-
Notifications
You must be signed in to change notification settings - Fork 708
/
Copy pathOverview.bs
1811 lines (1539 loc) · 65.2 KB
/
Overview.bs
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 Positioned Layout Module Level 3
Status: WD
Work Status: Refining
Shortname: css-position
Level: 3
Group: csswg
ED: https://drafts.csswg.org/css-position-3/
TR: https://www.w3.org/TR/css-position-3/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
Former Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com, w3cid 49885
Former Editor: Arron Eicholz, Microsoft, arronei@microsoft.com, w3cid 37934
!Feedback: <a href="http://wiki.csswg.org/spec/css-position/">in Wiki</a>
Abstract: This module contains defines coordinate-based positioning and offsetting schemes of <a href="https://www.w3.org/TR/CSS/">CSS</a>: [=relative positioning=], [=sticky positioning=], [=absolute positioning=], and [=fixed positioning=].
Link Defaults: css-transforms-1 (property) transform
</pre>
<pre class=link-defaults>
spec:css-writing-modes-4; type:dfn; text:end
spec:css2; type:value; for:float; text:none
spec:css-align-3; type:value; for:align-self; text:center
spec:css2; type:dfn; text:stacking context
</pre>
<style type="text/css">
.switch > dt { font: inherit; }
.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;
}
#abspos-auto ul { list-style-type: none; margin: 0; }
#abspos-auto caption { caption-side: bottom; }
</style>
<pre class=anchors>
urlPrefix: https://www.w3.org/TR/CSS2/visuren.html; spec: CSS2;
url: #normal-flow; text: normal flow; type: dfn;
url: #x1; text: viewport; type: dfn;
urlPrefix: https://www.w3.org/TR/CSS2/visudet.html; spec: CSS2;
url: #static-position; text: static position; type: dfn;
url: #static-position; text: static-position containing block; type: dfn;
</pre>
<pre class=link-defaults>
spec: css-align-3; type: value; text: stretch
</pre>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
The CSS layout algorithms, by default,
size and position boxes in relation to each other
so that nothing overlaps.
This specification defines several ways to violate these assumptions when needed,
moving elements around in ways that can make them overlap other content:
* [=Relative positioning=], which visually shifts a box relative to its laid-out location.
* [=Sticky positioning=], which visually shifts a box relative to its laid-out location
in order to keep it visible when a scrollable ancestor
would otherwise scroll it out of sight.
* [=Absolute positioning=], which ignores normal layout entirely,
pulling the element [=out of flow=] and
positioning it relative to its [=containing block=]
with no regard for other content.
* [=Fixed positioning=], which absolutely positions the box
and affixes it to the viewport or page frame
so that it is always visible.
These [=positioning schemes=],
controlled by the 'position' property and the [=inset properties=],
are powerful
but easy to misuse.
With appropriate care,
they allow many interesting and useful layouts
that couldn't otherwise be achieved with standard layout rules;
without, they allow a page to be laid out in an unusable overlapping jumble of content.
<h3 id="placement">
Module Interactions</h3>
This module replaces and extends the [=positioning scheme=] features
defined in [[!CSS2]] sections:
<ul>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#containing-block">9.1.2 Containing blocks</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#positioning-scheme">9.3 Positioning schemes</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#relative-positioning">9.4.3 Relative positioning</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#absolute-positioning">9.6 Absolute positioning</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">9.7 Relationships between display, position, and float</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visuren.html#comparison">9.8 Comparison of normal flow, floats, and absolute positioning</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#containing-block-details">10.1 Definition of "containing block"</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">10.3.7 Absolutely positioned, non-replaced elements</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-replaced-width">10.3.8 Absolutely positioned, replaced elements</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">10.6.4 Absolutely positioned, non-replaced elements</a>
<li>
<a href="https://www.w3.org/TR/CSS2/visudet.html#abs-replaced-height">10.6.5 Absolutely positioned replaced elements</a>
</ul>
It also replaces and supersedes the 'inset*' property definitions in [[CSS-LOGICAL-1]]
([[CSS-LOGICAL-1#inset-properties]]).
<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 [[!CSS2]]
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="position-property">
Choosing A Positioning Scheme: 'position' property</h2>
<pre class="propdef">
Name: position
Value: static | relative | absolute | sticky | fixed
Initial: static
Applies to: all elements except table-column-group and table-column
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
The 'position' property determines which of the <dfn export lt="positioning scheme">positioning schemes</dfn>
is used to calculate the position of a box.
Values other than ''static'' make the box a <dfn export lt="positioned box|positioned">positioned box</dfn>,
and cause it to establish an <dfn export>absolute positioning containing block</dfn> for its descendants.
Values have the following meanings:
<dl dfn-for="position" dfn-type="value">
<dt><dfn>static</dfn>
<dd>
The box is not a [=positioned box=],
and is laid out according to the rules of its parent [=formatting context=].
The [=inset properties=] do not apply.
<dt><dfn>relative</dfn>
<dd>
The box is laid out as for ''static'',
then offset from the resulting position.
This offsetting is a purely visual effect,
and does not affect the size or position of any other box,
except insofar as it increases the [=scrollable overflow area=] of its ancestors.
This [=positioning scheme=] is called
<dfn export dfn for="" lt="relative position | relatively position | relatively-positioned | relatively positioned box" local-lt="relative|relatively">relative positioning</dfn>.
<dt><dfn>sticky</dfn>
<dd>
Identical to ''relative'',
except that its offsets are automatically adjusted
in reference to the nearest ancestor [=scroll container's=] [=scrollport=]
(as modified by the [=inset properties=])
in whichever axes the [=inset properties=] are not both ''top/auto'',
to try to keep the box in view within its [=containing block=]
as the user scrolls.
This [=positioning scheme=] is called
<dfn export dfn for="" lt="sticky position | sticky-positioned | sticky-positioned box" local-lt="sticky">sticky positioning</dfn>.
<dt><dfn>absolute</dfn>
<dd>
The box is taken [=out of flow=]
such that it has no impact on the size or position of its siblings and ancestors,
and does not participate in its parent’s [=formatting context=].
Instead, the box is positioned and sized solely in reference to
its [=absolute positioning containing block=],
as modified by the box's [=inset properties=],
see [[#abspos-layout]].
It can overlap [=in-flow=] content
or other [=absolutely positioned=] elements,
and is included in the [=scrollable overflow area=]
of the box that generates is [=containing block=].
This [=positioning scheme=] is called
<dfn export dfn for="" lt="absolute position | absolutely position | absolutely-positioned | absolutely positioned box | absolutely positioned element" local-lt="absolute|absolutely">absolute positioning</dfn>.
<dt><dfn>fixed</dfn>
<dd>
Same as ''absolute'',
except the box is positioned and sized relative to a [=fixed positioning containing block=]
(usually the [=viewport=] in [=continuous media=], or the [=page area=] in [=paged media=]).
The box’s position is fixed with respect to this reference rectangle:
when attached to the [=viewport=]
it does not move when the document is scrolled,
and when attached to the [=page area=]
is replicated on every page when the document is paginated.
This [=positioning scheme=] is called
<dfn export dfn for="" lt="fixed position | fixed-positioned | fixed-positioned box" local-lt="fixed">fixed positioning</dfn>
and is considered a subset of [=absolute positioning=].
<div class="example">
Authors may wish to specify ''fixed'' in a media-dependent way.
For instance, an author may want a box to remain at the top of the <a>viewport</a> on the screen,
but not at the top of each printed page.
The two specifications may be separated by using an <a href="https://www.w3.org/TR/CSS2/media.html#at-media-rule">'@media'</a> rule, as in:
<pre highlight=css>
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
</pre>
</div>
</dl>
A 'position' value of ''position/absolute'' or ''position/fixed''
[=blockifies=] the box,
causes 'float' to compute to ''float/none'',
and forces the box to [=establish an independent formatting context=].
<h3 id="def-cb">
Containing Blocks of Positioned Boxes</h3>
The [=containing block=] of a ''position/static'', ''position/relative'', or ''position/sticky'' [=box=]
is as defined by its [=formatting context=].
For ''position/fixed'' and ''position/absolute'' boxes,
it is defined as follows:
<dl class=switch>
<dt id="absolute-cb">If the box has ''position: absolute'':
<dd>
The [=containing block=] is established
by the nearest ancestor box that establishes an [=absolute positioning containing block=],
in the following way:
<dl class=switch>
<dt>If the ancestor is not an [=inline box=],
<dd>
the [=containing block=] is formed by the [=padding edge=] of the ancestor,
unless otherwise specified
(for example, see [[CSS-GRID-1#abspos-items]]).
<dt>If the ancestor is an [=inline box=],
<dd>
the [=containing block=] is formed by
the [=block-start=] and [=inline-start=] [=content edges=] of the first [=box fragment=] of the ancestor,
and the [=block-end=] and [=inline-end=] [=content edges=] of the last [=box fragment=] of the ancestor.
If there are multiple fragments on the same line
(e.g. due to [[css-writing-modes-3#bidi-box-model|bidi reordering]]),
take the [=start=]most/[=end=]most fragment
as the first/last fragment.
However, since a [=containing block=] cannot have negative area:
if the [=block-end=] edge would be [=start=]wards of the [=block-start=] edge,
then the [=block-end=] edge of the [=containing block=]
instead uses the [=block-start=] edge;
and analogously for the [=inline axis=].
(Thus the [=containing block=] will have zero height and/or width as a result.)
</dl>
If no ancestor establishes one,
the [=absolute positioning containing block=] is
the [=initial containing block=].
Note: Properties that can cause a box to establish
an [=absolute positioning containing block=] include
'position', 'transform', 'will-change', 'contain'…
<dt id="fixed-cb">If the box has ''position: fixed'':
<dd>
The [=containing block=] is established
by the nearest ancestor box that establishes an <dfn export>fixed positioning containing block</dfn>,
with the bounds of the [=containing block=]
determined identically to the [=absolute positioning containing block=].
Note: Properties that can cause a box to establish
a [=fixed positioning containing block=] include
'transform', 'will-change', 'contain'…
If no ancestor establishes one,
the [=fixed positioning containing block=] is:
* in [=continuous media=],
the <a>viewport</a>;
as a result, [=fixed=] boxes do not move when the document is scrolled.
Note: In this respect, they are similar to
[[css-backgrounds-3#background-attachment|fixed background images]]
(''background-attachment: fixed'').
* in [=paged media=],
the [=page area=] of each page;
[=fixed positioned=] [=boxes=] are thus replicated on every page.
(They are fixed with respect to the page box only,
and are not affected by being seen through a [=viewport=];
as in the case of print preview, for example.)
Note: As a result, parts of [=fixed-positioned boxes=]
that extend outside the [=initial containing block=]/[=page area=]
cannot be scrolled to
and will not print.
</dl>
<div class="example">
With no positioning, the containing blocks (C.B.) in the following document:
<pre highlight=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>
</pre>
are established as follows:
<table class="cb-example-table">
<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>
If we position "div1":
<pre highlight=css>
#div1 { position: absolute; left: 50px; top: 50px }
</pre>
its containing block is no longer "body"; it becomes the <span>initial containing block</span>
(since there are no other positioned ancestor boxes).
If we position "em1" as well:
<pre highlight=css>
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
</pre>
the table of containing blocks becomes:
<table class="cb-example-table">
<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>
By positioning "em1", its containing block becomes the nearest positioned ancestor box
(i.e., that generated by "div1").
</div>
<h3 id="stacking">
Painting Order and Stacking Contexts</h3>
The 'z-index' property applies to all [=positioned boxes=].
When 'z-index' is ''z-index/auto'':
* [=fixed positioned|Fixed=] and [=sticky positioned|sticky=] [=positioned boxes=]
nonetheless form a [=stacking context=].
* [=relative positioned|Relative=] and [=absolute positioned|absolute=] [=positioned boxes=]
do not form a [=stacking context=],
but are painted as if those elements did generated new [=stacking contexts=],
except that their [=positioned=] descendants and any would-be child [=stacking contexts=]
take part in the current [=stacking context=].
See <a href="https://www.w3.org/TR/CSS2/visuren.html#layers">CSS2 § 9.9
Layered presentation</a>
and <a href="https://www.w3.org/TR/CSS2/zindex.html">Appendix E:
Elaborate description of Stacking Contexts</a>
for details about 'z-index', [=stacking contexts=], and painting order.
<h2 id="coords">
Positioning Coordinates</h2>
The precise location of a [=positioned box=] is controlled by
the <dfn>inset properties</dfn>:
the [=physical=] [=inset properties=] 'top', 'right', 'bottom', 'left';
the [=flow-relative=] [=inset properties=] 'inset-block-start', 'inset-inline-start', 'inset-block-end', and 'inset-inline-end';
and their [=shorthands=], 'inset-block', 'inset-inline', and 'inset'.
The interpretation of these [=inset properties=] varies by [=positioning scheme=]:
* for [=absolute positioning=], they represent
insets from the containing block.
* for [=relative positioning=], they represent
insets from the box’s original margin edge.
* for [=sticky positioning=], they represent
insets from the [=scrollport=] edge.
<h3 id="insets">
Box Insets: the 'top', 'right', 'bottom', 'left', 'inset-block-start', 'inset-inline-start', 'inset-block-end', and 'inset-inline-end' properties </h3>
<pre class="propdef">
Name: top, right, bottom, left, inset-block-start, inset-inline-start, inset-block-end, inset-inline-end
Value: auto | <<length-percentage>>
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to size of <a>containing block</a>; see prose
Computed value: the keyword ''top/auto'' or a computed <<length-percentage>> value
Animation type: by computed value type
</pre>
These [=inset properties=] represent an inward “inset”
on the corresponding side of the box
(with respect to the box’s own [=writing mode=];
see [[CSS-WRITING-MODES-3#abstract-box]]).
For example, 'top' represents a downward inset of the top edge.
The [=physical=] and [=flow-relative=] properties
interact as defined in [[!CSS-LOGICAL-1]].
Values have the following meanings:
<dl dfn-for="top,right,bottom,left, inset-block-start,inset-inline-start,inset-block-end,inset-inline-end,inset-block,inset-inline,inset" dfn-type="value">
<dt><dfn><length></dfn>
<dd>
The inset is a fixed distance from the reference edge.
Negative values are allowed.
<dt><dfn><percentage></dfn>
<dd>
The inset is a percentage
relative to the <a>containing block</a>’s size
in the corresponding axis
(e.g. width for 'left' or 'right', height for 'top' and 'bottom').
For [=sticky positioned=] boxes,
the inset is instead
relative to the relevant [=scrollport=]’s size.
Negative values are allowed.
<dt><dfn>auto</dfn></dt>
<dd>
Represents an unconstrained inset;
the exact meaning depends on the [=positioning scheme=].
</dl>
Note: For [=fixed positioned=] elements,
using large values or negative values
can easily move elements outside the <a>viewport</a>
and make the contents unreachable through scrolling or other means.
<h3 id="inset-shorthands">
Box Insets Shorthands: the 'inset-block', 'inset-inline', and 'inset' properties</h3>
<pre class="propdef">
Name: inset-block, inset-inline
Value: <'top'>{1,2}
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation type: by computed value type
</pre>
The 'inset-block' and 'inset-inline' properties are [=shorthand properties=]
for setting 'inset-block-start' + 'inset-block-end'
or 'inset-inline-start' + 'inset-inline-end',
respectively,
in a single declaration.
The first component value sets the [=CSS/start=] side,
the second sets the [=CSS/end=];
if omitted, the second value defaults to the first.
<pre class="propdef">
Name: inset
Value: <'top'>{1,4}
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation type: by computed value type
</pre>
The 'inset' property is a [=shorthand property=]
that sets all of the [=inset properties=]
in a single declaration,
assigning values to the longhands representing each side
exactly as the 'margin' property does for its longhands.
<div class=note>
By default, the 'inset' property values
are assigned to the corresponding <em>[=physical=]</em> [=longhand properties=]--
'top', 'right', 'bottom', and 'left'--
which for historical reasons do not have an <css>inset-</css> prefix.
This matches the behavior of other "4 values assigned to sides" properties,
such as 'margin'.
Allowing properties such as this to resolve to the [=flow-relative=] [=longhands=] is under discussion in [[CSS-LOGICAL-1]].
Yes, we understand it's a little confusing
that 'inset' doesn't expand to any 'inset-*' properties.
</div>
<h3 id="relpos-insets" oldids="relpos">
Relative Positioning</h3>
For a [=relatively positioned=] box,
the [=inset properties=] move the box inward
from the respective edge,
without changing its size.
'left' moves the box to the right, 'right' moves it to the left, etc.
Since boxes are not split or stretched as a result of [=relative positioning=]
opposing [=used values=] in a given axis must be negations of each other:
<ul>
<li>
If opposing [=inset properties=] in an axis both compute to ''top/auto''
(their [=initial values=]),
their [=used values=] are zero
(i.e., the boxes stay in their original position in that axis).
<li>
If only one is ''top/auto'',
its [=used value=] becomes the negation of the other,
and the box is shifted by the specified amount.
<li>
If neither is ''top/auto'',
the position is over-constrained;
(with respect to the [=writing mode=] of its [=containing block=])
the [=computed value|computed=] [=CSS/end=] side value is ignored,
and its [=used value=] becomes the negation of the [=CSS/start=] side.
</ul>
<div class="example">
The following three rules are equivalent,
and shift the box ''1em'' to the left:
<pre highlight=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 }
</pre>
</div>
If specified on
a ''table-row-group'', ''table-header-group'', ''table-footer-group'', or ''table-row'' [=box=]
the shift affects all the contents of the box,
including all <a value lt="table-cell">table cells</a> that originate in the affected row,
but not those that don't.
Note: Since 'position' does not apply to ''table-column-group'' or ''table-column'' boxes,
they are not affected by [=relative positioning=].
<h3 id="stickypos-insets" oldids="sticky-pos">
Sticky positioning</h3>
[=Sticky positioning=] is similar to [=relative positioning=]
except the offsets are automatically calculated
in reference to the [=nearest scrollport=].
For a [=sticky positioned=] [=box=],
the [=inset properties=]
represent insets from the respective edges of the [=nearest scrollport=],
defining the <dfn export>sticky view rectangle</dfn>
used to constrain the box’s position.
(For this purpose an ''top/auto'' value represents a zero inset.)
If this results in a [=sticky view rectangle=] size in any axis
less than the size of the [=border box=] of the [=sticky=] box in that axis,
then the effective [=end=]-edge inset in the affected axis is reduced
(possibly becoming negative)
to bring the [=sticky view rectangle=]’s size up to
the size of the [=border box=] in that axis
(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
<div class=example>
For example,
if the [=nearest scrollport=] is ''300px'' tall,
the [=sticky=] box's [=border box=] is ''200px'' tall,
and it has ''top: 20px'',
then the top-edge inset of the [=nearest scrollport=] is ''20px'',
and the bottom-edge inset is ''0px'',
yielding a [=sticky view rectangle=] that is ''280px'' tall.
But if the [=nearest scrollport=] were only ''100px'' tall,
then the effective bottom-edge inset becomes ''-120px'',
resulting in a [=sticky view rectangle=] that’s ''200px'' tall,
enough to fully contain the [=margin box=] of the [=sticky=] box.
</div>
For each side of the box,
if the corresponding [=inset property=] is not ''top/auto'',
and the corresponding [=border edge=] of the box would be outside
the corresponding edge of the [=sticky view rectangle=],
then the box must be visually shifted
([[#relpos-insets|as for relative positioning]])
to be inward of that [=sticky view rectangle=] edge,
insofar as it can while its [=position box=]
remains contained within its [=containing block=].
The <dfn>position box</dfn> is its [=margin box=],
except that for any side for which the distance between its [=margin edge=]
and the corresponding edge of its [=containing block=]
is less than its corresponding [=margin=],
that distance is used in place of that [=margin=].
Note: A sticky positioned element with
a non-''auto'' 'top' value and an ''auto'' 'bottom' value
will only ever be pushed down by sticky positioning;
it will never be offset upwards.
Note: Multiple [=sticky positioned=] boxes in the same container
are offset independently,
and therefore might overlap.
<h4 id="stickypos-scroll">
Scroll Position of Sticky-Positioned Boxes</h4>
For the purposes of any operation targeting the scroll position
of a sticky positioned element (or one of its descendants),
the sticky positioned element must be considered to be
at its offsetted position.
<div class="example">
For example, if a user clicks a link targeting a [=sticky-positioned=] element,
if the element's [=nearest scrollport=] is currently scrolled
such that the [=sticky positioned=] element is offset from its initial position,
the [=scroll container=] will be scrolled back
only the minimum necessary to bring it into its desired position in the [=scrollport=]
(rather than scrolling all the way back
to target its original, non-offsetted position).
</div>
<h3 id="abspos-insets" oldids="abs-pos,fixed-pos">
Absolute (and Fixed) Positioning</h3>
For an [=absolutely positioned=] box,
the [=inset properties=] effectively reduce the [=containing block=]
into which it is sized and positioned
by the specified amounts.
If only one [=inset property=] in a given axis is ''top/auto'',
it is set to zero.
If both [=inset properties=] in a given axis are ''top/auto'', then,
depending on the box’s [=self-alignment property=] in the relevant axis:
<dl class=switch>
<dt>for ''align-self/self-start'' alignment or its equivalent
<dd>
Set its start-edge [=inset property=] to the static position,
and its end-edge [=inset property=] to zero.
<dt>for ''align-self/self-end'' alignment or its equivalent
<dd>
Set its end-edge [=inset property=] to the static position,
and its start-edge [=inset property=] to zero.
<dt>for ''align-self/center'' alignment
<dd>
Let <var>start distance</var> be the distance from
the center of its [=static-position rectangle=]
to the start edge of its [=containing block=],
and <var>end distance</var> be the distance from
the center of its [=static-position rectangle=]
to the end edge of its [=containing block=].
If <var>start distance</var> is less than or equal to <var>end distance</var>,
then set the start-edge [=inset property=] to zero,
and set the end-edge [=inset property=] to
(<var>containing block size</var> - 2 × |<var>start distance</var>|);
otherwise,
set the end-edge [=inset property=] to zero
and the start-edge [=inset property=] to
(<var>containing block size</var> - 2 × |<var>end distance</var>|).
</dl>
For the rules above,
ignore [=overflow alignment=],
and treat ''align-self/normal'' as ''align-self/start''
and any [=baseline alignment|baseline=] or ''align-self/stretch'' alignment value
as its fallback alignment.
If these adjustments result in
an effective [=containing block=] size in any axis less than zero,
then the [=weaker inset=] in the affected axis
is reduced (possibly becoming negative)
to bring that size up to zero.
In the case that only one inset is ''auto'',
that is the <dfn>weaker inset</dfn>;
otherwise the [=weaker inset=] is the inset of the [=end=] edge
(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
Note: Sizing and positioning of the [=absolutely positioned box=]
into this <dfn>inset-modified containing block</dfn>
is as described in [[#abspos-layout]].
If its [=self-alignment property=] in an axis is ''align-self/normal'',
then the [=resolved value=] of its [=weaker inset=] in that axis
is the value necessary to match that edge of its [=inset-modified containing block=]
to the corresponding edge of its [=margin box=]
after [[#abspos-layout|layout]].
(Otherwise the [=resolved value=] is the [=used value=] described above.)
<h4 id="staticpos-rect">
Resolving Automatic Insets: the “Static Position Rectangle”</h4>
When both [=inset properties=] in a given axis are ''left/auto'',
they are resolved into a <dfn>static position</dfn>
by aligning the box into its <dfn>static-position rectangle</dfn>,
an [=alignment container=] derived
from the [=formatting context=] the box would have participated in
if it were ''position: static''
(independent of its actual containing block).
The [=static position=] represents
an approximation of the position the box would have had
if it were ''position: static''.
<dl>
<dt>Block Layout
<dd>
The [=static positions=] of a [=block-level box=]
are defined in [[CSS2]] Chapter 10.
The [=static-position rectangle=] is a zero-thickness rectangle spanning between
the inline-axis sides of the box’s [=static-position containing block=]
(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2§10.3.7</a>);
and positioned at its [=block-start=] [=static position=]
(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">CSS2§10.6.4</a>).
Note: In block layout the static-position rectangle corresponds to the position of the “hypothetical box”
described in <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2.1§10.3.7</a>.
Since it has no alignment properties,
CSS2.1 always uses a <a>block-start</a> <a>inline-start</a> alignment
of the absolutely-positioned box within the <a>static-position rectangle</a>.
<dt>Inline Layout
<dd>
The [=static positions=] of an [=inline-level box=]
are defined in [[CSS2]] Chapter 10.
The [=static-position rectangle=] is a zero-thickness rectangle spanning between
the [=line-over=]/[=line-under=] sides of the [=line box=]
that would have contained its “hypothetical box”
(see <a href="https://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">CSS2§10.3.7</a>);
and positioned at its [=inline-start=] [=static position=].
<dt>Flex Layout
<dd>
The [=static-position rectangle=] of the child of a [=flex container=]
corresponds to the [=content edges=] of the [=flex container=].
See <a href="https://www.w3.org/TR/css-flexbox-1/#abspos-items">static position of a grid container child</a> in [[!CSS-FLEXBOX-1]].
<dt>Grid Layout
<dd>
By default, the [=static-position rectangle=] of the child of a [=grid container=]
corresponds to the [=content edges=] of the [=grid container=].
However, if that [=grid container=] also establishes
the box’s actual [=containing block=],
then the [=grid area=] specified by the [=grid-placement properties=]
establishes its [=static-position rectangle=] instead.
See the <a href="https://www.w3.org/TR/css-grid-1/#static-position">static position of a grid container child</a> in [[!CSS-GRID-1]].
</dl>
Finding the [=static position=] and the [=static position rectangle=]
assumes that both 'float' and 'clear' have their initial values,
and can require assuming a different hypothetical value
for 'display' as well.
Additionally, the [=containing block=] of [=fixed positioned=] elements
is assumed to be the [=initial containing block=] instead of the [=viewport=],
and all [=scroll containers=] should be assumed
to be scrolled to their [=initial scroll position=].
Lastly, all ''margin/auto'' margins on the box itself
are treated as zero.
<h4 id="abspos-breaking" oldids="breaking">
Fragmenting Absolutely-positioned Elements</h4>
In a [=fragmented flow=], an [=absolutely positioned box=]
is positioned relative to its [=containing block=]
ignoring any [=fragmentation breaks=]
(as if the flow were continuous).
The box may subsequently be broken over several [=fragmentation containers=].
For absolutely positioned content in [=paged media=]
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
that 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>
Note: A [=block-level=] element that is split over several pages
can have a different width on each page,
and there may be device-specific limits.
User agents must not paginate the content of [=fixed-positioned boxes=].
Note: User agents might print invisible content in other ways.
See [[CSS-PAGE-3#content-outside-box]].
<h2 id="abspos-layout">
Absolute Positioning Layout Model</h2>
[=Absolute positioning=] not only takes a box [=out of flow=],
but also lays it out in its [=containing block=]
(after the final size of the [=containing block=] has been determined)
according to the <dfn export lt="absolute positioning layout | absolute positioning layout model">absolute positioning layout model</dfn>:
<ol>
<li>
First, its [=inset-modified containing block=] is calculated,
defining its [=available space=].
(See [[#abspos-insets]].)
Because an [=absolutely positioned box=] can have no effect on the size of its [=containing block=],
its [=available space=] is always [=definite=].
<li>
Next, its width and height are resolved against this [=definite=] [=available space=],
as its [=preferred size=] capped by its [=maximum size=] (if any), floored by its [=minimum size=].
See [[#abspos-auto-size]].
Percentages, however,
are resolved against the original [=containing block=] size.
<li>
Then, the value of any ''margin/auto'' margins are calculated,
see [[#abspos-margins]].
<li>
Lastly, its [=margin box=] is aligned within the [=inset-modified containing block=]
as defined by its [=self-alignment properties=].
</ol>
<h3 id="abspos-auto-size">
Automatic Sizes of Absolutely-Positioned Boxes</h3>
The [=automatic size=] of an [=absolutely positioned box=] is resolved against its [=inset-modified containing block=] as follows
(treating ''margin/auto'' [=margins=] as zero):
<dl class=switch>
: If its [=self-alignment property=] in the relevant axis is ''align-self/stretch''
: Or if it is ''align-self/normal''
and the box is [=non-replaced=],
not a [=table wrapper box=],
and has no ''inset/auto'' [=inset property|inset=] in the relevant axis
::
Its [=automatic size=] is its [=stretch-fit size=].
: Otherwise
::
Its [=automatic size=] is its [=fit-content size=].
</dl>
However, if the box has an aspect-ratio,
then an [=automatic size=] in the [=ratio-dependent axis=]
is instead resolved as a [=max-content size=].
When both axes have an [=automatic size=],
if only one axis has an ''inset/auto'' [=inset property|inset=]
then that axis is the [=ratio-dependent axis=],
else the [=block axis=] is the [=ratio-dependent axis=].
An [=automatic size=] in the [=ratio-determining axis=]
is determined as above.
The [=automatic minimum size=] of an absolutely-positioned box is always zero.
Note: To the extent that form controls can be resized,
they are expected to be treated as [=non-replaced=] here.
In HTML, all form controls are treated as [=non-replaced=].
<h3 id="abspos-margins">
Auto Margins of Absolutely-Positioned Boxes</h3>
If either [=inset property=] in the relevant axis is ''top/auto'',
then any ''margin/auto'' [=margins=] resolve to zero.
Otherwise, the <var>remaining space</var> is calculated
as the size of its [=inset-modified containing block=] in the relevant axis
minus the box's used size in the relevant axis,
and this <var>remaining space</var> is divided among
any ''margin/auto'' margins in the relevant axis.
However,
(all with respect to the [=writing mode=] of the [=containing block=]),
if in the [=inline axis=]
the <var>remaining space</var> is negative
and both margins are ''margin/auto'',
the [=inline-start|start=] margin resolves to zero
and the [=inline-end|end=] margin receives the <var>remaining space</var>.
<h2 id="abspos-old">
Old Absolute Positioning Layout Model</h2>
ISSUE: This section is being replaced with the new [[#abspos-layout]] section.
It is preserved here for comparison:
both models should yield the same result
in [=horizontal writing modes=] when the box’s [=self-alignment=] is ''align-self/normal''.
<h3 id="abs-non-replaced-width">
The Width of Absolutely-Positioned, Non-Replaced Elements</h3>
The constraint that determines the used values for these elements is:
<code>