drafts: css-overflow/Overview.src.html@0cd153c3469c (annotated)

css-overflow/Overview.src.html@0cd153c3469c (annotated)

css-overflow/Overview.src.html

Tue, 28 Jan 2014 01:13:53 -0800

author: L. David Baron <[email protected]>
date: Tue, 28 Jan 2014 01:13:53 -0800
changeset 9850: 0cd153c3469c
parent 8332: 31b5970ee580
child 9865: 625d8c1b183a
permissions: -rw-r--r--

[css-overflow] Add a section defining what overflow is and what types of overflow there are.

 <!DOCTYPE html>
 <html lang="en">
 <head>
   <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
   <title>CSS Overflow Module Level 3</title>
   <link rel=contents href="#contents">
   <link rel=index href="#index">
   <link rel="stylesheet" type="text/css" href="../default.css">
   <link rel="stylesheet" type="text/css"
         href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
 	<style>
 		table.source-demo-pair {
 			width: 100%;
 		}
 		.in-cards-demo {
 			width: 13em;
 			height: 8em;
 			padding: 4px;
 			border: medium solid blue;
 			margin: 6px;
 			font: medium/1.3 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.bouncy-columns-demo {
 			width: 6em;
 			height: 10em;
 			float: left;
 			margin: 1em;
 			font: medium/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.bouncy-columns-demo.one {
 			background: aqua; color: black;
 			transform: rotate(-3deg);
 		}
 		.bouncy-columns-demo.two {
 			background: yellow; color: black;
 			transform: rotate(3deg);
 		}
 		.article-font-inherit-demo {
 			font: 1em/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.article-font-inherit-demo.one {
 			width: 12em;
 			font-size: 1.5em;
 			margin-bottom: 1em;
 			height: 4em;
 		}
 		.article-font-inherit-demo.two {
 			width: 11em;
 			margin-left: 5em;
 			margin-right: 2em;
 		}
 		.dark-columns-demo {
 			width: 6em;
 			height: 10em;
 			float: left;
 			margin-right: 1em;
 			font: medium/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.dark-columns-demo.one {
 			background: aqua; color: black;
 		}
 		.dark-columns-demo.one :link {
 			color: blue;
 		}
 		.dark-columns-demo.one :visited {
 			color: purple;
 		}
 		.dark-columns-demo.two {
 			background: navy; color: white;
 		}
 		.dark-columns-demo.two :link {
 			color: aqua;
 		}
 		.dark-columns-demo.two :visited {
 			color: fuchsia;
 		}
 		.article-max-lines-demo {
 			font: 1em/1.25 Times New Roman, Times, serif;
 			white-space: nowrap;
 		}
 		.article-max-lines-demo.one::first-letter {
 			font-size: 2em;
 			line-height: 0.9;
 		}
 		.article-max-lines-demo.one {
 			font-size: 1.5em;
 			width: 16em;
 		}
 		.article-max-lines-demo.two {
 			width: 11.5em;
 			float: left; margin-right: 1em;
 		}
 		.article-max-lines-demo.three {
 			width: 11.5em;
 			float: left;
 		}
 	</style>
 </head>
 <div class="head">
 <!--logo-->
 <h1>CSS Overflow Module Level 3</h1>
 <h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
 <dl>
   <dt>This version:
     <dd><a href="[VERSION]">[VERSION]</a>
   <dt>Latest version:
     <dd><a href="http://www.w3.org/TR/css-overflow-3/">http://www.w3.org/TR/css-overflow-3/</a>
   <dt>Editor's draft:
     <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
     (<a href="https://dvcs.w3.org/hg/csswg/log/tip/css-overflow/Overview.src.html">change log</a>,
     <a href="https://dvcs.w3.org/hg/csswg/log/tip/css3-overflow/Overview.src.html">older change log</a>)
   <dt>Previous version:
     <dd><a href="http://www.w3.org/TR/2013/WD-css-overflow-3-20130418/">http://www.w3.org/TR/2013/WD-css-overflow-3-20130418/</a>
   <dt>Editors:
     <dd class="h-card vcard">
       <a class="p-name fn u-url url" rel="author"
          href="http://dbaron.org/">L. David Baron</a>,
       <a class="p-org org" href="http://www.mozilla.org/">Mozilla</a>
   <dt>Issue Tracking:</dt>
     <dd>Maintained in document (only editor's draft is current)
   <dt>Feedback:</dt>
     <dd><a href="mailto:[email protected]?subject=%5Bcss-overflow%5D%20feedback"
          >[email protected]</a>
          with subject line &ldquo;<kbd>[css-overflow]
          <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
          (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/"
            >archives</a>)
   <dt>Test suite:
     <dd>none yet
 </dl>
 <!--copyright-->
 <hr title="Separator for header">
 </div>
 <h2 class="no-num no-toc" id="abstract">Abstract</h2>
 	<p>
 		<a href="http://www.w3.org/TR/CSS/">CSS</a> is
 		a language for describing
 		the rendering of structured documents (such as HTML and XML)
 		on screen, on paper, in speech, etc.
 		This module contains the features of CSS
 		relating to new mechanisms of overflow handling in visual media (e.g., screen or paper).
 		In interactive media,
 		it describes features that allow the overflow
 		from a fixed size container
 		to be handled by pagination (displaying one page at a time).
 		It also describes features, applying to all visual media,
 		that allow the contents of an element
 		to be spread across multiple fragments,
 		allowing the contents to flow across multiple regions
 		or to have different styles for different fragments.
 	</p>
 <h2 class="no-num no-toc" id="status">Status of this document</h2>
 <!--status-->
 <p>The following features are at risk: &hellip;
 <h2 class="no-num no-toc" id="contents">
 Table of contents</h2>
 <!--toc-->
 <h2 id="intro">
 Introduction</h2>
 	<p>
 		In CSS Level 1 [[CSS1]], placing more content than would fit
 		inside an element with a specified size
 		was generally an authoring error.
 		Doing so caused the content to extend
 		outside the bounds of the element,
 		which would likely cause
 		that content to overlap with other elements.
 	</p>
 	<p>
 		CSS Level 2 [[CSS21]] introduced the 'overflow' property,
 		which allows authors to have overflow be handled by scrolling,
 		which means it is no longer an authoring error.
 		It also allows authors to specify
 		that overflow is handled by clipping,
 		which makes sense when the author's intent
 		is that the content not be shown.
 	</p>
 	<p>
 		However, scrolling is not the only way
 		to present large amounts of content,
 		and may even not be the optimal way.
 		After all, the codex replaced the scroll
 		as the common format for large written works
 		because of its advantages.
 	</p>
 	<p>
 		This specification introduces
 		a mechanism for Web pages to specify
 		that an element of a page should handle overflow
 		through pagination rather than through scrolling.
 	</p>
 	<p>
 		This specification also extends the concept of overflow
 		in another direction.
 		Instead of requiring that authors specify a single area
 		into which the content of an element must flow,
 		this specification allows authors to specify multiple fragments,
 		each with their own dimensions and styles,
 		so that the content of the element can flow from one to the next,
 		using as many as needed to place the content without overflowing.
 	</p>
 	<p>
 		In both of these cases, implementations must
 		break the content in the block-progression dimension.
 		Implementations must do this is described
 		in the CSS Fragmentation Module [[!CSS3-BREAK]].
 	</p>
 <h2 id="overflow-concepts">Types of overflow</h2>
 	<p>
 		CSS uses the term <dfn>overflow</dfn> to describe
 		the contents of a box
 		that extend outside that one of that box's edges
 		(i.e., its <i>content edge</i>, <i>padding edge</i>,
 		<i>border edge</i>, or <i>margin edge</i>).
 		The overflow might be described as the elements or features
 		that cause this overflow,
 		the non-rectangular region occupied by these features,
 		or, more commonly,
 		as the minimal rectangle that bounds that region.
 		A box's overflow is computed based on the boxes and styles
 		of the box and of all its descendants whose containing block chain
 		<span class="issue">undefined term?</span>
 		includes the box.
 	</p>
 	<p>
 		In most cases, any of these types of overflow
 		can be computed for any box
 		from the bounds and properties of that box,
 		and from the overflow (of that type)
 		of each of its children.
 		However, this is not always the case; for example,
 		when ''transform-style: preserve-3d'' [[CSS3-TRANSFORMS]] is used on
 		some of the children, their descendants with
 		''transform-style: preserve-3d'' must also be examined.
 	</p>
 <h3 id="ink-overflow">Ink overflow</h3>
 	<p>
 		The <dfn>ink overflow</dfn> of a box
 		is the part of that box and its contents that
 		creates a visual effect outside of
 		the box's border box.
 	</p>
 	<p>
 		Since some effects in CSS (for example, the blurs in
 		'text-shadow' [[CSS3TEXT]] and 'box-shadow' [[CSS3BG]])
 		do not define what visual extent they cover, the extent
 		of the <i>ink overflow</i> is undefined.
 	</p>
 	<p class="issue">
 		Should we try to define it at all and just leave pieces undefined?
 	</p>
 	<p>
 		The <dfn>ink overflow region</dfn> is the non-rectangular region
 		occupied by the <i>ink overflow</i>, and the
 		<dfn>ink overflow rectangle</dfn> is
 		the minimal rectangle whose axis is aligned to the box's axes
 		and contains the <i>ink overflow region</i>.
 		Note that the <i>ink overflow rectangle</i> is a rectangle
 		in the box's coordinate system, but might be non-rectangular
 		in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
 	</p>
 <h3 id="scrollable-overflow">Scrollable overflow</h3>
 	<p>
 		The <dfn>scrollable overflow</dfn> of a box is the
 		set of things extending outside of that box's padding edge
 		for which a scrolling mechanism needs to be provided.
 	</p>
 	<p>
 		Given the following definitions
 		<span class="issue">which belong in [[CSS3-TRANSFORMS]]</span>:
 	</p>
 	<dl>
 		<dt><dfn>3d-preserving child</dfn></dt>
 		<dd>
 			A child box C of a parent box P is a 3d-preserving
 			child if it has ''transform-style: preserve-3d''
 			and the user-agent is not required to flatten it
 			based on the <a href="http://www.w3.org/TR/css3-transforms/#transform-style-property">requirements</a> in [[!CSS3-TRANSFORMS]].
 		</dt>
 		<dt><dfn>non-3d-preserving child</dfn></dt>
 		<dd>
 			A child C of a box P is a non-3d-preserving-child if
 			it is not a <i>3d-preserving child</i>.
 		</dd>
 		<dt><dfn>3d-preserving descendant</dfn></dt>
 		<dd>
 			Box D is a 3d-preserving descendant of box A if A is
 			an ancestor of D, and D and all of the boxes (if any)
 			in the ancestor chain from D to A
 			are <i>3d-preserving child</i> boxes.
 		</dd>
 	</dl>
 	<p>The scrollable overflow of a box is the union of the following things,
 	all adjusted for transforms <span class="issue">undefined concept!</span> into the box's coordinate space:</p>
 	<ul>
 		<li>
 			for the box and all of its <i>3d-preserving descendant</i> boxes:
 			<ul>
 				<li>the box's own padding edge (for the box itself) or border edge (for <i>3d-preserving descendant</i> boxes)</li>
 				<li>the bounds <span class="issue">undefined term!</span> of any text directly in the box</li>
 				<li><span class="issue">MORE HERE!</span>
 			</ul>
 		<li>
 			for all the <i>non-3d-preserving child</i> boxes of the
 			box and its <i>3d-preserving descendant</i> boxes,
 			the scrollable overflow of the box
 		</li>
 	</ul>
 	<p class="issue">
 		I wrote this definition off the top of my head,
 		so it can't possibly be right.
 		It's missing tons of pieces!
 	</p>
 	<p>
 		The <dfn>scrollable overflow region</dfn> is the non-rectangular region
 		occupied by the <i>scrollable overflow</i>, and the
 		<dfn>scrollable overflow rectangle</dfn> is
 		the minimal rectangle whose axis is aligned to the box's axes
 		and contains the <i>scrollable overflow region</i>.
 		Note that the <i>scrollable overflow rectangle</i> is a rectangle
 		in the box's coordinate system, but might be non-rectangular
 		in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
 	</p>
 <h3 id="border-box-overflow">Border box overflow</h3>
 	<p class="issue">
 		This concept has been proposed for some uses, such as for
 		determining what the 'outline' property goes around, and
 		as the basis of a coordinate system for specifying clips and masks,
 		but it's not clear if it's needed.
 	</p>
 	<p>
 		The <dfn>border-box overflow</dfn> of a box is the
 		union of the box's border edge and the border edges of
 		the box's descendants.</p>
 	</p>
 	<p class="issue">
 		If needed, define more formally, as for scrollable overflow above.
 		(Maybe even share the definitions in an appropriate way!)
 	</p>
 	<p>
 		The <dfn>border-box overflow region</dfn> is the non-rectangular region
 		occupied by the <i>border-box overflow</i>, and the
 		<dfn>border-box overflow rectangle</dfn> is
 		the minimal rectangle whose axis is aligned to the box's axes
 		and contains the <i>border-box overflow region</i>.
 		Note that the <i>border-box overflow rectangle</i> is a rectangle
 		in the box's coordinate system, but might be non-rectangular
 		in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
 	</p>
 <h2 id="overflow-properties">Overflow properties</h2>
 	<p>
 		The ''overflow-x'' property specifies
 		the handling of overflow in the horizontal direction
 		(i.e., overflow from the left and right sides of the box),
 		and the ''overflow-y'' property specifies the handling
 		of overflow in the vertical direction
 		(i.e., overflow from the top and bottom sides of the box)
 	</p>
 	<table class=propdef>
 		<tr>
 			<th>Name:
 			<td><dfn>overflow-x</dfn>, <dfn>overflow-y</dfn>
 		<tr>
 			<th><a href="#values">Value</a>:
 			<td>visible | hidden | scroll | auto | paged-x | paged-y | paged-x-controls | paged-y-controls | fragments
 		<tr>
 			<th>Initial:
 			<td>visible
 		<tr>
 			<th>Applies to:
 			<td>block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		<tr>
 			<th>Inherited:
 			<td>no
 		<tr>
 			<th>Percentages:
 			<td>N/A
 		<tr>
 			<th>Media:
 			<td>visual
 		<tr>
 			<th>Computed&nbsp;value:
 			<td>see below
 		<tr>
 			<th>Animatable:
 			<td>no
 		<tr>
 			<th>Canonical order:
 			<td><abbr title="follows order of property value definition">per grammar</abbr>
 	</table>
 	<p>
 		The 'overflow' property is a shorthand property
 		that sets the specified values of both 'overflow-x' and 'overflow-y'
 		to the value specified for 'overflow'.
 	</p>
 	<table class=propdef>
 		<tr>
 			<th>Name:
 			<td><dfn>overflow</dfn>
 		<tr>
 			<th><a href="#values">Value</a>:
 			<td>visible | hidden | scroll | auto | paged-x | paged-y | paged-x-controls | paged-y-controls | fragments
 		<tr>
 			<th>Initial:
 			<td>see individual properties
 		<tr>
 			<th>Applies to:
 			<td>block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		<tr>
 			<th>Inherited:
 			<td>no
 		<tr>
 			<th>Percentages:
 			<td>N/A
 		<tr>
 			<th>Media:
 			<td>visual
 		<tr>
 			<th>Computed&nbsp;value:
 			<td>see individual properties
 		<tr>
 			<th>Animatable:
 			<td>no
 		<tr>
 			<th>Canonical order:
 			<td><abbr title="follows order of property value definition">per grammar</abbr>
 	</table>
 	<p>The values of these properties are:</p>
 	<dl>
 		<dt><dfn>visible</dfn>
 		<dd>
 			There is no special handling of overflow, that is, it
 			may be rendered outside the block container.
 		</dd>
 		<dt><dfn>hidden</dfn>
 		<dt><dfn>scroll</dfn>
 		<dt><dfn>auto</dfn>
 		<dd>
 			These values are collectively the <dfn>scrolling values</dfn>;
 			they are defined in the section on
 			<a href="#scrolling-overflow">scrolling and hidden overflow</a>.
 		</dd>
 		<dt><dfn>paged-x</dfn>
 		<dt><dfn>paged-y</dfn>
 		<dt><dfn>paged-x-controls</dfn>
 		<dt><dfn>paged-y-controls</dfn>
 		<dt><dfn>fragments</dfn>
 		<dd>
 			These values are collectively the <dfn>fragmenting values</dfn>;
 			they are defined in the sections on
 			<a href="#paginated-overflow">paginated overflow</a> and
 			<a href="#fragment-overflow">fragment overflow</a>.
 		</dd>
 	</dl>
 	<div id="overflow-computed-values">
 		<p>The computed values of 'overflow-x' and 'overflow-y'
 		are determined from the cascaded values [[!CSS3CASCADE]]
 		based on the following rules:</p>
 		<ol>
 			<li>
 				If one or both of the cascaded values are
 				<i>fragmenting values</i>, then:
 				<ol>
 					<li>
 						If one of the cascaded values is one of the
 						<i>fragmenting values</i>
 						and the other is not,
 						then the computed values are
 						the same as the cascaded values.
 					</li>
 					<li>
 						If both of the cascaded values are <i>fragmenting values</i>, then:
 						<ol>
 							<li>
 								for horizontal writing mode [[!CSS3-WRITING-MODES]],
 								the computed value for ''overflow-y'' is the cascaded value
 								and the computed value for ''overflow-x'' is ''hidden'', or
 							</li>
 							<li>
 								for vertical writing mode [[!CSS3-WRITING-MODES]],
 								the computed value for ''overflow-x'' is the cascaded value
 								and the computed value for ''overflow-y'' is ''hidden''.
 							</li>
 						</ol>
 					</li>
 				</ol>
 			</li>
 			<li>
 				Otherwise, if one cascaded values is
 				one of the <i>scrolling values</i>
 				and the other is ''visible'',
 				then computed values are the cascaded values
 				with ''visible'' changed to ''hidden''.
 			</li>
 			<li>
 				Otherwise, the computed values are as specified.
 			</li>
 		</ol>
 	</div>
 	<p class="issue">
 		Are all 4 of the ''paged-*'' values really needed?
 	</p>
 	<p>
 		When the <i>fragmenting values</i> are used,
 		the overflow from the fragments themselves
 		treats the fragmenting value as ''hidden''.
 		<span class="issue">Is this the right behavior?</span>
 		<span class="issue">Give example.</span>
 	</p>
 	<p class="issue">
 		[[CSS3-MARQUEE]] describes an 'overflow-style' property,
 		but it has not picked up implementation experience
 		that the working group is aware of.
 		Should this document treat 'overflow-style' as a defunct proposal,
 		or should this document describe the 'overflow-style' property
 		and attempt to revive it,
 		despite that implementations have implemented
 		'overflow-x' and 'overflow-y' instead?
 	</p>
 	<p class="issue">
 		There are <a href="http://lists.w3.org/Archives/Public/www-style/2012May/1197.html">discussions</a>
 		about how overflow, overflow-style, overflow-x and overflow-y
 		should work and interact with each other.
 		Until consensus on this topic is reached,
 		it is not completely clear which of these
 		should be used for
 		paged-x | paged-y | paged-x-controls | paged-y-controls | fragments
 	</p>
 <h2 id="scrolling-overflow">Scrolling and hidden overflow</h2>
 	<p class="issue">
 		Move material from [[CSS21]] and [[CSS3BOX]] here.
 	</p>
 <h2 id="paginated-overflow">Paginated overflow</h2>
 <p class="issue">overflow:paginate or overflow:pages (or paged-x, paged-y, paged-x-controls, paged-y-controls as [[CSS3GCPM]] has?)</p>
 <p class="issue">Ability to display N pages at once
 rather than just one page at once?</p>
 	<p class="issue">
 		The current implementation of paginated overflow uses
 		the 'overflow'/'overflow-x'/'overflow-y' properties
 		rather than the 'overflow-style' property as proposed
 		in the [[CSS3GCPM]] draft
 		(which also matches the [[CSS3-MARQUEE]] proposal).
 		We should probably switch away from 'overflow-style',
 		but that's not 100% clear.
 	</p>
 <h2 id="fragment-overflow">Fragment overflow</h2>
 	<p>
 		This section introduces and defines the meaning of
 		the new ''fragments'' value of the 'overflow' property.
 	</p>
 	<p>
 		When the computed value of 'overflow' for an element is ''fragments'',
 		and implementations would otherwise have created a box for the element,
 		then implementations must create a sequence of <dfn>fragment box</dfn>es
 		for that element.
 		(It is possible for an element with ''overflow: fragments''
 		to generate only one <i>fragment box</i>.
 		However, if an element's computed 'overflow' is not ''fragments'',
 		then its box is not a <i>fragment box</i>.)
 		Every <i>fragment box</i> is a fragmentation container,
 		and any overflow
 		that would cause that fragmentation container to fragment
 		causes another <i>fragment box</i> created as a next sibling
 		of the previous one.
 		<span class="issue">Or is it as though it's a next sibling of
 		the element?  Need to figure out exactly how this interacts with
 		other box-level fixup.</span>
 		Additionally, if the <i>fragment box</i> is also
 		a multi-column box (as defined in [[!CSS3COL]]
 		<span class="issue">though it defines <i>multi-column element</i></span>)
 		any content that would lead to the creation of <i>overflow columns</i> [[!CSS3COL]]
 		instead is flown into an additional fragment box.
 		However, fragment boxes may themselves be broken
 		(due to fragmentation in a fragmentation context outside of them,
 		such as pages, columns, or other fragment boxes);
 		such breaking leads to fragments of the same fragment box
 		rather than multiple fragment boxes.
 		(This matters because fragment boxes may be styled by their index;
 		such breaking leads to multiple fragments of a fragment box
 		with a single index.
 		This design choice is so that
 		breaking a fragment box across pages does not break
 		the association of indices to particular pieces of content.)
 		<span class="issue">Should a forced break that breaks to
 		an outer fragmentation context cause a new fragment of a single
 		fragment box or a new fragment box?</span>
 		<span class="issue">Should we find a term other than
 		<i>fragment box</i> here to make this a little less confusing?</span>
 	</p>
 	<p class="issue">
 		What if we want to be able to style the pieces of an element
 		split within another type of fragmentation context?
 		These rules prevent ever using ''::nth-fragment()'' for that,
 		despite that the name seems the most logical name for such a feature.
 	</p>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;title&gt;Breaking content into
   equal-sized cards&lt;/title&gt;
 &lt;style&gt;
   .in-cards {
     overflow: fragments;
     width: 13em;
     height: 8em;
     padding: 4px;
     border: medium solid blue;
     margin: 6px;
     font: medium/1.3 Times New
       Roman, Times, serif;
   }
 &lt;/style&gt;
 &lt;div class="in-cards"&gt;
   In this example, the text in the div
   is broken into a series of cards.
   These cards all have the same style.
   The presence of enough content to
   overflow one of the cards causes
   another one to be created.  The second
   card is created just like it's the
   next sibling of the first.
 &lt;/div&gt;</pre></td><td>
 			<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>
 			<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>
 		</td></tr></table>
 	</div>
 	<p class="issue">
 		We should specify that ''overflow: fragments'' does not apply
 		to at least some table parts,
 		and perhaps other elements as well.
 		We need to determine exactly which ones.
 	</p>
 	<p class="issue">
 		This specification needs to say which type of
 		fragmentation context is created
 		so that it's clear which values of the 'break' property
 		cause breaks within this context.
 		We probably want ''break: regions'' to apply.
 	</p>
 	<p class="issue">
 		This specification needs a processing model
 		that will apply in cases where the layout containing the
 		fragments has characteristics that use the intrinsic size of the fragments
 		to change the amount of space available for them,
 		such as [[CSS3-GRID-LAYOUT]].
 		There has already been some work on such a processing model
 		in [[CSS3-REGIONS]],
 		and the work done on a model there,
 		and the editors of that specification,
 		should inform what happens in this specification.
 	</p>
 <h3 id="fragment-styling">Fragment styling</h3>
 <h4 id="fragment-pseudo-element">The ::nth-fragment() pseudo-element</h4>
 	<p>
 		The ::nth-fragment() pseudo-element is a pseudo-element
 		that describes some of the <i>fragment box</i>es generated by an element.
 		The argument to the pseudo-element takes the same syntax
 		as the argument to the :nth-child() pseudo-class
 		defined in [[!SELECT]], and has the same meaning
 		except that the number is relative to
 		<i>fragment box</i>es generated by the element
 		instead of siblings of the element.
 	</p>
 	<p class="note">
 		Selectors that allow addressing fragments
 		by counting from the end rather than the start
 		are intentionally not provided.
 		Such selectors would interfere with determining
 		the number of fragments.
 	</p>
 	<p class="issue">
 		Depending on future discussions,
 		this ''::nth-fragment(<var>an+b</var>)'' syntax
 		may be replaced with
 		the new ''::fragment:nth(<var>an+b</var>)'' syntax.
 	</p>
 <h4 id="style-of-fragments">Styling of fragments</h4>
 	<p class="issue">
 		Should this apply to fragment overflow only,
 		or also to paginated overflow?
 		(If it applies,
 		then stricter property restrictions would be needed
 		for paginated overflow.)
 	</p>
 	<p>
 		In the absence of rules with ''::nth-fragment()'' pseudo-elements,
 		the computed style for each <i>fragment box</i>
 		is the computed style for the element
 		for which the <i>fragment box</i> was created.
 		However, the style for a <i>fragment box</i> is also influenced
 		by rules whose selector's <i>subject</i> [[!SELECT]]
 		has an ''::nth-fragment()'' pseudo-element,
 		if the 1-based number of the <i>fragment box</i> matches
 		that ''::nth-fragment()'' pseudo-element
 		and the selector (excluding the ''::nth-fragment()'' pseudo-element)
 		matches the element generating the fragments.
 	</p>
 	<p>
 		When determining the style of the <i>fragment box</i>,
 		these rules that match the fragment pseudo-element
 		cascade together with the rules that match the element,
 		with the fragment pseudo-element adding the specificity
 		of a pseudo-class to the specificity calculation.
 		<span class="issue">Does this need to be specified in
 		the cascading module as well?</span>
 	</p>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .bouncy-columns {
     overflow: fragments;
     width: 6em;
     height: 10em;
     float: left;
     margin: 1em;
     font: medium/1.25 Times New
       Roman, Times, serif;
   }
   .bouncy-columns::nth-fragment(1) {
     background: aqua; color: black;
     transform: rotate(-3deg);
   }
   .bouncy-columns::nth-fragment(2) {
     background: yellow; color: black;
     transform: rotate(3deg);
   }
 &lt;/style&gt;
 &lt;div class="bouncy-columns"&gt;
   <i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<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>
 			<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>
 			<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>
 		</td></tr></table>
 	</div>
 	<p>
 		Styling an ''::nth-fragment()'' pseudo-element with the 'overflow'
 		property does take effect;
 		if a <i>fragment box</i> has a
 		computed value of 'overflow' other than ''fragments''
 		then that fragment box is the last fragment.
 		However, overriding ''overflow'' on the first fragment
 		does not cause the <i>fragment box</i> not to exist;
 		whether there are fragment boxes at all is determined by
 		the computed value of overflow for the element.
 		<span class="issue">Need to reword this to refer to the
 		appropriate choice of ''overflow-x'' or ''overflow-y'',
 		and then point to rule about the handling of the other one
 		of ''overflow-x'' or ''overflow-y''.</span>
 	</p>
 	<p>
 		Styling an ''::nth-fragment()'' pseudo-element with the 'content'
 		property has no effect;
 		the computed value of 'content' for the fragment box
 		remains the same as the computed value of content for the element.
 	</p>
 	<p>
 		Specifying ''display: none'' for a <i>fragment box</i> causes
 		the fragment box with that index not to be generated.
 		However, in terms of the indices
 		used for matching ''::nth-fragment()'' pseudo-elements
 		of later fragment boxes,
 		it still counts as though it was generated.
 		However, since it is not generated, it does not contain any content.
 	</p>
 	<p>
 		Specifying other values of 'display', 'position',
 		or 'float' is permitted, but is not allowed to change
 		the computed value of 'display-inside'.
 		(Since 'overflow', 'overflow-x', and 'overflow-y' only
 		apply to block containers, flex containers, and grid containers
 		the computed value of 'display-inside' is always
 		''block'', ''flex'' or ''grid''.
 		<span class="issue">Need to specify exactly how this works,
 		but it depends on
 		having 'display-inside' and 'display-outside' specified.</span>
 	</p>
 	<p>
 		To match the model for other pseudo-elements
 		where the pseudo-elements live inside their corresponding element,
 		declarations in ''::nth-fragment()'' pseudo-elements override
 		declarations in rules without the pseudo-element.
 		The relative priority within such declarations is determined
 		by normal cascading order (see [[!CSS21]]).
 	</p>
 	<p>
 		Styles specified on ''::nth-fragment()'' pseudo-elements
 		do affect inheritance to content within the <i>fragment box</i>.
 		In other words, the content within the <i>fragment box</i> must
 		inherit from the fragment box's style (i.e., the pseudo-element style)
 		rather than directly from the element.
 		This means that elements split between fragment boxes may
 		have different styles for different parts of the element.
 	</p>
 	<p class="issue">
 		This inheritance rule allows specifying styles indirectly
 		(by using explicit ''inherit'' or using default inheritance
 		on properties that don't apply to '':first-letter'')
 		that can't be specified directly
 		(based on the rules in the next section).
 		This is a problem.
 		The restrictions that apply to styling inside fragments
 		should also apply to inheritance from fragments.
 	</p>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .article {
     overflow: fragments;
   }
   .article::nth-fragment(1) {
     font-size: 1.5em;
     margin-bottom: 1em;
     height: 4em;
   }
   .article::nth-fragment(2) {
     margin-left: 5em;
     margin-right: 2em;
   }
 &lt;/style&gt;
 &lt;div class="article"&gt;
   The &lt;code&gt;font-size&lt;/code&gt; property<i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<div class="article-font-inherit-demo one">The <code>font-size</code> property<br>specified on the fragment<br>is inherited into the</div>
 			<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>
 		</td></tr></table>
 	</div>
 <h4 id="style-in-fragments">Styling inside fragments</h4>
 	<p class="issue">
 		Should this apply to fragment overflow only,
 		or also to paginated overflow,
 		or even to pagination across pages?
 	</p>
 	<p>
 		The ''::nth-fragment()'' pseudo-element
 		can also be used to style
 		content inside of a <i>fragment box</i>.
 		Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements,
 		the ''::nth-fragment()'' pseudo-element can be applied
 		to parts of the selector other than the subject:
 		in particular, it can match ancestors of the subject.
 		However, the only CSS properties applied
 		by rules with such selectors
 		are those that apply
 		to the ''::first-letter'' pseudo-element.
 	</p>
 	<p>
 		To be more precise,
 		when a rule's selector has ''::nth-fragment()'' pseudo-elements
 		attached to parts of the selector other than the subject,
 		the declarations in that rule apply to
 		a fragment (or pseudo-element thereof) when:
 	</p>
 	<ol>
 		<li>
 			the declarations are for properties that apply to the
 			''::first-letter'' pseudo-element,
 		</li>
 		<li>
 			the declarations would apply to
 			that fragment (or pseudo-element thereof)
 			had those ''::nth-fragment()'' pseudo-elements been removed,
 			with a particular association between
 			each sequence of simple selectors and the element it matched,
 			and
 		</li>
 		<li>
 			for each removed ''::nth-fragment()'' pseudo-element,
 			the fragment lives within a <i>fragment box</i>
 			of the element associated in that association
 			with the selector that the pseudo-element was attached to,
 			and whose index matches the pseudo-element.
 		</li>
 	</ol>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .dark-columns {
     overflow: fragments;
     width: 6em;
     height: 10em;
     float: left;
     margin-right: 1em;
     font: medium/1.25 Times New
       Roman, Times, serif;
   }
   .dark-columns::nth-fragment(1) {
     background: aqua; color: black;
   }
   .dark-columns::nth-fragment(1) :link {
     color: blue;
   }
   .dark-columns::nth-fragment(1) :visited {
     color: purple;
   }
   .dark-columns::nth-fragment(2) {
     background: navy; color: white;
   }
   .dark-columns::nth-fragment(2) :link {
     color: aqua;
   }
   .dark-columns::nth-fragment(2) :visited {
     color: fuchsia;
   }
 &lt;/style&gt;
 &lt;div class="dark-columns"&gt;
   <i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<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>
 			<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>
 		</td></tr></table>
 	</div>
 <h3 id="max-lines">The 'max-lines' property</h3>
 	<p>
 		Authors may wish to style the opening lines of an element
 		with different styles
 		by putting those opening lines in a separate fragment.
 		However, since it may be difficult to predict the exact height
 		occupied by those lines
 		in order to restrict the first fragment to that height,
 		this specification introduces a 'max-lines' property
 		that forces a fragment to break
 		after a specified number of lines.
 		This forces a break after the given number of lines
 		contained within the element or its descendants,
 		as long as those lines are in the same block formatting context.
 	</p>
 	<table class=propdef>
 		<tr>
 			<th>Name:
 			<td><dfn>max-lines</dfn>
 		<tr>
 			<th><a href="#values">Value</a>:
 			<td>none | &lt;integer&gt;
 		<tr>
 			<th>Initial:
 			<td>none
 		<tr>
 			<th>Applies to:
 			<td>fragment boxes
 		<tr>
 			<th>Inherited:
 			<td>no
 		<tr>
 			<th>Animatable:
 			<td>as <a href="http://www.w3.org/TR/css3-transitions/#animatable-types">integer</a>
 		<tr>
 			<th>Percentages:
 			<td>N/A
 		<tr>
 			<th>Media:
 			<td>visual
 		<tr>
 			<th>Computed&nbsp;value:
 			<td>specified value
 		<tr>
 			<th>Canonical order:
 			<td><abbr title="follows order of property value definition">per grammar</abbr>
 	</table>
 	<dl>
 		<dt>none
 		<dd>
 			<p>
 				Breaks occur only as specified elsewhere.
 			</p>
 		</dd>
 		<dt>&lt;integer&gt;
 		<dd>
 			<p>
 				In addition to any breaks specified elsewhere,
 				a break is forced before any line that would exceed
 				the given number of lines
 				being placed inside the element
 				(excluding lines that are in
 				a different block formatting context from
 				the block formatting context to which
 				an unstyled child of the element would belong).
 			</p>
 			<p class="issue">
 				If there are multiple boundaries between this line
 				and the previous, where exactly (in terms of element
 				boundaries) is the break forced?
 			</p>
 			<p>
 				Only positive integers are accepted.
 				Zero or negative integers are a parse error.
 			</p>
 		</dd>
 	</dl>
 <p class="issue">Should this apply to fragment overflow only, or also
 to pagination?</p>
 	<div class="example">
 		<table class="source-demo-pair"><tr><td><pre>&lt;!DOCTYPE HTML&gt;
 &lt;style&gt;
   .article {
     overflow: fragments;
   }
   .article::first-letter {
     font-size: 2em;
     line-height: 0.9;
   }
   .article::nth-fragment(1) {
     font-size: 1.5em;
     max-lines: 3;
   }
   .article::nth-fragment(2) {
     column-count: 2;
   }
 &lt;/style&gt;
 &lt;div class="article"&gt;
   <i>...</i>
 &lt;/div&gt;</pre></td><td>
 			<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>
 			<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>
 			<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>
 		</td></tr></table>
 	</div>
 <h2 id="static-media">Overflow in static media</h2>
 	<p class="issue">
 		This specification should define useful behavior
 		for all values of 'overflow'
 		in static media (such as print).
 		Current implementation behavior is quite poor and
 		produces unexpected results when authors have not considered
 		what will happen when
 		the content they produce for interactive media
 		is printed.
 	</p>
 <h2 id="conformance">
 Conformance</h2>
 <h3 id="placement">
 Module interactions</h3>
   <p>This module extends the 'overflow'
   feature defined in [[CSS21]] section 11.1.1.  It defines additional
   overflow handling mechanisms that implementations must implement as
   described in this module in order to conform to this module.</p>
   <p>No properties in this module apply to the <code>::first-line</code> or
   <code>::first-letter</code> pseudo-elements.</p>
 <h3 id="values">
 Values</h3>
   <p>This specification follows the
   <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
   definition conventions</a> from [[!CSS21]]. Value types not defined in
   this specification are defined in CSS LevelÂ 2 RevisionÂ 1 [[!CSS21]].
   Other CSS modules may expand the definitions of these value types: for
   example [[CSS3COLOR]], when combined with this module, expands the
   definition of the &lt;color&gt; value type as used in this specification.</p>
   <p>In addition to the property-specific values listed in their definitions,
   all properties defined in this specification also accept the
   <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
   keyword as their property value. For readability it has not been repeated
   explicitly.
 <h3 id="conventions">
 Document conventions</h3>
   <p>Conformance requirements are expressed with a combination of
   descriptive assertions and RFC 2119 terminology. The key words â€œMUSTâ€,
   â€œMUST NOTâ€, â€œREQUIREDâ€, â€œSHALLâ€, â€œSHALL NOTâ€, â€œSHOULDâ€, â€œSHOULD NOTâ€,
   â€œRECOMMENDEDâ€, â€œMAYâ€, and â€œOPTIONALâ€ in the normative parts of this
   document are to be interpreted as described in RFC 2119.
   However, for readability, these words do not appear in all uppercase
   letters in this specification.
   <p>All of the text of this specification is normative except sections
   explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
   <p>Examples in this specification are introduced with the words â€œfor exampleâ€
   or are set apart from the normative text with <code>class="example"</code>,
   like this:
   <div class="example">
     <p>This is an example of an informative example.</p>
   </div>
   <p>Informative notes begin with the word â€œNoteâ€ and are set apart from the
   normative text with <code>class="note"</code>, like this:
   <p class="note">Note, this is an informative note.</p>
 <h3 id="conformance-classes">
 Conformance classes</h3>
   <p>Conformance to CSS Overflow Module Level 3
   is defined for three conformance classes:
   <dl>
     <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
       style sheet</a>.
     <dt><dfn>renderer</dfn></dt>
       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
       that interprets the semantics of a style sheet and renders
       documents that use them.
     <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
       that writes a style sheet.
   </dl>
   <p>A style sheet is conformant to CSS Overflow Module Level 3
   if all of its statements that use syntax defined in this module are valid
   according to the generic CSS grammar and the individual grammars of each
   feature defined in this module.
   <p>A renderer is conformant to CSS Overflow Module Level 3
   if, in addition to interpreting the style sheet as defined by the
   appropriate specifications, it supports all the features defined
   by CSS Overflow Module Level 3 by parsing them correctly
   and rendering the document accordingly. However, the inability of a
   UA to correctly render a document due to limitations of the device
   does not make the UA non-conformant. (For example, a UA is not
   required to render color on a monochrome monitor.)
   <p>An authoring tool is conformant to CSS Overflow Module Level 3
   if it writes style sheets that are syntactically correct according to the
   generic CSS grammar and the individual grammars of each feature in
   this module, and meet all other conformance requirements of style sheets
   as described in this module.
 <h3 id="partial">
 Partial implementations</h3>
   <p>So that authors can exploit the forward-compatible parsing rules to
   assign fallback values, CSS renderers <strong>must</strong>
   treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
   as appropriate</a>) any at-rules, properties, property values, keywords,
   and other syntactic constructs for which they have no usable level of
   support. In particular, user agents <strong>must not</strong> selectively
   ignore unsupported component values and honor supported values in a single
   multi-value property declaration: if any value is considered invalid
   (as unsupported values must be), CSS requires that the entire declaration
   be ignored.</p>
 <h3 id="experimental">
 Experimental implementations</h3>
   <p>To avoid clashes with future CSS features, the CSS2.1 specification
   reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
   syntax</a> for proprietary and experimental extensions to CSS.
   <p>Prior to a specification reaching the Candidate Recommendation stage
   in the W3C process, all implementations of a CSS feature are considered
   experimental. The CSS Working Group recommends that implementations
   use a vendor-prefixed syntax for such features, including those in
   W3C Working Drafts. This avoids incompatibilities with future changes
   in the draft.
   </p>
 <h3 id="testing">
 Non-experimental implementations</h3>
   <p>Once a specification reaches the Candidate Recommendation stage,
   non-experimental implementations are possible, and implementors should
   release an unprefixed implementation of any CR-level feature they
   can demonstrate to be correctly implemented according to spec.
   <p>To establish and maintain the interoperability of CSS across
   implementations, the CSS Working Group requests that non-experimental
   CSS renderers submit an implementation report (and, if necessary, the
   testcases used for that implementation report) to the W3C before
   releasing an unprefixed implementation of any CSS features. Testcases
   submitted to W3C are subject to review and correction by the CSS
   Working Group.
   <p>Further information on submitting testcases and implementation reports
   can be found from on the CSS Working Group's website at
   <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
   Questions should be directed to the
   <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">[email protected]</a>
   mailing list.
 <h3 id="cr-exit-criteria">
 CR exit criteria</h3>
   <p class=issue>[Change or remove the following CR exit criteria if
   the spec is not a module, but, e.g., a Note or a profile. This text was <a
   href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>
   <p>
   For this specification to be advanced to Proposed Recommendation,
   there must be at least two independent, interoperable implementations
   of each feature. Each feature may be implemented by a different set of
   products, there is no requirement that all features be implemented by
   a single product. For the purposes of this criterion, we define the
   following terms:
   <dl>
     <dt>independent <dd>each implementation must be developed by a
     different party and cannot share, reuse, or derive from code
     used by another qualifying implementation. Sections of code that
     have no bearing on the implementation of this specification are
     exempt from this requirement.
     <dt>interoperable <dd>passing the respective test case(s) in the
     official CSS test suite, or, if the implementation is not a Web
     browser, an equivalent test. Every relevant test in the test
     suite should have an equivalent test created if such a user
     agent (UA) is to be used to claim interoperability. In addition
     if such a UA is to be used to claim interoperability, then there
     must one or more additional UAs which can also pass those
     equivalent tests in the same way for the purpose of
     interoperability. The equivalent tests must be made publicly
     available for the purposes of peer review.
     <dt>implementation <dd>a user agent which:
     <ol class=inline>
       <li>implements the specification.
       <li>is available to the general public. The implementation may
       be a shipping product or other publicly available version
       (i.e., beta version, preview release, or â€œnightly buildâ€).
       Non-shipping product releases must have implemented the
       feature(s) for a period of at least one month in order to
       demonstrate stability.
       <li>is not experimental (i.e., a version specifically designed
       to pass the test suite and is not intended for normal usage
       going forward).
     </ol>
   </dl>
   <p>The specification will remain Candidate Recommendation for at least
   six months.
 <h2 class=no-num id="acknowledgments">
 Acknowledgments</h2>
 	<p>
 		Thanks especially to the feedback from
 		Rossen Atanassov,
 		Bert Bos,
 		Tantek Ã‡elik,
 		John Daggett,
 		fantasai,
 		Daniel Glazman,
 		Vincent Hardy,
 		H&aring;kon Wium Lie,
 		Peter Linss,
 		Robert O'Callahan,
 		Florian Rivoal,
 		Alan Stearns,
 		Steve Zilles,
 		and all the rest of the
 		<a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.
 	</p>
 <h2 class=no-num id="references">
 References</h2>
 <h3 class="no-num" id="normative-references">
 Normative references</h3>
 <!--normative-->
 <h3 class="no-num" id="other-references">
 Other references</h3>
 <!--informative-->
 <h2 class="no-num" id="index">
 Index</h2>
 <!--index-->
 <h2 class="no-num" id="property-index">
 Property index</h2>
 <!-- properties -->
 </body>
 </html>
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