-
Notifications
You must be signed in to change notification settings - Fork 709
/
Copy pathOverview.bs
3114 lines (2592 loc) · 116 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
F438
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
<style media="print" type="text/css">
img#edge { width: 80%; height: 70%;}
dt.label { display: run-in; }
</style>
<pre class='metadata'>
Title: CSS Inline Layout Module Level 3
Shortname: css-inline
Level: 3
Status: ED
Work Status: Revising
Group: csswg
TR: https://www.w3.org/TR/css-inline-3/
ED: https://drafts.csswg.org/css-inline-3/
Previous version: https://www.w3.org/TR/2020/WD-css-inline-3-20200827/
Previous version: https://www.w3.org/TR/2020/WD-css-inline-3-20200618/
!Issues list: <a href="https://github.com/w3c/csswg-drafts/issues?q=is%3Aissue+is%3Aopen+label%3Acss-inline-3">CSS3 Line Layout issues in GitHub</a>
Editor: Dave Cramer, Hachette Livre, dauwhe@gmail.com, w3cid 65283
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Former Editor: Steve Zilles, Adobe, szilles@adobe.com, w3cid 3129
Abstract: The CSS formatting model provides for a flow of elements and text inside of a container to be wrapped into lines. This module describes box model for this inline layout model and defines the block-axis alignment and sizing of inline-level content, extending the model in [[CSS2]]. It also adds a special layout mode for drop-caps.
Ignored Terms: line-height-shift-adjustment, text-script, after, before, alignment subtree
Link Defaults: css-fonts-3 (property) font-family, css-color-3 (property) color
At Risk: the 'initial-letter-wrap' property
</pre>
<pre class="link-defaults">
spec:css-align-3; type:dfn; text:alignment baseline
spec:css-box; type:dfn; text:content area
spec:css-break-3; type:dfn; text:fragment
spec:css-shapes-1; type:property; text:shape-margin
spec:svg2; type:dfn; text: current text position
spec:css-writing-modes-3; type:dfn; text:baseline
spec:css-fonts-4; type:property; text:font-language-override
spec:css-align-3; type:value; for:align-content; text:center
</pre>
<h2 id="intro">
Introduction</h2>
This module defines [=inline layout=], the CSS model
for laying out a mixed stream of text and [=inline-level=] boxes,
and defines controls for the [=block-axis=] alignment and sizing
of this content within each line.
It also adds a [[#initial-letter-styling|special layout mode for drop caps and similar initial letter styling]].
Note: Line-breaking, justification, and other aspects of
inline-axis positioning of [=inline-level=] content
are handled in the <a href="https://www.w3.org/TR/css-text/">CSS Text Module</a>.
ISSUE: Many aspects of layout here depend on font metrics.
While the relevant metrics exist in OpenType for Latin/Cyrillic/Greek
and for <abbr title="Chinese/Japanese/Korean">CJK</abbr>,
they are missing for many other writing systems.
For example, the visual top metric for Hebrew has no metric in the OpenType tables.
For this module to work well for the world,
we need fonts to provide the relevant metrics for all writing systems,
and that means both that OpenType needs to allow such metrics
and font designers need to provide accurate numbers.
See <a href="https://github.com/w3c/csswg-drafts/issues/5244">issue</a> and
<a href="https://lists.w3.org/Archives/Public/www-archive/2020Feb/att-0005/CSS-SC29-20200113.pdf">liaison statement</a>.
<h3 id="placement">
Module Interactions</h3>
This module
replaces and extends the CSS inline layout model and features defined in
[[!CSS2]] section 10.8.
<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="model">
Inline Layout Model</h2>
In <dfn>inline layout</dfn>,
a mixed, recursive stream of text and [=inline-level boxes=]
forming an <dfn>inline formatting context</dfn> within a [=block container=]
are laid out by [=fragmenting=] them into a stack of [=line boxes=].
Within each [=line box=],
[=inline-level boxes=] are [[#alignment|aligned to each other]] along the [=block axis=],
typically by the [=baselines=] of their text.
Any [=block container=] that directly contains
[=inline-level=] content--
such as [=inline boxes=], [=atomic inlines=], and [=text runs=]--
establishes an [=inline formatting context=]
to lay out its contents using [=inline layout=].
The [=block container=]’s [=content edges=] form the [=containing block=]
for each of the [=inline-level boxes=]
participating in its [=inline formatting context=].
The [=block container=] also generates
a <dfn export>root inline box</dfn>,
which is an <a lt="anonymous box">anonymous</a> [=inline box=] that holds
all of its [=inline-level=] contents.
(Thus, all text in an [=inline formatting context=] is directly contained
by an [=inline box=],
whether the [=root inline box=] or one of its descendants.)
The [=root inline box=] inherits from its parent [=block container=],
but is otherwise unstyleable.
In an [=inline formatting context=],
content is laid out along the [=inline axis=],
ordered according to the
<a href="https://www.w3.org/TR/css-writing-modes-3/#text-direction">Unicode bidirectional algorithm and its controls</a> [[CSS-WRITING-MODES-3]]
and distributed according to the typesetting controls in [[CSS-TEXT-3]].
[=Inline-axis=] [=margins=], [=borders=], and [=padding=]
are respected between [=inline-level boxes=]
(and their margins do not <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a>).
The resulting rectangular area that contains the boxes
that form a single line of [=inline-level content=]
is called a <dfn export>line box</dfn>.
Note: <a>Line boxes</a> and <a>inline boxes</a> and <a>inline-level boxes</a>
are each different things!
See [[CSS-DISPLAY-3]] for an in-depth discussion of box types and related terminology.
<h3 id="line-boxes">
Layout of Line Boxes</h3>
[=Line boxes=] are created as needed
to hold [=inline-level=] content
within an [=inline formatting context=].
When an [=inline box=] exceeds the [=logical width=] of a [=line box=],
or contains a <a spec="css-text-3">forced line break</a>,
it is split (see [[css-text-3#line-breaking]])
into several [=fragments=] [[css-break-3]],
which are partitioned across multiple [=line boxes=].
Like [=column boxes=] in [=multi-column layout=] [[CSS-MULTICOL-1]],
[=line boxes=] are [=fragmentation containers=]
generated by their [=formatting context=],
and are not part of the CSS [=box tree=].
Note: [=Inline boxes=] can also be
<a href="https://www.w3.org/TR/css-writing-modes-3/#bidi-box-model">split into several fragments
within the same line box due to bidirectional text processing</a>.
See [[CSS-WRITING-MODES-3]].
[=Line boxes=] are stacked
as the direct contents of the [=block container box=]
in its [=block flow direction=]
and aligned within this container as specified by 'align-content' [[CSS-ALIGN-3]].
Thus, an [=inline formatting context=] consists of
a stack of [=line boxes=].
[=Line boxes=] are stacked with no separation
(except as specified elsewhere,
e.g. for [=float=] <a href="https://www.w3.org/TR/CSS2/visuren.html#clearance">clearance</a>)
and they never overlap.
In general,
the [=line-left=] edge of a [=line box=] touches
the [=line-left=] edge of its [=containing block=]
and the [=line-right=] edge touches
the [=line-right=] edge of its [=containing block=],
and thus the [=logical width=] of a line box is equal to
the <a lt="inner size">inner</a> [=logical width=]
of its [=containing block=]
(i.e. the [=block container=]’s [=content box=]).
However, floating boxes or [=initial letter boxes=]
can come between the [=containing block=] edge and the [=line box=] edge,
reducing the space available to, and thus the [=logical width=] of,
any such impacted [=line boxes=].
(See [[CSS2/visuren#inline-formatting]]/[[CSS2/visuren#floats]]
and [[#initial-letter-styling]].)
The [=logical height=] of a [=line box=] is fitted to its contents
once they have been [[#alignment|block-axis aligned]].
This fit is controlled by 'line-height' and 'text-edge'.
The first/last line boxes in a [=block container=] may additionally
be trimmed by 'leading-trim'.
[=Line boxes=] that contain no text,
no [=preserved white space=],
no [=inline boxes=] with non-zero [=margins=], [=padding=], or [=borders=],
and no other [=in-flow=] content
(such as [=atomic inlines=] or [=ruby annotations=]),
and do not end with a [=forced line break=]
must be treated as zero-[=logical height|height=] [=line boxes=]
for the purposes of determining the positions of any elements inside of them
(such as [=absolutely positioned boxes=]),
and must be treated as not existing for any other purpose
(such as <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapsing margins</a>).
<figure>
<img src="images/box-model.png"
alt="diagram showing inline boxes split across line boxes as described above">
<figcaption>Inline Layout Box Model</figcaption>
</figure>
<h3 id="line-layout">
Layout Within Line Boxes</h3>
As described [[#model|above]],
user agents flow [=inline-level boxes=] into a stack of [=line boxes=].
Layout within each [=line box=] is performed,
sizing and positioning each [=box fragment=] and [=line box=] independently,
as follows:
1. The [=layout bounds=] of each [=inline-level box=] in the [=line box=]
are calculated.
<ul class=switch>
<li>
For [=atomic inlines=] such as [=replaced elements=] and [=inline blocks=]:
this is their [=margin box=].
<li>
For the [=root inline box=],
and for [=inline boxes=] with ''text-edge: leading'':
this derived from their used 'line-height',
ignoring any [=margin=]/[=border=]/[=padding=]
see [[#inline-height]].
<li>
For other [=inline boxes=]:
this is derived from their 'text-edge' metrics,
and includes any [=margin=]/[=border=]/[=padding=],
see [[#inline-height]]
</ul>
2. The [=inline-level boxes=]
are aligned in the [=block axis=]
according to 'dominant-baseline' and 'vertical-align'.
Those with [=line-relative values=] for 'baseline-shift'
are assumed to be aligned so as to minimize the line box height.
3. The [=line box=]’s [=logical height=] is calculated
to exactly include the aligned [=layout bounds=]
of all its [=inline-level boxes=].
4. The [=root inline box=]’s [=aligned subtree=]
and boxes with line-box–relative 'vertical-align' values
are aligned with respect to the [=line box=].
Issue: Define what to do for top/bottom/center aligned boxes that are taller than the rest of the content.
Note: Empty [=inline boxes=] still have
[=margins=], [=padding=], [=borders=], and a 'line-height',
and thus influence these calculations just like boxes with content.
<h3 id="paint-order">
Painting Order</h3>
Except as specified for [=positioned boxes=] (see [[!CSS-POSITION-3]])
[=inline-level boxes=] are painted in [=document order=];
the 'z-index' property does not generally apply.
<h2 id="css-metrics">
Baselines and Alignment Metrics</h2>
<h3 id="baseline-intro">
Introduction to Baselines</h3>
A <dfn export>baseline</dfn> is a line along the <a>inline axis</a> of a line box
along which individual glyphs of text are aligned.
[=Baselines=] guide the design of glyphs in a font
(for example, the bottom of most alphabetic glyphs
typically align with the alphabetic baseline),
and they guide the alignment of glyphs from different fonts or font sizes
when typesetting.
<figure>
<img alt="Picture of alphabetic text in two font sizes with the baseline and em-boxes"
width="480"
src="images/alphabetic-baseline-in-two-font-sizes.svg"></p>
<figcaption>
Alphabetic text in two font sizes with the baseline and em-boxes
</figcaption>
</figure>
Different writing systems prefer different [=baselines=].
<figure>
<img alt="Latin prefers the alphabetic baseline,
on top of which most letters rest,
though some letters have descenders that dangle below it.
Indic scripts are sometimes typeset with a hanging baseline,
since their glyph shapes appear to be hanging from a horizontal line.
Han-based systems, whose glyphs are designed to fill a square,
tend to align on their bottoms or through their centers."
src="images/script-preferred-baselines.gif"></p>
<figcaption>
Preferred baselines in various writing systems
</figcaption>
</figure>
A well-constructed font contains a <dfn>baseline table</dfn>,
which indicates the position of one or more [=baselines=]
within the font's design coordinate space.
(The design coordinate space is scaled with the font size.)
<figure>
<img alt=""
src="images/baselines.gif"></p>
<figcaption>
In a well-designed mixed-script font,
the glyphs are positioned in the coordinate space
to harmonize with one another when typeset together.
The baseline table is then constructed
to match the shape of the glyphs,
each baseline positioned to match the glyphs
from its preferred scripts.
</figcaption>
</figure>
The [=baseline table=] is a property of the font,
and the positions of the various baselines
apply to all glyphs in the font.
Different [=baseline tables=] can be provided for alignment
in horizontal and vertical text.
UAs should use the vertical tables in vertical [=typographic modes=]
and the horizontal tables otherwise.
Note: Fonts can have more than one [=baseline table=] in each axis;
the UA is responsible for choosing the appropriate table
in consideration of 'font-language-override' and the [=content language=].
<h3 id="baseline-types">
Baselines and Metrics</h3>
CSS uses the following text-based metrics
as [=baselines=]
for [=inline layout=] functions
such as alignment, box sizing, and initial letter layout.
Issue: The CSSWG would like to know which baseline values are necessary
for each property that uses them ('dominant-baseline', 'alignment-baseline', 'text-edge', 'initial-letter-align'):
if any can be dropped, or any need to be added.
See <a href="https://github.com/w3c/csswg-drafts/issues/859">Issue 859</a>.
<dl export>
<dt><dfn lt="alphabetic baseline" local-lt="alphabetic">alphabetic</dfn>
<dd>
Used in writing
Latin, Cyrillic, Greek, and many other scripts,
corresponds to the bottom of most, but not all, their characters,
(such as “m”, “Ш”, “Δ”).
Often represented as zero in font design coordinate systems;
assigned to <code>romn</code> in OpenType
and to <code>bsln</code> value zero in TrueType AAT.
<dt><dfn lt="cap-height baseline" local-lt="cap-height">cap-height</dfn>
<dd>
Corresponds to
the top of capital letters
(such as “T”, “Б”, “Σ”)
in Latin, Cyrillic, Greek, etc.
Calculated using <code>sCapHeight</code> in OpenType.
<dt><dfn lt="x-height baseline" local-lt="x-height">x-height</dfn>
<dd>
Corresponds to
the top of short lowercase letters
(such as “m”, “л”, “α”)
in Latin, Cyrillic, Greek, etc.
Calculated using <code>sxHeight</code> in OpenType.
<dt><dfn lt="x-middle baseline" local-lt="x-middle">x-middle</dfn>
<dd>
Corresponds to halfway between
the [=alphabetic=] and [=x-height=] baselines.
<dt><dfn lt="ideographic-over baseline" local-lt="ideographic-over">ideographic-over</dfn>
<dd>
Corresponds to
the [=line-over=] design edge of CJK (Han/Hangul/Kana) text.
Assigned to <code>idtp</code> in OpenType.
<dt><dfn lt="ideographic-under baseline" local-lt="ideographic-under">ideographic-under</dfn>
<dd>
Corresponds to
the [=line-under=] design edge of CJK (Han/Hangul/Kana) text.
Assigned to <code>ideo</code> in OpenType.
<dt><dfn lt="central baseline" local-lt="central">central</dfn>
<dd>
Corresponds to the ideographic central baseline,
halfway between the [=ideographic-under=] and [=ideographic-over=] baselines.
Assigned to <code>bsln</code> value 1 in TrueType AAT.
<dt><dfn lt="ideographic-ink-over baseline" local-lt="ideographic-ink-over">ideographic-ink-over</dfn>
<dd>
Corresponds to the [=line-over=] ink edge of CJK (Han/Hangul/Kana) text.
Assigned to <code>icft</code> in OpenType.
<dt><dfn lt="ideographic-ink-under baseline" local-lt="ideographic-ink-under">ideographic-ink-under</dfn>
<dd>
Corresponds to the line-under ink edge of CJK (Han/Hangul/Kana) text.
Assigned <code>icfb</code> in OpenType.
<dt><dfn lt="hanging baseline" local-lt="hanging">hanging</dfn>
<dd>
Corresponds to hanging baseline
from which characters in
Tibetan and similar unicameral scripts
with a strong but not absolute top edge seem to “hang”.
Assigned to <code>hang</code> in OpenType
and to <code>bsln</code> value 3 in TrueType AAT.
<dt><dfn lt="math baseline" local-lt="math">math</dfn>
<dd>
Corresponds to center baseline around which mathematical characters are designed.
Assigned to <code>math</code> in OpenType
and <code>bsln</code> value 4 in TrueType AAT.
<dt><dfn lt="text-over baseline" local-lt="text-over">text-over</dfn>
<dd>
Corresponds to the metric used as the [=line-over=] edge
of an [=inline box|inline=]’s [=content box=] per [[CSS2]].
<dt><dfn lt="text-under baseline" local-lt="text-under">text-under</dfn>
<dd>
Corresponds to the metric used as the [=line-under=] edge
of an [=inline box|inline=]’s [=content box=] per [[CSS2]].
<dt><dfn lt="em-over baseline" local-lt="em-over">em-over</dfn>
<dd>
Corresponds to a conceptual [=ascent=] normalized
to ensure 1em between [=em-over=] and [=em-under=].
See [[#baseline-synthesis-em]].
<dt><dfn lt="em-under baseline" local-lt="em-under">em-under</dfn>
<dd>
Corresponds to a conceptual [=descent=] normalized
to ensure 1em between [=em-over=] and [=em-under=].
See [[#baseline-synthesis-em]].
</dl>
Note: These metrics are optical design metrics,
and therefore do not necessarily correspond exactly
to actual glyph outlines.
In general, these metrics are taken from the appropriate font,
but if they are missing or need to be derived from a box rather than text,
they must be synthesized,
see [[#baseline-tables]] and [[#baseline-synthesis]].
<h4 id="ascent-descent">
Ascent and Descent Metrics</h4>
CSS assumes that every font has font metrics
that specify a characteristic height above the baseline--
called the <dfn local-lt="ascent">ascent metric</dfn>--
and a characteristic depth below it--
called the <dfn local-lt="descent">descent metric</dfn>--
which CSS uses for laying out text and boxes
in an [=inline formatting context=].
Note that these are metrics of the font as a whole
and need not correspond to the ascender and descender of any individual glyph.
Note: It is recommended that implementations that use OpenType or TrueType fonts
use the metrics <code>sTypoAscender</code> and <code>sTypoDescender</code>
from the font's OS/2 table
(after scaling to the current element's font size)
to find the [=ascent metric=] and [=descent metric=] for CSS layout.
In the absence of these metrics,
the "Ascent" and "Descent" metrics from the HHEA table should be used.
<!-- Note dependency from Canvas API fontBoundingBoxAscent/Descent
https://html.spec.whatwg.org/multipage/canvas.html#dom-textmetrics-fontboundingboxascent
-->
<h4 id="font-line-gap">
Line Gap Metrics</h4>
Font formats can allow for a font-recommended
“line gap” or “external leading” metric.
This metric is referred to as the <dfn>line gap metric</dfn>,
and may be incorporated into the [=line box=] [=logical height=] calculations
when 'line-height' is ''line-height/normal'' as described in [[#inline-height]].
Note: In OpenType, the [=line gap metric=] can be found
as <code>sTypoLineGap</code> or <code>hhea.lineGap</code>.
<h3 id="baseline-tables">
Baselines of Glyphs and Boxes</h3>
Each font, glyph, and [=inline-level box=]
is assumed to have a [=baseline=] coordinate
for each [=baseline=] type
indicating that [=baseline=]’s position on its [=block axis=].
The set of such [=baselines=] is called its <dfn>baseline set</dfn>.
The [=baseline=] from this set that is used to align
the box or glyph within its [=alignment context=]
is called its [=alignment baseline=];
the [=baseline=] used to align its content within itself
is called its [=dominant baseline=].
For an individual glyph,
the [=baseline set=] derives from the font’s [=baseline table=].
For an [=inline box=],
it derives from its [=first available font=]
regardless of whether the box actually contains any glyphs from that font.
If the requisite metrics are missing from a font,
the UA must synthesize them,
see [[#baseline-synthesis-fonts]].
For other [=boxes=],
its [=baseline set=] is nominally derived from its contents
in accordance with 'baseline-source'
and the rules of the [=formatting context=] in which it participates.
For an [=atomic inline box=] with no [=baseline set=]
in the [=inline formatting context=]’s [=inline axis=]
its [=alignment baselines=] are [=synthesize baseline|synthesized=]
from its [=margin box=],
see [[#baseline-synthesis-box]].
<h2 id="alignment">
Baseline Alignment</h2>
While most CSS [=formatting contexts=] position content
by aligning boxes with respect to their container’s edges,
[=inline layout=] positions boxes in the [=block axis=]
by aligning them with respect to each other
using their [=baselines=].
More specifically,
(unless using a [=line-relative shift value=])
each glyph or [=inline-level box=]
is aligned in the [=block axis=]
by positioning its [=alignment baseline=]
to match the <em>corresponding</em> [=baseline=] of its parent
(which is its [=alignment context=]),
and then is potentially shifted from that position
according to its [=post-alignment shift=].
Note: Baseline alignment always matches corresponding baselines:
alphabetic to alphabetic, hanging to hanging, mathematical to mathematical, etc.
When aligning a [=box=], the [=alignment baseline=] is chosen
according to its 'alignment-baseline' and 'baseline-source' values
(see shorthand 'vertical-align'),
and defaults to matching the parent’s 'dominant-baseline'.
For a glyph, the [=alignment baseline=] is always determined
by the parent’s [=dominant baseline=].
<div class="example">
<p>Given following sample markup:
<pre highlight=html><p><span class="outer">Ap <span class="inner">ਜੀ</span></span></p></pre>
<p>And the following style rule:
<pre highlight=css>.inner { font-size: 75%; }</pre>
<p>The [=baseline sets=] of the parent (<code>.outer</code>) and the child (<code>.inner</code>)
will not match up due to the font size difference.
The child box is aligned to its parent
by matching up their [=alphabetic baselines=].
<figure>
<img alt="In this example, the distance between each baseline in the baseline set
is compacted 75% in the span with a 75% font size.
Their alphabetic baselines, however, line up."
src="images/baseline-align-sizes.gif">
</figure>
<p>The [=alphabetic baseline=] is used here
because by default a box’s [=alignment baseline=] matches
the [=dominant baseline=] of its parent,
and in horizontal [=typographic mode=],
the [=dominant baseline=] itself defaults to the [=alphabetic baseline=].
<p>If we add ''vertical-align: super''
to the <code>.inner</code> element from the example above,
the same rules are used to align the <code>.inner</code> child to its parent;
but in addition to the baseline alignment,
the child is shifted to the superscript position.
<figure>
<p><img alt="In this example, the resulting alignment is equivalent to
shifting the parent baseline table upwards by its superscript offset,
and then aligning the child's alphabetic baseline
to the shifted position of the parent's alphabetic baseline."
src="images/baseline-align-super.gif">
</figure>
</div>
<h3 id="dominant-baseline-property">
Dominant Baselines: the 'dominant-baseline' property</h3>
<pre class="propdef">
Name: dominant-baseline
Value: auto | text-bottom | alphabetic | ideographic | middle | central | mathematical | hanging | text-top
Initial: auto
Applies to: block containers, inline boxes, table rows, grid containers, flex containers, and SVG <a>text content elements</a>
Inherited: yes
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
This property specifies the <dfn>dominant baseline</dfn>,
which is the default baseline type used to align content within the box.
For [=inline boxes=],
the [=dominant baseline=] is used to align the box’s text
(and, unless otherwise specified by 'vertical-align', any [=inline-level=] child boxes)
by aligning each glyph/box’s corresponding baseline to the box’s own [=dominant baseline=].
For other boxes, it indicates the default <a>alignment baseline</a>
of any boxes participating in <a>baseline alignment</a>
66C5
;
in the box’s <a>alignment context</a>;
see (''alignment-baseline: baseline'' and [[CSS-ALIGN-3]]).
Values have the following meanings:
<dl dfn-for=dominant-baseline dfn-type=value>
<dt><dfn>auto</dfn>
<dd>
Equivalent to ''dominant-baseline/alphabetic'' in <a>horizontal writing modes</a>
and in <a>vertical writing modes</a>
when 'text-orientation' is ''sideways''.
Equivalent to ''dominant-baseline/central'' in <a>vertical writing modes</a>
when 'text-orientation' is ''text-orientation/mixed'' or ''text-orientation/upright''.
However, in SVG text, the origin point of glyphs
(used for coordinate-based glyph positioning)
is always handled as for ''dominant-baseline/central''
in <a>vertical writing modes</a>.
<dt><dfn>text-bottom</dfn>
<dd>
Use the [=text-under baselines=].
<dt><dfn>alphabetic</dfn>
<dd>
Use the [=alphabetic baselines=].
<dt><dfn>ideographic</dfn>
<dd>
Use the [=ideographic-under baselines=].
<dt><dfn>middle</dfn>
<dd>
Use the [=x-middle baselines=];
except under ''text-orientation: upright''
(where the [=alphabetic=] and [=x-height=] baselines are essentially meaningless)
use the [=central baseline=].
<dt><dfn>central</dfn>
<dd>
Use the [=central baselines=].
<dt><dfn>mathematical</dfn>
<dd>
Use the [=math baselines=].
<dt><dfn>hanging</dfn>
<dd>
Use the [=hanging baselines=].
<dt><dfn>text-top</dfn>
<dd>
Use the [=text-over baselines=].
</dl>
See [[CSS-WRITING-MODES-3]] for an introduction to dominant baselines.
ISSUE: Define behavior for mixed vertical orientations that isn't nonsensical
when specified baseline isn't ''dominant-baseline/central''.
<h3 id="transverse-alignment">
Transverse Box Alignment: the 'vertical-align' property</h3>
<pre class="propdef shorthand">
Name: vertical-align
Value: [ first | last] || <<'alignment-baseline'>> || <<'baseline-shift'>>
Initial: baseline
Applies to: see individual properties
Inherited: no
Percentages: N/A
Animation type: see individual properties
</pre>
<p>This <a>shorthand</a> property specifies
how an inline-level box is aligned within the line
by specifying its [=alignment baseline=] type ('alignment-baseline'),
[=baseline alignment preference=] ('baseline-source'),
and [=post-alignment shift=] ('baseline-shift')
in a single declaration.
If ''baseline-source/first'' or ''baseline-source/last'' is specified,
it sets 'baseline-source'
(which is otherwise reset to ''baseline-source/auto'').
Other values are as for the corresponding longhand properties, see below.
<p class="advisement">
Authors should use this shorthand ('vertical-align') instead of its longhands,
unless specifically needing to cascade its longhands independently
or (on SVG elements) to support legacy SVG implementations.
Note: 'vertical-align' can also affect the alignment of table cells
when 'align-content' is ''align-content/normal''.
Specifically, ''vertical-align/top'' (''baseline-shift: top'') maps it to ''align-content/start'',
''vertical-align/bottom'' (''baseline-shift: bottom'') to ''align-content/end'',
and otherwise ''vertical-align/middle'' (''alignment-baseline: middle'') to ''align-content/center''.
See [[css-align-3#distribution-block]].
<h4 id="baseline-source">
Alignment Base
E870
line Source: the 'baseline-source' longhand</h4>
<pre class="propdef">
Name: baseline-source
Value: auto | first | last
Initial: auto
Applies to: inline-level boxes
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
When an inline-level box
has more than one possible source for baseline information
(such as for a multi-line inline block or inline flex container)
this property specifies whether the <a>first baseline set</a> or <a>last baseline set</a>
is preferred for alignment,
indicating the box’s <dfn export>baseline alignment preference</dfn>.
Values have the following meanings:
<dl dfn-for="baseline-source,vertical-align" dfn-type=value>
<dt><dfn>auto</dfn>
<dd>Specifies <a>last-baseline alignment</a> for ''inline-block'',
<a>first-baseline alignment</a> for everything else.
<dt><dfn>first</dfn>
<dd>Specifies <a>first-baseline alignment</a>.
<dt><dfn>last</dfn>
<dd>Specifies <a>last-baseline alignment</a>.
</dl>
<h4 id="alignment-baseline-property">
Alignment Baseline Type: the 'alignment-baseline' longhand</h4>
<pre class="propdef">
Name: alignment-baseline
Value: baseline | text-bottom | alphabetic | ideographic | middle | central | mathematical | text-top
Initial: baseline
Applies to: inline-level boxes, flex items, grid items, table cells, and SVG <a>text content elements</a>
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
This property specifies the box’s <dfn export>alignment baseline</dfn>:
the [=baseline=] used to align the box
prior to applying its [=post-alignment shift=]
(if applicable).
Values are defined as follows:
<dl dfn-for="alignment-baseline,vertical-align" dfn-type=value>
<dt><dfn>baseline</dfn>
<dd>
Use the <a>dominant baseline</a> choice of the parent.
<dt><dfn>text-bottom</dfn>
<dd>
Use the [=text-under baseline=].
<dt><dfn>alphabetic</dfn>
<dd>
Use the [=alphabetic baseline=].
<dt><dfn>ideographic</dfn>
<dd>
Use the [=ideographic-under baseline=].
<dt><dfn>middle</dfn>
<dd>
In general, use the [=x-middle baselines=];
except under ''text-orientation: upright''
(where the [=alphabetic=] and [=x-height=] baselines are essentially meaningless)
use the [=central baseline=] instead.
<dt><dfn>central</dfn>
<dd>
Use the [=central baseline=].
<dt><dfn>mathematical</dfn>
<dd>
Use the [=math baseline=].
<dt><dfn>text-top</dfn>
<dd>
Use the [=text-over baseline=].
</dl>
When performing [=baseline alignment=],
these values specify which [=baseline=] of the box is aligned
to the corresponding [=baseline=] of its [=alignment context=].
(In an [=inline formatting context=],
[=inline-level=] [=box fragments=] and glyphs
share an [=alignment context=] established by
their parent [=inline box=] [=box fragment|fragment=]
along its [=inline axis=].
For other [=formatting contexts=],
see [[css-align-3#baseline-terms]].)
In SVG text layout,
these values instead specify the [=baseline=] that is aligned
to the SVG <a>current text position</a>.
<h5 class="no-toc" id="alignment-baseline-svg-legacy">
Legacy Values for SVG</h5>
SVG implementations <em>may</em> support the following aliases
in order to support legacy content:
<pre dfn-for=alignment-baseline dfn-type=value>
<dfn>text-before-edge</dfn> = ''alignment-baseline/text-top''
<dfn>text-after-edge</dfn> = ''alignment-baseline/text-bottom''
</pre>
These values are not allowed in the 'vertical-align' shorthand.
<h4 id="baseline-shift-property">
Post-Alignment Shift: the 'baseline-shift' longhand</h4>
<pre class="propdef">
Name: baseline-shift
Value: <<length-percentage>> | sub | super | top | center | bottom
Initial: 0
Applies to: inline-level boxes and SVG <a>text content elements</a>
Inherited: no
Percentages: refer to the used value of 'line-height'
Computed value: the specified keyword or a computed <<length-percentage>> value
Animation type: by computed value type
</pre>
This property specifies the box’s <dfn>post-alignment shift</dfn>.
The <dfn local-lt="baseline-relative values">baseline-relative shift values</dfn>
<<length-percentage>>, ''baseline-shift/sub'', ''baseline-shift/super''
shift the box relative to its baseline-aligned position,
whereas the <dfn local-lt="line-relative values">line-relative shift values</dfn>
''baseline-shift/top'', ''baseline-shift/center'', and ''baseline-shift/bottom''
shift the [=inline box=] and its contents
relative to the bounds of its [=line box=].
<p class="advisement">
Authors should use the 'vertical-align' shorthand,
which has existed since CSS1,
instead of this 'baseline-shift' longhand
(except in SVG content,
where conversely 'baseline-shift' is more widely-supported in legacy user agents).
Values have the following meanings:
<dl dfn-for="baseline-shift,vertical-align" dfn-type=value>
<dt><dfn><<length>></dfn>
<dd>Raise (positive value) or lower (negative value) by the specified length.
<dt><dfn><<percentage>></dfn>
<dd>Raise (positive value) or lower (negative value) by the specified percentage of the 'line-height'.
<dt><dfn>sub</dfn>
<dd>
Lower by the offset appropriate for subscripts of the parent’s box.
The UA may use the parent’s font metrics to find this offset;
otherwise it defaults to dropping by
one fifth of the parent’s used 'font-size'.
<dt><dfn>super</dfn>
<dd>
Raise by the offset appropriate for superscripts of the parent’s box.
The UA may use the parent’s font metrics to find this offset;
otherwise it defaults to raising by
one third of the parent’s used 'font-size'.
<dt><dfn>top</dfn>
<dd>
Align the [=line-over=] edge of the [=aligned subtree=]
with the [=line-over=] edge of the [=line box=].
<dt><dfn>center</dfn>
<dd>
Align the center of the [=aligned subtree=]
with the center of the [=line box=].
<dt><dfn>bottom</dfn>
<dd>
Align the [=line-under=] edge of the [=aligned subtree=]
with the [=line-under=] edge of the [=line box=].
</dl>
The <dfn>aligned subtree</dfn> of an [=inline box=] contains
the [=layout bounds=] of that box
and the [=aligned subtrees=] of all child [=inline boxes=]
whose computed 'alignment-baseline' value
is not itself a [=line-relative shift value=].
The [=line-over=] edge of the [=aligned subtree=]
is the highest [=over=] edge of the [=layout bounds=] in the subtree,
and the [=line-under=] edge is analogously the lowest.
ISSUE: The [=line-relative shift values=] don't fit perfectly
in the dichotomy between 'alignment-baseline' and 'baseline-shift'.
There's <a href="https://github.com/w3c/csswg-drafts/issues/5180">decent</a> <a href="https://github.com/w3c/csswg-drafts/issues/5234">arguments</a>
for either option.
They're currently drafted here,
but if there's a strong argument to move them,
please file an issue for consideration.
<h5 class="no-toc" id="baseline-shift-svg-legacy">
Legacy Values for SVG</h5>
<p>User agents <em>may</em> additionally support
the keyword <dfn value for=baseline-shift>baseline</dfn> as computing to ''0''
if is necessary for them to support legacy SVG content.
This value is not allowed in the 'vertical-align' shorthand.
Issue: We would prefer to remove the ''baseline-shift/baseline'' value, and are looking for feedback from SVG user agents as to whether it's necessary.
<h2 id="line-height">
Logical Heights and Inter-line Spacing</h2>
The [=block-axis=] sizing of a [=line box=]
depends on the sizes and [[#alignment|alignment]] of its [=inline-level=] contents.
This sizing is controlled by
the 'line-height', 'text-edge', 'leading-trim', and 'inline-sizing' properties.
<h3 id="line-height-property">
Line Spacing: the 'line-height' property</h3>
<pre class="propdef">
Name: line-height
Value: n
3D21
ormal | <<number>> | <<length-percentage>>
Initial: normal
Applies to: non-replaced inline boxes and SVG <a>text content elements</a>
Inherited: yes
Percentages: computed relative to ''1em''
Computed value: the specified keyword, a number, or a computed <<length>> value
Animation type: by computed value type
</pre>
This property specifies the box’s <dfn>preferred line height</dfn>,
which is used in calculating its “[=layout bounds=]”,
i.e. its contribution to the [=logical height=] of its [=line box=].
(See [[#inline-height]].)
Note: Because it applies to the [=root inline box=]
when specified on a [=block container=],
'line-height' effectively establishes
the minimum height of the block’s [=line boxes=].
Values for this property have the following meanings:
<dl dfn-for=line-height dfn-type=value>
<dt><dfn>normal</dfn>
<dd>
Determine the [=preferred line height=]
automatically based on font metrics.
<dt><dfn><<length>></dfn>
<dd>
The specified length is used as the [=preferred line height=].
Negative values are illegal.
<dt><dfn><<number>></dfn>
<dd>
The [=preferred line height=] is this number
multiplied by the element's computed 'font-size'.
Negative values are illegal.
The [=computed value=] is the same as the [=specified value=].
<dt><dfn><<percentage>></dfn>
<dd>
The [=preferred line height=]
and [=computed value=] of the property
is this percentage of the element's computed 'font-size'.
Negative values are illegal.
</dl>
Note: Metrics from fonts other than the [=first available font=]
only impact the [=layout bounds=]
of an [=inline box=] with ''line-height: normal''.
<div class="example">
The three rules in the example below have the same used line height:
<pre><code highlight=css>
div { line-height: 1.2; font-size: 10pt } /* number */
div { line-height: 1.2em; font-size: 10pt } /* length */
div { line-height: 120%; font-size: 10pt } /* percentage */
</code></pre>
However, they inherit differently:
the first one inherits as a number,
which will lead to different line heights if descendants have different font sizes;
the last two as inherit as absolute lengths,
which will not be influenced by the font size on descendants.
</div>