-
Notifications
You must be signed in to change notification settings - Fork 792
Expand file tree
/
Copy pathOverview.html
More file actions
1060 lines (843 loc) · 47.9 KB
/
Overview.html
File metadata and controls
1060 lines (843 loc) · 47.9 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
4
10BC0
18
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
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang=en>
<head><meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Image Values Module Level 3</title>
<link href="../default.css" rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED" rel=stylesheet
type="text/css">
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS Image Values Module Level 3</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 5 May 2010</h2>
<dl>
<dt>This version:</dt>
<!-- <dd><a href="http://www.w3.org/TR/2010/CR-css3-images-20100505/">http://www.w3.org/TR/2010/CR-css3-images-20100505/</a></dd> -->
<dd><a
href="http://dev.w3.org/csswg/css3-images/">http://dev.w3.org/csswg/css3-images/</a>
<!-- <dt>Latest version:</dt>
<dd><a href="http://www.w3.org/TR/css3-images/">http://www.w3.org/TR/css3-images/</a></dd>
<dt>Previous version:</dt>
<dd><a href="http://www.w3.org/TR/2008/CR-css3-namespace-20080523/">http://www.w3.org/TR/2008/CR-css3-namespace-20080523/</a></dd> -->
<dt>Editor:
<dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>
<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2010 <a
href="http://www.w3.org/"><acronym title="World Wide Web
Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and
Mathematics">ERCIM</acronym></a>, <a
href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>This CSS Image Values and Replaced Content module has two parts: First,
it defines the syntax for <image> values in CSS. <image>
values can be a single URI to an image, a list of URIs denoting a series
of fallbacks, sprites (image slices), or gradients. Second, it defines
properties used to control the interaction of replaced content and the CSS
layout algorithms. These properties can affect the used image resolution
for bitmaps, the replaced object's orientation, and whether and how to
preserve the object's aspect ratio.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss3-images%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“css3-images” in the subject, preferably like this:
“[<!---->css3-images<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#intro"><span class=secno>1. </span>Introduction</a>
<li><a href="#conformance"><span class=secno>2. </span>Conformance</a>
<li><a href="#image"><span class=secno>3. </span>Image Values: the
<image> value type</a>
<li><a href="#url"><span class=secno>4. </span>Image References and Image
Slices: the ‘<code class=css>url()</code>’ notation</a>
<li><a href="#image-notation"><span class=secno>5. </span>Image Fallbacks:
the ‘<code class=css>image()</code>’ notation</a>
<li><a href="#image-sprites"><span class=secno>6. </span>Image Sprites</a>
<li><a href="#gradients-"><span class=secno>7. </span>Gradients </a>
<ul class=toc>
<li><a href="#linear-gradients"><span class=secno>7.1. </span>Linear
Gradients</a>
<ul class=toc>
<li><a href="#color-stop-syntax"><span class=secno>7.1.2.
</span><code>color-stop</code> Syntax</a>
</ul>
<li><a href="#radial-gradients"><span class=secno>7.2. </span>Radial
Gradients</a>
</ul>
<li><a href="#view-boxes-or-sizing-images-and-objects-"><span
class=secno>8. </span>View Boxes, or Sizing Images and Objects in CSS</a>
<li><a href="#resolution-units-the-ltresolutiongt-valu"><span
class=secno>9. </span>Resolution Units: the <resolution> value
type</a>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<ul class=toc>
<li class=no-num><a href="#normative-references">Normative
references</a>
<li class=no-num><a href="#informative-references">Informative
references</a>
</ul>
</ul>
<!--end-toc-->
<div>
<h2 id=intro><span class=secno>1. </span>Introduction</h2>
<p><em>This section is non-normative.</em></p>
<p>In CSS Levels 1 and 2, image values, such as those used in the
‘<code class=property>background-image</code>’ property,
could only be given by a single URI value. This module introduces
additional notations that allow a 2D image to be given as a list of URIs
denoting fallbacks, as a slice of a larger image (sprite), and as a
gradient.</p>
</div>
<div>
<h2 id=conformance><span class=secno>2. </span>Conformance</h2>
<p>A document or implementation cannot conform to CSS Image Values Level 3
alone, but can claim conformance to CSS Image Values Level 3 if it
satisfies the conformance requirements in this specification when
implementing CSS or another host language that normatively references
this specification.</p>
<p>Conformance to CSS Namespaces is defined for two classes:
<dl>
<dt><dfn id=style-sheet>style sheet</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
style sheet</a> (or a complete unit of another host language that
normatively references CSS Namespaces).
<dt><dfn id=interpreter>interpreter</dfn>
<dd>Someone or something that interprets the semantics of a style sheet.
(CSS <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user
agents</a> fall under this category.)
</dl>
<p>The conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this
document are to be interpreted as described in RFC 2119. However, for
readability, these words do not appear in all uppercase letters in this
specification. All of the text of this specification is normative except
sections explicitly marked as non-normative, examples, and notes. <a
href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a></p>
<p>Examples in this specification are introduced with the words "for
example" or are set apart from the normative text with
<code>class="example"</code>, like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word "Note" and are set apart from the
normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.</p>
</div>
<div>
<h2 id=image><span class=secno>3. </span>Image Values: the <image>
value type</h2>
<p>The <image> value type denotes a 2D image. It is defined as
<pre
class=prod><dfn id=ltimagegt><image></dfn> = <url> | <sprite> | <image-list> | <linear-gradient> | <radial-gradient></pre>
<p>Image values can be used in many CSS properties, including the
‘<code class=property>background-image</code>’, ‘<code
class=property>list-style-image</code>’, ‘<code
class=property>cursor</code>’ properties <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
</div>
<div>
<h2 id=url><span class=secno>4. </span>Image References and Image Slices:
the ‘<code class=css>url()</code>’ notation</h2>
<p>The simplest way to indicate an image is to reference an image file by
URI. This is done with the <a
href="http://www.w3.org/TR/CSS21/syndata.html#uri">‘<code
class=css>url()</code>’ notation</a>, defined in <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<div class=example>
<p>In the example below, a background image is specified with
‘<code class=css>url()</code>’ syntax:</p>
<pre>background-image: url(wavy.png);</pre>
</div>
<p>A portion of an image may be referenced (clipped out and used as a
standalone image) by use of fragment identifiers. <span class=issue>Need
a spec to reference here. Expecting to get one from <a
href="http://www.w3.org/2008/WebVideo/Fragments/">Media Fragments
WG</a>.</span>
<div class=example>
<p>For example,</p>
<pre>background-image: url('logos.png#xywh=10,30,60,20')</pre>
<p>uses the 60 pixel by 20 pixel rectangle of <code>logos.png</code>
beginning at the point 10 pixels in from the left, 30 pixels down from
the top.
<p class=note>Note that quotation marks are required here, because
unquoted commas are not allowed in ‘<code
class=css>url()</code>’ syntax.</p>
</div>
</div>
<div>
<h2 id=image-notation><span class=secno>5. </span>Image Fallbacks: the
‘<code class=css>image()</code>’ notation</h2>
<p>The ‘<code class=css>image()</code>’ notation allows an
author to specify multiple images, each one a fallback for the previous.
The UA must use only the first image that it can load and display. The
syntax for ‘<code class=css>image()</code>’ is defined as
<pre class=prod><dfn id=ltimage-listgt><image-list></dfn> = <!--
-->image( [ <image-decl> , ]* [ <image-decl> ] [ or <color> ]? )</pre>
<p>where <image-decl> is given by
<pre class=prod><dfn id=ltimage-declgt><image-decl></dfn> = <!--
-->[ <string> | <url-token> ] [ snap? && <resolution> ]?</pre>
<p><url-token> is given as
<code>[!#$%&*-~]|{nonascii}|{escape}</code> (i.e. the contents of
‘<code class=css>url()</code>’) using the productions in the
<a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">CSS2.1
tokenization</a>. <strong>The <url-token> must not contain
unescaped brackets, commas, white space characters, single quotes (') or
double quotes ("); if it does the ‘<code
class=css>image()</code>’ containing it is invalid.</strong>
<p>Each string or url-token represents the URI of an image. If a
resolution is given, then the image must be rendered at the specified
resolution. If the ‘<code class=css>snap</code>’ keyword is
also specified, and the image is a raster image, then the image must be
rendered at the resolution closest to the specified resolution that would
result in no pixel rounding. <span class=issue>I don't think "no pixel
rounding" is the right terminology here... basically we want to avoid
blurry images.</span></p>
<p>The optional color at the end of the list is the <dfn
id=fallback-color>fallback color</dfn>. It is used if, and only if, none
of the images can be loaded and displayed (e.g. because they are in
unsupported formats, or cannot be found, or are corrupted in some way, or
because image loading is disabled in the UA). It is treated as a
single-color image with no intrinsic dimensions.
<div class=example>
<p>For example, the rule below would tell the UA to load ‘<code
class=css>wavy.svg</code>’ if it can; failing that to load
‘<code class=css>wavy.png</code>’ and display it at 150dpi;
failing that to display ‘<code class=css>wavy.gif</code>’;
and finally, if none of the images can be loaded and displayed, to use
the color ‘<code class=css>blue</code>’ to create a
dimensionless background image.
<pre>background-image: image(wavy.svg, 'wavy.png' 150dpi, "wavy.gif" or blue);</pre>
<p>The ‘<code class=property>background-image</code>’
property specifies that dimensionless images must stretch to cover the
entire background positioning area <a href="#CSS3BG"
rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a>, so if none of the
specified images can be displayed the background will be painted blue.
As with any image, this fallback will be painted over the ‘<code
class=property>background-color</code>’ (if any).
</div>
<p>If the URL for the image ends in a file extension (a period followed by
letters) of four ascii letters ([a-zA-Z]) or less, then the UA
<em>may</em> skip trying to load images whose file extension matches an
image format it knows it does not support. For example, a UA that knows
it supports JPEG and GIF, but expects to encounter many PNG and SVG
images may choose to skip over images with filenames ending in
‘<code class=css>.png</code>’ or ‘<code
class=css>.svg</code>’ without loading them to check their MIME
type. However it must not skip over images with filenames ending in
extensions it doesn't recognize.
</div>
<div>
<h2 id=image-sprites><span class=secno>6. </span>Image Sprites</h2>
<p>The image sprites syntax is different from image slices noted above.
Image slice syntax allows you to specify the coordinates of a region in
the image and use that as a standalone image, but all the coordinates
must be specified inline. This is great for one-off cutouts, but becomes
unnecessarily verbose when the slices fit into a grid structure. The goal
of this section is to define a syntax that:
<ul>
<li>Makes it easy to specify an image slice and swap it out in later
subclass or :hover rules.
<li>Makes it easy to maintain a sprite image, so that adding or shifting
sections of the image don't require crawling through and modifying the
entire style sheet.
<li>Has straightforward handling of strict grids, but can also handle
more irregular sprite organization.
<li>(Any other use cases/requirements?)
</ul>
<div class=issue>
<p>As yet unwritten. Discussions/proposals include:</p>
<ul>
<li><a
href="http://lists.w3.org/Archives/Public/www-style/2009May/0157.html">Jorrit's
@sprite proposal</a>
<li><a
href="http://www.css3.info/summary-of-the-two-current-css-constants-proposals/#comment-212898">Adrian's
image-vars proposal</a>
<li><a
href="http://lists.w3.org/Archives/Public/www-style/2009Jan/0185.html">Discussion
from Jan 2008</a> and <a
href="http://lists.w3.org/Archives/Public/www-style/2009Jan/0284.html">Jorrit's
response</a>
</ul>
</div>
</div>
<div>
<h2 id=gradients-><span class=secno>7. </span>Gradients</h2>
<p>A gradient is a browser-generated image specified entirely in CSS,
which consists of smooth fades between several colors. There are two
basic kinds of gradients currently defined, linear and radial. These are
specified by the <code>linear-gradient()</code> and
<code>radial-gradient()</code> functions, and can be used any place an
image can currently be used.</p>
<p>In many places this specification references a box, such as "the box's
top-left corner" or "the box's right side". In all of these
circumstances, the "box" refers to the ‘<a
href="#css-view-box"><code class=css>CSS View Box</code></a>’ (see
the "View Boxes" section of this spec for clarification). A gradient's
‘<code class=css>Image View Box</code>’ has no intrinsic
dimensions. This means that, for example, if you use a gradient in a
background-image, the "box" will simply be the size of the background
area. If you use a gradient in a list-style-image, the "box" will be a
1em square.</p>
<div class=example>
<p>As with the other <image> types defined in this specification,
gradients can be used in any property that accepts images. For example:</p>
<ul>
<li><code>background: linear-gradient(white, gray);</code>
<li><code>list-style-image: radial-gradient(circle, #006, #00a 90%,
#0000af 100%, white 100%)</code>
</ul>
</div>
<div>
<h3 id=linear-gradients><span class=secno>7.1. </span>Linear Gradients</h3>
<p>A linear gradient is created by specifying a gradient-line and then
several colors placed along that line. The image is constructed by
creating an infinite canvas and painting it with lines perdendicular to
the gradient-line, with the color of the painted line being the color of
the gradient-line where the two intersect.</p>
<h4 class=no-toc id=linear-gradient-syntax><span class=secno>7.1.1.
</span><code>linear-gradient()</code> syntax</h4>
<pre
class=prod><code>linear-gradient([<bg-position> || <angle>,]? <color-stop>, <color-stop>[, <color-stop>]*);</code></pre>
<p>The first argument to the function specifies the
<strong>gradient-line</strong>, which gives the gradient a direction and
determines how color-stops are positioned. It may be omitted; if so, it
defaults to "<code>top</code>".</p>
<p>The gradient's color stops are typically placed between the
starting-point and ending-point on the gradient-line, but this is not
required. The starting-point and ending-point are merely arbitrary
distance markers - the starting-point defines where 0%, 0px, etc are
located when specifying color-stops, and the ending-point defines where
100% is located. Color stops are allowed to have positions b
4B74
efore 0% or
after 100%.</p>
<p><code><bg-position></code> is taken from the Backgrounds and
Borders Module, and has the same definition. <code><angle></code>
is from the Values Module, and has the same definition.</p>
<p>If only a <code><bg-position></code> is specified, it specifies
the starting-point of the gradient-line. The ending-point is obtained by
rotating the starting-point 180 degrees around the center of the box.</p>
<p>If only a <code><angle></code> is specified, it specifies the
direction of the gradient-line, and the starting-point and ending-point
of the gradient-line are implied by the value of the
<code><angle></code> as follows:
<ul>
<li>If the <code><angle></code> is greater than or equal to 0deg
and less then 90deg, the starting-point of the gradient-line is the
bottom-left corner of the gradient-box.
<li>If the <code><angle></code> is greater than or equal to 90deg
and less then 180deg, the starting-point of the gradient-line is the
bottom-right corner of the gradient-box.
<li>If the <code><angle></code> is greater than or equal to 180deg
and less then 270deg, the starting-point of the gradient-line is the
top-right corner of the gradient-box.
<li>If the <code><angle></code> is greater than or equal to 270deg
and less then 360deg, the starting-point of the gradient-line is the
top-left corner of the gradient-box.
</ul>
<p>The ending-point of the gradient-line is the point on the
gradient-line where a line drawn perpendicular to the gradient-line
would intersect the opposite corner from the starting-point.</p>
<p>If both a <code><bg-position></code> and
<code><angle></code> are specified, the starting-point of the
gradient-line is determined by the <code><bg-position></code>, the
gradient-line extends at the <code><angle></code>, and the
ending-point is determined in a way similar to the previous case - based
on the <code><angle></code>, determine a "starting corner" and an
opposing "ending corner", and set the ending-point of the gradient-line
as the point on the gradient-line where a line drawn perpendicular to
the gradient-line would intersect the "ending corner".</p>
<p class=note>Note that, in some cases, this may cause the gradient to
"reverse direction". For example, if you specify "-10px -10px 135deg" as
the first argument, it produces a display identical to if you had had
specified "-10px -10px 315deg", because the "ending corner" is "behind"
the starting-point of the gradient-line.</p>
<p>If both are omitted, it is equivalent to simply specifying "top" for
the first argument. That is, it defines a vertical gradient starting at
the top and ending at the bottom.</p>
<p>If both starting-point and ending-point are the same, then the
gradient is just a solid color, defined by the last color-stop in the
list. For example, "left top 90deg" will result in a solid-color
gradient (since it has 90deg, the "ending corner" is the top-left, which
is where it starts), as will "center" (to obtain the ending-point, you
rotate the starting-point around the center - rotating the center around
the center gives you the center).</p>
<div class=example> This example illustrates visually how to calculate
the gradient-line from the rules above. This shows the starting and
ending-point of the gradient-line, along with the actual gradien
C04C
t,
produced by an element with ‘<code class=css>background:
linear-gradient(45deg, white, black);</code>’.
<p><img alt src=gradient-line-example.png></p>
</div>
<h4 id=color-stop-syntax><span class=secno>7.1.2.
</span><code>color-stop</code> Syntax</h4>
<p>The <dfn id=ltcolor-stopgt><code><color-stop></code></dfn>
stands for:</p>
<pre
class=prod><code><color> [ <percentage> | <length> ]?</code></pre>
<p>Color-stops are points placed along the line defined by the
gradient-line at the beginning of the rule. Color-stops must be
specified in order. Percentages refer to the length of the
gradient-line, with 0% being at the starting point and 100% being at the
ending point. Lengths are measured from the starting-point in the
direction of the ending-point. With either measure, color-stops may be
placed before the starting-point or after the ending-point; this is
perfectly valid, as the starting-point and ending-point are just
convenient references, and do not directly interact in the display of
the gradient. At each color-stop, the line is the color of the
color-stop. Before the first color-stop, the line is the color of the
first color-stop. After the last color-stop, the line is the color of
the last color-stop. Between two color-stops, the colors are
interpolated as SVG gradients.</p>
<p>The following steps must be applied <em>in order</em> to process the
list of color-stops. After applying these rules, all color-stops will
have a definite position and they will be in ascending order:</p>
<ol>
<li>If the first color-stop does not have a position, its position
defaults to 0%. If the last color-stop does not have a position, its
position defaults to 100%.
<li>If a color-stop has a position that is less than the specified
position of any color-stop before it in the list, its position is
changed to be equal to the largest specified position of any color-stop
before it.
<li>If any color-stop still does not have a position, then, for each run
of adjacent color-stops without positions, set their positions so that
they are evenly spaced between the preceding and following color-stops
with positions.
</ol>
<p>If multiple color-stops have the same position, they produce an
infinitesimal transition from the one specified first in the rule to the
one specified last. In effect, the color suddenly changes at that
position rather than smoothly transitioning.</p>
<p class=note>It is recommended that authors not mix different types of
units, such as px, em, or %, in a single rule, as this can cause a
color-stop to unintentionally try to move before an earlier one. For
example, the rule <code>background-image: linear-gradient(red, yellow
100px, blue 50%)</code> would work as expected as long as the background
area is at least 200px tall. If it was 150px tall, however, the blue
color-stop would be specifying a position of "75px", which precedes the
yellow color-stop, and would be corrected to a position of 100px.</p>
<h4 class=no-toc id=linear-gradient-examples><span class=secno>7.1.3.
</span>Linear Gradient Examples</h4>
<p>All of the following <code>linear-gradient()</code> examples are
presumed to be backgrounds applied to a box that is 200px wide and 100px
tall.</p>
<div class=example>
<p>Below are various ways of specifying a basic vertical gradient:</p>
<pre><code>linear-gradient(top, yellow, blue);
linear-gradient(bottom, blue, yellow);
linear-gradient(-90deg, yellow, blue);
linear-gradient(270deg, yellow, blue);
linear-gradient(top, yellow 0%, blue 100%);
linear-gradient(center top, yellow, blue);
linear-gradient(yellow, blue);</code></pre>
<p><img alt="" src=linear1.png></p>
</div>
<div class=example>
<p>This gradient goes from the upper-left to the lower-right corner.</p>
<pre><code>linear-gradient(top left, yellow, blue);
linear-gradient(0 0, yellow, blue);
linear-gradient(top 0px left 0px, yellow, blue);
linear-gradient(bottom right, blue, yellow);</code></pre>
<p><img alt="" src=linear2.png></p>
</div>
<div class=example>
<p>This demonstrates the use of an angle in the gradient. Compare this
image with the previous example. In both gradients, the top-left of the
box is pure yellow, and the bottom-right of the box is pure blue. The
difference is in the angle that the gradient follows.</p>
<pre><code>linear-gradient(-45deg, yellow, blue);
linear-gradient(315deg, yellow, blue);</code></pre>
<p><img alt="" src=linear3.png></p>
</div>
<div class=example>
<p>This demonstrates a 3-color gradient, and how to specify the location
of a stop explicitly:</p>
<pre><code>linear-gradient(top, yellow, blue 20%, #0f0);</code></pre>
<p><img alt="" src=linear4.png></p>
</div>
<div class=example>
<p>Here we use the full background-position syntax to specify the
direction of the gradient. Notice that in this example a substantial
portion of the image is pure yellow and pure blue, because the starting
and ending points of the gradient-line are not on the box edge, but
rather somewhere within the box.</p>
<pre><code>linear-gradient(20px 30px, yellow, blue);
linear-gradient(left 20px top 30px, yellow, blue);</code></pre>
<p><img alt="" src=linear5.png></p>
</div>
</div>
<div>
<h3 id=radial-gradients><span class=secno>7.2. </span>Radial Gradients</h3>
<p>A radial gradient is specified in a manner very similar to a linear
gradient. You define a gradient-line by providing a starting-point and
an angle and place color-stops along this line, but also specify a
gradient-shape that determines where the gradient-line ends and the
overall appearance of the gradient image. The image is constructed by
creating an infinite canvas and painting it with concentric scaled
copies of the ending-shape, with the color of the painted shape being
the color of the gradient-line where the two intersect.</p>
<h4 class=no-toc id=radial-gradient-syntax><span class=secno>7.2.1.
</span><code>radial-gradient()</code> Syntax</h4>
<pre
class=prod><code>radial-gradient([<bg-position> || <angle>,]? [<shape> || <size>,]? <color-stop>, <color-stop>[, <color-stop>]*)</code></pre>
<p>The first argument to the function specifies the
<strong><code>gradient-line</code></strong>.
<code><bg-position></code> is taken from the Backgrounds and
Borders Module, and has the same definition. It specifies the
starting-point of the gradient-line, and the center of the
gradient-shape. If omitted, it defaults to <code>center</code>.
<code><angle></code> is from the Values Module, and has the same
definition. If omitted, it defaults to 0deg. The gradient-line extends
from the starting-point at this angle.</p>
<p>The second argument to the function specifies the
<strong><code>gradient-shape</code></strong>. <code><shape></code>
is defined as <code>[ circle | ellipse ]</code>. <code>circle</code>
indicates that the gradient-shape will be a circle with a constant
radius. <code>ellipse</code> indicates that the gradient-shape will be
an axis-aligned ellipse (that is, its major and minor radiuses will be
horizontal and vertical, not necessarily in that order). If omitted, it
defaults to <code>ellipse</code>.</p>
<p><code><size></code> is defined as <code>[ closest-side |
closest-corner | farthest-side | farthest-corner | contain | cover
]</code>. If <code><shape></code> is <code>circle</code> and
<code><size></code> is <code>closest-side</code>, the ending-shape
is a circle sized so that it exactly meets the side of the box closest
to its center. For example, if the box was 100px wide and 200px tall,
and the starting-point of the gradient-line was <code>'10% 10%'</code>,
then the closest side is the left side of the box (it is 10px from the
starting-point, while the top is 20px from it, and the right and bottom
sides are much further). The gradient-shape would be a circle with a
radius of 10px. If <code><shape></code> is <code>ellipse</code>
and <code><size></code> is <code>closest-side</code>, the
gradient-shape is an ellipse sized so that it exactly meets the vertical
and horizontal sides of the box closest to its center. Using the same
box and starting-point as the previous example, the gradient-shape would
be an ellipse with a 20px vertical radius and a 10px horizontal radius.
(If necessary, such as if the starting-point is outside of the box,
extend the sides of the box so that there is a line the ellipse can
meet.)</p>
<p><code>farthest-side</code> is identical to <code>closest-side</code>,
except that the gradient-shape is sized to meet the side of the box that
is farthest from its center (or the farthest vertical and horizontal
sides, in the case of <code>ellipse</code>). <code>closest-corner</code>
and <code>farthest-corner</code> size the gradient-shape so that it
exactly meets the closest or farthest corner of the box from its center,
respectively. If <code><shape></code> is <code>ellipse</code>, the
gradient-shape has the same ratio of width to height that it would if
<code>closest-side</code> or <code>farthest-side</code> were specified,
as appropriate. <code>contain</code> is a synonym for
<code>closest-side</code>, and <code>cover</code> is a synonym for
<code>farthest-corner</code>. If omitted, <code><size></code>
defaults to <code>cover</code>.</p>
<p>In certain circumstances the given parameters may define a degenerate
shape - a circle or ellipse with a radius of 0. In these instances the
gradient image is just a solid color equal to the color of the last
color-stop in the rule. The following combinations of values will
trigger this: <code>closest-side</code> if the starting-point is on a
box edge, <code>closest-corner</code> if the starting-point is on a box
corner, and <code>ellipse</code> and <code>closest-corner</code> if the
starting-point is on a box edge.</p>
<p>The ending-point of the gradient-line is determined by extending a
line from the starting-point in the direction specified by the
<code><angle></code> in the first argument. The ending-point is
where this line intersects the gradient-shape.</p>
<p><a href="#ltcolor-stopgt"><code><color-stop></code></a> is
defined identically to its treatment in <code>linear-gradient()</code>.
As in linear gradients, 0% is at the starting-point of the
gradient-line, and 100% is at the ending-point. Distances past 100% can
be specified, and simply indicate a color-stop placed on the
gradient-line a corresponding distance from the starting-point. Negative
distances are allowed in a radial gradient and work the same as in
linear gradients with respect to setting the color of the gradient-line,
but colors before the starting-point of the gradient-line are not
displayed. For example, <code>radial-gradient(red -50px, yellow
100px)</code> would produce an elliptical gradient which starts with a
reddish-orange color in the center (the color 1/3 between red and
yellow) and transitions to yellow at 100px wide.</p>
<h4 class=no-toc id=radial-gradient-examples><span class=secno>7.2.2.
</span>Radial Gradient Examples</h4>
<p>All of the following examples are applied to a box that is 200px wide
and 100px tall.</p>
<div class=example>
<p>These examples demonstrate the basic syntax for radial gradients:</p>
<pre><code>radial-gradient(yellow, green);
radial-gradient(center, ellipse cover, yellow 0%, green 100%);
radial-gradient(50% 50%, farthest-corner, yellow, green);</code></pre>
<p><img alt="" src=radial1.png></p>
<pre><code>radial-gradient(circle, yellow, green);</code></pre>
<p><img alt="" src=radial2.png></p>
<pre><code>radial-gradient(red, yellow, green);</code></pre>
<p><img alt="" src=radial3.png></p>
</div>
<div class=example>
<p>The next two images show a gradient originating from somewhere other
than the center of the box:</p>
<pre><code>radial-gradient(bottom left, farthest-side, red, yellow 50px, green);</code></pre>
<p><img alt="" src=radial4.png></p>
<pre><code>radial-gradient(bottom left 90deg, farthest-side, red, yellow 50px, green);</code></pre>
<p><img alt="" src=radial5.png></p>
<p>Pay attention to the differences in these two images. In both of them
the distance between the red color-stop and the yellow color-stop is
50px; the difference is the <em>direction</em> this distance is
measured. By default the angle is 0deg, which means that distances are
measured from the center going right. An angle of 90deg makes it
measure from the center going up. Specifying an angle between 0deg and
90deg would produce a gradient somewhere between these two examples.</p>
</div>
<div class=example>
<p>Here we illustrate a ‘<code
class=property>contain</code>’ gradient.</p>
<pre><code>radial-gradient(20px 30px, contain, red, yellow, green)</code></pre>
<p><img alt="" src=radial6.png></p>
<pre><code>radial-gradient(20px 30px, circle contain, red, yellow, green)</code></pre>
<p><img alt="" src=radial7.png></p>
</div>
</div>
<!-- Removed for lack of a use-case; we'll just leave it as a vendor-prefix for now.
<div>
<h3>Repeating Gradients</h3>
<p>In addition to the <code>linear-gradient()</code> and <code>radial-gradient()</code> functions, this specification defines <code>repeating-linear-gradient()</code> and <code>repeating-radial-gradient()</code> functions. These two functions take the same values and are interpreted the same as their respective non-repeating siblings defined previously. However, the color-stops are repeated infinitely in both directions, with their positions shifted by multiples of the difference between the last color-stop's position and the first color-stop's position. For example, <code>repeating-linear-gradient(red 10px, blue 50px)</code> is equivalent to <code>linear-gradient(..., red -30px, blue 10px, red 10px, blue 50px, red 50px, blue 90px, ...)</code>. Note that the last color-stop and first color-stop will always coincide at the boundaries of each group, which may produce sharp transitions if the gradient does not start and end with the same color.</p>
<div class=example>
<p>Repeating gradient syntax is basically identical to that of non-repeating gradients:</p>
<pre><code>repeating-linear-gradient(red, blue 20px, red 40px)</code></pre>
<p><img src="repeating1.png" alt=""></p>
<pre><code>repeating-radial-gradient(red, blue 20px, red 40px)</code></pre>
<p><img src="repeating2.png" alt=""></p>
<pre><code>repeating-radial-gradient(20px 30px, circle contain, red, yellow, green 100%, yellow 150%, red 200%)</code></pre>
<p><img src="repeating3.png" alt=""></p>
</div>
<p>If the difference in the first and last color-stops is 0 (for example, in the rule <code>linear-gradient(red 10px, blue 10px)</code>), the gradient defines a solid-color image with the color of the last color-stop in the rule (in this case, then, it would simply define a blue image).</p>
</div>
-->
</div>
<div>
<h2 id=view-boxes-or-sizing-images-and-objects-><span class=secno>8.
</span>View Boxes, or Sizing Images and Objects in CSS</h2>
<p>Images used in CSS may come from a number of sources, from defined
image formats (such as gif, jpeg, etc), dedicated markup formats (such as
SVG), and CSS-specific formats (such as the linear-gradient() value type
defined in this specification). As well, a document may conain many other
types of objects, such as video, plugins, or nested documents. These
images and objects (just "objects" hereafter) may offer many types of
sizing information to CSS, or none at all. This section defines
generically the size negotiation model between the image or replaced
content object and the CSS layout algorithms.</p>
<p>In order to define this handling, we must define a few terms, to make
it easier to refer to various concepts:</p>
<dl>
<dt><dfn id=intrinsic-dimensions>intrinsic dimensions</dfn>
<dd>The intrinsic dimensions are defined are the object's preferred,
natural size or aspect ratio, if any. There can be an <dfn
id=intrinsic-height>intrinsic height</dfn> and <dfn
id=intrinsic-width>intrinsic width</dfn>, defining a definite rectangle.
(Most bitmap images fall into this category.) There can be an <dfn
id=intrinsic-aspect-ratio>intrinsic aspect ratio</dfn> defining the
relation of the width to the height, but no definite size. (SVG images
designed to scale may fall into this category.) There can be just an
intrinsic height or width. Or there can be no intrinsic dimensions at
all, implying that the object has no preferred size or aspect ratio.
(Embedded documents are often assumed to have no intrinsic size, as are
CSS gradients, defined in this specification.)
<p>If an object (such as an icon) has multiple sizes, then the largest
size is used. If it has multiple aspect ratios of that size (or of no
size), then the aspect ratio closest to the aspect ratio of the default
image sizing area is use <span class=issue>This is pretty
arbitrary</span>.
<dt><dfn id=specified-size>specified size</dfn>
<dd>The specified size of an object is given by CSS, such as through the
`object-fit` or `background-size` properties. The specified size can be
a definite width and height, a set of constraints, or a combination
thereof.
<dt><dfn id=css-view-box>CSS View Box</dfn>
<dd>The CSS View Box is the result of transforming the intrinsic
dimensions into a concrete size, based on the specified size and the
default image sizing area. A CSS View Box always has a definite height
and width.
<dt><dfn id=default-image-sizing-area>default image sizing area</dfn>
<dd>The default image sizing area is a rectangle with a definite height
and width used to determine the size of the CSS View Box when both the
intrinsic dimensions and specified size are missing dimensions.
</dl>
<p>Images and objects in CSS are sized as follows:</p>
<ol>
<li>When an image or object is specified in a document, such as through
url() value in a background-image property or a @src attribute on an
<img> element, CSS queries the object for its intrinsic size.
<li>Using the intrinsic dimensions and the specified size, CSS then
computes a CSS View Box that defines the size of the region the object
will render in, as follows:
<ol>
<li>If the specified size is just a definite width and height, the CSS
View Box must be that width and height.
<li>If the specified size is just a definite width or just a definite
height, then the CSS View Box must have the same width or height, as
appropriate. The other dimension is calculated as follows:
<ol>
<li>If the object has an intrinsic aspect ratio, the CSS View Box
must have the same aspect ratio..
<li>Otherwise, if the object has an an intrinsic height or intrinsic
width (whichever is missing from the specified size), then the CSS
View Box must have that height or width.
<li>Otherwise, the CSS View Box must have the same width or height
(whichever is missing from the specified size) as the default image
sizing area.
</ol>
<li>If the specified size is neither a definite width nor height, and
has no additional contraints, the dimensions of the CSS View Box must
be computed as follows:
<ol>
<li>If the object has an intrinsic width and an intrinsic height, the
CSS View Box must have that same height and width.
<li>If the object has only an intrinsic width or intrinsic height,
and no intrinsic aspect ratio, the CSS View Box must have that
height or width, and the width or height (whichever is missing from
the intrinsic dimensions) of the default image sizing area.
<li>If the object has only an intrinsic aspect ratio, the CSS View
Box must have that aspect ratio, and additionally be as large as
possible without either its height or width exceeding the height or
width of the default image sizing area.
<li>Otherwise, the CSS View Box must be the size of the default image
sizing area.
</ol>
<li>If the specified size has additional constraints, the CSS View Box
must be sized to satisfy those constraints. (For example,
object-fit:contain specifies slightly more complex handling for sizing
replaced elements, and background-repeat:round can adjust the size
specified in background-size so that the image fits an even number of
times into the background positioning area.)
</ol>
</ol>
<p>After determining the size of the CSS View Box, CSS positions it within
the appropriate area according to any relevant properties, such as
object-position or background-position, and asks the object to render
itself using the dimensions of the CSS View Box. CSS does not define how
objects when the dimensions of the CSS View Box are different than the
object's intrinsic dimensions. The object may adjust itself to match the
dimensions of the CSS View Box in some way, or even render itself larger
or smaller than the CSS View Box to satisfy sizing constraints of its
own.</p>
</div>
<div>
<h2 id=resolution-units-the-ltresolutiongt-valu><span class=secno>9.
</span>Resolution Units: the <resolution> value type</h2>
<p>This specification defines the following units as part of the <dfn
id=ltresolutiongt><resolution></dfn> value type:
<dl>
<dt>dpi
<dd>dots per inch
<dt>dpcm
<dd>dots per centimeter
<dt>dppx
<dd>dots per ‘<code class=css>px</code>’ unit
</dl>
<p class=note>The default resolution of raster images in CSS is
‘<code class=css>1dppx</code>’.</p>
</div>
<h2 class=no-num id=acknowledgments>Acknowledgments</h2>
<h2 class=no-num id=references>References</h2>
<h3 class=no-num id=normative-references>Normative references</h3>
<!--begin-normative-->
<!-- Sorted by label -->
<dl class=bibliography>
<dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
<!---->
<dt id=CSS21>[CSS21]