><span class=secno>1.1. </span>Module
Interactions</a>
<li><a href="#values"><span class=secno>1.2. </span>Values</a>
</ul>
<li><a href="#viewport"><span class=secno>2. </span>The Viewport</a>
<li><a href="#containing-block"><span class=secno>3. </span>Containing
Blocks</a>
<ul class=toc>
<li><a href="#def-containing-blocks"><span class=secno>3.1.
</span>Definition of containing block</a>
</ul>
<li><a href="#normal-flow"><span class=secno>4. </span>Normal Flow</a>
<li><a href="#floats"><span class=secno>5. </span>Floats</a>
<li><a href="#positioning-schemes"><span class=secno>6. </span>Positioning
schemes</a>
<ul class=toc>
<li><a href="#relative-positioning"><span class=secno>6.1.
</span>Relative positioning</a>
<li><a href="#sticky-positioning"><span class=secno>6.2. </span><span
class=index-def id=sticky-positioning0
title="sticky positioning|sticky position|stickily positioned">Sticky
positioning</span></a>
<li><a href="#absolute-positioning"><span class=secno>6.3. </span><span
class=index-def id=absolute-positioning0
title="absolute positioning|absolute position|absolutely positioned">Absolute
positioning</span></a>
<li><a href="#center-positioning"><span class=secno>6.4. </span><span
class=index-def id=center-positioning0
title="center positioning|center position|center positioned">Center
positioning</span></a>
<li><a href="#page-positioning"><span class=secno>6.5. </span><span
class=index-def id=page-positioning0
title="page positioning|page position|page positioned">Page
positioning</span></a>
<li><a href="#fixed-positioning"><span class=secno>6.6. </span><span
class=index-def id=fixed-positioning0
title="fixed positioning|fixed position|fixed positioned">Fixed
positioning</span></a>
<li><a href="#position-property"><span class=secno>6.7. </span>Choosing
a positioning scheme: ‘<code class=property>position</code>’
property</a>
<li><a href="#box-offsets-trbl"><span class=secno>6.8. </span>Box
offsets: ‘<code class=property>top</code>’, ‘<code
class=property>right</code>’, ‘<code
class=property>bottom</code>’, ‘<code
class=property>left</code>’</a>
<li><a href="#logical-box-offsets-beaso"><span class=secno>6.9.
</span>Logical box offsets: ‘<code
class=property>offset-before</code>’, ‘<code
class=property>offset-end</code>’, ‘<code
class=property>offset-after</code>’, ‘<code
class=property>offset-start</code>’, and ‘<code
class=property>offset</code>’</a>
</ul>
<li><a href="#position-root"><span class=secno>7. </span>Choosing a
different containing block for positioning</a>
<ul class=toc>
<li><a href="#attachment-properties"><span class=secno>7.1.
</span>Attachment properties: ‘<code
class=property>attachment-type</code>’, ‘<code
class=property>attachment-top</code>’, ‘<code
class=property>attachment-right</code>’, ‘<code
class=property>attachment-bottom</code>’, ‘<code
class=property>attachment-left</code>’, and ‘<code
class=property>attachment</code>’</a>
</ul>
<li><a href="#size-and-position-details"><span class=secno>8.
</span>Sizing and positioning details</a>
<ul class=toc>
<li><a href="#abs-non-replaced-width"><span class=secno>8.1. </span>The
width of absolute, page or fixed positioned, non-replaced elements</a>
<li><a href="#center-non-replaced-width"><span class=secno>8.2.
</span>The width of a center positioned, non-replaced elements</a>
<li><a href="#abs-replaced-width"><span class=secno>8.3. </span>The
width of absolute, page or fixed positioned, replaced elements</a>
<li><a href="#center-replaced-width"><span class=secno>8.4. </span>The
width of a center positioned, replaced elements</a>
<li><a href="#abs-non-replaced-height"><span class=secno>8.5. </span>The
height of absolute, page or fixed positioned, non-replaced elements</a>
<li><a href="#center-non-replaced-height"><span class=secno>8.6.
</span>The height of a center positioned, non-replaced elements</a>
<li><a href="#abs-replaced-height"><span class=secno>8.7. </span>The
height of absolute, page or fixed positioned, replaced elements</a>
<li><a href="#center-replaced-height"><span class=secno>8.8. </span>The
height of a center positioned, replaced elements</a>
<li><a href="#root-height"><span class=secno>8.9. </span>‘<code
class=property>Auto</code>’ heights for block formatting context
roots</a>
</ul>
<li><a href="#dis-pos-flo"><span class=secno>9. </span>Relationships
between ‘<code class=property>display</code>’, ‘<code
class=property>position</code>’, and ‘<code
class=property>float</code>’</a>
<li><a href="#comparison"><span class=secno>10. </span>Comparison of
normal flow, floats, and positioning</a>
<ul class=toc>
<li><a href="#comp-normal-flow"><span class=secno>10.1. </span>Normal
flow</a>
<li><a href="#comp-relpos"><span class=secno>10.2. </span>Relative
positioning</a>
<li><a href="#comp-floating"><span class=secno>10.3. </span>Floating a
box</a>
<li><a href="#comp-abspos"><span class=secno>10.4. </span>Absolute
positioning</a>
<li><a href="#comp-pagepos"><span class=secno>10.5. </span>Page
positioning</a>
</ul>
<li><a href="#layered-presentation"><span class=secno>11. </span>Layered
presentation</a>
<li><a href="#det-stacking-context"><span class=secno>12. </span>Detailed
stacking context</a>
<ul class=toc>
<li><a href="#definitions-stack"><span class=secno>12.1.
</span>Definitions</a>
<li><a href="#painting-order"><span class=secno>12.2. </span>Painting
order</a>
<li><a href="#stacking-notes"><span class=secno>12.3. </span>Notes</a>
</ul>
<li><a href="#conformance"><span class=secno>13. </span>Conformance</a>
<ul class=toc>
<li><a href="#conventions"><span class=secno>13.1. </span>Document
Conventions</a>
<li><a href="#conformance-classes"><span class=secno>13.2.
</span>Conformance Classes</a>
<li><a href="#partial"><span class=secno>13.3. </span>Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>13.4. </span>Experimental
Implementations</a>
<li><a href="#testing"><span class=secno>13.5. </span>Non-Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>13.6. </span>CR Exit
Criteria</a>
</ul>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<ul class=toc>
<li class=no-num><a href="#normative-references">Normative
references</a>
<li class=no-num><a href="#other-references">Other references</a>
</ul>
<li class=no-num><a href="#index">Index</a>
<li class=no-num><a href="#property-index">Property index</a>
</ul>
<!--end-toc-->
<h2 id=intro><span class=secno>1. </span>Introduction</h2>
<p><em>This section is not normative.</em>
<p> CSS assumes the document layout is modeled as a tree of elements. The
unique element that has no parent is called the root element. This module
describes how any of the elements from the tree of elements can be
arranged independent of document order (i.e. taken out of "flow"). With a
positioned element the element may be placed anywhere within the content
not specifically respecting the tree of elements order.
<p> In <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>,
the visual formatting model explained how each element in the document
tree generates zero or more boxes according to the box model. This module
further explains and extends the positioning scheme. The layout of these
boxes is governed by:
<ul>
<li><a href="http://www.w3.org/TR/CSS2/box.html#box-dimensions">box
dimensions</a> and <a
href="http://www.w3.org/TR/CSS2/visuren.html#box-gen">type</a>.
<li>positioning scheme (<a href="#normal-flow0">normal flow</a>, float,
and absolute positioning).
<li>relationships between elements in the <a
href="http://www.w3.org/TR/CSS2/conform.html#doctree">document tree</a>.
<li>external information (e.g., viewport size, intrinsic dimensions of
images, etc.).
</ul>
<p>The properties defined in this module apply to both continuous media and
paged media.
<h3 id=placement><span class=secno>1.1. </span>Module Interactions</h3>
<p>This module replaced and extends the positioning scheme features defined
in <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
sections:
<ul>
<li><a href="http://www.w3.org/TR/CSS2/visuren.html#viewport">9.1.1 The
viewport</a>
<li><a
href="http://www.w3.org/TR/CSS2/visuren.html#containing-block">9.1.2
Containing blocks</a>
<li><a
href="http://www.w3.org/TR/CSS2/visuren.html#positioning-scheme">9.3
Positioning schemes</a>
<li><a
href="http://www.w3.org/TR/CSS2/visuren.html#relative-positioning">9.4.3
Relative positioning</a>
<li><a
href="http://www.w3.org/TR/CSS2/visuren.html#absolute-positioning">9.6
Absolute positioning</a>
<li><a href="http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">9.7
Relationships between ‘<code class=property>display</code>’, ‘<code
class=property>position</code>’, and ‘<code
class=property>float</code>’</a>
<li><a href="http://www.w3.org/TR/CSS2/visuren.html#comparison">9.8
Comparison of normal flow, floats, and absolute positioning</a>
<li><a href="http://www.w3.org/TR/CSS2/visuren.html#layers">9.9 Layered
presentation</a>
<li><a
href="http://www.w3.org/TR/CSS2/visudet.html#containing-block-details">10.1
Definition of "containing block"</a>
<li><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">10.3.7
Absolutely positioned, non-replaced elements</a>
<li><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-width">10.3.8
Absolutely positioned, replaced elements</a>
<li><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">10.6.4
Absolutely positioned, non-replaced elements</a>
<li><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-height">10.6.5
Absolutely positioned replaced elements</a>
<li><a href="http://www.w3.org/TR/CSS2/visudet.html#root-height">10.6.7
‘<code class=css>Auto</code>’ heights for block formatting context
roots</a>
<li><a href="http://www.w3.org/TR/CSS2/zindex.html">Appendix E. Elaborate
description of Stacking Contexts</a>
</ul>
<!-- End section" Module Interactions -->
<h3 id=values><span class=secno>1.2. </span>Values</h3>
<p>This specification follows the CSS property definition conventions from
<a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<p> Value types not defined in this specification are defined in CSS Level
2 Revision 1 <a href="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS modules may expand
the definitions of these value types: for example <a href="#ref-CSS3VAL"
rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>, when combined with this
module, expands the definition of the <length> value type as used in
this specification.
<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>
<!-- End section: Values -->
<!-- End section Introduction -->
<h2 id=viewport><span class=secno>2. </span>The <dfn id=viewport0
title=viewport>Viewport</dfn></h2>
<p> User agents for <a
href="http://www.w3.org/TR/CSS2/media.html#continuous-media-group">continuous
media</a> generally offer users a viewport (a window or other viewing area
on the screen) through which users consult a document. User agents may
change the document's layout when the viewport is resized (see the
<span>initial containing block</span>).
<p> When the viewport is smaller than the area of the canvas on which the
document is rendered, the user agent may offer a scrolling mechanism.
There is at most one viewport per <a
href="http://www.w3.org/TR/CSS2/intro.html#canvas">canvas</a>, but user
agents may render to more than one canvas (i.e., provide different views
of the same document).</p>
<!-- End section: The Viewport -->
<h2 id=containing-block><span class=secno>3. </span>Containing Blocks</h2>
<p> In CSS, many box positions and sizes are calculated with respect to the
edges of a rectangular box called a containing block. In general,
generated boxes act as containing blocks for descendant boxes; we say that
a box "establishes" the containing block for its descendants. The phrase
"a box's containing block" means "the containing block in which the box
lives," not the one it generates.
<p> Each box is given a position with respect to its containing block, but
it is not confined by this containing block; it may <a
href="http://www.w3.org/TR/CSS2/visufx.html#overflow">overflow</a>.
<h3 id=def-containing-blocks><span class=secno>3.1. </span>Definition of
<dfn id=containing-block0>containing block</dfn></h3>
<p> The position and size of an element's box(es) are sometimes computed
relative to a certain rectangle, called the containing block of the
element. The containing block of a <a href="#static"
title="'position'!!''static''">‘<code class=css>static</code>’</a> or
<a href="#relative" title="'position'!!''relative''">‘<code
class=css>relative</code>’</a> element is defined in the Box Model <a
href="#ref-CSS3BOX" rel=biblioentry>[CSS3BOX]<!--{{!CSS3BOX}}--></a>. The
containing block of a <a href="#sticky"
title="'position'!!'sticky'">‘<code class=property>sticky</code>’</a>
element is the same as for a <a href="#relative"
title="'position'!!''relative''">‘<code class=css>relative</code>’</a>
element. For <a href="#fixed" title="'position'!!''fixed''">‘<code
class=css>fixed</code>’</a>, <a href="#absolute"
title="'position'!!''absolute''">‘<code
class=css>absolute</code>’</a>, <a href="#center"
title="'position'!!''center''">‘<code class=css>center</code>’</a> and
<a href="#page" title="'position'!!''page''">‘<code
class=css>page</code>’</a>, it is defined as follows:
<ol>
<li> If the element has <a href="#fixed"
title="'position'!!''fixed''">‘<code class=css>positioned:
fixed</code>’</a>, the containing block is established by the <a
href="#viewport0">viewport</a> in the case of continuous media or the
page area in the case of paged media.
<li> If the element has <a href="#absolute"
title="'position'!!''absolute''">‘<code class=css>position:
absolute</code>’</a>, the containing block is established by the
nearest ancestor with a ‘<a href="#position"><code
class=property>position</code></a>’ other than <a href="#static"
title="'position'!!''static''">‘<code class=css>static</code>’</a>,
in the following way:
<ol>
<li> In the case that the ancestor is block-level, the containing block
is formed by the padding edge of the ancestor.
<li> In the case that the ancestor is inline-level, the containing block
depends on the ‘<code class=property>direction</code>’ property of
the ancestor:
<ol>
<li> If the ‘<code class=property>direction</code>’ is ‘<code
class=css>ltr</code>’, the top and left of the containing block are
the top and left content edges of the first box generated by the
ancestor, and the bottom and right are the bottom and right content
edges of the last box of the ancestor.
<li> If the ‘<code class=property>direction</code>’ is ‘<code
class=css>rtl</code>’, the top and right are the top and right
edges of the first box generated by the ancestor, and the bottom and
left are the bottom and left content edges of the last box of the
ancestor.
<p class=note> Note, in some cases when a line wraps it may seem as
if the left and right positions are swapped.
</ol>
</ol>
<li> I the element has <a href="#center"
title="'position'!!''center''">‘<code class=css>position:
center</code>’</a>, the containing block is established by the nearest
ancestor with a ‘<a href="#position"><code
class=property>position</code></a>’ other than <a href="#static"
title="'position'!!''static''">‘<code class=css>static</code>’</a>,
in the following way:
<ol>
<li> In the case that the ancestor is block-level, the containing block
is formed by the padding edge of the ancestor.
<li> In the case that the ancestor is inline-level, the containing block
depends on the content edges of the boxes generated by the ancestor.
The top most, right most, bottom most and left most content edges of
all the boxes generated by the ancestor determine the edges for the
containing block.
</ol>
<li> If the element has <a href="#page"
title="'position'!!''page''">‘<code class=css>position:
page</code>’</a> and the element is within paged media or within a
rerion, the containing block is the <span>initial containing
block</span>. This is the <a href="#viewport0">viewport</a> or the page
area when in paged media. In the case of CSS Regions <a
href="#ref-CSS3-REGIONS"
rel=biblioentry>[CSS3-REGIONS]<!--{{!CSS3-REGIONS}}--></a> this is the
individual region. When a page positioned element is not in paged media
or a region the containing block is determined per the absolute
positioning model.
<li>If there is no such ancestor, the containing block is the
<span>initial containing block</span>.
</ol>
<p> In paged media, an absolutely positioned element is positioned relative
to its containing block ignoring any page breaks (as if the document were
continuous). The element may subsequently be broken over several pages.
<p> For absolutely positioned content that resolves to a position on a page
other than the page being laid out (the current page), or resolves to a
position on the current page which has already been rendered for printing,
printers may place the content:
<ul>
<li>on the current page,
<li>on a subsequent page, or
<li>may omit it altogether.
</ul>
<p class=note> Note, a block-level element that is split over several pages
may have a different width on each page and that there may be
device-specific limits.
<div class=example>
<p>With no positioning, the containing blocks (C.B.) in the following
document:
<pre><code class=html>
<!DOCTYPE html>
<html>
<head>
<title>Illustration of containing blocks</title>
</head>
<body id="body">
<div id="div1">
<p id="p1">This is text in the first paragraph...</p>
<p id="p2">This is text <em id="em1"> in the
<strong id="strong1">second</strong> paragraph.</em></p>
</div>
</body>
</html>
</code></pre>
<p>are established as follows:
<table class=cb-example-table>
<tbody>
<tr>
<th>For box generated by
<th>C.B. is established by
<tr>
<td>html
<td>initial C.B. (UA-dependent)
<tr>
<td>body
<td>html
<tr>
<td>div1
<td>body
<tr>
<td>p1
<td>div1
<tr>
<td>p2
<td>div1
<tr>
<td>em1
<td>p2
<tr>
<td>strong1
<td>p2
</table>
<p>If we position "div1":
<pre><code class=css>
#div1 { position: absolute; left: 50px; top: 50px }
</code></pre>
<p> its containing block is no longer "body"; it becomes the <span>initial
containing block</span> (since there are no other positioned ancestor
boxes).
<p>If we position "em1" as well:
<pre><code class=css>
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
</code></pre>
<p>the table of containing blocks becomes:
<table class=cb-example-table>
<tbody>
<tr>
<th>For box generated by
<th>C.B. is established by
<tr>
<td>html
<td>initial C.B. (UA-dependent)
<tr>
<td>body
<td>html
<tr>
<td>div1
<td>initial C.B.
<tr>
<td>p1
<td>div1
<tr>
<td>p2
<td>div1
<tr>
<td>em1
<td>div1
<tr>
<td>strong1
<td>em1
</table>
<p> By positioning "em1", its containing block becomes the nearest
positioned ancestor box (i.e., that generated by "div1").
</div>
<!-- End section: Containing blocks -->
<h2 id=normal-flow><span class=secno>4. </span><dfn id=normal-flow0
title="normal flow">Normal Flow</dfn></h2>
<p> Boxes in the normal flow belong to a formatting context, which may be
block or inline, but not both simultaneously. See the CSS Basic Box Model
module <a href="#ref-CSS3BOX"
rel=biblioentry>[CSS3BOX]<!--{{!CSS3BOX}}--></a> for further details about
<a href="http://www.w3.org/TR/CSS2/visuren.html#normal-flow">normal
flow</a>.</p>
<!-- End section: Normal Flow -->
<h2 id=floats><span class=secno>5. </span><dfn id=floats0
title="float|floated box|floating box">Floats</dfn></h2>
<p> A float is a box that is positioned within content, often left or right
on the current line. The most interesting characteristic of a float (or
"floated" or "floating" box) is that inline-level content may flow along
its side (or be prohibited from doing so by the ‘<code
class=property>clear</code>’ property) or around the floated box. See
the CSS 2.1 <a href="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a><!--CSS Floats module [[!CSS3FLOATS]]-->
for details about <a
href="http://www.w3.org/TR/CSS2/visuren.html#floats">floating boxes</a>.</p>
<!-- End section: Floats -->
<h2 id=positioning-schemes><span class=secno>6. </span><dfn
id=positioning-schemes0 title="positioning schemes">Positioning
schemes</dfn></h2>
<p>In CSS, a box may be laid out according to three positioning schemes:
<ol>
<li> Normal flow<br>
In CSS, <a href="#normal-flow0">normal flow</a> includes <a
href="http://www.w3.org/TR/CSS2/visuren.html#block-formatting">block
formatting</a> of block-level boxes, <a
href="http://www.w3.org/TR/CSS2/visuren.html#inline-formatting">inline
formatting</a> of inline-level boxes, and <a
href="#relative-positioning0" title="relative positioning">relative</a>
and <span title="sticky positioning">sticky</span> positioning of
block-level and inline-level boxes.
<li> Floats<br>
In the <a href="#floats0" title=floats>float</a> model, a box is first
laid out according to the <a href="#normal-flow0">normal flow</a>, then
taken out of the flow and positioned, typically to the left or right.
Content may flow along the side of a <a href="#floats0"
title=floats>float</a>.
<li> Absolute positioning<br>
In the absolute positioning model, a box is removed from the <a
href="#normal-flow0">normal flow</a> entirely (it has no impact on later
siblings) and assigned a position with respect to a <a
href="#containing-block0">containing block</a>.
</ol>
<p> An element is called <em>out-of-flow</em> if it is <a href="#floats0"
title=floats>floated</a>, absolutely positioned, or is the root element.
An element is called <em>in-flow</em> if it is not out-of-flow. The
<em>flow of an element</em> A is the set consisting of A and all in-flow
elements whose nearest out-of-flow ancestor is A.
<h3 id=relative-positioning><span class=secno>6.1. </span><dfn
id=relative-positioning0
title="relative positioning|relative position|relatively positioned">Relative
positioning</dfn></h3>
<p> Once a box has been laid out according to the <a
href="#normal-flow0">normal flow</a> or <a href="#floats0"
title=floats>floated</a>, it may be offset relative to this position. This
is called relative positioning. Offsetting a box (B1) in this way has no
effect on the box (B2) that follows: B2 is given a position as if B1 were
not offset and B2 is not re-positioned after B1's offset is applied. This
implies that relative positioning may cause boxes to overlap. However, if
relative positioning causes an ‘<code class=css>overflow: auto</code>’
or ‘<code class=css>overflow: scroll</code>’ box to have overflow, the
user agent must allow the user to access this content (at its offset
position), which, through the creation of a scrolling mechanism, may
affect layout.
<p> A relatively positioned box keeps its <a href="#normal-flow0">normal
flow</a> size, including line breaks and the space originally reserved for
it.
<p> A relatively positioned box establishes a new a new <a
href="#containing-block0">containing block</a> for absolutely positioned
descendants. (This is a common use of relatively positioned boxes.) The
section on <a href="#containing-block0"
title="containing block">containing blocks</a> explains when a relatively
positioned box establishes a new <a href="#containing-block0">containing
block</a>.
<p> For relatively positioned elements, ‘<a href="#left"><code
class=property>left</code></a>’ and ‘<a href="#right"><code
class=property>right</code></a>’ move the box(es) horizontally, without
changing their size. ‘<a href="#left"><code
class=property>Left</code></a>’ moves the boxes to the right, and ‘<a
href="#right"><code class=property>right</code></a>’ moves them to the
left. Since boxes are not split or stretched as a result of ‘<a
href="#left"><code class=property>left</code></a>’ or ‘<a
href="#right"><code class=property>right</code></a>’, the used values
are always: left = -right.
<p> If both ‘<a href="#left"><code class=property>left</code></a>’ and
‘<a href="#right"><code class=property>right</code></a>’ are ‘<code
class=css>auto</code>’ (their initial values), the used values are
‘<code class=css>0</code>’ (i.e., the boxes stay in their original
position).
<p> If ‘<a href="#left"><code class=property>left</code></a>’ is
‘<code class=css>auto</code>’, its used value is minus the value of
‘<a href="#right"><code class=property>right</code></a>’ (i.e., the
boxes move to the left by the value of ‘<a href="#right"><code
class=property>right</code></a>’).
<p>If ‘<a href="#right"><code class=property>right</code></a>’ is
specified as ‘<code class=css>auto</code>’, its used value is minus
the value of ‘<a href="#left"><code class=property>left</code></a>’.
<p> If neither ‘<a href="#left"><code class=property>left</code></a>’
nor ‘<a href="#right"><code class=property>right</code></a>’ is
‘<code class=css>auto</code>’, the position is over-constrained, and
one of them has to be ignored. If the ‘<code
class=property>direction</code>’ property of the <a
href="#containing-block0">containing block</a> is ‘<code
class=css>ltr</code>’, the value of ‘<a href="#left"><code
class=property>left</code></a>’ wins and ‘<a href="#right"><code
class=property>right</code></a>’ becomes -‘<a href="#left"><code
class=property>left</code></a>’. If ‘<code
class=property>direction</code>’ of the containing block is ‘<code
class=css>rtl</code>’, ‘<a href="#right"><code
class=property>right</code></a>’ wins and ‘<a href="#left"><code
class=property>left</code></a>’ is ignored.
<div class=example>
<p>The following three rules are equivalent:
<pre><code class=css>
div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
</code></pre>
</div>
<p> The ‘<a href="#top"><code class=property>top</code></a>’ and ‘<a
href="#bottom"><code class=property>bottom</code></a>’ properties move
relatively positioned element(s) up or down without changing their size.
‘<a href="#top"><code class=property>Top</code></a>’ moves the boxes
down, and ‘<a href="#bottom"><code class=property>bottom</code></a>’
moves them up. Since boxes are not split or stretched as a result of ‘<a
href="#top"><code class=property>top</code></a>’ or ‘<a
href="#bottom"><code class=property>bottom</code></a>’, the used values
are always: top = -bottom.
<p>If ‘<a href="#top"><code class=property>top</code></a>’ and ‘<a
href="#bottom"><code class=property>bottom</code></a>’ are ‘<code
class=css>auto</code>’, their used values are both ‘<code
class=css>0</code>’.
<p> If one of them, ‘<a href="#top"><code
class=property>top</code></a>’ or ‘<a href="#bottom"><code
class=property>bottom</code></a>’, is ‘<code class=css>auto</code>’,