forked from w3c/csswg-drafts
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathOverview.bs
More file actions
1263 lines (1032 loc) · 56.4 KB
/
Copy pathOverview.bs
File metadata and controls
1263 lines (1032 loc) · 56.4 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
<pre class='metadata'>
Title: CSS Font Loading Module Level 3
Shortname: css-font-loading
Level: 3
Group: csswg
Status: WD
Date: 2023-04-06
Prepare for TR: yes
Work Status: Exploring
ED: https://drafts.csswg.org/css-font-loading/
TR: https://www.w3.org/TR/css-font-loading/
Previous Version: https://www.w3.org/TR/2014/WD-css-font-loading-3-20140522/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
Former Editor: John Daggett, Mozilla, jdaggett@mozilla.com
Abstract: This CSS module describes events and interfaces used for dynamically loading font resources.
Link Defaults: css-fonts-4 (descriptor) src, dom (interface) Event
Ignored Terms: EventHandler, InvalidModificationError, WorkerGlobalScope, Document, add(), src
</pre>
<pre class="link-defaults">
spec:dom; type:interface; text:EventTarget
</pre>
<h2 id="introduction">
Introduction</h2>
CSS allows authors to load custom fonts from the web via the ''@font-face'' rule.
While this is easy to use when authoring a stylesheet,
it's much more difficult to use dynamically via scripting.
Further, CSS allows the user agent to choose when to actually load a font;
if a font face isn't <em>currently</em> used by anything on a page,
most user agents will not download its associated file.
This means that later use of the font face will incur a delay
as the user agent finally notices a usage and begins downloading and parsing the font file.
This specification defines a scripting interface to font faces in CSS,
allowing font faces to be easily created (via the {{FontFace}} interface)
and loaded from script (via [[#font-face-source|document.fonts]]).
It also provides methods to track the loading status of an individual font,
or of all the fonts on an entire page.
Issue: Several things in this spec use normal ES objects to define behavior,
such as various things using Promises internally,
and FontFaceSet using a Set internally.
I believe the intention here is that these objects
(and their prototype chains) are pristine,
unaffected by anything the author has done.
Is this a good intention?
If so, how should I indicate this in the spec?
<h3 id="values">
Values</h3>
This specification uses {{Promise}}s,
which are defined in <a href="https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-promise-objects">ECMAScript 6</a>.
MDN has some <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise">good tutorial material introducing Promises</a>.
<h3 id='task-source'>
Task Sources</h3>
Whenever this specification queues a task,
it queues it onto the "font loading" [=task source=].
<!--
████████ ███████ ██ ██ ████████ ████████ ███ ██████ ████████
██ ██ ██ ███ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ████ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██ ██ ██ ██ ██████ ██ ██ ██ ██████
██ ██ ██ ██ ████ ██ ██ █████████ ██ ██
██ ██ ██ ██ ███ ██ ██ ██ ██ ██ ██ ██
██ ███████ ██ ██ ██ ██ ██ ██ ██████ ████████
-->
<h2 id="fontface-interface">
The <code>FontFace</code> Interface</h2>
The {{FontFace}} interface represents a single usable font face.
CSS ''@font-face'' rules implicitly define FontFace objects,
or they can be constructed manually from a url or a buffer source.
<xmp class="idl">
dictionary FontFaceDescriptors {
CSSOMString style = "normal";
CSSOMString weight = "normal";
CSSOMString stretch = "normal";
CSSOMString unicodeRange = "U+0-10FFFF";
CSSOMString featureSettings = "normal";
CSSOMString variationSettings = "normal";
CSSOMString display = "auto";
CSSOMString ascentOverride = "normal";
CSSOMString descentOverride = "normal";
CSSOMString lineGapOverride = "normal";
};
enum FontFaceLoadStatus { "unloaded", "loading", "loaded", "error" };
[Exposed=(Window,Worker)]
interface FontFace {
constructor(CSSOMString family, (CSSOMString or BufferSource) source,
optional FontFaceDescriptors descriptors = {});
attribute CSSOMString family;
attribute CSSOMString style;
attribute CSSOMString weight;
attribute CSSOMString stretch;
attribute CSSOMString unicodeRange;
attribute CSSOMString featureSettings;
attribute CSSOMString variationSettings;
attribute CSSOMString display;
attribute CSSOMString ascentOverride;
attribute CSSOMString descentOverride;
attribute CSSOMString lineGapOverride;
readonly attribute FontFaceLoadStatus status;
Promise<FontFace> load();
readonly attribute Promise<FontFace> loaded;
};
</xmp>
Issue: Clarify all mentions of "the document" to be clear about which document is being referenced,
since objects can move between documents.
<div dfn-type=attribute dfn-for=FontFace>
: <dfn>family</dfn>
: <dfn>style</dfn>
: <dfn>weight</dfn>
: <dfn>stretch</dfn>
: <dfn>unicodeRange</dfn>
::
These attributes all represent the corresponding aspects of a font face,
as defined by the descriptors defined in the CSS ''@font-face'' rule.
They are parsed the same as the corresponding ''@font-face'' descriptors.
They are used by the font matching algorithm,
but otherwise have no effect.
For example, a {{FontFace}} with a {{FontFace/style}} of <code>"italic"</code>
<em>represents</em> an italic font face;
it does not <strong>make</strong> the font face italic.
On getting, return the string associated with this attribute.
On setting, [=CSS/parse=] the string according to the grammar for the corresponding ''@font-face'' descriptor.
If it does not match the grammar,
throw a {{SyntaxError}};
otherwise, set the attribute to the serialization of the parsed value.
: <dfn>featureSettings</dfn>
: <dfn>variationSettings</dfn>
: <dfn>display</dfn>
: <dfn>ascentOverride</dfn>
: <dfn>descentOverride</dfn>
: <dfn>lineGapOverride</dfn>
::
These attributes have the same meaning,
and are parsed the same as,
the corresponding descriptors in the CSS ''@font-face'' rules.
They turn on or off specific features in fonts that support them.
Unlike the previous attributes,
these attributes actually affect the font face.
On getting, return the string associated with this attribute.
On setting, [=CSS/parse=] the string according to the grammar for the corresponding ''@font-face'' descriptor.
If it does not match the grammar,
throw a {{SyntaxError}};
otherwise, set the attribute to the serialization of the parsed value.
: <dfn>status</dfn>
::
This attribute reflects the current status of the font face.
It must be "unloaded" for a newly-created {{FontFace}}.
It can change due to an author explicitly requesting a font face to load,
such as through the {{FontFace/load()}} method on {{FontFace}},
or implicitly by the user agent,
due to it detecting that the font face is needed to draw some text on the screen.
: <dfn>loaded</dfn>
::
This attribute reflects the {{[[FontStatusPromise]]}} of the font face.
</div>
All {{FontFace}} objects contain an internal <dfn attribute for=FontFace>\[[FontStatusPromise]]</dfn> slot,
which tracks the status of the font.
It starts out pending,
and fulfills or rejects when the font is successfully loaded and parsed, or hits an error.
All {{FontFace}} objects also contain
internal <dfn attribute for=FontFace>\[[Urls]]</dfn> and <dfn attribute for=FontFace>\[[Data]]</dfn> slots,
of which one is <code>null</code> and the other is not <code>null</code>
(the non-null one is set by the constructor,
based on which data is passed in).
<!--
██████ ███████ ██ ██ ██████ ████████ ████████ ██ ██ ██████ ████████ ███████ ████████
██ ██ ██ ██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ████ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██████ ██ ████████ ██ ██ ██ ██ ██ ██ ████████
██ ██ ██ ██ ████ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ███████ ██ ██ ██████ ██ ██ ██ ███████ ██████ ██ ███████ ██ ██
-->
<h3 id='font-face-constructor'>
The Constructor</h3>
A {{FontFace}} can be constructed either
from a URL pointing to a font face file,
or from a {{BufferSource}} containing the binary representation of a font face.
When the <dfn constructor for=FontFace>FontFace(family, source, descriptors)</dfn> method is called,
execute these steps:
1. Let <var>font face</var> be a fresh {{FontFace}} object.
Set <var>font face's</var> {{FontFace/status}} attribute to <code>"unloaded"</code>,
Set its internal {{[[FontStatusPromise]]}} slot to a fresh pending {{Promise}} object.
[=CSS/Parse=] the {{family!!argument}} argument,
and the members of the {{descriptors!!argument}} argument,
according to the grammars of the corresponding descriptors of the CSS ''@font-face'' rule.
If the {{source!!argument}} argument is a {{CSSOMString}},
parse it according to the grammar of the CSS ''@font-face/src'' descriptor of the ''@font-face'' rule.
If any of them fail to parse correctly,
reject <var>font face's</var> {{[[FontStatusPromise]]}} with a DOMException named "SyntaxError",
set <var>font face’s</var> corresponding attributes to the empty string,
and set <var>font face’s</var> {{FontFace/status}} attribute to "error".
Otherwise, set <var>font face's</var> corresponding attributes to the serialization of the parsed values.
Note: Note that this means that passing a naked url as the source argument,
like <code>"http://example.com/myFont.woff"</code>,
won't work - it needs to be at least wrapped in a ''url()'' function,
like <code>"url(http://example.com/myFont.woff)"</code>.
In return for this inconvenience,
you get to specify multiple fallbacks,
specify the type of font each fallback is,
and refer to local fonts easily.
Issue: Need to define the base url,
so relative urls can resolve.
Should it be the url of the document?
Is that correct for workers too,
or should they use their worker url?
Is that always defined?
Return <var>font face</var>.
If <var>font face’s</var> {{FontFace/status}} is "error",
terminate this algorithm;
otherwise,
complete the rest of these steps asynchronously.
2. If the {{source!!argument}} argument was a {{CSSOMString}},
set <var>font face's</var> internal {{[[Urls]]}} slot to the string.
If the {{source}} argument was a {{BufferSource}},
set <var>font face's</var> internal {{[[Data]]}} slot to the passed argument.
3. If <var>font face's</var> {{[[Data]]}} slot is not <code>null</code>,
queue a task to run the following steps synchronously:
1. Set <var>font face's</var> {{FontFace/status}} attribute to "loading".
2. For each {{FontFaceSet}} <var>font face</var> is in:
1. If the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list is empty,
<a>switch the FontFaceSet to loading</a>.
2. Append <var>font face</var> to the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list.
Asynchronously, attempt to parse the data in it as a font.
When this is completed,
successfully or not,
queue a task to run the following steps synchronously:
1. If the load was successful,
<var>font face</var> now represents the parsed font;
fulfill <var>font face's</var> {{[[FontStatusPromise]]}} with <var>font face</var>,
and set its {{FontFace/status}} attribute to "loaded".
For each {{FontFaceSet}} <var>font face</var> is in:
1. Add <var>font face</var> to the {{FontFaceSet}}’s {{[[LoadedFonts]]}} list.
2. Remove <var>font face</var> from the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list.
If <var>font</var> was the last item in that list
(and so the list is now empty),
<a>switch the FontFaceSet to loaded</a>.
2. Otherwise,
reject <var>font face's</var> {{[[FontStatusPromise]]}} with a DOMException named "SyntaxError"
and set <var>font face's</var> {{FontFace/status}} attribute to "error".
For each {{FontFaceSet}} <var>font face</var> is in:
1. Add <var>font face</var> to the {{FontFaceSet}}’s {{[[FailedFonts]]}} list.
2. Remove <var>font face</var> from the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list.
If <var>font</var> was the last item in that list
(and so the list is now empty),
<a>switch the FontFaceSet to loaded</a>.
Note: Newly constructed FontFace objects are not automatically added
to the FontFaceSet associated with a document
or a context for a worker thread.
This means that while newly constructed fonts can be preloaded,
they cannot actually be used until they are explicitly added to a FontFaceSet.
See the following section for a more complete description of FontFaceSet.
<!--
██ ███████ ███ ████████ ███ ███
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ █████████ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
████████ ███████ ██ ██ ████████ ███ ███
-->
<h3 id='font-face-load'>
The <code>load()</code> method</h3>
The {{FontFace/load()}} method of {{FontFace}}
forces a url-based font face to request its font data and load.
For fonts constructed from a buffer source,
or fonts that are already loading or loaded,
it does nothing.
When the <dfn method for=FontFace>load()</dfn> method is called,
execute these steps:
<ol>
<li>
Let <var>font face</var> be the {{FontFace}} object on which this method was called.
<li>
If <var>font face's</var> {{[[Urls]]}} slot is <code>null</code>,
or its {{FontFace/status}} attribute is anything other than <code>"unloaded"</code>,
return <var>font face's</var> {{[[FontStatusPromise]]}}
and abort these steps.
<li>
Otherwise,
set <var>font face's</var> {{FontFace/status}} attribute to "loading",
return <var>font face's</var> {{[[FontStatusPromise]]}},
and continue executing the rest of this algorithm asynchronously.
<li>
Using the value of <var>font face's</var> {{[[Urls]]}} slot,
attempt to load a font as defined in [[!CSS-FONTS-3]],
as if it was the value of a ''@font-face'' rule's 'src' descriptor.
<li>
When the load operation completes,
successfully or not,
queue a task to run the following steps synchronously:
<ol>
<li>
If the attempt to load fails,
reject <var>font face's</var> {{[[FontStatusPromise]]}} with
a DOMException whose name is "NetworkError"
and set <var>font face's</var> {{FontFace/status}} attribute to "error".
For each {{FontFaceSet}} <var>font face</var> is in:
1. Add <var>font face</var> to the {{FontFaceSet}}’s {{[[FailedFonts]]}} list.
2. Remove <var>font face</var> from the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list.
If <var>font</var> was the last item in that list
(and so the list is now empty),
<a>switch the FontFaceSet to loaded</a>.
<li>
Otherwise,
<var>font face</var> now represents the loaded font;
fulfill <var>font face's</var> {{[[FontStatusPromise]]}} with <var>font face</var>
and set <var>font face's</var> {{FontFace/status}} attribute to "loaded".
For each {{FontFaceSet}} <var>font face</var> is in:
1. Add <var>font face</var> to the {{FontFaceSet}}’s {{[[LoadedFonts]]}} list.
2. Remove <var>font face</var> from the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list.
If <var>font</var> was the last item in that list
(and so the list is now empty),
<a>switch the FontFaceSet to loaded</a>.
</ol>
</ol>
User agents can initiate font loads on their own,
whenever they determine that a given font face is necessary to render something on the page.
When this happens,
they must act as if they had called the corresponding {{FontFace}}’s {{FontFace/load()}} method described here.
Note: Some UAs utilize a "font cache"
which avoids having to download the same font multiple times
on a page or on multiple pages within the same origin.
Multiple {{FontFace}} objects can be mapped to the same entry in the font cache,
which means that a {{FontFace}} object might start loading unexpectedly,
even if it's not in a {{FontFaceSet}},
because some other {{FontFace}} object pointing to the same font data
(perhaps on a different page entirely!)
has been loaded.
<h3 id='font-face-css-connection'>
Interaction with CSS’s ''@font-face'' Rule</h3>
A CSS ''@font-face'' rule automatically defines a corresponding {{FontFace}} object,
which is automatically placed in the document's <a>font source</a>
when the rule is parsed.
This {{FontFace}} object is <dfn>CSS-connected</dfn>.
The {{FontFace}} object corresponding to a ''@font-face'' rule
has its {{FontFace/family}}, {{FontFace/style}}, {{FontFace/weight}}, {{FontFace/stretch}}, {{FontFace/unicodeRange}}, {{FontFace/variant}}, and {{FontFace/featureSettings}} attributes
set to the same value as the corresponding descriptors in the ''@font-face'' rule.
There is a two-way connection between the two:
any change made to a ''@font-face'' descriptor is immediately reflected in the corresponding {{FontFace}} attribute,
and vice versa.
Issue: When a FontFace is transferred between documents, it's no longer CSS-connected.
The internal {{[[Urls]]}} slot of the {{FontFace}} object is set to the value of the ''@font-face'' rule's 'src' descriptor,
and reflects any changes made to the 'src' descriptor.
Otherwise, a {{FontFace}} object created by a CSS ''@font-face'' rule is identical to one created manually.
If a ''@font-face'' rule is removed from the document, its corresponding {{FontFace}} object is no longer <a>CSS-connected</a>.
The connection is not restorable by any means
(but adding the ''@font-face'' back to the stylesheet will create a brand new {{FontFace}} object which <em>is</em> <a>CSS-connected</a>).
If a ''@font-face'' rule has its '@font-face/src' descriptor changed to a new value,
the original connected {{FontFace}} object must stop being <a>CSS-connected</a>.
A new {{FontFace}} reflecting its new '@font-face/src' must be created
and <a>CSS-connected</a> to the ''@font-face''.
(This will also remove the old and add the new {{FontFace}} objects from any <a>font sources</a> they appear in.)
<h3 id='discovery'>
Discovery of information about a font</h3>
A {{FontFace}} object includes a variety of read-only information about the contents of the font file.
<xmp class="idl">
[Exposed=(Window,Worker)]
interface FontFaceFeatures {
/* The CSSWG is still discussing what goes in here */
};
[Exposed=(Window,Worker)]
interface FontFaceVariationAxis {
readonly attribute DOMString name;
readonly attribute DOMString axisTag;
readonly attribute double minimumValue;
readonly attribute double maximumValue;
readonly attribute double defaultValue;
};
[Exposed=(Window,Worker)]
interface FontFaceVariations {
readonly setlike<FontFaceVariationAxis>;
};
[Exposed=(Window,Worker)]
interface FontFacePalette {
iterable<DOMString>;
readonly attribute unsigned long length;
getter DOMString (unsigned long index);
readonly attribute boolean usableWithLightBackground;
readonly attribute boolean usableWithDarkBackground;
};
[Exposed=(Window,Worker)]
interface FontFacePalettes {
iterable<FontFacePalette>;
readonly attribute unsigned long length;
getter FontFacePalette (unsigned long index);
};
partial interface FontFace {
readonly attribute FontFaceFeatures features;
readonly attribute FontFaceVariations variations;
readonly attribute FontFacePalettes palettes;
};
</xmp>
Note: This read-only data is intended to help authors know which values are accepted by
'font-feature-settings!!property',
'font-variation-settings!!property',
and ''@font-palette-values''.
<!--
████████ ███████ ██ ██ ████████ ████████ ███ ██████ ████████ ██████ ████████ ████████
██ ██ ██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ████ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ██ ██ ██ ██ ██████ ██ ██ ██ ██████ ██████ ██████ ██
██ ██ ██ ██ ████ ██ ██ █████████ ██ ██ ██ ██ ██
██ ██ ██ ██ ███ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ███████ ██ ██ ██ ██ ██ ██ ██████ ████████ ██████ ████████ ██
-->
<h2 id="FontFaceSet-interface">
The <code>FontFaceSet</code> Interface</h2>
<xmp class="idl">
dictionary FontFaceSetLoadEventInit : EventInit {
sequence<FontFace> fontfaces = [];
};
[Exposed=(Window,Worker)]
interface FontFaceSetLoadEvent : Event {
constructor(CSSOMString type, optional FontFaceSetLoadEventInit eventInitDict = {});
[SameObject] readonly attribute FrozenArray<FontFace> fontfaces;
};
enum FontFaceSetLoadStatus { "loading", "loaded" };
[Exposed=(Window,Worker)]
interface FontFaceSet : EventTarget {
setlike<FontFace>;
FontFaceSet add(FontFace font);
boolean delete(FontFace font);
undefined clear();
// events for when loading state changes
attribute EventHandler onloading;
attribute EventHandler onloadingdone;
attribute EventHandler onloadingerror;
// check and start loads if appropriate
// and fulfill promise when all loads complete
Promise<sequence<FontFace>> load(CSSOMString font, optional CSSOMString text = " ");
// return whether all fonts in the fontlist are loaded
// (does not initiate load if not available)
boolean check(CSSOMString font, optional CSSOMString text = " ");
// async notification that font loading and layout operations are done
readonly attribute Promise<FontFaceSet> ready;
// loading state, "loading" while one or more fonts loading, "loaded" otherwise
readonly attribute FontFaceSetLoadStatus status;
};
</xmp>
<div dfn-for="FontFaceSet">
: <dfn attribute>ready</dfn>
:: This attribute reflects the {{FontFaceSet}}'s {{[[ReadyPromise]]}} slot.
See [[#font-face-set-ready]] for more details on this {{Promise}} and its use.
: <dfn dfn export>iteration order</dfn>
:: When iterated over,
all <a>CSS-connected</a> {{FontFace}} objects must come first,
in document order of their connected ''@font-face'' rules,
followed by the non-<a>CSS-connected</a> {{FontFace}} objects,
in insertion order.
: <dfn dfn>set entries</dfn>
:: If a {{FontFaceSet}} is a <a>font source</a>,
its <a spec=webidl for>set entries</a> are initialized as specified in [[#document-font-face-set]].
Otherwise, its <a spec=webidl>set entries</a> are initially empty.
: <dfn method>add(font)</dfn>
::
When the {{add()}} method is called,
execute the following steps:
1. If <var>font</var> is already in the {{FontFaceSet}}’s [=FontFaceSet/set entries=],
skip to the last step of this algorithm immediately.
2. If <var>font</var> is <a>CSS-connected</a>,
throw an {{InvalidModificationError}} exception
and exit this algorithm immediately.
3. Add the <var>font</var> argument to the {{FontFaceSet}}’s [=FontFaceSet/set entries=].
2. If <var>font</var>’s {{FontFace/status}} attribute is "loading":
1. If the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list is empty,
<a>switch the FontFaceSet to loading</a>.
2. Append <var>font</var> to the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list.
3. Return the {{FontFaceSet}}.
: <dfn method>delete(font)</dfn>
::
When the {{delete()}} method is called,
execute the following steps:
1. If <var>font</var> is <a>CSS-connected</a>,
return <code class='lang-javascript'>false</code>
and exit this algorithm immediately.
2. Let <var>deleted</var> be the result of
8DF9
removing <var>font</var> from the {{FontFaceSet}}’s [=FontFaceSet/set entries=].
3. If <var>font</var> is present in the {{FontFaceSet}}’s {{[[LoadedFonts]]}}, or {{[[FailedFonts]]}} lists,
remove it.
4. If <var>font</var> is present in the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list,
remove it.
If <var>font</var> was the last item in that list
(and so the list is now empty),
<a>switch the FontFaceSet to loaded</a>.
5. Return <var>deleted</var>.
: <dfn method>clear()</dfn>
::
When the {{clear()}} method is called,
execute the following steps:
1. Remove all non-<a>CSS-connected</a> items from the {{FontFaceSet}}’s [=FontFaceSet/set entries=],
its {{[[LoadedFonts]]}} list,
and its {{[[FailedFonts]]}} list.
2. If the {{FontFaceSet}}’s {{[[LoadingFonts]]}} list is non-empty,
remove all items from it,
then <a>switch the FontFaceSet to loaded</a>.
</div>
{{FontFaceSet}} objects also have internal
<dfn attribute for=FontFaceSet>\[[LoadingFonts]]</dfn>,
<dfn attribute for=FontFaceSet>\[[LoadedFonts]]</dfn>,
and <dfn attribute for=FontFaceSet>\[[FailedFonts]]</dfn> slots,
all of which are initialized to empty lists,
and a <dfn attribute for=FontFaceSet>\[[ReadyPromise]]</dfn> slot,
which is initialized to a fresh pending {{Promise}}.
Because font families are loaded only when they are used,
content sometimes needs to understand when the loading of fonts occurs.
Authors can use the events and methods defined here to allow greater control over actions
that are dependent upon the availability of specific fonts.
A {{FontFaceSet}} is <dfn export for="FontFaceSet">pending on the environment</dfn> if any of the following are true:
* the document is still loading
* the document has pending stylesheet requests
* the document has pending layout operations which might cause the user agent to request a font,
or which depend on recently-loaded fonts
Note: The idea is that once a {{FontFaceSet}} stops being <a>pending on the environment</a>,
as long as nothing further changes the document,
an author can depend on sizes/positions of things being "correct" when measured.
If the above conditions do not fully capture this guarantee,
they need to be amended to do so.
<!--
████████ ██ ██ ████████ ██ ██ ████████ ██████
██ ██ ██ ██ ███ ██ ██ ██ ██
██ ██ ██ ██ ████ ██ ██ ██
██████ ██ ██ ██████ ██ ██ ██ ██ ██████
██ ██ ██ ██ ██ ████ ██ ██
██ ██ ██ ██ ██ ███ ██ ██ ██
████████ ███ ████████ ██ ██ ██ ██████
-->
<h3 id='FontFaceSet-events'>
Events</h3>
Font load events make it easy to respond to the font-loading behavior of the entire document,
rather than having to listen to each font specifically.
The <dfn event for="FontFaceSet">loading</dfn> event
fires when the document begins loading fonts,
while the <dfn event for="FontFaceSet">loadingdone</dfn>
and <dfn event for="FontFaceSet">loadingerror</dfn> events
fire when the document is done loading fonts,
containing the fonts that successfully loaded
or failed to load,
respectively.
The following are the event handlers (and their corresponding event handler event types)
that must be supported by <code>FontFaceSet</code> objects as IDL attributes:
<table class="data" id="eventhandlers">
<thead>
<tr>
<th>Event handler
<th>Event handler event type
<tbody>
<tr>
<th>{{onloading}}
<td>{{loading!!event}}
<tr>
<th>{{onloadingdone}}
<td>{{loadingdone}}
<tr>
<th>{{onloadingerror}}
<td>{{loadingerror}}
</table>
To <dfn>fire a font load event</dfn> named <var>e</var>
at a {{FontFaceSet}} <var>target</var>
with optional <var>font faces</var>
means to
<a href="https://www.w3.org/TR/html5/webappapis.html#event-firing">fire a simple event</a> named <var>e</var>
using the {{FontFaceSetLoadEvent}} interface that also meets these conditions:
<ol>
<li>
The {{FontFaceSetLoadEvent/fontfaces}} attribute is initialized to
the result of filtering <var>font faces</var> to only contain {{FontFace}} objects contained in <var>target</var>.
</ol>
When asked to <dfn>switch the FontFaceSet to loading</dfn> for a given {{FontFaceSet}},
the user agent must run the following steps:
<ol>
<li>
Let <var>font face set</var> be the given {{FontFaceSet}}.
<li>
Set the {{FontFaceSet/status}} attribute of <var>font face set</var> to "loading".
<li>
If <var>font face set's</var> {{[[ReadyPromise]]}} slot currently holds a fulfilled promise,
replace it with a fresh pending promise.
<li>
Queue a task to <a>fire a font load event</a> named {{loading!!event}} at <var>font face set</var>.
</ol>
When asked to <dfn>switch the FontFaceSet to loaded</dfn> for a given {{FontFaceSet}},
the user agent must run the following steps:
1. Let <var>font face set</var> be the given {{FontFaceSet}}.
2. If <var>font face set</var> is <a>pending on the environment</a>,
mark it as <dfn export for=FontFaceSet>stuck on the environment</dfn>,
and exit this algorithm.
3. Set <var>font face set’s</var> {{FontFaceSet/status}} attribute to "loaded".
4. Fulfill <var>font face set's</var> {{[[ReadyPromise]]}} attribute's value with <var>font face set</var>.
5. Queue a task to perform the following steps synchronously:
1. Let <var>loaded fonts</var> be the (possibly empty) contents of <var>font face set's</var> {{[[LoadedFonts]]}} slot.
2. Let <var>failed fonts</var> be the (possibly empty) contents of <var>font face set's</var> {{[[FailedFonts]]}} slot.
3. Reset the {{[[LoadedFonts]]}} and {{[[FailedFonts]]}} slots to empty lists.
4. <a>Fire a font load event</a> named {{loadingdone}} at <var>font face set</var>
with <var>loaded fonts</var>.
5. If <var>font face set's</var> <var>failed fonts</var> is non-empty,
<a>fire a font load event</a> named {{loadingerror}} at <var>font face set</var>
with <var>failed fonts</var>.
Whenever a {{FontFaceSet}} goes from <a>pending on the environment</a> to not <a>pending on the environment</a>,
the user agent must run the following steps:
1. If the {{FontFaceSet}} is <a>stuck on the environment</a> and its {{[[LoadingFonts]]}} list is empty,
<a>switch the FontFaceSet to loaded</a>.
2. If the {{FontFaceSet}} is <a>stuck on the environment</a>,
unmark it as such.
If asked to <dfn export>find the matching font faces</dfn>
from a FontFaceSet <var>source</var>,
for a given font string <var>font</var>
optionally some sample text <var>text</var>,
and optionally an <var>allow system fonts</var> flag,
run the following steps:
<ol>
<li>
Parse <var>font</var>
using the CSS value syntax of the 'font' property.
If a syntax error occurs,
return a syntax error.
If the parsed value is a <a>CSS-wide keyword</a>,
return a syntax error.
Absolutize all relative lengths against the initial values of the corresponding properties.
(For example, a relative font weight like ''bolder'' is evaluated against the initial value ''font-weight/normal''.)
<li>
If <var>text</var> was not explicitly provided,
let it be a string containing a single space character (U+0020 SPACE).
<li>
Let <var>font family list</var> be the list of font families parsed from <var>font</var>,
and <var>font style</var> be the other font style attributes parsed from <var>font</var>.
<li>
Let <var>available font faces</var> be the [=available font faces=] within <var>source</var>.
If the <var>allow system fonts</var> flag is specified,
add all system fonts to <var>available font faces</var>.
<li>
Let <var>matched font faces</var> initially be an empty list.
<li>
For each family in <var>font family list</var>,
use the font matching rules to select the font faces from <var>available font faces</var>
that match the <var>font style</var>,
and add them to <var>matched font faces</var>.
The use of the {{FontFace/unicodeRange}} attribute means that this may be more than just a single font face.
<li>
If <var>matched font faces</var> is empty,
set the <var>found faces</var> flag to false.
Otherwise, set it to true.
<li>
For each font face in <var>matched font faces</var>,
if its defined 'unicode-range!!descriptor' does not include the codepoint of at least one character in <var>text</var>,
remove it from the list.
Note: Therefore, if <var>text</var> is the empty string, every font will be removed.
<li>
Return <var>matched font faces</var> and the <var>found faces</var> flag.
</ol>
<!--
██ ███████ ███ ████████ ███ ███
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ █████████ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
████████ ███████ ██ ██ ████████ ███ ███
-->
<h3 id='font-face-set-load'>
The <code>load()</code> method</h3>
The {{FontFaceSet/load()}} method of {{FontFaceSet}} will determine whether all fonts in the given font list
have been loaded and are available.
If any fonts are downloadable fonts and have not already been loaded,
the user agent will initiate the load of each of these fonts.
It returns a Promise,
which is fulfilled when all of the fonts are loaded and ready to be used,
or rejected if any font failed to load properly.
When the <dfn method for="FontFaceSet" lt="load(font, text)">load</dfn>(
<span dfn-for="FontFaceSet/load(font, text)">
<dfn argument>font</dfn>,
<dfn argument>text</dfn>
</span>
) method is called,
execute these steps:
<ol>
<li>
Let <var>font face set</var> be the {{FontFaceSet}} object this method was called on.
Let <var>promise</var> be a newly-created promise object.
<li>
Return <var>promise</var>.
Complete the rest of these steps asynchronously.
<li>
<a>Find the matching font fac
E0A7
es</a> from <var>font face set</var>
using the {{FontFaceSet/load()/font}} and {{FontFaceSet/load()/text}} arguments passed to the function,
and let <var>font face list</var> be the return value
(ignoring the <var>found faces</var> flag).
If a syntax error was returned,
reject <var>promise</var> with a SyntaxError exception
and terminate these steps.
<li>
Queue a task to run the following steps synchronously:
<ol>
<li>
For all of the font faces in the <var>font face list</var>,
call their {{FontFace/load()}} method.
<li>
Resolve <var>promise</var> with the result of
waiting for all of the {{[[FontStatusPromise]]}}s of each font face in the <var>font face list</var>, in order.
</ol>
</ol>
<!--
██████ ██ ██ ████████ ██████ ██ ██ ███ ███
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ █████████ ██████ ██ █████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██████ ██ ██ ████████ ██████ ██ ██ ███ ███
-->
<h3 id='font-face-set-check'>
The <code>check()</code> method</h3>
The {{FontFaceSet/check()}} method of {{FontFaceSet}} will determine whether you can "safely"
render some provided text with a particular font list,
such that it won't cause a "font swap" later.
If the given text/font combo will render without attempting to use any unloaded or currently-loading fonts,
this method will return true;
otherwise, it returns false.
<div class=note>
Two special cases in this method's behavior should be noted,
as they are non-obvious:
* If the specified fonts exist,
but all possible faces are ruled out due to their 'unicode-range!!descriptor' not covering the provided text,
the method returns <code>true</code>,
as the text will be rendered in the UA's fallback font instead,
and won't trigger any font loads.
* Likewise, if none of the specified fonts exist (for example, names are mis-spelled),
the method also returns <code>true</code>,
because using this font list will not trigger any loads;
instead, fallback will occur.
</div>
When the <dfn method for="FontFaceSet" lt="check(font, text)|check(font)">check</dfn>(
<span data-dfn-for="FontFaceSet/check(font, text)"><dfn argument>font</dfn>, <dfn argument>text</dfn>)</span>
method is called,
execute these steps:
<ol>
<li>
Let <var>font face set</var> be the {{FontFaceSet}} object this method was called on.
<li>
<a>Find the matching font faces</a> from <var>font face set</var>
using the {{FontFaceSet/check()/font}} and {{FontFaceSet/check()/text}} arguments passed to the function,
and including system fonts,
and let <var>font face list</var> be the returned list of font faces,
and <var>found faces</var> be the returned <var>found faces</var> flag.
If a syntax error was returned,
throw a SyntaxError exception
and terminate these steps.
<li>
If <var>font face list</var> is empty,
or all fonts in the <var>font face list</var> either have a {{FontFace/status}} attribute of "loaded" or are system fonts,
return <code>true</code>.
Otherwise, return <code>false</code>.
</ol>
<!--
████████ ████████ ███ ████████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ████
████████ ██████ ██ ██ ██ ██ ██
██ ██ ██ █████████ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██
██ ██ ████████ ██ ██ ████████ ██
-->
<h3 id='font-face-set-ready'>
The <code>ready</code> attribute</h3>
Because the number of fonts loaded depends on the how many fonts are used for a given piece of text,
in some cases whether fonts need to be loaded or not may not be known.
The {{FontFaceSet/ready}} attribute contains a {{Promise}} which is resolved when the document is done loading fonts,
which provides a way for authors to avoid having to keep track of which fonts have or haven't been loaded
before examining content which may be affected by loading fonts.
Note: Authors should note that a given <var>ready promise</var> is only fulfilled once,
but further fonts may be loaded after it fulfills.
This is similar to listening for a {{loadingdone}} event to fire,
but the callbacks passed to the {{FontFaceSet/ready}} promise will <strong>always</strong> get called,
even when no font loads occur because the fonts in question are already loaded.
It's a simple, easy way to synchronize code to font loads
without the need to keep track of what fonts are needed and precisely when they load.
Note: Note that the user agent may need to iterate over multiple font loads before the <var>ready promise</var> is fulfilled.
This can occur with font fallback situations,
where one font in the fontlist is loaded
but doesn't contain a particular glyph
and other fonts in the fontlist need to be loaded.
The <var>ready promise</var> is only fulfilled after layout operations complete
and no additional font loads are necessary.
Note: Note that the Promise returned by this {{FontFaceSet/ready}} attribute is only ever fulfilled,
never rejected,
unlike the Promise returned by the {{FontFace}} {{FontFace/load()}} method.
<h3 id='font-face-set-css'>
Interaction with CSS Font Loading and Matching</h3>
When the font matching algorithm in [[CSS-FONTS-3]] is run automatically by the user-agent,
the set of font faces it matches over must be precisely the set of fonts in the <a>font source</a> for the document,
plus any local font faces.
When a user-agent needs to load a font face,
it must do so by calling the {{FontFace/load()}} method
of the corresponding {{FontFace}} object.
(This means it must run the same algorithm,
not literally call the value currently stored in the <code>load</code> property of the object.)
<div class="example" highlight=js>
Fonts are available when they are added to a {{FontFaceSet}}.
Adding a new ''@font-face'' rule to a stylesheet
also adds a new {{FontFace}} to the {{FontFaceSet}} of the {{Document}} object.
Adding a new ''@font-face'' rule:
<pre>
document.styleSheets[0].insertRule(
"@font-face { font-family: newfont; src: url(newfont.woff); }", 0);
document.body.style.fontFamily = "newfont, serif";
</pre>
Constructing a new {{FontFace}} object and adding it to <code>document.fonts</code>:
<pre>
var f = new FontFace("newfont", "url(newfont.woff)");
document.fonts.add(f);
document.body.style.fontFamily = "newfont, serif";
</pre>
In both cases, the loading of the font resource “newfont.woff” will be initiated by the layout engine,
just as other ''@font-face'' rule fonts are loaded.
Omitting the addition to <code>document.fonts</code> means the font would never be loaded
and text would be displayed in the default serif font:
<pre>
var f = new FontFace("newfont", "url(newtest.woff)", {});
/* new {{FontFace}} not added to {{FontFaceSet}},