-
Notifications
You must be signed in to change notification settings - Fork 707
/
Copy pathOverview.bs
1390 lines (1206 loc) · 46.1 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 Fonts Module Level 5
Shortname: css-fonts
Level: 5
Status: ED
Prepare for TR: no
Work Status: exploring
Group: CSSWG
ED: https://drafts.csswg.org/css-fonts-5/
TR: https://www.w3.org/TR/css-fonts-5/
Previous Version: https://www.w3.org/TR/2021/WD-css-fonts-5-20211221/
!Delta Spec: yes
Former Editor: Myles C. Maxfield, Formerly of Apple Inc., mmaxfield@apple.com, w3cid 77180
Editor: Chris Lilley, W3C, https://svgees.us/, w3cid 1438
Abstract: This specification defines modifications to the existing <a href="https://drafts.csswg.org/css-fonts-4/">CSS Fonts 4</a> specification along with additional features.
Repository: w3c/csswg-drafts
Inline Github Issues: title
Default Highlight: css
WPT Path Prefix: css/css-fonts/
WPT Display: closed
</pre>
<pre class="link-defaults">
spec:css-color-4; type:property; text:color
spec:css-values; type:value; text:ex
spec:css22; type:value; for:/; text:block
spec:html; type:element; text:font
spec:css-fonts-4; type:property; text:font-size
spec:css-fonts-4; type:descriptor; text:font-language-override
spec:css-fonts-4; type:property; text:font-family
spec:css-fonts-4; type:property; text:font-style
spec:css-fonts-4; type:property; text:font-weight
spec:css-fonts-4; type:descriptor; text:src
spec:css-cascade-5; type:dfn; text:computed value
spec:css-cascade-5; type:dfn; text:used value
</pre>
<pre class=biblio>
{
"avar2": {
"title": "avar — Axis Variations Table Version 2",
"href": "https://github.com/harfbuzz/boring-expansion-spec/blob/main/avar2.md",
"publisher": "HarfBuzz",
"date": "2022"
}
}
</pre>
<h2 id="introduction">
Introduction</h2>
The CSS Fonts Level 4 specification ([[!CSS-FONTS-4]]) describes the
controls CSS provides for selecting and using fonts within documents,
including support for variable fonts and color fonts.
The ideas here are additions or modifications to the properties and
rules defined in CSS Fonts Level 4.
This specification is currently a delta to the CSS Fonts Level 4 specification.
Do not assume that if something is not here, it has been dropped.
<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="basic-font-props">
Basic Font Properties</h2>
Issue(126):
<h3 id="font-family-prop">
Font family: the 'font-family!!property' property</h3>
<h4 id="generic-font-families">
Generic font families</h4>
In addition to the [[css-fonts-4#generic-font-families]]
in CSS Fonts Level 4, the following new generic font families are also defined.
Issue(4910):
<dl dfn-for="font-family,<generic-family>" dfn-type=value>
<dt id="xxx-def"><dfn>xxx</dfn>
<dd>
Placeholder text for the xxx generic font family.
</dl>
Issue(4566):
<h3 id="font-weight-prop">Font weight: the 'font-weight!!property' property</h3>
Issue(2690):
<h3 id="font-style-prop">
Font style: the 'font-style!!property' property</h3>
Issue(4044):
<h3 id="font-size-adjust-prop">
Relative sizing: the 'font-size-adjust' property</h3>
<pre class="propdef">
Name: font-size-adjust
Value: none | [ ex-height | cap-height | ch-width | ic-width | ic-height ]? [ from-font | <<number [0,∞]>> ]
Initial: none
Applies to: all elements and text
Inherited: yes
Percentages: N/A
Computed value: the keyword ''font-size-adjust/none'', or a pair of a metric keyword and a <<number>>
Animation type: discrete if the keywords differ, otherwise by computed value type
</pre>
For any given font size, the apparent size and effective legibility of text
varies across fonts as a function of their design.
For example,
for bicameral scripts such as Latin or Cyrillic that
distinguish between upper and lowercase letters,
the relative height of lowercase letters
compared to their uppercase counterparts
is a determining factor of legibility.
In situations where font fallback occurs,
fallback fonts might not share
the same ratios as the desired font family
for key typographic metrics,
and will thus appear to be a different size
and possibly be less readable.
<wpt>
font-size-adjust-ic-height.html
</wpt>
Issue(8792):
The 'font-size-adjust' property provides a way
to preserve the readability and apparent size of text
when font fallback occurs.
It does this by adjusting the used font size
so that the specified metric is the same
regardless of the font used.
Values have the following meanings:
<dl dfn-type=value dfn-for="font-size-adjust">
<dt><dfn>none</dfn>
<dd>
No special 'font-size!!property' adjustment is applied.
<dt><dfn>ex-height | cap-height | ch-width | ic-width | ic-height</dfn>
<dd>
Specifies the font metric to normalize,
defaulting to ''font-size-adjust/ex-height'':
<dl>
<dt><dfn>ex-height</dfn>
<dd>
Normalize the <dfn dfn>aspect value</dfn> of the fonts,
using the x-height divided by the font size.
<dt><dfn>cap-height</dfn>
<dd>
Normalize the cap-height of the fonts,
using the cap-height by the font size.
<dt><dfn>ch-width</dfn>
<dd>
Normalize the horizontal narrow pitch of the fonts,
using the advance width of “0” (ZERO, U+0030)
divided by the font size.
<dt><dfn>ic-width</dfn>
<dd>
Normalize the horizontal wide pitch of the font,
using the advance width of “水” (CJK water ideograph, U+6C34)
divided by the font size.
<dt><dfn>ic-height</dfn>
<dd>
Normalize the vertical wide pitch of the font,
using the advance height of “水” (CJK water ideograph, U+6C34)
divided by the font size.
</dl>
ISSUE(6384):
If the required font metric cannot be derived from a font,
then that font’s size is not adjusted.
<dt><dfn><<number [0,∞]>></dfn>
<dd>
Each font’s [=used value|used=] size is normalized
to match the chosen font metric to
this specified proportion of the [=computed value|computed=] 'font-size!!property'.
In other words,
for each glyph,
the adjusted font size to use <var>u</var>
is calculated as:
<pre><var>u</var> = ( <var>m</var> / <var>m′</var> ) <var>s</var></pre>
where:
<pre>
<var>s</var> = computed 'font-size!!property' value
<var>m</var> = metric as specified by the 'font-size-adjust' property
<var>m′</var> = metric as specified in the actual font
<var>u</var> = adjusted font-size to use
</pre>
Negative values are invalid.
<dt><dfn>from-font</dfn>
<dd>
Computes to the <<number>> corresponding to
the specified metric of the [=first available font=].
ISSUE(6384): Shouldn't this be the first available font
that can provide the required metric?
</dl>
<wpt>
animations/font-size-adjust-composition.html
animations/font-size-adjust-interpolation.html
font-size-adjust-013.html
font-size-adjust-014.html
font-size-adjust-metrics-override.html
font-size-adjust-text-orientation.html
font-size-adjust-units-001.html
parsing/font-size-adjust-computed.html
parsing/font-size-adjust-invalid.html
parsing/font-size-adjust-valid.html
</wpt>
<div class="example">
The style defined below defines Verdana as the desired font family,
but if Verdana is not available Futura or Times will be used.
One paragraph also has font-size-adjust specified.
<pre>
p {
font-family: Verdana, Futura, Times;
}
p.adj {
font-size-adjust: 0.545;
}
<p>Lorem ipsum dolor sit amet, ...</p>
<p class="adj">Lorem ipsum dolor sit amet, ...</p>
</pre>
Verdana has a relatively high aspect value of 0.545,
meaning lowercase letters are relatively tall
compared to uppercase letters,
so at small sizes text appears legible.
Times has a lower aspect value of 0.447,
and so if fallback occurs,
the text will be less legible at small sizes than Verdana
unless font-size-adjust is also specified.
Note: For text which uses diacritics,
too large an x-height will actually decrease legibility
as the diacritics become cramped.
How text rendered in each of these fonts compares is shown below,
the columns show text rendered in Verdana, Futura and Times.
The same font-size value is used across cells
within each row and red lines are included to show the differences in x-height.
In the upper half, each row is rendered in the same font-size value.
The same is true for the lower half,
but in this half the 'font-size-adjust' property is also set
to 0.545,
so that the actual font size is adjusted
to preserve the x-height of Verdana across each row.
Note how small text remains relatively legible across each row in the lower half.
<figure>
<img alt="text with and without 'font-size-adjust'" src="images/fontsizeadjust.png" >
<figcaption>Text with and without the use of 'font-size-adjust'</figcaption>
</figure>
</div>
The value of 'font-size-adjust' affects the [=used value|used=] value of 'font-size!!property'
but does not affect the [=computed value|computed=] value.
Therefore it can affect the size of relative units
that are based on font metrics
such as <code>ex</code> and <code>ch</code>
but does not affect the size of <code>em</code> units.
Since numeric values of 'line-height'
refer to the [=computed value|computed=] size of 'font-size!!property',
'font-size-adjust' also does not affect the [=used value|used=] value of 'line-height'.
Note: Since 'font-size-adjust' does not factor into the 'line-height',
specifying a line height too tightly can result in overlapping lines of text.
For example, when a fallback font with a low [=aspect value=]
is normalized to match one with a high [=aspect value=],
its ascenders and descenders are likely to extend outside the line box
with ''line-height: 1''.
The 'font-size-adjust' adjustment applies to any font that is selected
but in typical usage it would be based
on the corresponding metric value
of the first (most desired) font in the 'font-family' list.
If this is specified accurately,
the <code>(<var>m</var>/<var>m′</var>)</code> term in the adjustment formula
will resolve to <code>1</code> for the first font
and no adjustment occurs for that font;
and the rest of the fonts will resolve to match.
If the value is specified inaccurately,
text rendered using the first font in the family list
will display differently in older user agents
that don't support 'font-size-adjust'.
<div class="example">
Authors can calculate the <a>aspect value</a> for a given font
by comparing spans with the same content
but different 'font-size-adjust' properties.
If the same font-size is used, the spans will match
when the 'font-size-adjust' value is accurate for the given font.
Two spans with borders are used to determine the <a>aspect value</a> of a font.
The 'font-size!!property' is the same for both spans
but the 'font-size-adjust' property is specified only for the right span.
Starting with a value of 0.5,
the aspect value can be adjusted
until the borders around the two letters line up.
<pre>
p {
font-family: Futura;
font-size: 500px;
}
span {
border: solid 1px red;
}
.adjust {
font-size-adjust: 0.5;
}
<p><span>b</span><span class="adjust">b</span></p>
</pre>
<figure>
<img alt="Futura with an aspect value of 0.5" src="images/beforefontsizeadjust.png" >
<figcaption>Futura with an <a>aspect value</a> of 0.5</figcaption>
</figure>
The box on the right is a bit bigger than the one on the left, so the <a>aspect value</a> of this font is something less than 0.5.
Adjust the value until the boxes align.
</div>
Note: If the specified metric has been overridden in ''@font-face'',
e.g. by 'size-adjust',
then the overridden metric will be used in the 'font-size-adjust' calculation.
Consequently, applying 'font-size-adjust' and 'size-adjust' together
means that 'size-adjust' appears to have no effect.
<h2 id="font-resources">
Font Resources</h2>
<h3 id="font-face-rule">
The <dfn id="at-font-face-rule">''@font-face''</dfn> rule</h3>
Note: Descriptors are applied per-font rather than per-element.
Multiple fonts can be used within an individual element
e.g. for characters not supported by the [=first available font=].
<h3 id="src-desc">Font reference: the 'src' descriptor</h3>
<!-- same as CSS Fonts 4, keep the two in sync -->
<h4 id="font-face-src-parsing">Parsing the 'src!!descriptor' descriptor</h4>
The 'src!!descriptor' descriptor value must be parsed
according to section [[css-syntax#parse-comma-separated-list-of-component-values]].
Then each component value is parsed according to this grammar:
<pre><<url>> [ format(<<font-format>>)]? [ tech( <<font-tech>>#)]? | local(<<family-name>>)</pre>
<pre class="prod"><dfn id="font-format-values"><font-format></dfn>
= [<<string>> | collection | embedded-opentype | opentype
| svg | truetype | woff | woff2 ]</pre>
<pre class="prod"><dfn id="font-tech-values"><font-tech></dfn>
= [<<font-features-tech>> | <<color-font-tech>>
| variations | palettes | incremental ]</pre>
<pre class="prod"><dfn id="font-features-tech-values"><font-features-tech></dfn>
= [features-opentype | features-aat | features-graphite]</pre>
<pre class="prod"><dfn id="color-font-tech-values"><color-font-tech></dfn>
= [color-COLRv0 | color-COLRv1 | color-SVG | color-sbix | color-CBDT ]</pre>
If a component value is parsed correctly
and is a supported [[css-fonts-4#font-format-definitions]] or
[[css-fonts-4#font-tech-definitions]],
add it to the list of supported sources.
If parsing a component value results in a parsing error
or its format or tech are unsupported,
do not add it to the list of supported sources.
If there are no supported entries at the end of this process,
the value for the 'src!!descriptor' descriptor is a parse error.
These parsing rules allow for graceful fallback of fonts
for user agents which don't support a particular font tech
or font format.
<div class="example" id="ex-incremental">
For example,
when incremental transfer is not supported,
a woff2 compressed version of the font
is supplied,
for optimal performance.
Then, for
<a href="https://w3c.github.io/IFT/Overview.html">incremental transfer</a>,
the raw uncompressed OpenType font is provided,
portions of which will be loaded on demand.
<pre>
@font-face {
font-family: "MyIncrementallyLoadedWebFont";
src: url("FallbackURLForBrowsersWhichDontSupportIncrementalLoading.woff2") format("woff2");
src: url("MyIncrementallyLoadedWebFont.otf") format(opentype) tech(incremental);
}
</pre>
</div>
<h3 id="font-size-desc">
Font property descriptors: the 'font-size!!descriptor'
</h3>
Issue(806):
Issue(731):
<pre class='descdef'>
Name: font-size
Value: auto | [<<number>>]{1,2}
Initial: auto
For: @font-face
</pre>
<dl dfn-type="value" dfn-for="@font-face/font-size">
<dt><dfn>auto</dfn></dt>
<dd>
The font matches any font size
</dd>
<dt><dfn><<number>></dfn></dt>
<dd>
If a single <<number>> is given
the font matches that specific font size, only.
If two <<number>>s are given,
they specify a range of font sizes which will match.
</dd>
</dl>
<h3 id="size-adjust-desc">
Glyph Size Multiplier:
the 'size-adjust' descriptor</h3>
<pre class='descdef'>
Name: size-adjust
Value: <<percentage [0,∞]>>
For: @font-face
Initial: 100%
</pre>
The 'size-adjust' descriptor defines a multiplier
for glyph outlines and metrics associated with this font,
to allow the author to harmonize the designs of various fonts
when rendered at the same 'font-size!!property'.
All metrics associated with this font--
including glyph advances, baseline tables, and overrides provided by ''@font-face'' descriptors--
are scaled by the given percentage,
as are the rendered glyph images.
Consequently,
any values derived from font metrics
(such as ''ex'' and ''ch'' units,
or the ''text-decoration-thickness/from-font'' value of 'text-decoration-thickness')
are also affected when sourced from this font.
However, the [=computed value|computed=] 'font-size!!property'
(and thus any values that derive from it,
such as ''em'' units, percentages in 'text-underline-offset', etc.)
remains unaffected.
Note: The 'size-adjust' descriptor functions similarly to
the 'font-size-adjust' property,
which essentially calculates an adjustment per font by matching ex heights,
but likewise does not affect the [=computed value|computed=] 'font-size!!property'.
<wpt>
font-size-adjust-metrics-override.html
size-adjust-01.html
size-adjust-02.html
size-adjust-03.html
</wpt>
<!-- should this stay in Fonts 4 or be here in Fonts 5? -->
<h3 id="font-metrics-override-desc">
Line Height Font Metrics Overrides:
the 'ascent-override', 'descent-override', and 'line-gap-override' descriptors</h3>
<pre class='descdef'>
Name: ascent-override
Value: [ normal | <<percentage [0,∞]>> ]{1,2}
For: @font-face
Initial: normal
</pre>
<pre class='descdef'>
Name: descent-override
Value: [ normal | <<percentage [0,∞]>> ]{1,2}
For: @font-face
Initial: normal
</pre>
<pre class='descdef'>
Name: line-gap-override
Value: [ normal | <<percentage [0,∞]>> ]{1,2}
For: @font-face
Initial: normal
</pre>
The 'ascent-override', 'descent-override', and 'line-gap-override' descriptors specify the
<a spec="CSS-INLINE-3">ascent metric</a>,
<a spec="CSS-INLINE-3">descent metric</a>, and
<a spec="CSS-INLINE-3">line gap metric</a>
of the font, respectively.
The first value provides the value for the x axis,
and the second value provides the value for the y axis
(defaulting to ''ascent-override/normal'' if omitted).
<dl dfn-for="ascent-override!!descriptor,descent-override!!descriptor,line-gap-override!!descriptor" dfn-type=value>
<dt><dfn>normal</dfn>
<dd>
The corresponding metric value is obtained from the font as usual,
as if this descriptor were absent from the <code>@font-face</code> block.
Note: Since there are multiple sources of such metrics in some font formats,
this can result in text layout that varies across UAs/platforms.
<dt><dfn><<percentage>></dfn>
<dd>
The corresponding metric is replaced by
the given percentage multiplied by the effective font size after adjustment.
</dl>
<wpt>
ascent-descent-override.html
font-size-adjust-metrics-override.html
line-gap-override.html
metrics-override-normal-keyword.html
</wpt>
The 'font-size-adjust' property is applied after the 'size-adjust' descriptor.
Note: The consequence of applying 'font-size-adjust' after 'size-adjust' is that
'size-adjust' appears to have no effect.
Note: None of these descriptors affect the [=computed value|computation=] of
'font-size!!property', 'line-height', or [=font-relative lengths=].
They can, however, affect the behavior of ''line-height: normal''
and more generally the baseline alignment of [=inline-level=] content.
Note: Since these metrics are only applicable in the [=block axis=],
the y-axis value will only be used
when [=typesetting upright=] in vertical [=typographic modes=].
<div class="example">
The percentage is resolved against different font sizes for different elements.
<pre>
@font-face {
font-family: overridden-font;
ascent-override: 50%;
...
}
<span style="font-family: overridden-font; font-size: 20px;">
Outer span content
<span style="font-size: 150%;">Inner span content</span>
</span>
</pre>
The outer span uses an <a spec="CSS-INLINE-3" lt="ascent metric">ascent</a> value of
10px, whereas the inner span uses 15px.
</div>
<div class="example">
We may override the metrics of a local fallback font to match the primary font, which
is a web font. This reduces layout shifting when switching from fallback to the
primary font.
<pre>
@font-face {
font-family: cool-web-font;
src: url("https://example.com/font.woff");
}
@font-face {
font-family: fallback-to-local;
src: local(Some Local Font);
/* Override metric values to match cool-web-font */
ascent-override: 125%;
descent-override: 25%;
line-gap-override: 0%;
size-adjust: 96%;
}
<div style="font-family: cool-web-font, fallback-to-local">Title goes here</div>
<img src="https://example.com/largeimage" alt="A large image that you don't want to shift">
</pre>
The image will not be shifted as much when the user agent finishes loading and
switches to use the web font (assuming the override values are similar to the web font's
natural metrics).
</div>
<h3 id="font-sup-sub-override-desc">
Superscript and subscript metrics overrides:
the 'superscript-position-override', 'subscript-position-override','superscript-size-override' and 'subscript-size-override' descriptors</h3>
<pre class='descdef'>
Name: superscript-position-override
Value: [ normal | from-font | <<percentage>> ]{1,2}
For: @font-face
Initial: normal
</pre>
<pre class='descdef'>
Name: subscript-position-override
Value: [ normal | from-font | <<percentage>> ]{1,2}
For: @font-face
Initial: normal
</pre>
<pre class='descdef'>
Name: superscript-size-override
Value: [ normal | from-font | <<percentage [0,∞]>> ]{1,2}
For: @font-face
Initial: normal
</pre>
<pre class='descdef'>
Name: subscript-size-override
Value: [ normal | from-font | <<percentage [0,∞]>> ]{1,2}
For: @font-face
Initial: normal
</pre>
The 'superscript-position-override', 'subscript-position-override',
'superscript-size-override', and 'subscript-size-override' descriptors
specify the superscript offset, subscript offset, superscript size, and subscript size
metrics of the font, respectively,
which are used to synthesize glyphs when required by 'font-variant-position'.
The first value provides the value for the x axis,
and the second value provides the value for the y axis
(defaulting to the first value if omitted).
<dl dfn-for="superscript-position-override!!descriptor,subscript-position-override!!descriptor,superscript-size-override!!descriptor,subscript-size-override!!descriptor" dfn-type=value>
<dt><dfn>normal</dfn>
<dd>
The UA determines what metrics value to use,
whether derived from the font or from some heuristic.
<dt><dfn>from-font</dfn>
<dd>
The corresponding metric in the font data is used, if any.
(If the metric is missing, same as ''superscript-position-override/normal''.)
<dt><dfn><<percentage>></dfn>
<dd>
The corresponding metric is replaced by
the given percentage multiplied by the used font size.
</dl>
Note: Since these metrics are only applicable in the [=block axis=],
the y-axis value will only be used
when [=typesetting upright=] in [=vertical typographic modes=].
<h2 id="font-rend-props">
Font Feature Properties</h2>
<h3 id="font-language-override-prop">Font language override: the 'font-language-override' property</h3>
Issue(5484):
<h2 id="font-feature-variation-resolution">
Font Feature and Variation Resolution</h3>
Issue(5635):
<h2 id="font-variation-props">
Font Variation Properties</h2>
<h3 id="font-optical-sizing-def">
Optical sizing control: the 'font-optical-sizing' property</h3>
Issue(5466):
<!-- This might get moved here -->
<!--
<h2 id="color-font-support">
Color Font Support</h2>
-->
<h2 id="font-technologies-formats">
Font Technologies and Formats
</h2>
<h3 id="font-tech-definitions">Font tech</h3>
This section expands upon [[css-fonts-4#font-tech-definitions]].
The 'avar2' tech refers to support for version 2 of OpenType axis variations [[!avar2]],
for flexible variation axis remapping.
<div class="example" id="ex-avar2">
For example, a font using variation axis remapping is loaded only if supported:
<pre>
@font-face {
font-family: 'Roboto Flex';
src: url('RobotoFlex-VF-avar2.woff2') format(woff2) tech(variations, avar2);
src: url('RobotoFlex-VF.woff2') format(woff2) tech(variations);
font-weight: 100 1000;
font-stretch: 25% 151%;
}
</pre>
</div>
<!--
███████ ████████ ██ ████████ ██████ ████████ ██ ██ ███████ ████████ ████████ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ███ ███ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ████ ████ ██ ██ ██ ██ ██ ██
██ ██ ████████ ██ ██████ ██ ██ ██ ███ ██ ██ ██ ██ ██ ██████ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
███████ ████████ ██████ ████████ ██████ ██ ██ ██ ███████ ████████ ████████ ████████
-->
<h2 id="object-model">
Object Model</h2>
This section expands upon [[css-fonts-4#object-model]].
The contents of ''@font-face'' and ''@font-feature-values'' rules
can be accessed via the following extensions to the CSS Object Model.
<wpt>
idlharness.html
</wpt>
<h3 id="om-fontface">
The <code id="cssfontfacerule2">CSSFontFaceRule</code> interface</h3>
The <dfn id="cssfontfacerule-interface">CSSFontFaceRule</dfn> interface represents a <<@font-face>> rule.
<pre class="idl">
[Exposed=Window]
interface CSSFontFaceDescriptors : CSSStyleDeclaration {
attribute [LegacyNullToEmptyString] CSSOMString src;
attribute [LegacyNullToEmptyString] CSSOMString fontFamily;
attribute [LegacyNullToEmptyString] CSSOMString font-family;
attribute [LegacyNullToEmptyString] CSSOMString fontStyle;
attribute [LegacyNullToEmptyString] CSSOMString font-style;
attribute [LegacyNullToEmptyString] CSSOMString fontWeight;
attribute [LegacyNullToEmptyString] CSSOMString font-weight;
attribute [LegacyNullToEmptyString] CSSOMString fontStretch;
attribute [LegacyNullToEmptyString] CSSOMString font-stretch;
attribute [LegacyNullToEmptyString] CSSOMString fontWidth;
attribute [LegacyNullToEmptyString] CSSOMString font-width;
attribute [LegacyNullToEmptyString] CSSOMString fontSize;
attribute [LegacyNullToEmptyString] CSSOMString font-size;
attribute [LegacyNullToEmptyString] CSSOMString sizeAdjust;
attribute [LegacyNullToEmptyString] CSSOMString size-adjust;
attribute [LegacyNullToEmptyString] CSSOMString unicodeRange;
attribute [LegacyNullToEmptyString] CSSOMString unicode-range;
attribute [LegacyNullToEmptyString] CSSOMString fontFeatureSettings;
attribute [LegacyNullToEmptyString] CSSOMString font-feature-settings;
attribute [LegacyNullToEmptyString] CSSOMString fontVariationSettings;
attribute [LegacyNullToEmptyString] CSSOMString font-variation-settings;
attribute [LegacyNullToEmptyString] CSSOMString fontNamedInstance;
attribute [LegacyNullToEmptyString] CSSOMString font-named-instance;
attribute [LegacyNullToEmptyString] CSSOMString fontDisplay;
attribute [LegacyNullToEmptyString] CSSOMString font-display;
attribute [LegacyNullToEmptyString] CSSOMString fontLanguageOverride;
attribute [LegacyNullToEmptyString] CSSOMString font-language-override;
attribute [LegacyNullToEmptyString] CSSOMString ascentOverride;
attribute [LegacyNullToEmptyString] CSSOMString ascent-override;
attribute [LegacyNullToEmptyString] CSSOMString descentOverride;
attribute [LegacyNullToEmptyString] CSSOMString descent-override;
attribute [LegacyNullToEmptyString] CSSOMString lineGapOverride;
attribute [LegacyNullToEmptyString] CSSOMString line-gap-override;
attribute [LegacyNullToEmptyString] CSSOMString superscriptPositionOverride;
attribute [LegacyNullToEmptyString] CSSOMString superscript-position-override;
attribute [LegacyNullToEmptyString] CSSOMString subscriptPositionOverride;
attribute [LegacyNullToEmptyString] CSSOMString subscript-position-override;
attribute [LegacyNullToEmptyString] CSSOMString superscriptSizeOverride;
attribute [LegacyNullToEmptyString] CSSOMString superscript-size-override;
attribute [LegacyNullToEmptyString] CSSOMString subscriptSizeOverride;
attribute [LegacyNullToEmptyString] CSSOMString subscript-size-override;
};
[Exposed=Window]
interface CSSFontFaceRule : CSSRule {
[SameObject, PutForwards=cssText] readonly attribute CSSFontFaceDescriptors style;
};
</pre>
<h2 id="security">Security Considerations</h2>
No new security considerations have been reported on this specification.
<h2 id="privacy">Privacy Considerations</h2>
No new privacy considerations have been reported on this specification.
<h2 id="acknowledgments">
Acknowledgments</h2>
Firstly, the editors would like to thank all of the
<a href="https://www.w3.org/TR/css-fonts-4/#acknowledgments">contributors to the previous level of this module</a>.
Secondly, we would like to acknowledge
Dave Crossland from Google,
Bernhard Fey from RealObjects,
Xiaocheng Hu from Google,
Jonathan Kew from Mozilla,
and
Laurence Penney
for their contributions to the improvements in this Level 5.
<h2 id="changes">
Changes
</h2>
<h3 id="changes-20260206">Changes since the <a href="https://www.w3.org/TR/2024/WD-css-fonts-5-20240206/">WD of 6 February 2024</a></h3>
<ul>
<!-- to 18 Dec 2024 -->
<li>Added the avar2 font tech</li>
<li>Updated the incremental example to conform to the latest IFT specification</li>
</ul>
<h3 id="changes-20211221">Changes since the <a href="https://www.w3.org/TR/2021/WD-css-fonts-5-20211221/">WD of 21 December 2021</a> </h3>
<ul>
<!-- to 31 Jan 2024 -->
<li>Corrected the initial value for the font-size descriptor</li>
<li>Set a range for the grammar of font-size-adjust number</li>
<li>Changed grammar of local() to use family-name instead for font-face-name</li>
<li>Updated to current incremental font download keywords</li>
<li>Made feature-* plural for consistency with CSS Conditional 5, and 11.1 Font tech</li>
<li>Normalized infinity to ∞ in range notation </li>
<li>Fixed a typo in the tech() function</li>
</ul>
<h3 id="changes-20210729">Changes since the <a href="https://www.w3.org/TR/2021/WD-css-fonts-5-20210729/">WD of 2021-07-29</a></h3>
<ul>
<li>Renamed technology to tech</li>
<li>Security and Privacy are now separate sections</li>
<li>Fixed font-technology grammar</li>
<li>Copied section on parsing src descriptor from CSS Fonts 4</li>
</ul>
<h3 id="changes-20210629">Changes since the <a href="https://www.w3.org/TR/2021/WD-css-fonts-5-20210629/">FPWD of 2021-06-29</a></h3>
<ul>
<li>Removed scary warning notice</li>
<li>Added incremental font technology to supports</li>
</ul>
<!-- WPT for Levels 4 and 3 of CSS Fonts, ignore -->
<wpt hidden>
alternates-order.html
animations/font-palette-animation-not-specified-endpoints.html
animations/font-palette-interpolation.html
animations/font-size-interpolation-001.html
animations/font-size-interpolation-002.html
animations/font-size-interpolation-003.html
animations/font-stretch-interpolation.html
animations/font-style-interpolation.html
animations/font-variation-settings-composition.html
animations/font-variation-settings-interpolation.html
animations/multiple-elements-font-palette-animation.html
animations/system-fonts.html
calc-in-font-variation-settings.html
cjk-kerning.html
downloadable-font-in-iframe-print.html
downloadable-font-print.html
fallback-remote-to-data-url.html
fallback-url-to-local.html
first-available-font-001.html
first-available-font-002.html
first-available-font-003.html
first-available-font-004.html
first-available-font-005.html
first-available-font-006.html
first-available-font-007.html
font-colorization.html
font-default-01.html
font-default-02.html
font-default-03.html
font-default-04.html
font-display/font-display-change.html
font-display/font-display-failure-fallback.html
font-display/font-display-preload.html
font-display/font-display.html
font-face-range-order.html
font-face-stretch-auto-static.html
font-face-stretch-auto-variable.html
font-face-stretch-default-variable.html
font-face-style-auto-static.html
font-face-style-auto-variable.html
font-face-style-default-variable.html
font-face-unicode-range-2.html
font-face-unicode-range-nbsp.html
font-face-unicode-range.html
font-face-weight-auto-static.html
font-face-weight-auto-variable.html
font-face-weight-default-variable.html
font-family-name-000.xht
font-family-name-001.xht
font-family-name-002.xht
font-family-name-003.xht
font-family-name-004.xht
font-family-name-005.xht
font-family-name-006.xht
font-family-name-007.xht
font-family-name-008.xht
font-family-name-009.xht
font-family-name-010.xht
font-family-name-011.xht
font-family-name-012.xht
font-family-name-013.xht
font-family-name-014.xht
font-family-name-015.xht
font-family-name-016.xht
font-family-name-017.xht
font-family-name-018.xht
font-family-name-019.xht
font-family-name-020.xht
font-family-name-021.xht
font-family-name-022.xht
font-family-name-023.xht
font-family-name-024.xht
font-family-name-025.html
font-family-src-quoted.html
font-feature-resolution-001.html
font-feature-resolution-002.html
font-feature-settings-descriptor-01.html
font-feature-settings-serialization-001.html
font-feature-settings-tibetan.html
font-features-across-space-1.html
font-features-across-space-2.html
font-features-across-space-3.html
font-kerning-01.html
font-kerning-02.html
font-kerning-03.html
font-kerning-04.html
font-kerning-05.html
font-language-override-01.html
font-language-override-02.html
font-language-override-03.html
font-palette-10.html
font-palette-11.html
font-palette-12.html
font-palette-13.html
font-palette-14.html
font-palette-15.html
font-palette-16.html
font-palette-17.html
font-palette-18.html