css3-overflow/Overview.src.html@b139c2dad64d
css3-overflow/Overview.src.html
Thu, 16 Aug 2012 14:40:05 -0700
- author
- L. David Baron <dbaron@dbaron.org>
- date
- Thu, 16 Aug 2012 14:40:05 -0700
- changeset 6486
- b139c2dad64d
- parent 6485
- d71ab862ce20
- child 6487
- cd908039c977
- permissions
- -rw-r--r--
Specify cascading rules.
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 <h3 id="fragment-styling">Fragment styling</h3>
336 <h4 id="fragment-pseudo-element">The ::nth-fragment() pseudo-element</h4>
338 <p>
339 The ::nth-fragment() pseudo-element is a pseudo-element
340 that describes some of the <i>fragment box</i>es generated by an element.
341 The argument to the pseudo-element takes the same syntax
342 as the argument to the :nth-child() pseudo-class
343 defined in [[!SELECT]], and has the same meaning
344 except that the number is relative to
345 <i>fragment box</i>es generated by the element
346 instead of siblings of the element.
347 </p>
349 <p class="note">
350 Selectors that allow addressing fragments
351 by counting from the end rather than the start
352 are intentionally not provided.
353 Such selectors would interfere with determining
354 the number of fragments.
355 </p>
357 <h4 id="style-of-fragments">Styling of fragments</h4>
359 <p class="issue">
360 Should this apply to fragment overflow only,
361 or also to paginated overflow?
362 (If it applies,
363 then stricter property restrictions would be needed
364 for paginated overflow.)
365 </p>
367 <p>
368 In the absence of rules with ''::nth-fragment()'' pseudo-elements,
369 the computed style for each <i>fragment box</i>
370 is the computed style for the element
371 for which the <i>fragment box</i> was created.
372 However, the style for a <i>fragment box</i> is also influenced
373 by rules whose selector's <i>subject</i> [[!SELECT]]
374 has an ''::nth-fragment()'' pseudo-element,
375 if the 1-based number of the <i>fragment box</i> matches
376 that ''::nth-fragment()'' pseudo-element
377 and the selector (excluding the ''::nth-fragment()'' pseudo-element)
378 matches the element generating the fragments.
379 </p>
381 <p>
382 When determining the style of the <i>fragment box</i>,
383 these rules that match the fragment pseudo-element
384 cascade together with the rules that match the element,
385 with the fragment pseudo-element adding the specificity
386 of a pseudo-class to the specificity calculation.
387 <span class="issue">Does this need to be specified in
388 the cascading module as well?</span>
389 </p>
391 <div class="example">
392 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
393 <style>
394 .bouncy-columns {
395 overflow: fragments;
396 width: 6em;
397 height: 10em;
398 float: left;
399 margin: 1em;
400 font: medium/1.25 Times New
401 Roman, Times, serif;
402 }
403 .bouncy-columns::nth-fragment(1) {
404 background: aqua; color: black;
405 transform: rotate(-3deg);
406 }
407 .bouncy-columns::nth-fragment(2) {
408 background: yellow; color: black;
409 transform: rotate(3deg);
410 }
411 </style>
412 <div class="bouncy-columns">
413 <i>...</i>
414 </div></pre></td><td>
415 <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>
416 <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>
417 <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>
418 </td></tr></table>
419 </div>
421 <p>
422 Styling an ''::nth-fragment()'' pseudo-element with the 'overflow'
423 property has no effect;
424 the computed value of 'overflow' for the fragment box
425 remains the same as the computed value of overflow for the element.
426 </p>
428 <p>
429 Styling an ''::nth-fragment()'' pseudo-element with the 'content'
430 property has no effect;
431 the computed value of 'content' for the fragment box
432 remains the same as the computed value of content for the element.
433 </p>
435 <p>
436 Specifying ''display: none'' for a <i>fragment box</i> causes
437 the fragment box with that index not to be generated.
438 However, in terms of the indices
439 used for matching ''::nth-fragment()'' pseudo-elements
440 of later fragment boxes,
441 it still counts as though it was generated.
442 However, since it is not generated, it does not contain any content.
443 </p>
445 <p class="issue">
446 Would it make more sense to forbid ''display:none''?
447 Or perhaps to forbid 'display', 'position', 'float',
448 and similar (in addition to 'overflow')?
449 </p>
451 <p>
452 To match the model for other pseudo-elements
453 where the pseudo-elements live inside their corresponding element,
454 declarations in ''::nth-fragment()'' pseudo-elements override
455 declarations in rules without the pseudo-element.
456 The relative priority within such declarations is determined
457 by normal cascading order (see [[!CSS21]]).
458 </p>
460 <p>
461 Styles specified on ''::nth-fragment()'' pseudo-elements
462 do affect inheritance to content within the <i>fragment box</i>.
463 In other words, the content within the <i>fragment box</i> must
464 inherit from the fragment box's style (i.e., the pseudo-element style)
465 rather than directly from the element.
466 This means that elements split between fragment boxes may
467 have different styles for different parts of the element.
468 </p>
470 <p class="issue">
471 This inheritance rule allows specifying styles indirectly
472 (by using explicit ''inherit'' or using default inheritance
473 on properties that don't apply to '':first-letter'')
474 that can't be specified directly
475 (based on the rules in the next section).
476 This is a problem.
477 The restrictions that apply to styling inside fragments
478 should also apply to inheritance from fragments.
479 </p>
481 <div class="example">
482 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
483 <style>
484 .article {
485 overflow: fragments;
486 }
487 .article::nth-fragment(1) {
488 font-size: 1.5em;
489 margin-bottom: 1em;
490 height: 4em;
491 }
492 .article::nth-fragment(n+2) {
493 /* 2 and up */
494 margin-left: 5em;
495 margin-right: 2em;
496 }
497 </style>
498 <div class="article">
499 The <code>font-size</code> property<i>...</i>
500 </div></pre></td><td>
501 <div class="article-font-inherit-demo one">The <code>font-size</code> property<br>specified on the fragment<br>is inherited into the</div>
502 <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>
503 </td></tr></table>
504 </div>
506 <h4 id="style-in-fragments">Styling inside fragments</h4>
508 <p class="issue">
509 Should this apply to fragment overflow only,
510 or also to paginated overflow,
511 or even to pagination across pages?
512 </p>
514 <p>
515 The ''::nth-fragment()'' pseudo-element
516 can also be used to style
517 content inside of a <i>fragment box</i>.
518 Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements,
519 the ''::nth-fragment()'' pseudo-element can be applied
520 to parts of the selector other than the subject:
521 in particular, it can match ancestors of the subject.
522 However, the only CSS properties applied
523 by rules with such selectors
524 are those that apply
525 to the ''::first-letter'' pseudo-element.
526 </p>
528 <p>
529 To be more precise,
530 when a rule's selector has ''::nth-fragment()'' pseudo-elements
531 attached to parts of the selector other than the subject,
532 the declarations in that rule apply to
533 a fragment (or pseudo-element thereof) when:
534 </p>
535 <ol>
536 <li>
537 the declarations are for properties that apply to the
538 ''::first-letter'' pseudo-element,
539 </li>
540 <li>
541 the declarations would apply to
542 that fragment (or pseudo-element thereof)
543 had those ''::nth-fragment()'' pseudo-elements been removed,
544 with a particular association between
545 each sequence of simple selectors and the element it matched,
546 and
547 </li>
548 <li>
549 for each removed ''::nth-fragment()'' pseudo-element,
550 the fragment lives within a <i>fragment box</i>
551 of the element associated in that association
552 with the selector that the pseudo-element was attached to,
553 and whose index matches the pseudo-element.
554 </li>
555 </ol>
557 <div class="example">
558 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
559 <style>
560 .dark-columns {
561 overflow: fragments;
562 width: 6em;
563 height: 10em;
564 float: left;
565 margin-right: 1em;
566 font: medium/1.25 Times New
567 Roman, Times, serif;
568 }
569 .dark-columns::nth-fragment(1) {
570 background: aqua; color: black;
571 }
572 .dark-columns::nth-fragment(1) :link {
573 color: blue;
574 }
575 .dark-columns::nth-fragment(1) :visited {
576 color: purple;
577 }
578 .dark-columns::nth-fragment(2) {
579 background: navy; color: white;
580 }
581 .dark-columns::nth-fragment(2) :link {
582 color: aqua;
583 }
584 .dark-columns::nth-fragment(2) :visited {
585 color: fuchsia;
586 }
587 </style>
588 <div class="dark-columns">
589 <i>...</i>
590 </div></pre></td><td>
591 <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>
592 <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>
593 </td></tr></table>
594 </div>
597 <h3 id="max-lines">The 'max-lines' property</h3>
599 <p>
600 Authors may wish to style the opening lines of an element
601 with different styles
602 by putting those opening lines in a separate fragment.
603 However, since it may be difficult to predict the exact height
604 occupied by those lines
605 in order to restrict the first fragment to that height,
606 this specification introduces a 'max-lines' property
607 that forces a fragment to break
608 after a specified number of lines.
609 This forces a break after the given number of lines
610 contained within the element or its descendants,
611 as long as those lines are in the same block formatting context.
612 </p>
614 <table class=propdef>
615 <tr>
616 <th>Name:
617 <td><dfn>max-lines</dfn>
618 <tr>
619 <th><a href="#values">Value</a>:
620 <td>none | <integer>
621 <tr>
622 <th>Initial:
623 <td>none
624 <tr>
625 <th>Applies to:
626 <td>fragment boxes
627 <tr>
628 <th>Inherited:
629 <td>no
630 <tr>
631 <th>Animatable:
632 <td>as <a href="http://www.w3.org/TR/css3-transitions/#animatable-types">integer</a>
633 <tr>
634 <th>Percentages:
635 <td>N/A
636 <tr>
637 <th>Media:
638 <td>visual
639 <tr>
640 <th>Computed value:
641 <td>specified value
642 <tr>
643 <th>Canonical order:
644 <td><abbr title="follows order of property value definition">per grammar</abbr>
645 </table>
647 <dl>
648 <dt>none
649 <dd>
650 <p>
651 Breaks occur only as specified elsewhere.
652 </p>
653 </dd>
655 <dt><integer>
656 <dd>
657 <p>
658 In addition to any breaks specified elsewhere,
659 a break is forced before any line that would exceed
660 the given number of lines
661 being placed inside the element
662 (excluding lines that are in
663 a different block formatting context from
664 the block formatting context to which
665 an unstyled child of the element would belong).
666 </p>
668 <p class="issue">
669 If there are multiple boundaries between this line
670 and the previous, where exactly (in terms of element
671 boundaries) is the break forced?
672 </p>
674 <p>
675 Only positive integers are accepted.
676 Zero or negative integers are a parse error.
677 </p>
678 </dd>
679 </dl>
681 <p class="issue">Should this apply to fragment overflow only, or also
682 to pagination?</p>
684 <div class="example">
685 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
686 <style>
687 .article {
688 overflow: fragments;
689 }
690 .article::first-letter {
691 font-size: 2em;
692 line-height: 0.9;
693 }
694 .article::nth-fragment(1) {
695 font-size: 1.5em;
696 max-lines: 3;
697 }
698 .article::nth-fragment(n+2) {
699 /* 2 and up */
700 column-count: 2;
701 }
702 </style>
703 <div class="article">
704 <i>...</i>
705 </div></pre></td><td>
706 <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>
707 <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>
708 <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>
709 </td></tr></table>
710 </div>
712 <h2 id="conformance">
713 Conformance</h2>
715 <h3 id="placement">
716 Module interactions</h3>
718 <p>This module extends the 'overflow'
719 feature defined in [[CSS21]] section 11.1.1. It defines additional
720 overflow handling mechanisms that implementations must implement as
721 described in this module in order to conform to this module.</p>
723 <p>No properties in this module apply to the <code>::first-line</code> or
724 <code>::first-letter</code> pseudo-elements.</p>
726 <h3 id="values">
727 Values</h3>
729 <p>This specification follows the
730 <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
731 definition conventions</a> from [[!CSS21]]. Value types not defined in
732 this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
733 Other CSS modules may expand the definitions of these value types: for
734 example [[CSS3COLOR]], when combined with this module, expands the
735 definition of the <color> value type as used in this specification.</p>
737 <p>In addition to the property-specific values listed in their definitions,
738 all properties defined in this specification also accept the
739 <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
740 keyword as their property value. For readability it has not been repeated
741 explicitly.
744 <h3 id="conventions">
745 Document conventions</h3>
747 <p>Conformance requirements are expressed with a combination of
748 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
749 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
750 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
751 document are to be interpreted as described in RFC 2119.
752 However, for readability, these words do not appear in all uppercase
753 letters in this specification.
755 <p>All of the text of this specification is normative except sections
756 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
758 <p>Examples in this specification are introduced with the words “for example”
759 or are set apart from the normative text with <code>class="example"</code>,
760 like this:
762 <div class="example">
763 <p>This is an example of an informative example.</p>
764 </div>
766 <p>Informative notes begin with the word “Note” and are set apart from the
767 normative text with <code>class="note"</code>, like this:
769 <p class="note">Note, this is an informative note.</p>
771 <h3 id="conformance-classes">
772 Conformance classes</h3>
774 <p>Conformance to CSS Overflow Module Level 3
775 is defined for three conformance classes:
776 <dl>
777 <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
778 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
779 style sheet</a>.
780 <dt><dfn>renderer</dfn></dt>
781 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
782 that interprets the semantics of a style sheet and renders
783 documents that use them.
784 <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
785 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
786 that writes a style sheet.
787 </dl>
789 <p>A style sheet is conformant to CSS Overflow Module Level 3
790 if all of its statements that use syntax defined in this module are valid
791 according to the generic CSS grammar and the individual grammars of each
792 feature defined in this module.
794 <p>A renderer is conformant to CSS Overflow Module Level 3
795 if, in addition to interpreting the style sheet as defined by the
796 appropriate specifications, it supports all the features defined
797 by CSS Overflow Module Level 3 by parsing them correctly
798 and rendering the document accordingly. However, the inability of a
799 UA to correctly render a document due to limitations of the device
800 does not make the UA non-conformant. (For example, a UA is not
801 required to render color on a monochrome monitor.)
803 <p>An authoring tool is conformant to CSS Overflow Module Level 3
804 if it writes style sheets that are syntactically correct according to the
805 generic CSS grammar and the individual grammars of each feature in
806 this module, and meet all other conformance requirements of style sheets
807 as described in this module.
809 <h3 id="partial">
810 Partial implementations</h3>
812 <p>So that authors can exploit the forward-compatible parsing rules to
813 assign fallback values, CSS renderers <strong>must</strong>
814 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
815 as appropriate</a>) any at-rules, properties, property values, keywords,
816 and other syntactic constructs for which they have no usable level of
817 support. In particular, user agents <strong>must not</strong> selectively
818 ignore unsupported component values and honor supported values in a single
819 multi-value property declaration: if any value is considered invalid
820 (as unsupported values must be), CSS requires that the entire declaration
821 be ignored.</p>
823 <h3 id="experimental">
824 Experimental implementations</h3>
826 <p>To avoid clashes with future CSS features, the CSS2.1 specification
827 reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
828 syntax</a> for proprietary and experimental extensions to CSS.
830 <p>Prior to a specification reaching the Candidate Recommendation stage
831 in the W3C process, all implementations of a CSS feature are considered
832 experimental. The CSS Working Group recommends that implementations
833 use a vendor-prefixed syntax for such features, including those in
834 W3C Working Drafts. This avoids incompatibilities with future changes
835 in the draft.
836 </p>
838 <h3 id="testing">
839 Non-experimental implementations</h3>
841 <p>Once a specification reaches the Candidate Recommendation stage,
842 non-experimental implementations are possible, and implementors should
843 release an unprefixed implementation of any CR-level feature they
844 can demonstrate to be correctly implemented according to spec.
846 <p>To establish and maintain the interoperability of CSS across
847 implementations, the CSS Working Group requests that non-experimental
848 CSS renderers submit an implementation report (and, if necessary, the
849 testcases used for that implementation report) to the W3C before
850 releasing an unprefixed implementation of any CSS features. Testcases
851 submitted to W3C are subject to review and correction by the CSS
852 Working Group.
854 <p>Further information on submitting testcases and implementation reports
855 can be found from on the CSS Working Group's website at
856 <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
857 Questions should be directed to the
858 <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
859 mailing list.
861 <h3 id="cr-exit-criteria">
862 CR exit criteria</h3>
864 <p class=issue>[Change or remove the following CR exit criteria if
865 the spec is not a module, but, e.g., a Note or a profile. This text was <a
866 href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>
868 <p>
869 For this specification to be advanced to Proposed Recommendation,
870 there must be at least two independent, interoperable implementations
871 of each feature. Each feature may be implemented by a different set of
872 products, there is no requirement that all features be implemented by
873 a single product. For the purposes of this criterion, we define the
874 following terms:
876 <dl>
877 <dt>independent <dd>each implementation must be developed by a
878 different party and cannot share, reuse, or derive from code
879 used by another qualifying implementation. Sections of code that
880 have no bearing on the implementation of this specification are
881 exempt from this requirement.
883 <dt>interoperable <dd>passing the respective test case(s) in the
884 official CSS test suite, or, if the implementation is not a Web
885 browser, an equivalent test. Every relevant test in the test
886 suite should have an equivalent test created if such a user
887 agent (UA) is to be used to claim interoperability. In addition
888 if such a UA is to be used to claim interoperability, then there
889 must one or more additional UAs which can also pass those
890 equivalent tests in the same way for the purpose of
891 interoperability. The equivalent tests must be made publicly
892 available for the purposes of peer review.
894 <dt>implementation <dd>a user agent which:
896 <ol class=inline>
897 <li>implements the specification.
899 <li>is available to the general public. The implementation may
900 be a shipping product or other publicly available version
901 (i.e., beta version, preview release, or “nightly build”).
902 Non-shipping product releases must have implemented the
903 feature(s) for a period of at least one month in order to
904 demonstrate stability.
906 <li>is not experimental (i.e., a version specifically designed
907 to pass the test suite and is not intended for normal usage
908 going forward).
909 </ol>
910 </dl>
912 <p>The specification will remain Candidate Recommendation for at least
913 six months.
915 <h2 class=no-num id="acknowledgments">
916 Acknowledgments</h2>
918 <p>
919 Thanks especially to the feedback from
920 Håkon Wium Lie,
921 Florian Rivoal,
922 Alan Stearns,
923 and all the rest of the
924 <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
925 </p>
927 <h2 class=no-num id="references">
928 References</h2>
930 <h3 class="no-num" id="normative-references">
931 Normative references</h3>
932 <!--normative-->
934 <h3 class="no-num" id="other-references">
935 Other references</h3>
936 <!--informative-->
938 <h2 class="no-num" id="index">
939 Index</h2>
940 <!--index-->
942 <h2 class="no-num" id="property-index">
943 Property index</h2>
944 <!-- properties -->
946 </body>
947 </html>
948 <!-- Keep this comment at the end of the file
949 Local variables:
950 mode: sgml
951 sgml-declaration:"~/SGML/HTML4.decl"
952 sgml-default-doctype-name:"html"
953 sgml-minimize-attributes:t
954 sgml-nofill-elements:("pre" "style" "br")
955 sgml-live-element-indicator:t
956 sgml-omittag:nil
957 sgml-shorttag:nil
958 sgml-namecase-general:t
959 sgml-general-insert-case:lower
960 sgml-always-quote-attributes:t
961 sgml-indent-step:nil
962 sgml-indent-data:t
963 sgml-parent-document:nil
964 sgml-exposed-tags:nil
965 sgml-local-catalogs:nil
966 sgml-local-ecat-files:nil
967 End:
968 -->
