css3-overflow/Overview.src.html@cd908039c977
css3-overflow/Overview.src.html
Thu, 16 Aug 2012 14:41:43 -0700
- author
- L. David Baron <dbaron@dbaron.org>
- date
- Thu, 16 Aug 2012 14:41:43 -0700
- changeset 6487
- cd908039c977
- parent 6486
- b139c2dad64d
- child 6488
- 5eeaa30b7d12
- permissions
- -rw-r--r--
Mention issue of not applying to table parts, as discussed at face-to-face meeting afternoon of 2012-08-13.
1 <!DOCTYPE html>
2 <html lang="en">
3 <head>
4 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
5 <title>CSS Overflow Module Level 3</title>
6 <link rel=contents href="#contents">
7 <link rel=index href="#index">
8 <link rel="stylesheet" type="text/css" href="../default.css">
9 <link rel="stylesheet" type="text/css"
10 href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
11 <style>
12 table.source-demo-pair {
13 width: 100%;
14 }
16 .in-cards-demo {
17 width: 13em;
18 height: 8em;
20 padding: 4px;
21 border: medium solid blue;
22 margin: 6px;
24 font: medium/1.3 Times New Roman, Times, serif;
25 white-space: nowrap;
26 }
28 .bouncy-columns-demo {
29 width: 6em;
30 height: 10em;
31 float: left;
32 margin: 1em;
33 font: medium/1.25 Times New Roman, Times, serif;
34 white-space: nowrap;
35 }
36 .bouncy-columns-demo.one {
37 background: aqua; color: black;
38 transform: rotate(-3deg);
39 }
40 .bouncy-columns-demo.two {
41 background: yellow; color: black;
42 transform: rotate(3deg);
43 }
45 .article-font-inherit-demo {
46 font: 1em/1.25 Times New Roman, Times, serif;
47 white-space: nowrap;
48 }
49 .article-font-inherit-demo.one {
50 width: 12em;
51 font-size: 1.5em;
52 margin-bottom: 1em;
53 height: 4em;
54 }
55 .article-font-inherit-demo.two {
56 width: 11em;
57 margin-left: 5em;
58 margin-right: 2em;
59 }
61 .dark-columns-demo {
62 width: 6em;
63 height: 10em;
64 float: left;
65 margin-right: 1em;
66 font: medium/1.25 Times New Roman, Times, serif;
67 white-space: nowrap;
68 }
69 .dark-columns-demo.one {
70 background: aqua; color: black;
71 }
72 .dark-columns-demo.one :link {
73 color: blue;
74 }
75 .dark-columns-demo.one :visited {
76 color: purple;
77 }
78 .dark-columns-demo.two {
79 background: navy; color: white;
80 }
81 .dark-columns-demo.two :link {
82 color: aqua;
83 }
84 .dark-columns-demo.two :visited {
85 color: fuchsia;
86 }
88 .article-max-lines-demo {
89 font: 1em/1.25 Times New Roman, Times, serif;
90 white-space: nowrap;
91 }
92 .article-max-lines-demo.one::first-letter {
93 font-size: 2em;
94 line-height: 0.9;
95 }
96 .article-max-lines-demo.one {
97 font-size: 1.5em;
98 width: 16em;
99 }
100 .article-max-lines-demo.two {
101 width: 11.5em;
102 float: left; margin-right: 1em;
103 }
104 .article-max-lines-demo.three {
105 width: 11.5em;
106 float: left;
107 }
108 </style>
109 </head>
111 <div class="head">
112 <!--logo-->
114 <h1>CSS Overflow Module Level 3</h1>
116 <h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
117 <dl>
118 <dt>This version:
119 <dd><a href="[VERSION]">http://www.w3.org/TR/[YEAR]/ED-css3-overflow-[CDATE]/</a>
121 <dt>Latest version:
122 <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>
124 <dt>Editor's draft:
125 <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
127 <!--
128 <dt>Previous version:
129 <dd><a href="http://www.w3.org/PreviousVersionURI">
130 http://www.w3.org/PreviousVersionURI</a>
131 -->
133 <dt>Issue Tracking:</dt>
134 <dd>Maintained in document (only editor's draft is current)
136 <dt>Feedback:</dt>
137 <dd><a href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a> with subject line “<kbd>[[SHORTNAME]] <var>… message topic …</var></kbd>”
139 <dt>Editors:
140 <dd class="h-card vcard">
141 <a class="p-name fn u-url url" rel="author"
142 href="http://dbaron.org/">L. David Baron</a>,
143 <a class="p-org org" href="http://www.mozilla.org/">Mozilla</a>
144 </dl>
146 <!--copyright-->
148 <hr title="Separator for header">
149 </div>
151 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
153 <p>
154 <a href="http://www.w3.org/TR/CSS/">CSS</a> is
155 a language for describing
156 the rendering of structured documents (such as HTML and XML)
157 on screen, on paper, in speech, etc.
158 This module contains the features of CSS
159 relating to new mechanisms of overflow handling in visual media (e.g., screen or paper).
160 In interactive media,
161 it describes features that allow the overflow
162 from a fixed size container
163 to be handled by pagination (displaying one page at a time).
164 It also describes features, applying to all visual media,
165 that allow the contents of an element
166 to be spread across multiple fragments,
167 allowing the contents to flow across multiple regions
168 or to have different styles for different fragments.
169 </p>
171 <h2 class="no-num no-toc" id="status">Status of this document</h2>
173 <!--status-->
175 <p>The following features are at risk: …
177 <h2 class="no-num no-toc" id="contents">
178 Table of contents</h2>
180 <!--toc-->
182 <h2 id="intro">
183 Introduction</h2>
185 <p>
186 In CSS Level 1 [[CSS1]], placing more content than would fit
187 inside an element with a specified size
188 was generally an authoring error.
189 Doing so caused the content to extend
190 outside the bounds of the element,
191 which would likely cause
192 that content to overlap with other elements.
193 </p>
195 <p>
196 CSS Level 2 [[CSS21]] introduced the 'overflow' property,
197 which allows authors to have overflow be handled by scrolling,
198 which means it is no longer an authoring error.
199 It also allows authors to specify
200 that overflow is handled by clipping,
201 which makes sense when the author's intent
202 is that the content not be shown.
203 </p>
205 <p>
206 However, scrolling is not the only way
207 to present large amounts of content,
208 and may even not be the optimal way.
209 After all, the codex replaced the scroll
210 as the common format for large written works
211 because of its advantages.
212 </p>
214 <p>
215 This specification introduces
216 a mechanism for Web pages to specify
217 that an element of a page should handle overflow
218 through pagination rather than through scrolling.
219 </p>
221 <p>
222 This specification also extends the concept of overflow
223 in another direction.
224 Instead of requiring that authors specify a single area
225 into which the content of an element must flow,
226 this specification allows authors to specify multiple fragments,
227 each with their own dimensions and styles,
228 so that the content of the element can flow from one to the next,
229 using as many as needed to place the content without overflowing.
230 </p>
232 <p>
233 In both of these cases, implementations must
234 break the content in the block-progression dimension.
235 Implementations must do this is described
236 in the CSS Fragmentation Module [[!CSS3-BREAK]].
237 </p>
239 <h2 id="scrolling-overflow">Scrolling and hidden overflow</h2>
241 <p class="issue">
242 Move material from [[CSS21]] and [[CSS3BOX]] here.
243 </p>
245 <h2 id="paginated-overflow">Paginated overflow</h2>
247 <p class="issue">overflow:paginate or overflow:pages (or paged-x, paged-y, paged-x-controls, paged-y-controls as css3-gcpm has?)</p>
249 <p class="issue">Ability to display N pages at once
250 rather than just one page at once?</p>
252 <h2 id="fragment-overflow">Fragment overflow</h2>
254 <p>
255 This section introduces and defines the meaning of
256 the new ''fragments'' value of the 'overflow' property.
257 </p>
259 <p>
260 When the computed value of 'overflow' for an element is ''fragments'',
261 and implementations would otherwise have created a box for the element,
262 then implementations must create at least one box for that element.
263 Each box created for the element is called a <dfn>fragment box</dfn>
264 for that element.
265 (If an element with ''overflow: fragments'' generates only one box,
266 that box is a <i>fragment box</i>.
267 However, if an element's computed 'overflow' is not ''fragments'',
268 then its box is not a <i>fragment box</i>.)
269 Every <i>fragment box</i> is a fragmentation container,
270 and for each <i>fragment box</i> which ends with a fragmentation break,
271 (which could happen
272 because breakable content overflows in the block dimension
273 or because of a forced break),
274 there must be another <i>fragment box</i> created as a next sibling
275 of the previous one.
276 <span class="issue">Or is it as though it's a next sibling of
277 the element? Need to figure out exactly how this interacts with
278 other box-level fixup.</span>
279 (Breakable content might overflow in the box dimension either
280 because of a specified size on the <i>fragment box</i>
281 or because the <i>fragment box</i> is within a fragmentation context
282 in which it is being broken.
283 In other words, a single <i>fragment box</i> is never broken
284 across columns or pages;
285 the pieces that are in separate columns or pages
286 are always distinct <i>fragment box</i>es.)
287 </p>
289 <p class="issue">
290 We also want ''::nth-fragment()'' pseudo-elements
291 to be able to apply to the pieces of an element
292 split within a fragmentation context.
293 Should we require that authors who want to use
294 ''::nth-fragment()'' in this way specify ''overflow:fragments''
295 (even if they don't specify a constrained height),
296 or should it work automatically for all elements
297 even if they don't have ''overflow: fragments''?
298 </p>
300 <div class="example">
301 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
302 <title>Breaking content into
303 equal-sized cards</title>
304 <style>
305 .in-cards {
306 overflow: fragments;
308 width: 13em;
309 height: 8em;
311 padding: 4px;
312 border: medium solid blue;
313 margin: 6px;
315 font: medium/1.3 Times New
316 Roman, Times, serif;
317 }
318 </style>
319 <div class="in-cards">
320 In this example, the text in the div
321 is broken into a series of cards.
322 These cards all have the same style.
323 The presence of enough content to
324 overflow one of the cards causes
325 another one to be created. The second
326 card is created just like it's the
327 next sibling of the first.
328 </div></pre></td><td>
329 <div class="in-cards-demo">In this example, the text in the<br>div is broken into a series of<br>cards. These cards all have the<br>same style. The presence of<br>enough content to overflow<br>one of the cards causes another</div>
330 <div class="in-cards-demo">one to be created. The second<br>card is created just like it's the<br>next sibling of the first.</div>
331 </td></tr></table>
332 </div>
334 <p class="issue">
335 We should specify that ''overflow: fragments'' does not apply
336 to at least some table parts,
337 and perhaps other elements as well.
338 We need to determine exactly which ones.
339 </p>
341 <h3 id="fragment-styling">Fragment styling</h3>
343 <h4 id="fragment-pseudo-element">The ::nth-fragment() pseudo-element</h4>
345 <p>
346 The ::nth-fragment() pseudo-element is a pseudo-element
347 that describes some of the <i>fragment box</i>es generated by an element.
348 The argument to the pseudo-element takes the same syntax
349 as the argument to the :nth-child() pseudo-class
350 defined in [[!SELECT]], and has the same meaning
351 except that the number is relative to
352 <i>fragment box</i>es generated by the element
353 instead of siblings of the element.
354 </p>
356 <p class="note">
357 Selectors that allow addressing fragments
358 by counting from the end rather than the start
359 are intentionally not provided.
360 Such selectors would interfere with determining
361 the number of fragments.
362 </p>
364 <h4 id="style-of-fragments">Styling of fragments</h4>
366 <p class="issue">
367 Should this apply to fragment overflow only,
368 or also to paginated overflow?
369 (If it applies,
370 then stricter property restrictions would be needed
371 for paginated overflow.)
372 </p>
374 <p>
375 In the absence of rules with ''::nth-fragment()'' pseudo-elements,
376 the computed style for each <i>fragment box</i>
377 is the computed style for the element
378 for which the <i>fragment box</i> was created.
379 However, the style for a <i>fragment box</i> is also influenced
380 by rules whose selector's <i>subject</i> [[!SELECT]]
381 has an ''::nth-fragment()'' pseudo-element,
382 if the 1-based number of the <i>fragment box</i> matches
383 that ''::nth-fragment()'' pseudo-element
384 and the selector (excluding the ''::nth-fragment()'' pseudo-element)
385 matches the element generating the fragments.
386 </p>
388 <p>
389 When determining the style of the <i>fragment box</i>,
390 these rules that match the fragment pseudo-element
391 cascade together with the rules that match the element,
392 with the fragment pseudo-element adding the specificity
393 of a pseudo-class to the specificity calculation.
394 <span class="issue">Does this need to be specified in
395 the cascading module as well?</span>
396 </p>
398 <div class="example">
399 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
400 <style>
401 .bouncy-columns {
402 overflow: fragments;
403 width: 6em;
404 height: 10em;
405 float: left;
406 margin: 1em;
407 font: medium/1.25 Times New
408 Roman, Times, serif;
409 }
410 .bouncy-columns::nth-fragment(1) {
411 background: aqua; color: black;
412 transform: rotate(-3deg);
413 }
414 .bouncy-columns::nth-fragment(2) {
415 background: yellow; color: black;
416 transform: rotate(3deg);
417 }
418 </style>
419 <div class="bouncy-columns">
420 <i>...</i>
421 </div></pre></td><td>
422 <div class="bouncy-columns-demo one">In this<br>example, the<br>text in the div<br>is broken into<br>a series of<br>columns. The<br>author<br>probably</div>
423 <div class="bouncy-columns-demo two">intended the<br>text to fill two<br>columns. But<br>if it happens to<br>fill three<br>columns, the<br>third column is<br>still created. It</div>
424 <div class="bouncy-columns-demo">just doesn't<br>have any<br>fragment-specific<br>styling because<br>the author<br>didn't give it<br>any.</div>
425 </td></tr></table>
426 </div>
428 <p>
429 Styling an ''::nth-fragment()'' pseudo-element with the 'overflow'
430 property has no effect;
431 the computed value of 'overflow' for the fragment box
432 remains the same as the computed value of overflow for the element.
433 </p>
435 <p>
436 Styling an ''::nth-fragment()'' pseudo-element with the 'content'
437 property has no effect;
438 the computed value of 'content' for the fragment box
439 remains the same as the computed value of content for the element.
440 </p>
442 <p>
443 Specifying ''display: none'' for a <i>fragment box</i> causes
444 the fragment box with that index not to be generated.
445 However, in terms of the indices
446 used for matching ''::nth-fragment()'' pseudo-elements
447 of later fragment boxes,
448 it still counts as though it was generated.
449 However, since it is not generated, it does not contain any content.
450 </p>
452 <p class="issue">
453 Would it make more sense to forbid ''display:none''?
454 Or perhaps to forbid 'display', 'position', 'float',
455 and similar (in addition to 'overflow')?
456 </p>
458 <p>
459 To match the model for other pseudo-elements
460 where the pseudo-elements live inside their corresponding element,
461 declarations in ''::nth-fragment()'' pseudo-elements override
462 declarations in rules without the pseudo-element.
463 The relative priority within such declarations is determined
464 by normal cascading order (see [[!CSS21]]).
465 </p>
467 <p>
468 Styles specified on ''::nth-fragment()'' pseudo-elements
469 do affect inheritance to content within the <i>fragment box</i>.
470 In other words, the content within the <i>fragment box</i> must
471 inherit from the fragment box's style (i.e., the pseudo-element style)
472 rather than directly from the element.
473 This means that elements split between fragment boxes may
474 have different styles for different parts of the element.
475 </p>
477 <p class="issue">
478 This inheritance rule allows specifying styles indirectly
479 (by using explicit ''inherit'' or using default inheritance
480 on properties that don't apply to '':first-letter'')
481 that can't be specified directly
482 (based on the rules in the next section).
483 This is a problem.
484 The restrictions that apply to styling inside fragments
485 should also apply to inheritance from fragments.
486 </p>
488 <div class="example">
489 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
490 <style>
491 .article {
492 overflow: fragments;
493 }
494 .article::nth-fragment(1) {
495 font-size: 1.5em;
496 margin-bottom: 1em;
497 height: 4em;
498 }
499 .article::nth-fragment(n+2) {
500 /* 2 and up */
501 margin-left: 5em;
502 margin-right: 2em;
503 }
504 </style>
505 <div class="article">
506 The <code>font-size</code> property<i>...</i>
507 </div></pre></td><td>
508 <div class="article-font-inherit-demo one">The <code>font-size</code> property<br>specified on the fragment<br>is inherited into the</div>
509 <div class="article-font-inherit-demo two">descendants of the fragment.<br>This means that inherited<br>properties can be used<br>reliably on a fragment, as in<br>this example.</div>
510 </td></tr></table>
511 </div>
513 <h4 id="style-in-fragments">Styling inside fragments</h4>
515 <p class="issue">
516 Should this apply to fragment overflow only,
517 or also to paginated overflow,
518 or even to pagination across pages?
519 </p>
521 <p>
522 The ''::nth-fragment()'' pseudo-element
523 can also be used to style
524 content inside of a <i>fragment box</i>.
525 Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements,
526 the ''::nth-fragment()'' pseudo-element can be applied
527 to parts of the selector other than the subject:
528 in particular, it can match ancestors of the subject.
529 However, the only CSS properties applied
530 by rules with such selectors
531 are those that apply
532 to the ''::first-letter'' pseudo-element.
533 </p>
535 <p>
536 To be more precise,
537 when a rule's selector has ''::nth-fragment()'' pseudo-elements
538 attached to parts of the selector other than the subject,
539 the declarations in that rule apply to
540 a fragment (or pseudo-element thereof) when:
541 </p>
542 <ol>
543 <li>
544 the declarations are for properties that apply to the
545 ''::first-letter'' pseudo-element,
546 </li>
547 <li>
548 the declarations would apply to
549 that fragment (or pseudo-element thereof)
550 had those ''::nth-fragment()'' pseudo-elements been removed,
551 with a particular association between
552 each sequence of simple selectors and the element it matched,
553 and
554 </li>
555 <li>
556 for each removed ''::nth-fragment()'' pseudo-element,
557 the fragment lives within a <i>fragment box</i>
558 of the element associated in that association
559 with the selector that the pseudo-element was attached to,
560 and whose index matches the pseudo-element.
561 </li>
562 </ol>
564 <div class="example">
565 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
566 <style>
567 .dark-columns {
568 overflow: fragments;
569 width: 6em;
570 height: 10em;
571 float: left;
572 margin-right: 1em;
573 font: medium/1.25 Times New
574 Roman, Times, serif;
575 }
576 .dark-columns::nth-fragment(1) {
577 background: aqua; color: black;
578 }
579 .dark-columns::nth-fragment(1) :link {
580 color: blue;
581 }
582 .dark-columns::nth-fragment(1) :visited {
583 color: purple;
584 }
585 .dark-columns::nth-fragment(2) {
586 background: navy; color: white;
587 }
588 .dark-columns::nth-fragment(2) :link {
589 color: aqua;
590 }
591 .dark-columns::nth-fragment(2) :visited {
592 color: fuchsia;
593 }
594 </style>
595 <div class="dark-columns">
596 <i>...</i>
597 </div></pre></td><td>
598 <div class="dark-columns-demo one">In this<br><a href="http://en.wiktionary.org/wiki/example">example</a>, the<br>text flows<br>from one<br>light-colored<br>fragment into<br>another<br>dark-colored</div>
599 <div class="dark-columns-demo two">fragment. We<br>therefore want<br>different styles<br>for <a href="http://www.w3.org/Provider/Style/IntoContext.html">hyperlinks</a><br>in the different<br>fragments.</div>
600 </td></tr></table>
601 </div>
604 <h3 id="max-lines">The 'max-lines' property</h3>
606 <p>
607 Authors may wish to style the opening lines of an element
608 with different styles
609 by putting those opening lines in a separate fragment.
610 However, since it may be difficult to predict the exact height
611 occupied by those lines
612 in order to restrict the first fragment to that height,
613 this specification introduces a 'max-lines' property
614 that forces a fragment to break
615 after a specified number of lines.
616 This forces a break after the given number of lines
617 contained within the element or its descendants,
618 as long as those lines are in the same block formatting context.
619 </p>
621 <table class=propdef>
622 <tr>
623 <th>Name:
624 <td><dfn>max-lines</dfn>
625 <tr>
626 <th><a href="#values">Value</a>:
627 <td>none | <integer>
628 <tr>
629 <th>Initial:
630 <td>none
631 <tr>
632 <th>Applies to:
633 <td>fragment boxes
634 <tr>
635 <th>Inherited:
636 <td>no
637 <tr>
638 <th>Animatable:
639 <td>as <a href="http://www.w3.org/TR/css3-transitions/#animatable-types">integer</a>
640 <tr>
641 <th>Percentages:
642 <td>N/A
643 <tr>
644 <th>Media:
645 <td>visual
646 <tr>
647 <th>Computed value:
648 <td>specified value
649 <tr>
650 <th>Canonical order:
651 <td><abbr title="follows order of property value definition">per grammar</abbr>
652 </table>
654 <dl>
655 <dt>none
656 <dd>
657 <p>
658 Breaks occur only as specified elsewhere.
659 </p>
660 </dd>
662 <dt><integer>
663 <dd>
664 <p>
665 In addition to any breaks specified elsewhere,
666 a break is forced before any line that would exceed
667 the given number of lines
668 being placed inside the element
669 (excluding lines that are in
670 a different block formatting context from
671 the block formatting context to which
672 an unstyled child of the element would belong).
673 </p>
675 <p class="issue">
676 If there are multiple boundaries between this line
677 and the previous, where exactly (in terms of element
678 boundaries) is the break forced?
679 </p>
681 <p>
682 Only positive integers are accepted.
683 Zero or negative integers are a parse error.
684 </p>
685 </dd>
686 </dl>
688 <p class="issue">Should this apply to fragment overflow only, or also
689 to pagination?</p>
691 <div class="example">
692 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
693 <style>
694 .article {
695 overflow: fragments;
696 }
697 .article::first-letter {
698 font-size: 2em;
699 line-height: 0.9;
700 }
701 .article::nth-fragment(1) {
702 font-size: 1.5em;
703 max-lines: 3;
704 }
705 .article::nth-fragment(n+2) {
706 /* 2 and up */
707 column-count: 2;
708 }
709 </style>
710 <div class="article">
711 <i>...</i>
712 </div></pre></td><td>
713 <div class="article-max-lines-demo one">The max-lines property allows<br>authors to use a larger font for the first<br>few lines of an article. Without the</div>
714 <div class="article-max-lines-demo two">max-lines property, authors<br>might have to use the<br>'height' property instead, but<br>that would leave a slight gap<br>if the author miscalculated<br>how much height a given<br>number of lines would<br>occupy (which might be</div>
715 <div class="article-max-lines-demo three">particularly hard if the author<br>didn't know what text would<br>be filling the space, exactly<br>what font would be used, or<br>exactly which platform's font<br>rendering would be used to<br>display the font).</div>
716 </td></tr></table>
717 </div>
719 <h2 id="conformance">
720 Conformance</h2>
722 <h3 id="placement">
723 Module interactions</h3>
725 <p>This module extends the 'overflow'
726 feature defined in [[CSS21]] section 11.1.1. It defines additional
727 overflow handling mechanisms that implementations must implement as
728 described in this module in order to conform to this module.</p>
730 <p>No properties in this module apply to the <code>::first-line</code> or
731 <code>::first-letter</code> pseudo-elements.</p>
733 <h3 id="values">
734 Values</h3>
736 <p>This specification follows the
737 <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
738 definition conventions</a> from [[!CSS21]]. Value types not defined in
739 this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
740 Other CSS modules may expand the definitions of these value types: for
741 example [[CSS3COLOR]], when combined with this module, expands the
742 definition of the <color> value type as used in this specification.</p>
744 <p>In addition to the property-specific values listed in their definitions,
745 all properties defined in this specification also accept the
746 <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
747 keyword as their property value. For readability it has not been repeated
748 explicitly.
751 <h3 id="conventions">
752 Document conventions</h3>
754 <p>Conformance requirements are expressed with a combination of
755 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
756 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
757 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
758 document are to be interpreted as described in RFC 2119.
759 However, for readability, these words do not appear in all uppercase
760 letters in this specification.
762 <p>All of the text of this specification is normative except sections
763 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
765 <p>Examples in this specification are introduced with the words “for example”
766 or are set apart from the normative text with <code>class="example"</code>,
767 like this:
769 <div class="example">
770 <p>This is an example of an informative example.</p>
771 </div>
773 <p>Informative notes begin with the word “Note” and are set apart from the
774 normative text with <code>class="note"</code>, like this:
776 <p class="note">Note, this is an informative note.</p>
778 <h3 id="conformance-classes">
779 Conformance classes</h3>
781 <p>Conformance to CSS Overflow Module Level 3
782 is defined for three conformance classes:
783 <dl>
784 <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
785 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
786 style sheet</a>.
787 <dt><dfn>renderer</dfn></dt>
788 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
789 that interprets the semantics of a style sheet and renders
790 documents that use them.
791 <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
792 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
793 that writes a style sheet.
794 </dl>
796 <p>A style sheet is conformant to CSS Overflow Module Level 3
797 if all of its statements that use syntax defined in this module are valid
798 according to the generic CSS grammar and the individual grammars of each
799 feature defined in this module.
801 <p>A renderer is conformant to CSS Overflow Module Level 3
802 if, in addition to interpreting the style sheet as defined by the
803 appropriate specifications, it supports all the features defined
804 by CSS Overflow Module Level 3 by parsing them correctly
805 and rendering the document accordingly. However, the inability of a
806 UA to correctly render a document due to limitations of the device
807 does not make the UA non-conformant. (For example, a UA is not
808 required to render color on a monochrome monitor.)
810 <p>An authoring tool is conformant to CSS Overflow Module Level 3
811 if it writes style sheets that are syntactically correct according to the
812 generic CSS grammar and the individual grammars of each feature in
813 this module, and meet all other conformance requirements of style sheets
814 as described in this module.
816 <h3 id="partial">
817 Partial implementations</h3>
819 <p>So that authors can exploit the forward-compatible parsing rules to
820 assign fallback values, CSS renderers <strong>must</strong>
821 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
822 as appropriate</a>) any at-rules, properties, property values, keywords,
823 and other syntactic constructs for which they have no usable level of
824 support. In particular, user agents <strong>must not</strong> selectively
825 ignore unsupported component values and honor supported values in a single
826 multi-value property declaration: if any value is considered invalid
827 (as unsupported values must be), CSS requires that the entire declaration
828 be ignored.</p>
830 <h3 id="experimental">
831 Experimental implementations</h3>
833 <p>To avoid clashes with future CSS features, the CSS2.1 specification
834 reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
835 syntax</a> for proprietary and experimental extensions to CSS.
837 <p>Prior to a specification reaching the Candidate Recommendation stage
838 in the W3C process, all implementations of a CSS feature are considered
839 experimental. The CSS Working Group recommends that implementations
840 use a vendor-prefixed syntax for such features, including those in
841 W3C Working Drafts. This avoids incompatibilities with future changes
842 in the draft.
843 </p>
845 <h3 id="testing">
846 Non-experimental implementations</h3>
848 <p>Once a specification reaches the Candidate Recommendation stage,
849 non-experimental implementations are possible, and implementors should
850 release an unprefixed implementation of any CR-level feature they
851 can demonstrate to be correctly implemented according to spec.
853 <p>To establish and maintain the interoperability of CSS across
854 implementations, the CSS Working Group requests that non-experimental
855 CSS renderers submit an implementation report (and, if necessary, the
856 testcases used for that implementation report) to the W3C before
857 releasing an unprefixed implementation of any CSS features. Testcases
858 submitted to W3C are subject to review and correction by the CSS
859 Working Group.
861 <p>Further information on submitting testcases and implementation reports
862 can be found from on the CSS Working Group's website at
863 <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
864 Questions should be directed to the
865 <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
866 mailing list.
868 <h3 id="cr-exit-criteria">
869 CR exit criteria</h3>
871 <p class=issue>[Change or remove the following CR exit criteria if
872 the spec is not a module, but, e.g., a Note or a profile. This text was <a
873 href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>
875 <p>
876 For this specification to be advanced to Proposed Recommendation,
877 there must be at least two independent, interoperable implementations
878 of each feature. Each feature may be implemented by a different set of
879 products, there is no requirement that all features be implemented by
880 a single product. For the purposes of this criterion, we define the
881 following terms:
883 <dl>
884 <dt>independent <dd>each implementation must be developed by a
885 different party and cannot share, reuse, or derive from code
886 used by another qualifying implementation. Sections of code that
887 have no bearing on the implementation of this specification are
888 exempt from this requirement.
890 <dt>interoperable <dd>passing the respective test case(s) in the
891 official CSS test suite, or, if the implementation is not a Web
892 browser, an equivalent test. Every relevant test in the test
893 suite should have an equivalent test created if such a user
894 agent (UA) is to be used to claim interoperability. In addition
895 if such a UA is to be used to claim interoperability, then there
896 must one or more additional UAs which can also pass those
897 equivalent tests in the same way for the purpose of
898 interoperability. The equivalent tests must be made publicly
899 available for the purposes of peer review.
901 <dt>implementation <dd>a user agent which:
903 <ol class=inline>
904 <li>implements the specification.
906 <li>is available to the general public. The implementation may
907 be a shipping product or other publicly available version
908 (i.e., beta version, preview release, or “nightly build”).
909 Non-shipping product releases must have implemented the
910 feature(s) for a period of at least one month in order to
911 demonstrate stability.
913 <li>is not experimental (i.e., a version specifically designed
914 to pass the test suite and is not intended for normal usage
915 going forward).
916 </ol>
917 </dl>
919 <p>The specification will remain Candidate Recommendation for at least
920 six months.
922 <h2 class=no-num id="acknowledgments">
923 Acknowledgments</h2>
925 <p>
926 Thanks especially to the feedback from
927 Håkon Wium Lie,
928 Florian Rivoal,
929 Alan Stearns,
930 and all the rest of the
931 <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
932 </p>
934 <h2 class=no-num id="references">
935 References</h2>
937 <h3 class="no-num" id="normative-references">
938 Normative references</h3>
939 <!--normative-->
941 <h3 class="no-num" id="other-references">
942 Other references</h3>
943 <!--informative-->
945 <h2 class="no-num" id="index">
946 Index</h2>
947 <!--index-->
949 <h2 class="no-num" id="property-index">
950 Property index</h2>
951 <!-- properties -->
953 </body>
954 </html>
955 <!-- Keep this comment at the end of the file
956 Local variables:
957 mode: sgml
958 sgml-declaration:"~/SGML/HTML4.decl"
959 sgml-default-doctype-name:"html"
960 sgml-minimize-attributes:t
961 sgml-nofill-elements:("pre" "style" "br")
962 sgml-live-element-indicator:t
963 sgml-omittag:nil
964 sgml-shorttag:nil
965 sgml-namecase-general:t
966 sgml-general-insert-case:lower
967 sgml-always-quote-attributes:t
968 sgml-indent-step:nil
969 sgml-indent-data:t
970 sgml-parent-document:nil
971 sgml-exposed-tags:nil
972 sgml-local-catalogs:nil
973 sgml-local-ecat-files:nil
974 End:
975 -->
