css3-overflow/Overview.src.html@dcb554a16adc
css3-overflow/Overview.src.html
Thu, 16 Aug 2012 15:09:23 -0700
- author
- L. David Baron <dbaron@dbaron.org>
- date
- Thu, 16 Aug 2012 15:09:23 -0700
- changeset 6492
- dcb554a16adc
- parent 6491
- 64dd301ea467
- child 6493
- bcbba88e4047
- permissions
- -rw-r--r--
Define overflow columns as creating new fragment boxes, 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 a sequence of <dfn>fragment box</dfn>es
263 for that element.
264 (It is possible for an element with ''overflow: fragments''
265 to generate only one <i>fragment box</i>.
266 However, if an element's computed 'overflow' is not ''fragments'',
267 then its box is not a <i>fragment box</i>.)
268 Every <i>fragment box</i> is a fragmentation container,
269 and any overflow
270 that would cause that fragmentation container to fragment
271 causes another <i>fragment box</i> created as a next sibling
272 of the previous one.
273 <span class="issue">Or is it as though it's a next sibling of
274 the element? Need to figure out exactly how this interacts with
275 other box-level fixup.</span>
276 Additionally, if the <i>fragment box</i> is also
277 a multi-column box (as defined in [[!CSS3COL]]
278 <span class="issue">though it defines <i>multi-column element</i></span>)
279 any content that would lead to the creation of <i>overflow columns</i> [[!CSS3COL]]
280 instead is flown into an additional fragment box.
281 However, fragment boxes may themselves be broken
282 (due to fragmentation in a fragmentation context outside of them,
283 such as pages, columns, or other fragment boxes);
284 such breaking leads to fragments of the same fragment box
285 rather than multiple fragment boxes.
286 (This matters because fragment boxes may be styled by their index;
287 such breaking leads to multiple fragments of a fragment box
288 with a single index.
289 This design choice is so that
290 breaking a fragment box across pages does not break
291 the association of indices to particular pieces of content.)
292 <span class="issue">Should a forced break that breaks to
293 an outer fragmentation context cause a new fragment of a single
294 fragment box or a new fragment box?</span>
295 <span class="issue">Should we find a term other than
296 <i>fragment box</i> here to make this a little less confusing?</span>
297 </p>
299 <p class="issue">
300 What if we want to be able to style the pieces of an element
301 split within another type of fragmentation context?
302 These rules prevent ever using ''::nth-fragment()'' for that,
303 despite that the name seems the most logical name for such a feature.
304 </p>
306 <div class="example">
307 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
308 <title>Breaking content into
309 equal-sized cards</title>
310 <style>
311 .in-cards {
312 overflow: fragments;
314 width: 13em;
315 height: 8em;
317 padding: 4px;
318 border: medium solid blue;
319 margin: 6px;
321 font: medium/1.3 Times New
322 Roman, Times, serif;
323 }
324 </style>
325 <div class="in-cards">
326 In this example, the text in the div
327 is broken into a series of cards.
328 These cards all have the same style.
329 The presence of enough content to
330 overflow one of the cards causes
331 another one to be created. The second
332 card is created just like it's the
333 next sibling of the first.
334 </div></pre></td><td>
335 <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>
336 <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>
337 </td></tr></table>
338 </div>
340 <p class="issue">
341 We should specify that ''overflow: fragments'' does not apply
342 to at least some table parts,
343 and perhaps other elements as well.
344 We need to determine exactly which ones.
345 </p>
347 <p class="issue">
348 This specification needs to say which type of
349 fragmentation context is created
350 so that it's clear which values of the 'break' property
351 cause breaks within this context.
352 We probably want ''break: regions'' to apply.
353 </p>
355 <h3 id="fragment-styling">Fragment styling</h3>
357 <h4 id="fragment-pseudo-element">The ::nth-fragment() pseudo-element</h4>
359 <p>
360 The ::nth-fragment() pseudo-element is a pseudo-element
361 that describes some of the <i>fragment box</i>es generated by an element.
362 The argument to the pseudo-element takes the same syntax
363 as the argument to the :nth-child() pseudo-class
364 defined in [[!SELECT]], and has the same meaning
365 except that the number is relative to
366 <i>fragment box</i>es generated by the element
367 instead of siblings of the element.
368 </p>
370 <p class="note">
371 Selectors that allow addressing fragments
372 by counting from the end rather than the start
373 are intentionally not provided.
374 Such selectors would interfere with determining
375 the number of fragments.
376 </p>
378 <p class="issue">
379 Depending on future discussions,
380 this ''::nth-fragment(<var>an+b</var>)'' syntax
381 may be replaced with
382 the new ''::fragment:nth(<var>an+b</var>)'' syntax.
383 </p>
385 <h4 id="style-of-fragments">Styling of fragments</h4>
387 <p class="issue">
388 Should this apply to fragment overflow only,
389 or also to paginated overflow?
390 (If it applies,
391 then stricter property restrictions would be needed
392 for paginated overflow.)
393 </p>
395 <p>
396 In the absence of rules with ''::nth-fragment()'' pseudo-elements,
397 the computed style for each <i>fragment box</i>
398 is the computed style for the element
399 for which the <i>fragment box</i> was created.
400 However, the style for a <i>fragment box</i> is also influenced
401 by rules whose selector's <i>subject</i> [[!SELECT]]
402 has an ''::nth-fragment()'' pseudo-element,
403 if the 1-based number of the <i>fragment box</i> matches
404 that ''::nth-fragment()'' pseudo-element
405 and the selector (excluding the ''::nth-fragment()'' pseudo-element)
406 matches the element generating the fragments.
407 </p>
409 <p>
410 When determining the style of the <i>fragment box</i>,
411 these rules that match the fragment pseudo-element
412 cascade together with the rules that match the element,
413 with the fragment pseudo-element adding the specificity
414 of a pseudo-class to the specificity calculation.
415 <span class="issue">Does this need to be specified in
416 the cascading module as well?</span>
417 </p>
419 <div class="example">
420 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
421 <style>
422 .bouncy-columns {
423 overflow: fragments;
424 width: 6em;
425 height: 10em;
426 float: left;
427 margin: 1em;
428 font: medium/1.25 Times New
429 Roman, Times, serif;
430 }
431 .bouncy-columns::nth-fragment(1) {
432 background: aqua; color: black;
433 transform: rotate(-3deg);
434 }
435 .bouncy-columns::nth-fragment(2) {
436 background: yellow; color: black;
437 transform: rotate(3deg);
438 }
439 </style>
440 <div class="bouncy-columns">
441 <i>...</i>
442 </div></pre></td><td>
443 <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>
444 <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>
445 <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>
446 </td></tr></table>
447 </div>
449 <p>
450 Styling an ''::nth-fragment()'' pseudo-element with the 'overflow'
451 property does take effect;
452 if a <i>fragment box</i> has a
453 computed value of 'overflow' other than ''fragments''
454 then that fragment box is the last fragment.
455 However, overriding ''overflow'' on the first fragment
456 does not cause the <i>fragment box</i> not to exist;
457 whether there are fragment boxes at all is determined by
458 the computed value of overflow for the element.
459 </p>
461 <p>
462 Styling an ''::nth-fragment()'' pseudo-element with the 'content'
463 property has no effect;
464 the computed value of 'content' for the fragment box
465 remains the same as the computed value of content for the element.
466 </p>
468 <p>
469 Specifying ''display: none'' for a <i>fragment box</i> causes
470 the fragment box with that index not to be generated.
471 However, in terms of the indices
472 used for matching ''::nth-fragment()'' pseudo-elements
473 of later fragment boxes,
474 it still counts as though it was generated.
475 However, since it is not generated, it does not contain any content.
476 </p>
478 <p class="issue">
479 Would it make more sense to forbid ''display:none''?
480 Or perhaps to forbid 'display', 'position', 'float',
481 and similar (in addition to 'overflow')?
482 </p>
484 <p>
485 To match the model for other pseudo-elements
486 where the pseudo-elements live inside their corresponding element,
487 declarations in ''::nth-fragment()'' pseudo-elements override
488 declarations in rules without the pseudo-element.
489 The relative priority within such declarations is determined
490 by normal cascading order (see [[!CSS21]]).
491 </p>
493 <p>
494 Styles specified on ''::nth-fragment()'' pseudo-elements
495 do affect inheritance to content within the <i>fragment box</i>.
496 In other words, the content within the <i>fragment box</i> must
497 inherit from the fragment box's style (i.e., the pseudo-element style)
498 rather than directly from the element.
499 This means that elements split between fragment boxes may
500 have different styles for different parts of the element.
501 </p>
503 <p class="issue">
504 This inheritance rule allows specifying styles indirectly
505 (by using explicit ''inherit'' or using default inheritance
506 on properties that don't apply to '':first-letter'')
507 that can't be specified directly
508 (based on the rules in the next section).
509 This is a problem.
510 The restrictions that apply to styling inside fragments
511 should also apply to inheritance from fragments.
512 </p>
514 <div class="example">
515 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
516 <style>
517 .article {
518 overflow: fragments;
519 }
520 .article::nth-fragment(1) {
521 font-size: 1.5em;
522 margin-bottom: 1em;
523 height: 4em;
524 }
525 .article::nth-fragment(2) {
526 margin-left: 5em;
527 margin-right: 2em;
528 }
529 </style>
530 <div class="article">
531 The <code>font-size</code> property<i>...</i>
532 </div></pre></td><td>
533 <div class="article-font-inherit-demo one">The <code>font-size</code> property<br>specified on the fragment<br>is inherited into the</div>
534 <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>
535 </td></tr></table>
536 </div>
538 <h4 id="style-in-fragments">Styling inside fragments</h4>
540 <p class="issue">
541 Should this apply to fragment overflow only,
542 or also to paginated overflow,
543 or even to pagination across pages?
544 </p>
546 <p>
547 The ''::nth-fragment()'' pseudo-element
548 can also be used to style
549 content inside of a <i>fragment box</i>.
550 Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements,
551 the ''::nth-fragment()'' pseudo-element can be applied
552 to parts of the selector other than the subject:
553 in particular, it can match ancestors of the subject.
554 However, the only CSS properties applied
555 by rules with such selectors
556 are those that apply
557 to the ''::first-letter'' pseudo-element.
558 </p>
560 <p>
561 To be more precise,
562 when a rule's selector has ''::nth-fragment()'' pseudo-elements
563 attached to parts of the selector other than the subject,
564 the declarations in that rule apply to
565 a fragment (or pseudo-element thereof) when:
566 </p>
567 <ol>
568 <li>
569 the declarations are for properties that apply to the
570 ''::first-letter'' pseudo-element,
571 </li>
572 <li>
573 the declarations would apply to
574 that fragment (or pseudo-element thereof)
575 had those ''::nth-fragment()'' pseudo-elements been removed,
576 with a particular association between
577 each sequence of simple selectors and the element it matched,
578 and
579 </li>
580 <li>
581 for each removed ''::nth-fragment()'' pseudo-element,
582 the fragment lives within a <i>fragment box</i>
583 of the element associated in that association
584 with the selector that the pseudo-element was attached to,
585 and whose index matches the pseudo-element.
586 </li>
587 </ol>
589 <div class="example">
590 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
591 <style>
592 .dark-columns {
593 overflow: fragments;
594 width: 6em;
595 height: 10em;
596 float: left;
597 margin-right: 1em;
598 font: medium/1.25 Times New
599 Roman, Times, serif;
600 }
601 .dark-columns::nth-fragment(1) {
602 background: aqua; color: black;
603 }
604 .dark-columns::nth-fragment(1) :link {
605 color: blue;
606 }
607 .dark-columns::nth-fragment(1) :visited {
608 color: purple;
609 }
610 .dark-columns::nth-fragment(2) {
611 background: navy; color: white;
612 }
613 .dark-columns::nth-fragment(2) :link {
614 color: aqua;
615 }
616 .dark-columns::nth-fragment(2) :visited {
617 color: fuchsia;
618 }
619 </style>
620 <div class="dark-columns">
621 <i>...</i>
622 </div></pre></td><td>
623 <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>
624 <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>
625 </td></tr></table>
626 </div>
629 <h3 id="max-lines">The 'max-lines' property</h3>
631 <p>
632 Authors may wish to style the opening lines of an element
633 with different styles
634 by putting those opening lines in a separate fragment.
635 However, since it may be difficult to predict the exact height
636 occupied by those lines
637 in order to restrict the first fragment to that height,
638 this specification introduces a 'max-lines' property
639 that forces a fragment to break
640 after a specified number of lines.
641 This forces a break after the given number of lines
642 contained within the element or its descendants,
643 as long as those lines are in the same block formatting context.
644 </p>
646 <table class=propdef>
647 <tr>
648 <th>Name:
649 <td><dfn>max-lines</dfn>
650 <tr>
651 <th><a href="#values">Value</a>:
652 <td>none | <integer>
653 <tr>
654 <th>Initial:
655 <td>none
656 <tr>
657 <th>Applies to:
658 <td>fragment boxes
659 <tr>
660 <th>Inherited:
661 <td>no
662 <tr>
663 <th>Animatable:
664 <td>as <a href="http://www.w3.org/TR/css3-transitions/#animatable-types">integer</a>
665 <tr>
666 <th>Percentages:
667 <td>N/A
668 <tr>
669 <th>Media:
670 <td>visual
671 <tr>
672 <th>Computed value:
673 <td>specified value
674 <tr>
675 <th>Canonical order:
676 <td><abbr title="follows order of property value definition">per grammar</abbr>
677 </table>
679 <dl>
680 <dt>none
681 <dd>
682 <p>
683 Breaks occur only as specified elsewhere.
684 </p>
685 </dd>
687 <dt><integer>
688 <dd>
689 <p>
690 In addition to any breaks specified elsewhere,
691 a break is forced before any line that would exceed
692 the given number of lines
693 being placed inside the element
694 (excluding lines that are in
695 a different block formatting context from
696 the block formatting context to which
697 an unstyled child of the element would belong).
698 </p>
700 <p class="issue">
701 If there are multiple boundaries between this line
702 and the previous, where exactly (in terms of element
703 boundaries) is the break forced?
704 </p>
706 <p>
707 Only positive integers are accepted.
708 Zero or negative integers are a parse error.
709 </p>
710 </dd>
711 </dl>
713 <p class="issue">Should this apply to fragment overflow only, or also
714 to pagination?</p>
716 <div class="example">
717 <table class="source-demo-pair"><tr><td><pre><!DOCTYPE HTML>
718 <style>
719 .article {
720 overflow: fragments;
721 }
722 .article::first-letter {
723 font-size: 2em;
724 line-height: 0.9;
725 }
726 .article::nth-fragment(1) {
727 font-size: 1.5em;
728 max-lines: 3;
729 }
730 .article::nth-fragment(2) {
731 column-count: 2;
732 }
733 </style>
734 <div class="article">
735 <i>...</i>
736 </div></pre></td><td>
737 <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>
738 <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>
739 <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>
740 </td></tr></table>
741 </div>
743 <h2 id="conformance">
744 Conformance</h2>
746 <h3 id="placement">
747 Module interactions</h3>
749 <p>This module extends the 'overflow'
750 feature defined in [[CSS21]] section 11.1.1. It defines additional
751 overflow handling mechanisms that implementations must implement as
752 described in this module in order to conform to this module.</p>
754 <p>No properties in this module apply to the <code>::first-line</code> or
755 <code>::first-letter</code> pseudo-elements.</p>
757 <h3 id="values">
758 Values</h3>
760 <p>This specification follows the
761 <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
762 definition conventions</a> from [[!CSS21]]. Value types not defined in
763 this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
764 Other CSS modules may expand the definitions of these value types: for
765 example [[CSS3COLOR]], when combined with this module, expands the
766 definition of the <color> value type as used in this specification.</p>
768 <p>In addition to the property-specific values listed in their definitions,
769 all properties defined in this specification also accept the
770 <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
771 keyword as their property value. For readability it has not been repeated
772 explicitly.
775 <h3 id="conventions">
776 Document conventions</h3>
778 <p>Conformance requirements are expressed with a combination of
779 descriptive assertions and RFC 2119 terminology. The key words “MUST”,
780 “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
781 “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
782 document are to be interpreted as described in RFC 2119.
783 However, for readability, these words do not appear in all uppercase
784 letters in this specification.
786 <p>All of the text of this specification is normative except sections
787 explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
789 <p>Examples in this specification are introduced with the words “for example”
790 or are set apart from the normative text with <code>class="example"</code>,
791 like this:
793 <div class="example">
794 <p>This is an example of an informative example.</p>
795 </div>
797 <p>Informative notes begin with the word “Note” and are set apart from the
798 normative text with <code>class="note"</code>, like this:
800 <p class="note">Note, this is an informative note.</p>
802 <h3 id="conformance-classes">
803 Conformance classes</h3>
805 <p>Conformance to CSS Overflow Module Level 3
806 is defined for three conformance classes:
807 <dl>
808 <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
809 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
810 style sheet</a>.
811 <dt><dfn>renderer</dfn></dt>
812 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
813 that interprets the semantics of a style sheet and renders
814 documents that use them.
815 <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
816 <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
817 that writes a style sheet.
818 </dl>
820 <p>A style sheet is conformant to CSS Overflow Module Level 3
821 if all of its statements that use syntax defined in this module are valid
822 according to the generic CSS grammar and the individual grammars of each
823 feature defined in this module.
825 <p>A renderer is conformant to CSS Overflow Module Level 3
826 if, in addition to interpreting the style sheet as defined by the
827 appropriate specifications, it supports all the features defined
828 by CSS Overflow Module Level 3 by parsing them correctly
829 and rendering the document accordingly. However, the inability of a
830 UA to correctly render a document due to limitations of the device
831 does not make the UA non-conformant. (For example, a UA is not
832 required to render color on a monochrome monitor.)
834 <p>An authoring tool is conformant to CSS Overflow Module Level 3
835 if it writes style sheets that are syntactically correct according to the
836 generic CSS grammar and the individual grammars of each feature in
837 this module, and meet all other conformance requirements of style sheets
838 as described in this module.
840 <h3 id="partial">
841 Partial implementations</h3>
843 <p>So that authors can exploit the forward-compatible parsing rules to
844 assign fallback values, CSS renderers <strong>must</strong>
845 treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
846 as appropriate</a>) any at-rules, properties, property values, keywords,
847 and other syntactic constructs for which they have no usable level of
848 support. In particular, user agents <strong>must not</strong> selectively
849 ignore unsupported component values and honor supported values in a single
850 multi-value property declaration: if any value is considered invalid
851 (as unsupported values must be), CSS requires that the entire declaration
852 be ignored.</p>
854 <h3 id="experimental">
855 Experimental implementations</h3>
857 <p>To avoid clashes with future CSS features, the CSS2.1 specification
858 reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
859 syntax</a> for proprietary and experimental extensions to CSS.
861 <p>Prior to a specification reaching the Candidate Recommendation stage
862 in the W3C process, all implementations of a CSS feature are considered
863 experimental. The CSS Working Group recommends that implementations
864 use a vendor-prefixed syntax for such features, including those in
865 W3C Working Drafts. This avoids incompatibilities with future changes
866 in the draft.
867 </p>
869 <h3 id="testing">
870 Non-experimental implementations</h3>
872 <p>Once a specification reaches the Candidate Recommendation stage,
873 non-experimental implementations are possible, and implementors should
874 release an unprefixed implementation of any CR-level feature they
875 can demonstrate to be correctly implemented according to spec.
877 <p>To establish and maintain the interoperability of CSS across
878 implementations, the CSS Working Group requests that non-experimental
879 CSS renderers submit an implementation report (and, if necessary, the
880 testcases used for that implementation report) to the W3C before
881 releasing an unprefixed implementation of any CSS features. Testcases
882 submitted to W3C are subject to review and correction by the CSS
883 Working Group.
885 <p>Further information on submitting testcases and implementation reports
886 can be found from on the CSS Working Group's website at
887 <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
888 Questions should be directed to the
889 <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
890 mailing list.
892 <h3 id="cr-exit-criteria">
893 CR exit criteria</h3>
895 <p class=issue>[Change or remove the following CR exit criteria if
896 the spec is not a module, but, e.g., a Note or a profile. This text was <a
897 href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>
899 <p>
900 For this specification to be advanced to Proposed Recommendation,
901 there must be at least two independent, interoperable implementations
902 of each feature. Each feature may be implemented by a different set of
903 products, there is no requirement that all features be implemented by
904 a single product. For the purposes of this criterion, we define the
905 following terms:
907 <dl>
908 <dt>independent <dd>each implementation must be developed by a
909 different party and cannot share, reuse, or derive from code
910 used by another qualifying implementation. Sections of code that
911 have no bearing on the implementation of this specification are
912 exempt from this requirement.
914 <dt>interoperable <dd>passing the respective test case(s) in the
915 official CSS test suite, or, if the implementation is not a Web
916 browser, an equivalent test. Every relevant test in the test
917 suite should have an equivalent test created if such a user
918 agent (UA) is to be used to claim interoperability. In addition
919 if such a UA is to be used to claim interoperability, then there
920 must one or more additional UAs which can also pass those
921 equivalent tests in the same way for the purpose of
922 interoperability. The equivalent tests must be made publicly
923 available for the purposes of peer review.
925 <dt>implementation <dd>a user agent which:
927 <ol class=inline>
928 <li>implements the specification.
930 <li>is available to the general public. The implementation may
931 be a shipping product or other publicly available version
932 (i.e., beta version, preview release, or “nightly build”).
933 Non-shipping product releases must have implemented the
934 feature(s) for a period of at least one month in order to
935 demonstrate stability.
937 <li>is not experimental (i.e., a version specifically designed
938 to pass the test suite and is not intended for normal usage
939 going forward).
940 </ol>
941 </dl>
943 <p>The specification will remain Candidate Recommendation for at least
944 six months.
946 <h2 class=no-num id="acknowledgments">
947 Acknowledgments</h2>
949 <p>
950 Thanks especially to the feedback from
951 Håkon Wium Lie,
952 Florian Rivoal,
953 Alan Stearns,
954 and all the rest of the
955 <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
956 </p>
958 <h2 class=no-num id="references">
959 References</h2>
961 <h3 class="no-num" id="normative-references">
962 Normative references</h3>
963 <!--normative-->
965 <h3 class="no-num" id="other-references">
966 Other references</h3>
967 <!--informative-->
969 <h2 class="no-num" id="index">
970 Index</h2>
971 <!--index-->
973 <h2 class="no-num" id="property-index">
974 Property index</h2>
975 <!-- properties -->
977 </body>
978 </html>
979 <!-- Keep this comment at the end of the file
980 Local variables:
981 mode: sgml
982 sgml-declaration:"~/SGML/HTML4.decl"
983 sgml-default-doctype-name:"html"
984 sgml-minimize-attributes:t
985 sgml-nofill-elements:("pre" "style" "br")
986 sgml-live-element-indicator:t
987 sgml-omittag:nil
988 sgml-shorttag:nil
989 sgml-namecase-general:t
990 sgml-general-insert-case:lower
991 sgml-always-quote-attributes:t
992 sgml-indent-step:nil
993 sgml-indent-data:t
994 sgml-parent-document:nil
995 sgml-exposed-tags:nil
996 sgml-local-catalogs:nil
997 sgml-local-ecat-files:nil
998 End:
999 -->
