-
Notifications
You must be signed in to change notification settings - Fork 790
Expand file tree
/
Copy pathOverview.bs
More file actions
2335 lines (2002 loc) · 95.1 KB
/
Overview.bs
File metadata and controls
2335 lines (2002 loc) · 95.1 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
<h1>CSS Positioned Layout Module Level 3</h1>
<pre class='metadata'>
Status: ED
Work Status: Exploring
Shortname: css-position
Level: 1
Group: csswg
ED: http://dev.w3.org/csswg/css-position/
TR: http://www.w3.org/TR/css3-positioning/
Previous Version: http://www.w3.org/TR/2015/WD-css3-positioning-20150203/
Previous Version: http://www.w3.org/TR/2012/WD-css3-positioning-20120207/
Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com
Editor: Arron Eicholz, Microsoft, arronei@microsoft.com
!Issues list: <a href="http://wiki.csswg.org/spec/css-position/">in Wiki</a>
Abstract: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 ([[!CSS21]]), which builds on CSS level 1 [[CSS1]]. The main extensions compared to level 2 are the ability to position elements based on CSS Region boxes, the ability to specify different containing blocks for elements and sticky positioning. <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.
Ignored Terms: div, dl, dfn, attachment-type, attachment-top, attachment-right, attachment-bottom, attachment-left, attachment, scrolling box
Link Defaults: css-transforms-1 (property) transform
</pre>
<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>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
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.
In [[!CSS21]], 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>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>
The properties defined in this module apply to both continuous media and paged
media.
<h3 id="placement">
Module Interactions</h3>
This module replaced and extends the positioning scheme features defined in
[[!CSS21]] 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 display, position, and float</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 posi
8097
tioned, 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 Auto 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">
Values</h3>
This specification follows the CSS property definition conventions 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 [[!CSS3VAL]], when combined with this module, expands
the definition of the <length> value type as used in this specification.
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.
<!-- End section: Values -->
<!-- End section Introduction -->
<h2 id="vp">
The <dfn lt="viewport">Viewport</dfn></h2>
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>).
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).
<!-- End section: The Viewport -->
<h2 id="cb">
Containing Blocks</h2>
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.
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-cb">
Definition of <dfn lt="containing block">containing block</dfn></h3>
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 ''position/static'' or
''position/relative'' element is defined
in the Box Model [[!CSS3BOX]]. The containing block of a
''position/sticky'' element is the same as for a
''position/relative'' element. For
''position/fixed'' and
''position/absolute'', it is defined as follows:
<ol>
<li>
If the element has ''position: fixed'', the containing block is established by the
<a>viewport</a> in the case of continuous media or the page area in
the case of paged media.
<li>
If the element has ''position: absolute'',
the containing block is established by the nearest ancestor with a 'position'
other than ''position/static'', 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 'direction' property of the ancestor:
<ol>
<li>
If the 'direction' is ''ltr'', 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 'direction' is ''rtl'', 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.
</p>
</ol>
</ol>
<li>
<li>
If there is no such ancestor, the containing block is the <a>initial
containing block</a>.
</ol>
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.
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>
<li>on a subsequent page, or</li>
<li>may omit it altogether.</li>
</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.
</p>
<div class="example">
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>
are established as follows:
<table class="cb-example-table">
<tr><th>For box generated by </th>
<th>C.B. is established by</th></tr>
<tr><td>html</td><td>initial C.B. (UA-dependent)</td></tr>
<tr><td>body</td><td>html</td></tr>
<tr><td>div1</td><td>body</td></tr>
<tr><td>p1</td><td>div1</td></tr>
<tr><td>p2</td><td>div1</td></tr>
<tr><td>em1</td><td>p2</td></tr>
<tr><td>strong1</td><td>p2</td></tr>
</table>
If we position "div1":
<pre><code class="css">
#div1 { position: absolute; left: 50px; top: 50px }
</code></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><code class="css">
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
</code></pre>
the table of containing blocks becomes:
<table class="cb-example-table">
<tr><th>For box generated by</th>
<th>C.B. is established by</th></tr>
<tr><td>html</td><td>initial C.B. (UA-dependent)</td></tr>
<tr><td>body</td><td>html</td></tr>
<tr><td>div1</td><td>initial C.B.</td></tr>
<tr><td>p1</td><td>div1</td></tr>
<tr><td>p2</td><td>div1</td></tr>
<tr><td>em1</td><td>div1</td></tr>
<tr><td>strong1</td><td>em1</td></tr>
</table>
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="nf">
<dfn>Normal Flow</dfn></h2>
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
[[!CSS3BOX]] for further details about <a href="http://www.w3.org/TR/CSS2/visuren.html#normal-flow">normal flow</a>.
<!-- End section: Normal Flow -->
<h2 id="floats">
<dfn lt="float|floated box|floating box">Floats</dfn></h2>
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 'clear' property) or around the floated box.
See the CSS 2.1 [[!CSS21]]<!--CSS Floats module [[!CSS3FLOATS]]--> for details
about <a href="http://www.w3.org/TR/CSS2/visuren.html#floats">floating boxes</a>.
<!-- End section: Floats -->
<h2 id="pos-sch">
<dfn>Positioning schemes</dfn></h2>
In CSS, a box may be laid out according to three positioning schemes:
<ol>
<li>
Normal flow<br>
In CSS, <a>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 lt="relative positioning" spec="css21">relative</a> and <a lt="sticky positioning">sticky</a> positioning of block-level and inline-level boxes.
<li>
Floats<br>
In the <a>float</a> model, a box is first laid out according to the <a>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>float</a>.
<li>
Absolute positioning<br>
In the absolute positioning model, a box is removed from the <span>normal
flow</span> entirely (it has no impact on later siblings) and assigned a
position with respect to a <a>containing block</a>.
</ol>
An element is called <em>out-of-flow</em> if it is <a>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="rel-pos">
Relative positioning</h3>
Once a box has been laid out according to the <a>normal flow</a> or
<a>floated</a>, it may be offset relative to this position.
This is called <dfn export lt="relative position|relatively position|relpos">relative positioning</dfn>. 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 'overflow: auto' or 'overflow: scroll' 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.
A relatively positioned box keeps its <a>normal flow</a> size, including
line breaks and the space originally reserved for it.
A relatively positioned box establishes a new a new <a>containing block</a> for
absolutely positioned descendants. (This is a common use of relatively positioned
boxes.) The section on <a>containing blocks</a>
explains when a relatively positioned box establishes a new <a>containing block</a>.
For relatively positioned elements, 'left' and 'right' move the box(es) horizontally,
without changing their size. 'Left' moves the boxes to the right, and 'right' moves
them to the left. Since boxes are not split or stretched as a result of 'left' or
'right', the used values are always: left = -right.
If both 'left' and 'right' are <css>auto</css> (their initial values), the used values
are ''0'' (i.e., the boxes stay in their original position).
If 'left' is <css>auto</css>, its used value is minus the value of 'right' (i.e., the
boxes move to the left by the value of 'right').
If 'right' is specified as <css>auto</css>, its used value is minus the value of 'left'.
If neither 'left' nor 'right' is <css>auto</css>, the position is over-constrained, and
one of them has to be ignored. If the 'direction' property of the <a>containing
block</a> is ''ltr'', the value of 'left' wins and 'right' becomes -'left'.
If 'direction' of the containing block is ''rtl'', 'right' wins and 'left' is
ignored.
<div class="example">
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>
The 'top' and 'bottom' properties move relatively positioned element(s) up or
down without changing their size. 'Top' moves the boxes down, and 'bottom' moves
them up. Since boxes are not split or stretched as a result of 'top' or 'bottom',
the used values are always: top = -bottom.
If 'top' and 'bottom' are <css>auto</css>, their used values are both ''0''.
If one of them, 'top' or 'bottom', is <css>auto</css>, the <css>auto</css> value becomes the
negative of the other.
If neither 'top' and 'bottom' is <css>auto</css>, 'bottom' is ignored (i.e., the used
value of 'bottom' will be minus the value of 'top').
<p class="note">
Note, Although relative positioning could be used as a form of superscripting
and subscripting, the line height is not automatically adjusted to take the
positioning into consideration. See the description of <a href="http://www.w3.org/TR/CSS2/visudet.html#line-height">line height calculations</a> for more
information.
</p>
Examples of relative positioning are provided in the section comparing <a>normal
flow</a>, <a>floats</a>, and absolute positioning.
<!-- End section: Relative positioning -->
<h3 id="sticky-pos">
Sticky positioning</h3>
A <dfn export lt="sticky position|stickily position|stickypos">stickily positioned</dfn> box is positioned similarly to a <a>relatively
positioned</a> box, but the offset is computed with reference to the nearest
ancestor with a <a>scrolling box</a>, or the viewport if no ancestor has
a <a>scrolling box</a>.
Specifically, once a box has been laid out according to the <a>normal flow</a>
or <a>floated</a>, its sticky offset is computed as described
below. 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 sticky positioning may cause
boxes to overlap. However, if sticky positioning causes an ''overflow: auto'' or
''overflow: scroll'' 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.
A stickily positioned box keeps its <a>normal flow</a> size, including line
breaks and the space originally reserved for it.
A stickily positioned box establishes a new a new <a>containing block</a> for
absolutely positioned descendants, just as relative positioning does. The section
on <a>containing blocks</a> explains when a
stickily positioned box establishes a new <a>containing block</a>.
For stickily positioned elements, 'left', 'right', 'top' and 'bottom' are
offsets from the respective edges of its flow box which are used to constrain
the element's offset. Percentage values of 'left' and 'right' refer to the
width of its flow box; percentage values of 'top' and 'bottom' refer to the
height of its flow box.
<p class="issue">
Describe which element font-size-relative units are resolved against
</p>
The offset of a stickily positioned box is computed as follows:
<ol>
<li>
A rectangle is computed relative to the containing block of the stickily
positioned element, by insetting its flow box rectangle on each side by offsets
computed from the 'left', 'right', 'top' and 'bottom' properties of the stickily
positioned element.
<p class="issue">
Say what happens if this rectangle is empty
</p>
<li>
The intersection is taken between the resulting rectangle, and the containing
block of the stickily positioned element. The result, termed the the
<em>sticky-constraint rectangle</em>, is a rectangle used to constrain the
location of the stickily positioned element.
<li>
If the stickily positioned element's 'top' style is not <css>auto</css>, and the stickily
positioned element projects above the top of the <em>sticky-constraint
rectangle</em>, the stickily positioned element is moved down until it is fully
contained in the <em>sticky-constraint rectangle</em>.
<li>
If the stickily positioned element's 'bottom' style is not <css>auto</css>, and the stickily
positioned element projects below the bottom of the <em>sticky-constraint
rectangle</em>, the stickily positioned element is moved up until it is fully
contained in the <em>sticky-constraint rectangle</em>.
<li>
If the stickily positioned element's 'left' style is not <css>auto</css>, and the stickily
positioned element projects outside the left of the <em>sticky-constraint
rectangle</em>, the stickily positioned element is moved right until it is fully
contained in the <em>sticky-constraint rectangle</em>.
<li>
If the stickily positioned element's 'right' style is not <css>auto</css>, and the stickily
positioned element projects outside the right of the <em>sticky-constraint
rectangle</em>, the stickily positioned element is moved left until it is fully
contained in the <em>sticky-constraint rectangle</em>.
</ol>
When computing containement of the stickily positioned element within its
containing block, margins on the stickily positioned element are taken into
account.
<p class="issue">
Say what happens if it already overflows the containing block
</p>
<p class="issue">
Do marins collapse between the stickily positioned element and its containing
block element?
</p>
Intersection between the stickily positioned element and the bottom of the
<em>sticky-constraint rectangle</em> limits movement in any direction, so the
offset never pushes the stickily positioned element outside of its containing
block. However, when the element is free to move within its containing block
as the page is scrolled, it appears to be pinned to the relevant flow root
edges, similarly to a fixed position element.
<p class="note">
Note that a stickily positioned element with non-auto 'top' style and auto
'bottom' style will only ever be pushed down by sticky positioning; it will
never be offset upwards.
</p>
<p class="note">
Multiple stickily positioned elements in the same container are offset
independently. Sticky position offsets may cause them to overlap.
</p>
<p class="issue">
Does the margin on the stickily positioned element affect its distance from
the flow root edge?
</p>
<p class="issue">
Sticky positioning should really be defined in terms of the nearest scrollable
ancestor, but there is currently no such term defined elsewhere in CSS. CSSOM
View refers to "scrolling boxes." CSS Overflow has yet to pull in the relevant
text from CSS Box, and CSS Box has an old, confusing definition of "flow root"
which is almost (but probably not quite) what we want here. This spec refers
to "flow root," since that's the closest thing currently specced somewhere,
but this is not optimal.
</p>
<div class=example>
The following example is the same as the previous one, but now it is explained:
<pre>EM { font-style: italic }</pre>
</div>
<!-- End section: Sticky positioning -->
<h3 id="abs-pos">
Absolute positioning</h3>
In the absolute positioning model, a box is explicitly offset with respect to
its <a>containing block</a>. It is removed from the <a>normal flow</a> entirely
(it has no impact on later siblings). An absolutely positioned box establishes
a new <a>containing block</a> for normal flow children and absolutely (but not
fixed) positioned descendants. However, the contents of an absolutely
positioned element do not flow around any other boxes. They may obscure the
contents of another box (or be obscured themselves), depending on the stack
levels of the overlapping boxes.
References in this specification to an absolutely positioned element (or its
box) imply that the element’s 'position' property has the value
''absolute'' or ''fixed''.
<!-- End section: Absolute positioning -->
<h3 id="fixed-pos">
Fixed positioning</h3>
Fixed positioning is similar to absolute positioning. The only difference is
that for a fixed positioned box, the <a>containing block</a> is established
by the <a>viewport</a>. For <a href="http://www.w3.org/TR/CSS2/media.html#continuous-media-group">continuous media</a>,
fixed boxes do not move when the document is scrolled. In this respect, they are
22CE
similar
to <a href="http://www.w3.org/TR/CSS2/colors.html#background-properties">fixed background images</a>.
For <a href="http://www.w3.org/TR/CSS2/media.html#paged-media-group">paged
media</a>, boxes with fixed positions are repeated on every page. This is useful
for placing, for instance, a signature at the bottom of each page. Boxes with
fixed position that are larger than the page area are clipped. Parts of the
fixed position box that are not visible in the <span>initial containing
block</span> will not print.
<div class="example">
Authors may use fixed positioning to create the following presentation:
<figure>
<img src="images/frame.png" alt="Example of frame layout">
<figcaption>
A page layout with a static header and footer at the top and bottom,
a sidebar to one side,
and main content filling the remaining center space.
</figcaption>
</figure>
This might be achieved with the following HTML document and style rules:
<pre class="lang-html">
<!DOCTYPE html>
<html>
<head>
<title>A frame document with CSS</title>
<style type="text/css" media="screen">
body { height: 8.5in } /* Required for percentage heights below */
#header {
position: fixed;
width: 100%;
height: 15%;
top: 0;
right: 0;
bottom: auto;
left: 0;
}
#sidebar {
position: fixed;
width: 10em;
height: auto;
top: 15%;
right: auto;
bottom: 100px;
left: 0;
}
#main {
position: fixed;
width: auto;
height: auto;
top: 15%;
right: 0;
bottom: 100px;
left: 10em;
}
#footer {
position: fixed;
width: 100%;
height: 100px;
top: auto;
right: 0;
bottom: 0;
left: 0;
}
</style>
</head>
<body>
<div id="header"> ... </div>
<div id="sidebar"> ... </div>
<div id="main"> ... </div>
<div id="footer"> ... </div>
</body>
</html>
</pre>
</div>
<!-- End section: Fixed positioning -->
<!-- End section: Positioning schemes -->
<h3 id="position-property">
Choosing a positioning scheme: 'position' property</h3>
The 'position' property determines which of the positioning algorithms is used to
calculate the position of a box.
<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
Animatable: no
Percentages: N/A
Media: visual
Computed value: specified value
</pre>
The values of this property have the following meanings:
<dl dfn-for="position" dfn-type="value">
<dt><dfn>static</dfn></dt>
<dd>
The box is a normal box, laid out according to the <a>normal flow</a>. The
'top', 'right', 'bottom', and 'left' properties do not apply.
<dt><dfn>relative</dfn></dt>
<dd>
The box’s position is calculated according to the <a>normal flow</a> (this
is called the position in <a>normal flow</a>). Then the box is offset relative
to its normal position and in all cases, including table elements, does not affect
the position of any following boxes. When a box B is relatively positioned, the
position of the following box is calculated as though B were not offset. The effect
of ''position: relative'' on table elements is defined as follows:
<ul>
<li>
table-row-group, table-header-group, table-footer-group and table-row offset
relative to its normal position within the table. If table-cells span
multiple rows, only the cells originating in the <a>relative positioned</a> row is
offset.
<li>
table-column-group, table-column do not offset the respective column and has
no visual affect when 'position': ''relative'' is applied.
<li>
table-caption and table-cell offset relative to its normal position within
the table. If a table cell spans multiple columns or rows the full spanned
cell is offset.
</ul>
<dt><dfn>absolute</dfn></dt>
<dd>
The box’s position (and possibly size) is specified with the 'top', 'right',
'bottom', and 'left' properties. These properties specify offsets with respect
to the box’s <a>containing block</a>. Absolutely positioned boxes
are taken out of the normal flow. This means they have no impact on the layout
of later siblings. Though absolutely positioned boxes may have margins, those
margins do not <a href="http://www.w3.org/TR/CSS2/box.html#collapsing-margins">
collapse</a> with any other margins.
<dt><dfn>sticky</dfn></dt>
<dd>
The box’s position is calculated according to the <a>normal flow</a>
(this is called the position in <a>normal flow</a>). Then the box is offset
relative to its flow root and containing block and in all cases, including
table elements, does not affect the position of any following boxes. When a
box B is stickily positioned, the position of the following box is calculated
as though B were not offset. The effect of 'position': ''sticky'' on table
elements is the same as for 'position': ''relative''
<dt><dfn>fixed</dfn></dt>
<dd>
The box’s position is calculated according to the "absolute" model, but
in addition, the box is fixed with respect to some reference. As with the
"absolute" model, the box’s margins do not collapse with any other margins.
In the case of handheld, projection, screen, tty, and tv media types, the box
is fixed with respect to the <a>viewport</a> and does not move when scrolled.
In the case of the print media type, the box is rendered on every page, and
is fixed with respect to the page box, even if the page is seen through a
<a>viewport</a> (in the case of a print-preview, for example). For other media
types, the presentation is undefined. 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="http://www.w3.org/TR/CSS2/media.html#at-media-rule">'@media'</a>
rule, as in:
<div class="example">
<pre><code class="css">
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
</code></pre>
</div>
User agents must not paginate the content of fixed boxes.
<p class="note">
Note that user agents may print invisible content in other ways. See
<a href="http://www.w3.org/TR/CSS2/page.html#outside-page-box">"Content outside the page box"</a>.
</p>
</dd>
</dl>
In previous versions of CSS user agents may treat position ''fixed'' as ''static''
on the root element. This specification removes that option and it is now required
that newer user agents treat ''fixed'' value on the root element as defined for
all other elements.
<!-- End section: Choosing a positioning scheme: 'position' property -->
<h3 id="box-offsets-trbl">
Box offsets: 'top', 'right', 'bottom', 'left'</h3>
An element is said to be <a>positioned</a> if its 'position' property has a value other than ''static''.
Positioned elements generate positioned boxes, and may be laid out according
to the following four physical properties:
<pre class="propdef">
Name: top
Value: auto | <length> | <percentage>
Initial: auto
Applies to: positioned elements
Inherited: no
Animatable: <length>, <percentage>
Percentages: refer to height of <a>containing block</a>
Media: visual
Computed value: For 'position': ''relative'', see Relative positioning.<br/> For 'position': ''sticky'', see Sticky positioning.<br/> For 'position': ''static'', ''top/auto''.<br/> Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, ''top/auto''.
</pre>
This property specifies how far an absolutely positioned box’s top margin
edge is offset below the top edge of the box’s <a>containing block</a>.
For relatively positioned boxes, the offset is with respect to the top edge of
the box itself (i.e., the box is given a position in the <a>normal flow</a>,
and then offset from that position according to this property).
For stickily positioned boxes, the value is used to compute the <em>sticky-constraint
rectangle</em> as described in Sticky positioning.
For absolutely positioned elements whose <a>containing
block</a> is based on a block-level element, this property is an offset from
the padding edge of that element.
<pre class="propdef">
Name: right
Value: auto | <length> | <percentage>
Initial: auto
Applies to: positioned elements
Inherited: no
Animatable: <length>, <percentage>
Percentages: refer to height of <a>containing block</a>
Media: visual
Computed value: For 'position': ''relative'', see Relative positioning.<br/> For 'position': ''sticky'', see Sticky positioning.<br/> For 'position': ''static'', ''right/auto''.<br/> Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, ''right/auto''.
</pre>
Similar to 'top', but specifies how far a box’s right margin edge is offset
to the left of the right edge of the box’s <a>containing block</a>.
For relatively positioned boxes, the offset is with respect to the right edge
of the box itself (i.e., the box is given a position in the <a>normal flow</a>,
and then offset from that position according to this property).
For stickily positioned boxes, the value is used to compute the <em>sticky-constraint
rectangle</em> as described in Sticky positioning.
For absolutely positioned elements whose <a>containing
block</a> is based on a block-level element, this property is an offset from
the padding edge of that element.
<pre class="propdef">
Name: bottom
Value: auto | <length> | <percentage>
Initial: auto
Applies to: positioned elements
Inherited: no
Animatable: <length>, <percentage>
Percentages: refer to height of <a>containing block</a>
Media: visual
Computed value: For 'position': ''relative'', see Relative positioning.<br/> For 'position': ''sticky'', see Sticky positioning.<br/> For 'position': ''static'', ''bottom/auto''.<br/> Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, ''bottom/auto''.
</pre>
Similar to 'top', but specifies how far a box’s bottom margin edge is offset
above the bottom edge of the box’s <a>containing block</a>.
For relatively positioned boxes, the offset is with respect to the bottom edge
of the box itself (i.e., the box is given a position in the <a>normal flow</a>,
and then offset from that position according to this property).
For stickily positioned boxes, the value is used to compute the <em>sticky-constraint
rectangle</em> as described in Sticky positioning.
For absolutely positioned elements whose <a>containing
block</a> is based on a block-level element, this property is an offset from
the padding edge of that element.
<pre class="propdef">
Name: left
Value: auto | <length> | <percentage>
Initial: auto
Applies to: positioned elements
Inherited: no
Animatable: <length>, <percentage>
Percentages: refer to height of <a>containing block</a>
Media: visual
Computed value: For 'position': ''relative'', see Relative positioning.<br/> For 'position': ''sticky'', see Sticky positioning.<br/> For 'position': ''static'', ''left/auto''. Otherwise: if specified as a '<length>', the corresponding absolute length; if specified as a '<percentage>', the specified value; otherwise, ''left/auto''.
</pre>
Similar to 'top', but specifies how far a box’s left margin edge is offset
to the right of the left edge of the box’s <a>containing block</a>.
For relatively positioned boxes, the offset is with respect to the left edge of
the box itself (i.e., the box is given a position in the <a>normal flow</a>,
and then offset from that position according to this property).
For stickily positioned boxes, the value is used to compute the <em>sticky-constraint
rectangle</em> as described in Sticky positioning.
For absolutely positioned elements whose <a>containing
block</a> is based on a block-level element, this property is an offset from
the padding edge of that element.
The values for the four properties have the following meanings:
<dl dfn-for="top,right,bottom,left, offset-start, offset-end, offset-before, offset-after" dfn-type="value">
<dt><dfn><length></dfn>
<dd>
The offset is a fixed distance from the reference edge. Negative values are
allowed.
<dt><dfn><percentage></dfn>
<dd>
The offset is a percentage of the <a>containing block’s</a> width (for 'left' or 'right')
or height (for 'top' and 'bottom').
For stickily positioned elements, the offset is a percentage of the flow root's
width (for 'left' or 'right') or height (for 'top' or 'bottom').
Negative values are allowed.
<dt><dfn>auto</dfn></dt>
<dd>
For non-replaced elements, the effect of this value depends on which of related
properties have the value ''top/auto'' as well. See the sections on the
'width' and 'height' of absolutely positioned, non-replaced elements for details.
For replaced elements, the effect of this value depends only on the intrinsic
dimensions of the replaced content. See the sections on the 'width' and 'height'
of absolutely positioned, replaced elements for details.
</dl>
<p class="note">
Note, for fixed positioned elements using large values or
negative values may easily move elements outside the <a>viewport</a> and make the
contents unreachable through scrolling or other means. Authors should be aware that
fixed position elements are always relative to the <span>initial
containing block</span>.
</p>
<!-- End section: Box offsets: 'top', 'right', 'bottom', 'left' -->
<h3 id="logical-box-offsets-beaso">
Logical box offsets: 'offset-before', 'offset-end', 'offset-after' and 'offset-start'</h3>
Logical offset properties allow for offseting positioned boxes based on the
'writing-mode' and 'direction' properties. When both the physical property and
equivalent logical property (based on 'writing-mode' and 'direction') are
specified the physical property computes to the computed value of the coresponding
logical property.
Positioned elements generate positioned boxes, and may be laid out according to
the following four logical properties taking into account the 'writing-mode'
and 'direction' of the <a>containing block</a>:
<pre class="propdef">
Name: offset-before, offset-after, offset-start, offset-end
Value: auto | <length> | <percentage>
Initial: auto
Applies to: positioned elements
Inherited: no
Animatable: <length>, <percentage>
You can’t perform that action at this time.