Thu, 16 Aug 2012 14:43:26 -0700
Mention need for prose to make break:regions apply, 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 <p class="issue">
342 This specification needs to say which type of
343 fragmentation context is created
344 so that it's clear which values of the 'break' property
345 cause breaks within this context.
346 We probably want ''break: regions'' to apply.
347 </p>
349 <h3 id="fragment-styling">Fragment styling</h3>
351 <h4 id="fragment-pseudo-element">The ::nth-fragment() pseudo-element</h4>
353 <p>
354 The ::nth-fragment() pseudo-element is a pseudo-element
355 that describes some of the <i>fragment box</i>es generated by an element.
356 The argument to the pseudo-element takes the same syntax
357 as the argument to the :nth-child() pseudo-class
358 defined in [[!SELECT]], and has the same meaning
359 except that the number is relative to
360 <i>fragment box</i>es generated by the element
361 instead of siblings of the element.
362 </p>
364 <p class="note">
365 Selectors that allow addressing fragments
366 by counting from the end rather than the start
367 are intentionally not provided.
368 Such selectors would interfere with determining
369 the number of fragments.
370 </p>
372 <h4 id="style-of-fragments">Styling of fragments</h4>
374 <p class="issue">
375 Should this apply to fragment overflow only,
376 or also to paginated overflow?
377 (If it applies,
378 then stricter property restrictions would be needed
379 for paginated overflow.)
380 </p>
382 <p>
383 In the absence of rules with ''::nth-fragment()'' pseudo-elements,
384 the computed style for each <i>fragment box</i>
385 is the computed style for the element
386 for which the <i>fragment box</i> was created.
387 However, the style for a <i>fragment box</i> is also influenced
388 by rules whose selector's <i>subject</i> [[!SELECT]]
389 has an ''::nth-fragment()'' pseudo-element,
390 if the 1-based number of the <i>fragment box</i> matches
391 that ''::nth-fragment()'' pseudo-element
392 and the selector (excluding the ''::nth-fragment()'' pseudo-element)
393 matches the element generating the fragments.
394 </p>
396 <p>
397 When determining the style of the <i>fragment box</i>,
398 these rules that match the fragment pseudo-element
399 cascade together with the rules that match the element,
400 with the fragment pseudo-element adding the specificity
401 of a pseudo-class to the specificity calculation.
402 <span class="issue">Does this need to be specified in
403 the cascading module as well?</span>
404 </p>
406 <div class="example">
407 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
408 <style>
409 .bouncy-columns {
410 overflow: fragments;
411 width: 6em;
412 height: 10em;
413 float: left;
414 margin: 1em;
415 font: medium/1.25 Times New
416 Roman, Times, serif;
417 }
418 .bouncy-columns::nth-fragment(1) {
419 background: aqua; color: black;
420 transform: rotate(-3deg);
421 }
422 .bouncy-columns::nth-fragment(2) {
423 background: yellow; color: black;
424 transform: rotate(3deg);
425 }
426 </style>
427 <div class="bouncy-columns">
428 <i>...</i>
429 </div></pre></td><td>
430 <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>
431 <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>
432 <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>
433 </td></tr></table>
434 </div>
436 <p>
437 Styling an ''::nth-fragment()'' pseudo-element with the 'overflow'
438 property has no effect;
439 the computed value of 'overflow' for the fragment box
440 remains the same as the computed value of overflow for the element.
441 </p>
443 <p>
444 Styling an ''::nth-fragment()'' pseudo-element with the 'content'
445 property has no effect;
446 the computed value of 'content' for the fragment box
447 remains the same as the computed value of content for the element.
448 </p>
450 <p>
451 Specifying ''display: none'' for a <i>fragment box</i> causes
452 the fragment box with that index not to be generated.
453 However, in terms of the indices
454 used for matching ''::nth-fragment()'' pseudo-elements
455 of later fragment boxes,
456 it still counts as though it was generated.
457 However, since it is not generated, it does not contain any content.
458 </p>
460 <p class="issue">
461 Would it make more sense to forbid ''display:none''?
462 Or perhaps to forbid 'display', 'position', 'float',
463 and similar (in addition to 'overflow')?
464 </p>
466 <p>
467 To match the model for other pseudo-elements
468 where the pseudo-elements live inside their corresponding element,
469 declarations in ''::nth-fragment()'' pseudo-elements override
470 declarations in rules without the pseudo-element.
471 The relative priority within such declarations is determined
472 by normal cascading order (see [[!CSS21]]).
473 </p>
475 <p>
476 Styles specified on ''::nth-fragment()'' pseudo-elements
477 do affect inheritance to content within the <i>fragment box</i>.
478 In other words, the content within the <i>fragment box</i> must
479 inherit from the fragment box's style (i.e., the pseudo-element style)
480 rather than directly from the element.
481 This means that elements split between fragment boxes may
482 have different styles for different parts of the element.
483 </p>
485 <p class="issue">
486 This inheritance rule allows specifying styles indirectly
487 (by using explicit ''inherit'' or using default inheritance
488 on properties that don't apply to '':first-letter'')
489 that can't be specified directly
490 (based on the rules in the next section).
491 This is a problem.
492 The restrictions that apply to styling inside fragments
493 should also apply to inheritance from fragments.
494 </p>
496 <div class="example">
497 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
498 <style>
499 .article {
500 overflow: fragments;
501 }
502 .article::nth-fragment(1) {
503 font-size: 1.5em;
504 margin-bottom: 1em;
505 height: 4em;
506 }
507 .article::nth-fragment(n+2) {
508 /* 2 and up */
509 margin-left: 5em;
510 margin-right: 2em;
511 }
512 </style>
513 <div class="article">
514 The <code>font-size</code> property<i>...</i>
515 </div></pre></td><td>
516 <div class="article-font-inherit-demo one">The <code>font-size</code> property<br>specified on the fragment<br>is inherited into the</div>
517 <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>
518 </td></tr></table>
519 </div>
521 <h4 id="style-in-fragments">Styling inside fragments</h4>
523 <p class="issue">
524 Should this apply to fragment overflow only,
525 or also to paginated overflow,
526 or even to pagination across pages?
527 </p>
529 <p>
530 The ''::nth-fragment()'' pseudo-element
531 can also be used to style
532 content inside of a <i>fragment box</i>.
533 Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements,
534 the ''::nth-fragment()'' pseudo-element can be applied
535 to parts of the selector other than the subject:
536 in particular, it can match ancestors of the subject.
537 However, the only CSS properties applied
538 by rules with such selectors
539 are those that apply
540 to the ''::first-letter'' pseudo-element.
541 </p>
543 <p>
544 To be more precise,
545 when a rule's selector has ''::nth-fragment()'' pseudo-elements
546 attached to parts of the selector other than the subject,
547 the declarations in that rule apply to
548 a fragment (or pseudo-element thereof) when:
549 </p>
550 <ol>
551 <li>
552 the declarations are for properties that apply to the
553 ''::first-letter'' pseudo-element,
554 </li>
555 <li>
556 the declarations would apply to
557 that fragment (or pseudo-element thereof)
558 had those ''::nth-fragment()'' pseudo-elements been removed,
559 with a particular association between
560 each sequence of simple selectors and the element it matched,
561 and
562 </li>
563 <li>
564 for each removed ''::nth-fragment()'' pseudo-element,
565 the fragment lives within a <i>fragment box</i>
566 of the element associated in that association
567 with the selector that the pseudo-element was attached to,
568 and whose index matches the pseudo-element.
569 </li>
570 </ol>
572 <div class="example">
573 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
574 <style>
575 .dark-columns {
576 overflow: fragments;
577 width: 6em;
578 height: 10em;
579 float: left;
580 margin-right: 1em;
581 font: medium/1.25 Times New
582 Roman, Times, serif;
583 }
584 .dark-columns::nth-fragment(1) {
585 background: aqua; color: black;
586 }
587 .dark-columns::nth-fragment(1) :link {
588 color: blue;
589 }
590 .dark-columns::nth-fragment(1) :visited {
591 color: purple;
592 }
593 .dark-columns::nth-fragment(2) {
594 background: navy; color: white;
595 }
596 .dark-columns::nth-fragment(2) :link {
597 color: aqua;
598 }
599 .dark-columns::nth-fragment(2) :visited {
600 color: fuchsia;
601 }
602 </style>
603 <div class="dark-columns">
604 <i>...</i>
605 </div></pre></td><td>
606 <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>
607 <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>
608 </td></tr></table>
609 </div>
612 <h3 id="max-lines">The 'max-lines' property</h3>
614 <p>
615 Authors may wish to style the opening lines of an element
616 with different styles
617 by putting those opening lines in a separate fragment.
618 However, since it may be difficult to predict the exact height
619 occupied by those lines
620 in order to restrict the first fragment to that height,
621 this specification introduces a 'max-lines' property
622 that forces a fragment to break
623 after a specified number of lines.
624 This forces a break after the given number of lines
625 contained within the element or its descendants,
626 as long as those lines are in the same block formatting context.
627 </p>
629 <table class=propdef>
630 <tr>
631 <th>Name:
632 <td><dfn>max-lines</dfn>
633 <tr>
634 <th><a href="#values">Value</a>:
635 <td>none | <integer>
636 <tr>
637 <th>Initial:
638 <td>none
639 <tr>
640 <th>Applies to:
641 <td>fragment boxes
642 <tr>
643 <th>Inherited:
644 <td>no
645 <tr>
646 <th>Animatable:
647 <td>as <a href="http://www.w3.org/TR/css3-transitions/#animatable-types">integer</a>
648 <tr>
649 <th>Percentages:
650 <td>N/A
651 <tr>
652 <th>Media:
653 <td>visual
654 <tr>
655 <th>Computed value:
656 <td>specified value
657 <tr>
658 <th>Canonical order:
659 <td><abbr title="follows order of property value definition">per grammar</abbr>
660 </table>
662 <dl>
663 <dt>none
664 <dd>
665 <p>
666 Breaks occur only as specified elsewhere.
667 </p>
668 </dd>
670 <dt><integer>
671 <dd>
672 <p>
673 In addition to any breaks specified elsewhere,
674 a break is forced before any line that would exceed
675 the given number of lines
676 being placed inside the element
677 (excluding lines that are in
678 a different block formatting context from
679 the block formatting context to which
680 an unstyled child of the element would belong).
681 </p>
683 <p class="issue">
684 If there are multiple boundaries between this line
685 and the previous, where exactly (in terms of element
686 boundaries) is the break forced?
687 </p>
689 <p>
690 Only positive integers are accepted.
691 Zero or negative integers are a parse error.
692 </p>
693 </dd>
694 </dl>
696 <p class="issue">Should this apply to fragment overflow only, or also
697 to pagination?</p>
699 <div class="example">
700 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
701 <style>
702 .article {
703 overflow: fragments;
704 }
705 .article::first-letter {
706 font-size: 2em;
707 line-height: 0.9;
708 }
709 .article::nth-fragment(1) {
710 font-size: 1.5em;
711 max-lines: 3;
712 }
713 .article::nth-fragment(n+2) {
714 /* 2 and up */
715 column-count: 2;
716 }
717 </style>
718 <div class="article">
719 <i>...</i>
720 </div></pre></td><td>
721 <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>
722 <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>
723 <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>
724 </td></tr></table>
725 </div>
727 <h2 id="conformance">
728 Conformance</h2>
730 <h3 id="placement">
731 Module interactions</h3>
733 <p>This module extends the 'overflow'
734 feature defined in [[CSS21]] section 11.1.1. It defines additional
735 overflow handling mechanisms that implementations must implement as
736 described in this module in order to conform to this module.</p>
738 <p>No properties in this module apply to the <code>::first-line</code> or
739 <code>::first-letter</code> pseudo-elements.</p>
741 <h3 id="values">
742 Values</h3>
744 <p>This specification follows the
745 <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
746 definition conventions</a> from [[!CSS21]]. Value types not defined in
747 this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
748 Other CSS modules may expand the definitions of these value types: for
749 example [[CSS3COLOR]], when combined with this module, expands the
750 definition of the <color> value type as used in this specification.</p>
752 <p>In addition to the property-specific values listed in their definitions,
753 all properties defined in this specification also accept the
754 <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
755 keyword as their property value. For readability it has not been repeated
756 explicitly.
759 <h3 id="conventions">
760 Document conventions</h3>
762 <p>Conformance requirements are expressed with a combination of
763 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
764 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
765 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
766 document are to be interpreted as described in RFC 2119.
767 However, for readability, these words do not appear in all uppercase
768 letters in this specification.
770 <p>All of the text of this specification is normative except sections
771 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
773 <p>Examples in this specification are introduced with the words “for example”
774 or are set apart from the normative text with <code>class="example"</code>,
775 like this:
777 <div class="example">
778 <p>This is an example of an informative example.</p>
779 </div>
781 <p>Informative notes begin with the word “Note” and are set apart from the
782 normative text with <code>class="note"</code>, like this:
784 <p class="note">Note, this is an informative note.</p>
786 <h3 id="conformance-classes">
787 Conformance classes</h3>
789 <p>Conformance to CSS Overflow Module Level 3
790 is defined for three conformance classes:
791 <dl>
792 <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
793 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
794 style sheet</a>.
795 <dt><dfn>renderer</dfn></dt>
796 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
797 that interprets the semantics of a style sheet and renders
798 documents that use them.
799 <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
800 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
801 that writes a style sheet.
802 </dl>
804 <p>A style sheet is conformant to CSS Overflow Module Level 3
805 if all of its statements that use syntax defined in this module are valid
806 according to the generic CSS grammar and the individual grammars of each
807 feature defined in this module.
809 <p>A renderer is conformant to CSS Overflow Module Level 3
810 if, in addition to interpreting the style sheet as defined by the
811 appropriate specifications, it supports all the features defined
812 by CSS Overflow Module Level 3 by parsing them correctly
813 and rendering the document accordingly. However, the inability of a
814 UA to correctly render a document due to limitations of the device
815 does not make the UA non-conformant. (For example, a UA is not
816 required to render color on a monochrome monitor.)
818 <p>An authoring tool is conformant to CSS Overflow Module Level 3
819 if it writes style sheets that are syntactically correct according to the
820 generic CSS grammar and the individual grammars of each feature in
821 this module, and meet all other conformance requirements of style sheets
822 as described in this module.
824 <h3 id="partial">
825 Partial implementations</h3>
827 <p>So that authors can exploit the forward-compatible parsing rules to
828 assign fallback values, CSS renderers <strong>must</strong>
829 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
830 as appropriate</a>) any at-rules, properties, property values, keywords,
831 and other syntactic constructs for which they have no usable level of
832 support. In particular, user agents <strong>must not</strong> selectively
833 ignore unsupported component values and honor supported values in a single
834 multi-value property declaration: if any value is considered invalid
835 (as unsupported values must be), CSS requires that the entire declaration
836 be ignored.</p>
838 <h3 id="experimental">
839 Experimental implementations</h3>
841 <p>To avoid clashes with future CSS features, the CSS2.1 specification
842 reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
843 syntax</a> for proprietary and experimental extensions to CSS.
845 <p>Prior to a specification reaching the Candidate Recommendation stage
846 in the W3C process, all implementations of a CSS feature are considered
847 experimental. The CSS Working Group recommends that implementations
848 use a vendor-prefixed syntax for such features, including those in
849 W3C Working Drafts. This avoids incompatibilities with future changes
850 in the draft.
851 </p>
853 <h3 id="testing">
854 Non-experimental implementations</h3>
856 <p>Once a specification reaches the Candidate Recommendation stage,
857 non-experimental implementations are possible, and implementors should
858 release an unprefixed implementation of any CR-level feature they
859 can demonstrate to be correctly implemented according to spec.
861 <p>To establish and maintain the interoperability of CSS across
862 implementations, the CSS Working Group requests that non-experimental
863 CSS renderers submit an implementation report (and, if necessary, the
864 testcases used for that implementation report) to the W3C before
865 releasing an unprefixed implementation of any CSS features. Testcases
866 submitted to W3C are subject to review and correction by the CSS
867 Working Group.
869 <p>Further information on submitting testcases and implementation reports
870 can be found from on the CSS Working Group's website at
871 <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
872 Questions should be directed to the
873 <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
874 mailing list.
876 <h3 id="cr-exit-criteria">
877 CR exit criteria</h3>
879 <p class=issue>[Change or remove the following CR exit criteria if
880 the spec is not a module, but, e.g., a Note or a profile. This text was <a
881 href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>
883 <p>
884 For this specification to be advanced to Proposed Recommendation,
885 there must be at least two independent, interoperable implementations
886 of each feature. Each feature may be implemented by a different set of
887 products, there is no requirement that all features be implemented by
888 a single product. For the purposes of this criterion, we define the
889 following terms:
891 <dl>
892 <dt>independent <dd>each implementation must be developed by a
893 different party and cannot share, reuse, or derive from code
894 used by another qualifying implementation. Sections of code that
895 have no bearing on the implementation of this specification are
896 exempt from this requirement.
898 <dt>interoperable <dd>passing the respective test case(s) in the
899 official CSS test suite, or, if the implementation is not a Web
900 browser, an equivalent test. Every relevant test in the test
901 suite should have an equivalent test created if such a user
902 agent (UA) is to be used to claim interoperability. In addition
903 if such a UA is to be used to claim interoperability, then there
904 must one or more additional UAs which can also pass those
905 equivalent tests in the same way for the purpose of
906 interoperability. The equivalent tests must be made publicly
907 available for the purposes of peer review.
909 <dt>implementation <dd>a user agent which:
911 <ol class=inline>
912 <li>implements the specification.
914 <li>is available to the general public. The implementation may
915 be a shipping product or other publicly available version
916 (i.e., beta version, preview release, or “nightly build”).
917 Non-shipping product releases must have implemented the
918 feature(s) for a period of at least one month in order to
919 demonstrate stability.
921 <li>is not experimental (i.e., a version specifically designed
922 to pass the test suite and is not intended for normal usage
923 going forward).
924 </ol>
925 </dl>
927 <p>The specification will remain Candidate Recommendation for at least
928 six months.
930 <h2 class=no-num id="acknowledgments">
931 Acknowledgments</h2>
933 <p>
934 Thanks especially to the feedback from
935 Håkon Wium Lie,
936 Florian Rivoal,
937 Alan Stearns,
938 and all the rest of the
939 <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
940 </p>
942 <h2 class=no-num id="references">
943 References</h2>
945 <h3 class="no-num" id="normative-references">
946 Normative references</h3>
947 <!--normative-->
949 <h3 class="no-num" id="other-references">
950 Other references</h3>
951 <!--informative-->
953 <h2 class="no-num" id="index">
954 Index</h2>
955 <!--index-->
957 <h2 class="no-num" id="property-index">
958 Property index</h2>
959 <!-- properties -->
961 </body>
962 </html>
963 <!-- Keep this comment at the end of the file
964 Local variables:
965 mode: sgml
966 sgml-declaration:"~/SGML/HTML4.decl"
967 sgml-default-doctype-name:"html"
968 sgml-minimize-attributes:t
969 sgml-nofill-elements:("pre" "style" "br")
970 sgml-live-element-indicator:t
971 sgml-omittag:nil
972 sgml-shorttag:nil
973 sgml-namecase-general:t
974 sgml-general-insert-case:lower
975 sgml-always-quote-attributes:t
976 sgml-indent-step:nil
977 sgml-indent-data:t
978 sgml-parent-document:nil
979 sgml-exposed-tags:nil
980 sgml-local-catalogs:nil
981 sgml-local-ecat-files:nil
982 End:
983 -->