csswg-drafts/css3-images/Overview.html at d7f94b4a9715a9543cf854eea73bb5845e802afe · w3c/csswg-drafts · GitHub

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
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 and Replaced Content Module Level 3</title>
  <link href="http://dev.w3.org/csswg/default.css" rel=stylesheet
  type="text/css">
  <link href="http://www.w3.org/StyleSheets/TR/W3C-ED" rel=stylesheet
  type="text/css">

  <style type="text/css">
  p { text-indent: 0 !important; margin: .5em 0; }
  </style>

 <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 and Replaced Content Module Level 3</h1>

   <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 9 June 2011</h2>

   <dl>
    <dt>Latest Version:

    <dd><a
     href="http://dev.w3.org/csswg/css3-images/">http://dev.w3.org/csswg/css3-images/</a>


    <dt>Latest Published Version:

    <dd><a
     href="http://www.w3.org/TR/css3-images/">http://www.w3.org/TR/css3-images/</a>

    <dt>Previous Version:

    <dd><a
     href="http://www.w3.org/TR/2011/WD-css3-images-20110217/">http://www.w3.org/TR/2011/WD-css3-images-20110217/</a>

    <dd><a
     href="http://www.w3.org/TR/2009/WD-css3-images-20090723/">http://www.w3.org/TR/2009/WD-css3-images-20090723/</a>

    <dt>Editor:

    <dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>
     (Invited Expert)

    <dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a> (Google)
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2011 <a
    href="http://www.w3.org/"><acronym title="World Wide Web
    Consortium">W3C</acronym></a><sup>&reg;</sup> (<a
    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 <a href="#ltimage"><i>&lt;image></i></a> values
   in CSS. <a href="#ltimage"><i>&lt;image></i></a> values can be a single
   URI to an image, a list of URIs denoting a series of fallbacks, a
   reference to an element in the document, 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.</p>
  <!-- ====================================================================== -->

  <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
   &#8220;css3-images&#8221; in the subject, preferably like this:
   &#8220;[<!---->css3-images<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

  <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="#resolution-units"><span class=secno>3. </span> Resolution
    Units: the &lt;resolution&gt; value type</a>

   <li><a href="#image"><span class=secno>4. </span> Image Values: the
    &lt;image> value type</a>
    <ul class=toc>
     <li><a href="#url"><span class=secno>4.1. </span> Image References and
      Image Slices: the &lsquo;<code class=css>url()</code>&rsquo;
      notation</a>

     <li><a href="#image-notation"><span class=secno>4.2. </span> Image
      Annotations and Fallbacks: the &lsquo;<code
      class=css>image()</code>&rsquo; notation</a>

     <li><a href="#element-reference"><span class=secno>4.3. </span> Using
      Elements as Images: the &lsquo;<code class=css>element()</code>&rsquo;
      notation</a>

     <li><a href="#cross-fade-function"><span class=secno>4.4. </span>
      Combining images: the &lsquo;<code class=css>cross-fade()</code>&rsquo;
      notation</a>
    </ul>

   <li><a href="#gradients"><span class=secno>5. </span> Gradients</a>
    <ul class=toc>
     <li><a href="#linear-gradients"><span class=secno>5.1. </span> Linear
      Gradients</a>

     <li><a href="#radial-gradients"><span class=secno>5.2. </span> Radial
      Gradients</a>

     <li><a href="#repeating-gradients"><span class=secno>5.3. </span>
      Repeating Gradients</a>

     <li><a href="#color-stop-syntax"><span class=secno>5.4. </span> Gradient
      Color-Stops</a>
    </ul>

   <li><a href="#sizing"><span class=secno>6. </span> Sizing Images and
    Objects in CSS</a>
    <ul class=toc>
     <li><a href="#sizing-terms"><span class=secno>6.1. </span> Object-Sizing
      Terminology</a>

     <li><a href="#object-negotiation"><
23DA
span class=secno>6.2. </span>
      CSS⇋Object Negotiation</a>

     <li><a href="#default-sizing"><span class=secno>6.3. </span> Concrete
      Object Size Resolution</a>

     <li><a href="#object-fit"><span class=secno>6.4. </span> Sizing Objects:
      the &lsquo;<code class=property>object-fit</code>&rsquo; property</a>

     <li><a href="#object-position"><span class=secno>6.5. </span>
      Positioning Objects: the &lsquo;<code
      class=property>object-position</code>&rsquo; property</a>
    </ul>

   <li><a href="#image-processing"><span class=secno>7. </span> Image
    Processing</a>
    <ul class=toc>
     <li><a href="#image-resolution"><span class=secno>7.1. </span>
      Overriding Image Resolutions: the &lsquo;<code
      class=property>image-resolution</code>&rsquo; property</a>

     <li><a href="#image-orientation"><span class=secno>7.2. </span>
      Orienting an Image on the Page: the &lsquo;<code
      class=property>image-orientation</code>&rsquo; property</a>

     <li><a href="#image-rendering"><span class=secno>7.3. </span>
      Determining How to Scale an Image: the &lsquo;<code
      class=property>image-rendering</code>&rsquo; property</a>
    </ul>

   <li><a href="#serialization"><span class=secno>8. </span>
    Serialization</a>
    <ul class=toc>
     <li><a href="#serializing-resolution"><span class=se
E7E6
cno>8.1. </span>
      Serializing a &lt;resolution></a>

     <li><a href="#serializing-url-notation"><span class=secno>8.2. </span>
      Serializing the &lsquo;<code class=css>url()</code>&rsquo; notation</a>


     <li><a href="#serializing-image-notation"><span class=secno>8.3. </span>
      Serializing the &lsquo;<code class=css>image()</code>&rsquo; /
      <i>&lt;image-list></i> notation</a>

     <li><a href="#serializing-element-notation"><span class=secno>8.4.
      </span> Serializing the &lsquo;<code class=css>element()</code>&rsquo;
      / <i>&lt;element-reference></i> notation</a>

     <li><a href="#serializing-cross-fade"><span class=secno>8.5. </span>
      Serializing the &lsquo;<code class=css>cross-fade()</code>&rsquo; /
      <i>&lt;image-combination></i> notation</a>

     <li><a href="#serializing-gradients"><span class=secno>8.6. </span>
      Serializing Gradients</a>

     <li><a href="#serializing-properties"><span class=secno>8.7. </span>
      Serializing new properties</a>
    </ul>

   <li><a href="#interpolation"><span class=secno>9. </span>
    Interpolation</a>
    <ul class=toc>
     <li><a href="#interpolating-images"><span class=secno>9.1. </span>
      Interpolating <i>&lt;image></i></a>

     <li><a href="#interpolating-image-combinations"><span class=secno>9.2.
      </span> Interpolating <i>&lt;image-combination></i></a>

     <li><a href="#interpolating-gradients"><span class=secno>9.3. </span>
      Interpolating <i>&lt;gradient></i></a>
    </ul>

   <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-->
  <!-- ====================================================================== -->

  <h2 id=intro><span class=secno>1. </span> Introduction</h2>

  <p><em>This section is non-normative.</em>

  <p>In CSS Levels 1 and 2, image values, such as those used in the
   &lsquo;<code class=property>background-image</code>&rsquo; 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 reference to an element in the document, and as a
   gradient.</p>
  <!-- ====================================================================== -->

  <h2 id=conformance><span class=secno>2. </span> Conformance</h2>

  <p>A document or implementation cannot conform to CSS Image Values &
   Replaced Content Level 3 alone, but can claim conformance to CSS Image
   Values & Replaced Content Level 3 if it satisfies the conformance
   requirements in this specification when implementing CSS or another host
   language that normatively references this specification.

  <p>Conformance to CSS Image Values & Replaced Content Level 3 is defined
   for three classes:

  <dl>
   <dt><dfn id=minimal>minimal</dfn>

   <dd>A device that does not implement CSS Transitions, CSS Animations, nor
    CSSOM may ignore the chapters on Serializing and Interpolating values for
    the purpose of claiming conformance.

   <dt><dfn id=transition-capable>transition-capable</dfn>

   <dd>A device that implements CSS Transitions or CSS Animations must
    conform to the <a href="#minimal"><i>minimal</i></a> class, and
    additionally must implement the chapter on Interpolation.

   <dt><dfn id=cssom-capable>CSSOM-capable</dfn>

   <dd>A device that implements CSSOM must conform to the <a
    href="#minimal"><i>minimal</i></a> class, and additionally must implement
    the chapter on Serialization.
  </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>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>
  <!-- ====================================================================== -->

  <h2 id=resolution-units><span class=secno>3. </span> Resolution Units: the
   &lt;resolution&gt; value type</h2>

  <p>This specification defines the following units as part of the <dfn
   id=ltresolution>&lt;resolution></dfn> value type:

  <dl>
   <dt><dfn id=dpi>dpi</dfn>

   <dd>dots per inch

   <dt><dfn id=dpcm>dpcm</dfn>

   <dd>dots per centimeter

   <dt><dfn id=dppx>dppx</dfn>

   <dd>dots per &lsquo;<code class=css>px</code>&rsquo; unit
  </dl>

  <p>The &lt;resolution> unit represents the size of a single "dot" of an
   image. For raster images, a dot is an image pixel. A &lt;resolution>
   defines how many of these dots fit in a CSS &lsquo;<code
   class=property>in</code>&rsquo;, &lsquo;<code
   class=property>cm</code>&rsquo;, or &lsquo;<code
   class=property>px</code>&rsquo; so that images can be sized accordingly.
   The default resolution of raster images in CSS is &lsquo;<code
   class=css>1dppx</code>&rsquo;, which is equivalent to &lsquo;<code
   class=css>96dpi</code>&rsquo;. The concept of "resolution" does not apply
   to vector image formats like SVG; generally, this means that any attempt
   to change the resolution of a vector image is simply meaningless.
   Individual uses of the &lt;resolution> value define precisely what effects
   they have on raster and vector images.

  <div class=example>
   <p>Printers tend to have substantially higher resolution than computer
    monitors; due to this, an image that looks fine on the screen may look
    pixellated when printed out. A &lt;resolution> may be used in the
    &lsquo;<a href="#image-resolution0"><code
    class=property>image-resolution</code></a>&rsquo; property to embed a
    high-resolution image into the document that maintains an appropriate
    size, ensuring attractive display both on screen and on paper:</p>

   <pre class=css><code>
img.high-res {
	image-resolution: 300dpi;
}</code></pre>

   <p>With this set, an image meant to be 5 inches wide that was saved at
    300dpi will actually display as 5 inches wide; without this set, the
    image would display as approximately 15.6 inches wide since the image is
    15000 image pixels across, and by default there are 96 image pixels per
    inch.</p>
  </div>
  <!-- ====================================================================== -->

  <h2 id=image><span class=secno>4. </span> Image Values: the &lt;image>
   value type</h2>

  <p>The &lt;image> value type denotes a 2D image. It is defined as

  <pre
   class=prod><dfn id=ltimage>&lt;image></dfn> = &lt;url> | &lt;image-list> | &lt;element-reference> | &lt;image-combination> | &lt;gradient></pre>

  <p>Image values can be used in many CSS properties, including the
   &lsquo;<code class=property>background-image</code>&rsquo;, &lsquo;<code
   class=property>list-style-image</code>&rsquo;, &lsquo;<code
   class=property>cursor</code>&rsquo; properties <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
   <!-- ====================================================================== -->


  <h3 id=url><span class=secno>4.1. </span> Image References and Image
   Slices: the &lsquo;<code class=css>url()</code>&rsquo; notation</h3>

  <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">&lsquo;<code
   class=css>url()</code>&rsquo; 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 &lsquo;<code
    class=css>url()</code>&rsquo; 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 <a
   href="http://www.w3.org/TR/media-frags/#naming-space">media fragment
   identifiers</a>. <a href="#MEDIA-FRAGS"
   rel=biblioentry>[MEDIA-FRAGS]<!--{{!MEDIA-FRAGS}}--></a>

  <div class=example>
   <p>For example, given the following image* and CSS:</p>
   <a href=sprites.svg> <img alt="[9 circles, with 0 to 8 eighths filled in]"
   <pre>background-image: url('sprites.svg#xywh=40,0,20,20')</pre>

   <p>...the background of the element will be the portion of the image that
    starts at (40px,0px) and is 20px wide and tall, which is just the circle
    with a quarter filled in.</p>

   <p><small>* SVG-in-&lt;img> support required. Click the picture to view
    the SVG directly.</small></p>
  </div>

  <p class=note>Note however that a UA that doesn't understand the media
   fragments notation will display the entirety an image specified with
   &lsquo;<code class=css>url</code>&rsquo;. However, since URLs with media
   fragment identifiers can also be used in the &lsquo;<code
   class=css>image()</code>&rsquo; notation defined below, authors can take
   advantage of CSS's forward-compatible parsing rules to provide a fallback
   when using an image fragment URL:

  <div class=example>
   <p>In the example below, the &lsquo;<code class=css>image()</code>&rsquo;
    notation is used together with the media fragment syntax, so that UAs
    that don't support media fragments fail to parse the second declaration
    and use the first.</p>

   <pre>
<!-- -->background-image: url('swirl.png'); /* old UAs */
<!-- -->background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */</pre>
  </div>
  <!-- ====================================================================== -->

  <h3 id=image-notation><span class=secno>4.2. </span> Image Annotations and
   Fallbacks: the &lsquo;<code class=css>image()</code>&rsquo; notation</h3>
  <p>The &lsquo;<code class=css>image()</code>&rsquo; notation allows an
   author to tag an image with a few types of useful processing instructions
   which can affect the rendering of the image and to declare fallback images
   to be used if the original image can't be decoded or is a type that the
   browser doesn't recognize. The author can specify the desired resolution
   the image should be rendered at, declare the directionality of an image so
   that it can be automatically be reversed if used in text with a different
   directionality.

  <p>So that authors can take advantage of CSS's forwards-compatible parsing
   rules to provide a fallback for image slices, implementations that support
   the &lsquo;<code class=css>image()</code>&rsquo; notation <em>must</em>
   support the <code>xywh=#,#,#,#</code> form of media fragment identifiers
   for images. <a href="#MEDIA-FRAGS"
   rel=biblioentry>[MEDIA-FRAGS]<!--{{!MEDIA-FRAGS}}--></a>

  <p>The &lsquo;<code class=css>image()</code>&rsquo; notation is defined as:


  <pre class=prod><dfn id=ltimage-list>&lt;image-list></dfn> =
	image( [ &lt;image-decl> , ]* [ &lt;image-decl> | &lt;color> ] )</pre>

  <p>where &lt;image-decl> is given by:

  <pre class=prod><dfn id=ltimage-decl>&lt;image-decl></dfn> =
	&lt;string> [ [ snap? && &lt;resolution> ] || [ ltr | rtl ] ]</pre>

  <p>Each <a href="#ltimage-decl"><i>&lt;image-decl></i></a> represents the
   external image referenced by the URI given as the &lt;string> argument.

  <p>If the image is a raster image and a &lt;resolution> is given, the image
   must be rendered at that resolution. This must override the default
   resolution given by the &lsquo;<a href="#image-resolution0"><code
   class=property>image-resolution</code></a>&rsquo; property. <span
   class=note>Recall that the default resolution of raster images is
   &lsquo;<code class=css>1dppx</code>&rsquo;, so that one image pixel
   corresponds to one CSS &lsquo;<code class=css>px</code>&rsquo;
   unit.</span> If the &lsquo;<code class=css>snap</code>&rsquo; keyword is
   also specified, and the specified resolution would make one image pixel
   larger than one device pixel, the image must be rendered at the specified
   resolution, rounded to the nearest value that would map one image pixel to
   an integer number of device pixels; if the specified resolution would make
   one image pixel smaller than one device pixel, the image must be rendered
   at the specified resolution, rounded to the nearest value that would map
   an integer number of image pixels to one device pixel. If the image is a
   vector image, specifying a resolution has no effect.

  <p>If a directional keyword (&lsquo;<code class=css>ltr</code>&rsquo; or
   &lsquo;<code class=css>rtl</code>&rsquo;) is given, the image itself gains
   that directionality. If the image is used in a property on an element with
   opposite directionality, is must be mirrored in the inline dimension when
   rendered.

  <p>Multiple arguments can be given separated by commas, in which case the
   function represents the first &lt;image-decl> representing an image that
   the browser can successfully load and display. The final argument can
   specify a &lt;color> to serve as an ultimate fallback; this can be used,
   e.g. for &lsquo;<code class=property>background-image</code>&rsquo;, to
   ensure adequate contrast if none of the preceding &lt;image-decl>s can be
   used. If the final argument is a &lt;color>, it represents a solid-color
   image of the given color with no <a
   href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a>.

  <div class=example>
   <p>The rule below would tell the UA to load &lsquo;<code
    class=css>wavy.svg</code>&rsquo; if it can; failing that to load
    &lsquo;<code class=css>wavy.png</code>&rsquo; and display it at 150dpi;
    failing that to display &lsquo;<code class=css>wavy.gif</code>&rsquo;;
    and finally, if none of the images can be loaded and displayed, to use
    the color &lsquo;<code class=css>rgba(0,0,255,0.5)</code>&rsquo; to
    create a dimensionless background image. For example, the browser might
    not understand how to render SVG images, the PNG may be malformed, and
    the GIF might not exist on the server and return 404 error instead of an
    image.</p>

   <pre>background-image: image(url(wavy.svg), 'wavy.png' 150dpi, "wavy.gif", rgba(0,0,255,0.5));</pre>

   <p>The &lsquo;<code class=property>background-size</code>&rsquo; property
    specifies that dimensionless images by default 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
    semi-transparent blue. As with any image, this fallback will be painted
    over the &lsquo;<code class=property>background-color</code>&rsquo; (if
    any).</p>
  </div>
  <!-- ====================================================================== -->

  <h3 id=element-reference><span class=secno>4.3. </span> Using Elements as
   Images: the &lsquo;<code class=css>element()</code>&rsquo; notation</h3>

  <p>The &lsquo;<code class=css>element()</code>&rsquo; function allows an
   author to reference an element in the document that should be used as an
   image. As the referenced element changes, for example, by the user typing
   into a &lt;textarea> element or a script drawing into a &lt;canvas>
   element in HTML, the image produced by the &lsquo;<code
   class=css>element()</code>&rsquo; function stays in sync, allowing dynamic
   effects such as script-animated background images or previews of the next
   slide in a slideshow. The syntax for &lsquo;<code
   class=css>element()</code>&rsquo; is defined as:

  <pre
   class=prod><dfn id=ltelement-reference>&lt;element-reference></dfn> = element( [&lt;id-selector> | &lt;identifier> ] )</pre>

  <p>where &lt;id-selector> is an ID selector <a href="#SELECT"
   rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a>, and &lt;identifier> is an
   identifer <a href="#CSS3VAL"
   rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>.

  <p>If the argument to the &lsquo;<code class=css>element()</code>&rsquo;
   the selector. If it's an identifier, the function references the element
   whose <dfn id=css-element-reference-identifier>CSS element reference
   identifier</dfn> is the given identifier. (CSS does not define how an
   element acquires a <a href="#css-element-reference-identifier"><i>CSS
   element reference identifier</i></a>; that is determined by the host
   language.) If no element in the document matches the selector, or no
   element has the identifier as its <a
   href="#css-element-reference-identifier"><i>CSS element reference
   identifier</i></a>, the function represents a fully transparent image with
   no intrinsic dimensions, equivalent to <code>image(transparent)</code>. If
   the document changes so that which element is matched, or whether an
   element is matched at all, changes, the image represented by the function
   must change accordingly.

  <p>If the &lsquo;<code class=css>element()</code>&rsquo; function refers to
   an element, then it represents an image with width and height equal to the
   width and height of the margin box of the referenced element. The image
   must be constructed by rendering the referenced element and its
   descendants at the same size that the element would be in its document,
   over an infinite transparent black background, positioned so that the
   edges of the margin box of the element is flush with the edges of the
   image. <span class=note>If the element has decorations or descendants that
   extend outside the margin box, these will be clipped to the margin box in
   the generated image by default. &lsquo;<code
   class=css>background-repeat:extend</code>&rsquo; may allow the author to
   override this behavior so that decorations and descendants outside the
   margin box are still painted.</span> If the referenced element or an
   ancestor of the referenced element has a transform applied to it, the
   transform must be ignored for the purpose of constructing this image
   (transforms on descendants must be unaffected).

  <p>If the argument passed to &lsquo;<code class=css>element()</code>&rsquo;
   isn't an ID selector or an ident, it is a syntax error.

  <p>Implementations may either re-use existing bitmap data generated for the
   referenced element or regenerate the display of the element to maximize
   quality at the image's size (for example, if the implementation detects
   that the referenced element is an SVG fragment); in the latter case, the
   layout of the referenced element in the image must not be changed by the
   regeneration process. That is, the image must look identical to the
   referenced element, modulo rasterization quality.

  <div class=example> TODO: copy an example from the MozHacks article</div>
  <!-- ====================================================================== -->

  <h4 class="no-num no-toc" id=element-cycles> Detecting and Resolving
   Circular Relationships Introduced by &lsquo;<code
   class=css>element()</code>&rsquo;</h4>

  <p>The &lsquo;<code class=css>element()</code>&rsquo; function can produce
   nonsensical circular relationships, such as an element using itself as its
   own background. These relationships can be easily and reliably detected
   and resolved, however, by keeping track of a dependency graph and using
   common cycle-detection algorithms.

  <p>Populate the dependency graph initially by having every element depend
   on each of its children. Then, whenever a property on an element A uses
   the &lsquo;<code class=css>element()</code>&rsquo; function to refer to an
   element B, add an edge to the graph by having A depend on B. If a
   dependency cycle is detected, any &lsquo;<code
   class=css>element()</code>&rsquo; functions that produced a dependency in
   the cycle represent a fully transparent image with no intrinsic
   dimensions.

  <p class=issue>Someone else needs to review this and make sure that I'm not
   missing any cycles.</p>
  <!-- ====================================================================== -->

  <h3 id=cross-fade-function><span class=secno>4.4. </span> Combining images:
   the &lsquo;<code class=css>cross-fade()</code>&rsquo; notation</h3>

  <p>When transitioning between images, CSS requires a way to explicitly
   refer to the intermediate image that is a combination of the start and end
   images. This is accomplished with the &lsquo;<code
   class=css>cross-fade()</code>&rsquo; function, which indicates the two
   images to be combined and how far along in the transition the combination
   is.

  <p class=note>Authors can also use the &lsquo;<code
   class=css>cross-fade()</code>&rsquo; function for many simple image
   manipulations, such as tinting an image with a solid color or highlighting
   a particular area of the page by combining an image with a radial
   gradient.

  <p>The syntax for &lsquo;<code class=css>cross-fade()</code>&rsquo; is
   defined as:

  <pre
   class=prod><dfn id=ltimage-combination>&lt;image-combination></dfn> = cross-fade( &lt;image>, &lt;image>, &lt;percentage> )</pre>

  <p>The function represents an image generated by combining the first and
   second image (referred to in this section as the "start" and "end" images,
   respectively). The percentage represents how far along the transformation
   is, with 0% representing the start image, 100% representing the end image,
   and percentages between that representing corresponding combinations of
   the two images. The <code>&lt;percentage></code> must be between 0% and
   100% inclusive; any other value is invalid.

  <p>More precisely, given &lsquo;<code
   class=css>cross-fade(A,B,p)</code>&rsquo;, where <var>A</var> and
   <var>B</var> are images and <var>p</var> is a percentage between 0% and
   100%, the function represents an image with width equal to
   <code>width<sub>A</sub> &times; (1-<var>p</var>) + width<sub>B</sub>
   &times; p</code> and height equal to <code>height<sub>A</sub> &times;
   (1-<var>p</var>) + height<sub>B</sub> &times; p</code>. The contents of
   the image must be constructed by first scaling <var>A</var> and
   <var>B</var> to the size of the generated image, then applying
   <code>dissolve(<var>A</var>,1-<var>p</var>) plus
   dissolve(<var>B</var>,<var>p</var>)</code>. <span class=note>The
   "dissolve()" function and "plus" compositing operator are defined in the
   literature by Porter-Duff.</span></p>
  <!-- ====================================================================== -->

  <h2 id=gradients><span class=secno>5. </span> Gradients</h2>

  <p>A gradient is an image that smoothly fades from one color to another.
   These are commonly used for subtle shading in background images, buttons,
   and many other things. The two functions described in this section allow
   an author to specify such an image in a terse syntax, so that the UA can
   generate the image automatically when rendering the page. Gradients are a
   type of image, and can be used anywhere an image can, such as in the
   &lsquo;<code class=property>background-image</code>&rsquo; or &lsquo;<code
   class=property>list-style-image</code>&rsquo; properties. Gradients have
   no <a href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a>. The
   syntax of a <a href="#ltgradient"><i>&lt;gradient></i></a> is:
  <pre
   class=prod><dfn id=ltgradient>&lt;gradient></dfn> = [ &lt;linear-gradient> | &lt;radial-gradient> | &lt;repeating-linear-gradient> | &lt;repeating-radial-gradient> ]</pre>

  <p>where <a href="#ltlinear-gradient"><i>&lt;linear-gradient></i></a>, <a
   href="#ltradial-gradient"><i>&lt;radial-gradient></i></a>, <a
   href="#ltrepeating-linear-gradient"><i>&lt;repeating-linear-gradient></i></a>,
   and <a
   href="#ltrepeating-radial-gradient"><i>&lt;repeating-radial-gradient></i></a>
   are defined in their applicable sections below.

  <div class=example>
   <p>As with the other <a href="#ltimage"><i>&lt;image></i></a> 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>

  <p class=note>In many places this section 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 a rectangle with the dimensions of the
   <a href="#concrete-object-size"><i>concrete object size</i></a>. A
   gradient has no intrinsic dimensions. This means that, for example, if you
   use a gradient in the &lsquo;<code
   class=property>background-image</code>&rsquo; property (with &lsquo;<code
   class=property>background-size</code>&rsquo; at the default value and
   &lsquo;<code class=property>background-repeat</code>&rsquo; not equal to
   &lsquo;<code class=property>round</code>&rsquo;), the "box" will simply be
   the size of the background sizing area. If you use a gradient in a
   list-style-image, the "box" will be a 1em square.</p>
  <!-- ====================================================================== -->

  <h3 id=linear-gradients><span class=secno>5.1. </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. This produces a smooth fade
   from each color to the next, progressing in the specified direction.</p>
  <!-- ====================================================================== -->

  <h4 class=no-toc id=linear-gradient-syntax><span class=secno>5.1.1. </span>
   linear-gradient() syntax</h4>

  <pre
   class=prod><code><dfn id=ltlinear-gradient>&lt;linear-gradient></dfn> = linear-gradient(
	[[
		[ [ top | bottom ] || [ left | right ] ]
		|
		&lt;angle>
	],]?
	&lt;color-stop>[, &lt;color-stop>]+
	);</code></pre>

  <p>The first argument to the function specifies the <dfn
   id=gradient-line>gradient-line</dfn>, which gives the gradient a direction
   and determines how color-stops are positioned. It may be omitted; if so,
   it defaults to &lsquo;<code class=css>bottom</code>&rsquo;.

  <p>The <a href="#gradient-line"><i>gradient-line</i></a> may be specified
   in two different ways. The first is by specifying the angle the <a
   href="#gradient-line"><i>gradient-line</i></a> should assume; for the
   purposes of this argument, 0deg points upwards, 90deg points toward the
   right, and positive angles go clockwise. The starting-point and
   ending-point of the <a href="#gradient-line"><i>gradient-line</i></a> are
   determined by extending a line in both directions from the center of the
   box at the angle specified. In the direction of the angle, the
   ending-point is the point on the <a
   href="#gradient-line"><i>gradient-line</i></a> where a line drawn
   perpendicular to the <a href="#gradient-line"><i>gradient-line</i></a>
   would intersect the corner of the box in that direction. The
   starting-point is determined identically, except in the opposite direction
   of the angle.

  <p>The second way is to simply provide one or two keywords representing the
   side or corner of the box that the <a
   href="#gradient-line"><i>gradient-line</i></a> should point towards.
   Similar to the previous case, the starting-point and ending-point of the
   <a href="#gradient-line"><i>gradient-line</i></a> are determined by
   extending a line in both directions from the center of the box. If a
   single keyword is specified, the line assumes the necessary angle to place
   the ending-point of the <a href="#gradient-line"><i>gradient-line</i></a>
   in the center of the side specified by the keyword; if two keywords are
   specified, the line assumes the necessary angle to place the ending-point
   of the <a href="#gradient-line"><i>gradient-line</i></a> in the specified
   corner of the box. The starting-point of the gradient-line is then where
   the line intersects the opposite side or corner of the box.

  <p class=note>This means that the keyword &lsquo;<code
   class=property>bottom</code>&rsquo; (the default) makes the gradient point
   towards the bottom, equivalent to specifying &lsquo;<code
   class=css>180deg</code>&rsquo;. The keywords &lsquo;<code class=css>top
   left</code>&rsquo; make the gradient point towards the top-left corner of
   the box, which is equivalent to an angle somewhere between 0deg and 90deg
   (exclusive).

  <p class=note>The behavior of keywords is now exactly opposite what it has
   been for a long time. Previously, specifying &lsquo;<code
   class=property>top</code>&rsquo; made it start at the top and point down,
   but feedback suggests that this was confusingly different from how
   &lt;angle>s worked: giving an &lt;angle> makes the gradient point towards
   the specified direction, while giving keywords made the gradient point
   <em>away</em> from the specified side/corner.

  <div class=example>
   <div style="
8096
;overflow: hidden"> <img alt="[An image showing a box with a
    background shading gradually from white in the bottom-left corner to
    black in the top-right corner. There is a line, illustrating the
    gradient-line, angled at 45 degrees and passing through the center of the
    box. The starting-point and ending-point of the gradient-line are
    indicated by the intersection of the gradient-line with two additional
    lines that pass through the bottom-left and top-right corners of the
    box.]" src=gradient-diagram.png style="float: right; margin-left: 1em;">
    <p>This example illustrates visually how to calculate the <a
     href="#gradient-line"><i>gradient-line</i></a> from the rules above.
     This shows the starting and ending-point of the <a
     href="#gradient-line"><i>gradient-line</i></a>, along with the actual
     gradient, produced by an element with &lsquo;<code class=css>background:
     linear-gradient(45deg, white, black);</code>&rsquo;.</p>

    <p>Notice how, though the starting-point and ending-point are outside of
     the box, they're positioned precisely right so that the gradient is pure
     white <em>exactly</em> at the corner, and pure black <em>exactly</em> at
     the opposite corner. That's intentional, and will always be true for
     linear gradients.</p>
   </div>
  </div>

  <p>The gradient's color stops are typically placed between the
   starting-point and ending-point on the <a
   href="#gradient-line"><i>gradient-line</i></a>, but this isn't required -
   the <a href="#gradient-line"><i>gradient-line</i></a> extends infinitely
   in both directions. 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 before 0%
   or after 100%.</p>
  <!-- ====================================================================== -->

  <h4 class=no-toc id=linear-gradient-examples><span class=secno>5.1.2.
   </span> Linear Gradient Examples</h4>

  <p>All of the following &lsquo;<code
   class=css>linear-gradient()</code>&rsquo; examples are presumed to be
   backgrounds applied to a box that is 200px wide and 100px tall.

  <div class=example>
   <p>Below are various ways of specifying a basic vertical gradient:</p>

   <pre><code>linear-gradient(yellow, blue);
linear-gradient(bottom, yellow, blue);
linear-gradient(180deg, yellow, blue);
linear-gradient(top, blue, yellow);
linear-gradient(bottom, yellow 0%, blue 100%);</code></pre>

   <p><img alt="" src=linear1.png></p>
  </div>

  <div class=example>
   <p>This gradient goes from the top-left to the bottom-right corner.</p>

   <pre><code>linear-gradient(bottom right, yellow, blue);</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(135deg, yellow, blue);
linear-gradient(-45deg, blue, yellow);</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(yellow, blue 20%, #0f0);</code></pre>

   <p><img alt="" src=linear4.png></p>
  </div>
  <!-- ====================================================================== -->

  <h3 id=radial-gradients><span class=secno>5.2. </span> Radial Gradients</h3>

  <p>In a radial gradient, rather than colors smoothly fading from one side
   of the box to the other as with linear gradients, they instead emerge from
   a single point and smoothly spread outward in a circular or elliptical
   shape.

  <p>A radial gradient is specified by first pinpointing the center of the
   gradient, where the 0% ellipse will be, then specifying the size and shape
   of the 100% ellipse, ending with a list of color-stops just like a
   linear-gradient. Between the center and the ending-ellipse, and past the
   ending-ellipse, concentric ellipses are drawn and colored according to the
   specified color-stops.</p>
  <!-- ====================================================================== -->

  <h4 class=no-toc id=radial-gradient-syntax><span class=secno>5.2.1. </span>
   radial-gradient() Syntax</h4>

  <pre class=prod>
<dfn id=ltradial-gradient>&lt;radial-gradient></dfn> = radial-gradient(
	[&lt;bg-position&gt;,]?
	[[
		[&lt;shape&gt; || &lt;size&gt;]
		|
		[&lt;length> | &lt;percentage>]{2}
	],]?
	&lt;color-stop&gt;[, &lt;color-stop&gt;]+
)
	</pre>

  <p>The first argument to the function specifies the center of the ellipse.
   <i>&lt;bg-position></i> is taken from the Backgrounds and Borders Module,
   and has the same definition. It specifies the center of the gradient. If
   omitted, it defaults to &lsquo;<code class=css>center</code>&rsquo;.
   Color-stop positions are measured along an imaginary line extending from
   the center of the gradient to the right.

  <p>The second argument to the function specifies the size and shape of the
   ending-ellipse. This can be specified in two ways, with different
   characteristics:

  <dl>
   <dt>Implicitly

   <dd>
    <p>The size and shape of the ending-ellipse can be defined
     <em>implicitly</em> with a size and shape keyword. The <a
     href="#ltshape"><i>&lt;shape></i></a> is defined as</p>

    <pre><code><dfn id=ltshape>&lt;shape></dfn> = [ circle | ellipse ]</code></pre>

    <p>&lsquo;<code class=css>circle</code>&rsquo; in
735C
dicates that the
     ending-ellipse will be a circle with a constant radius. &lsquo;<code
     class=css>ellipse</code>&rsquo; 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).</p>

    <p>The <a href="#ltsize"><i>&lt;size></i></a> keyword is defined as</p>

    <pre><code><dfn id=ltsize>&lt;size></dfn> = [ closest-side | closest-corner | farthest-side | farthest-corner | contain | cover ]</code></pre>

    <p>If <a href="#ltshape"><i>&lt;shape></i></a> is &lsquo;<code
     class=css>circle</code>&rsquo; and <a
     href="#ltsize"><i>&lt;size></i></a> is &lsquo;<code
     class=css>closest-side</code>&rsquo;, 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 center of the
     gradient was &lsquo;<code class=css>10% 10%</code>&rsquo;, 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 thus be a circle with
     a radius of 10px. If <a href="#ltshape"><i>&lt;shape></i></a> is
     &lsquo;<code class=css>ellipse</code>&rsquo; and <a
     href="#ltsize"><i>&lt;size></i></a> is &lsquo;<code
     class=css>closest-side</code>&rsquo;, 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>&lsquo;<code class=css>farthest-side</code>&rsquo; is identical to
     &lsquo;<code class=css>closest-side</code>&rsquo;, 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, if the
     shape is &lsquo;<code class=css>ellipse</code>&rsquo;). &lsquo;<code
     class=css>closest-corner</code>&rsquo; and &lsquo;<code
     class=css>farthest-corner</code>&rsquo; size the gradient-shape so that
     it exactly meets the closest or farthest corner of the box from its
     center, respectively. If <a href="#ltshape"><i>&lt;shape></i></a> is
     &lsquo;<code class=css>ellipse</code>&rsquo;, the gradient-shape has the
     same ratio of width to height that it would if &lsquo;<code
     class=css>closest-side</code>&rsquo; or &lsquo;<code
     class=css>farthest-side</code>&rsquo; were specified, as appropriate.
     &lsquo;<code class=css>contain</code>&rsquo; is a synonym for
     &lsquo;<code class=css>closest-side</code>&rsquo;, and &lsquo;<code
     class=css>cover</code>&rsquo; is a synonym for &lsquo;<code
     class=css>farthest-corner</code>&rsquo;.</p>

   <dt>Explicitly

   <dd>
    <p>Alternately, the ending-shape's size and shape can be defined
     explicitly, by providing two lengths or percentages. These measure the
     length of the horizontal and vertical axes of the ellipse, respectively.
     (The axis length is the length from the center of the ellipse to the
     edge, similar to the radius of a circle, not the diameter.)</p>

    <p>Percentages used in the first value are relative to the width of the
     box, while percentages used in the second value are relative to the