csswg-drafts/css3-regions/Overview.src.html at 9b532164c3b91889da6b0d735d83c776e303f2ae · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

<html dir="ltr" lang="en">
<head profile="http://www.w3.org/2006/03/hcard">
    <meta http-equiv="X-UA-Compatible" co
F2A9
ntent="IE=edge">
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

    <title>CSS Regions Module</title>
    <link href="../default.css" rel="stylesheet" type="text/css">
    <link id="st" href="alternate-spec-style.css" rel="stylesheet" type=
    "text/css" title="additional spec styles">
    <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel="stylesheet"
    type="text/css">
</head>

<body>
    <div class="head" id="div-head">
        <!--begin-logo-->

        <p><a href="http://www.w3.org/"><img alt="W3C" height="48" src=
        "http://www.w3.org/Icons/w3c_home" width="72"></a> <!--end-logo--></p>

        <h1 id="css-regions-module">CSS Regions Module</h1>

        <h2 class="no-num no-toc" id="longstatus-date">[LONGSTATUS]
        [DATE]</h2>

        <dl>
            <dt>This version:</dt>

            <dd><a href=
            "[VERSION]">http://www.w3.org/csswg/css3-regions
            <!--http://www.w3.org/TR/[YEAR]/WD-[SHORTNAME]-[CDATE]/--></a></dd>

            <dt>Latest version:</dt>

            <dd><a href=
            "http://www.w3.org/TR/css3-regions">http://www.w3.org/TR/css3-regions</a></dd>

            <dt>Previous version:</dt>

            <dd>none</dd>

            <dt>Editors:</dt>

            <dd class="vcard"><span class="fn">Vincent Hardy</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">vhardy@adobe.com</span></dd>

            <dt>Authors and Former Editors:</dt>

            <dd class="vcard"><span class="fn">Stephen Zilles</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">szilles@adobe.com</span></dd>

            <dd class="vcard"><span class="fn">Alexandru Chiculita</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">achicu@adobe.com</span></dd>

            <dd class="vcard"><span class="fn">Andrei Bucur</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">abucur@adobe.com</span></dd>

            <dd class="vcard"><span class="fn">Mihnea Ovidenie</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">mihnea@adobe.com</span></dd>

            <dd class="vcard"><span class="fn">Peter Sorotokin</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">psorotok@adobe.com</span></dd>

            <dd class="vcard"><span class="fn">Virgil Palanciuc</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">virgilp@adobe.com</span></dd>

            <dd class="vcard"><span class="fn">Arno Gourdol</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">agourdol@adobe.com</span></dd>
        </dl><!--begin-copyright--><!--end-copyright-->
        <hr title="Separator for header">
    </div>

    <h2 class="no-num no-toc" id="abstract">Abstract</h2>

    <p>The CSS Regions module allows content to flow across multiple areas
    called regions. The regions do not necessarily follow the document order.
    The CSS Regions module provides an advanced content flow mechanism, which
    can be combined with positioning schemes as defined by other CSS modules
    such as the Multi-Column Module [[!CSS3COL]] or the Grid Layout Module
    [[CSS3-GRID-LAYOUT]] to position the regions where content flows.</p>

    <h2 class="no-num no-toc" id="status-of-this-document">Status of this
    document</h2><!--begin-status-->

    <p><em>This section describes the status of this document at the time of
    its publication. Other documents may supersede this document. A list of
    current W3C publications and the latest revision of this technical report
    can be found in the <a href="http://www.w3.org/TR/">W3C technical reports
    index at http://www.w3.org/TR/.</a></em></p>

    <p>This is a public copy of the editors' draft. It is provided for
    discussion only and may change at any moment. Its publication here does not
    imply endorsement of its contents by W3C. Don't cite this document other
    than as work in progress.</p>

    <p>The archived public mailing list www-style@w3.org is preferred for
    discussion of this specification. When sending e-mail, please put the text
    "css3-regions" in the subject, preferably like this: "[css3-regions]
    …summary of comment…"</p>

    <p>This draft is related to the drafts about Multi-column Layout <a href=
    '#CSSMULTICOL'>[CSSMULTICOL]</a>, Grid Layout <a href=
    '#CSS3GRID'>[CSS3GRID]</a>, Flexible Box Layout <a href=
    '#CSS3FLEXBOX'>[CSS3FLEXBOX]</a>, and Template Layout <a href=
    '#CSSTEMPLATELAYOUT'>[CSSTEMPLATELAYOUT]</a>.</p><!--end-status-->

    <h2 class="no-num no-toc" id="table-of-contents">Table of contents</h2>
    <!--begin-toc--><!--end-toc-->

    <h2 id="introduction">Introduction</h2>

    <p>Capturing the complex layouts of a typical magazine, newspaper, or
    textbook requires capabilities beyond those available in existing CSS
    modules. This is the purpose of the CSS Regions module.</p>

    <p>&nbsp;</p>

    <p>The CSS Regions module can be seen as an extension of the concept of
    multi-column elements. With CSS Multi-column layout [[!CSS3COL]], columns
    share the same dimensions and define column boxes organized in rows.
    Content flows from one column to the next.</p>

    <p>&nbsp;</p>

    <p>The multi-column model is an example of flowing content from one area to
    another, where the areas are the multi-column element's column boxes and
    the flow is made of the multi-column element's children.</p>

    <p>&nbsp;</p>

    <p>However, for more complex layouts, content needs to flow from one area
    of the page to the next without limitation of the areas' sizes and
    positions. These arbitrary areas are the target of specific content flows.
    In this document these areas are called regions, and the content flows are
    called named flows. Regions are based on the rectangular geometry of the
    CSS box model. Elements in a named flow are taken out of the normal visual
    formatting and rendered in a chain of regions.</p>

    <h3 id="named-flows-and-regions">Named flows and regions</h3>

    <p>Consider the layout illustrated in figure 1.</p>

    <div class="figure">
        <img src="images/regions-intro.svg" width="400" alt=
        "multiple chained regions which should receive content from a flow">

        <p class="caption">Layout requiring sophisticated content flow</p>
    </div>

    <p>The designer's intent is to position an image in region 'A' and to flow
    an article's text from region '1', to region '2', '3' and '4'.</p>

    <p>The following code snippet shows the content to flow between the regions
    1, 2, 3 and 4.</p>
    <pre>
&lt;div <em>id="article"</em>&gt;
&lt;h1&gt;Introduction&lt;/h1&gt;
    &lt;p&gt;This is an example ...&lt;/p&gt;

&lt;h1&gt;More Details&lt;/h1&gt;
    &lt;p&gt;This illustrates ...&lt;/p&gt;
    &lt;p&gt;Then, the example ...&lt;/p&gt;
    &lt;p&gt;Finally, this ...&lt;/p&gt;
&lt;/div&gt;

</pre>

    <p>CSS layout facilities can position and size regions as needed. However,
    there is no existing mechanism to associate the content with the regions so
    that content flows as intended. Figure 2 shows an example of the intended
    visual rendering of the content.</p>

    <div class="figure">
        <img src="images/regions-intro-rendering.png" width="450" alt=
        "Sample rendering showing a single thread of text flowing through a chain of regions">

        <p class="caption">Sample rendering for desired layout</p>
    </div>

    <p>Since the CSS Regions module is independent of the layout of regions and
    the mechanism used to create them, the following assumes there is a CSS
    selector for the regions and, for the purpose of the example, the selectors
    for regions 1, 2, 3 and 4 show as ''&lt;region1_sel&gt;'',
    ''&lt;region2_sel&gt;'', ''&lt;region3_sel&gt;'' and
    ''&lt;region4_sel&gt;'' respectively. Such a selector could be an id
    selector (e.g., ''#region_1'') or a grid cell selector (e.g.,
    ''#myGrid::grid-cell([cell-name])'') if using the CSS Grid Layout module,
    for example.</p>

    <div class="example">
        <pre>
&lt;style&gt;

#article {
    <strong>flow: article_flow;</strong>
}

&lt;region1_sel&gt;, &lt;region2_sel&gt;,
&lt;region3_sel&gt;, &lt;region4_sel&gt; {
    <strong>content: from-flow(article_flow);</strong>
}
&lt;/style&gt;
</pre>
    </div>

    <p>The above stylesheet directs the ''#article'' element to a named flow
    called 'article_flow' by setting the 'flow' property. Then, content is
    "poured" from that named flow into the desired regions by setting the
    regions' 'content' property to ''from-flow(article_flow)''.</p>

    <h3 id="region-styling">Regions Styling</h3>

    <p>Region styling allows content to be styled depending on the region it
    flows into. It is a form of context-based styling, similar to Media Queries [[MEDIAQ]] which enable or disable
    selectors depending on the rendering context. With region styling,
    additional selectors may apply depending on the region into which content
    flows.</p>

    <p>In our example, the designer wants to make text flowing into region 1
    larger, bold and dark blue. In addition, <code class=
    "html">&lt;h1&
1917
;gt;</code> headers should be run-ins and crimson.</p>

    <p>This design can be expressed with region styling as shown below.</p>

    <div class="example">
        <pre>
&lt;style&gt;
/*
 * Default article styling.
 */
#article {
    color:#777;
    text-align: justify;
}

#article h1 {
    border-left: 1px solid #777;
    padding-left: 2ex;
}

/*
 * Additional styling to apply to content when it falls into
 * region_1
 */
<strong>@region &lt;region1_sel&gt; {</strong>
    #article::region-lines {
        font-weight: bold;
        font-size: larger;
    }

    #article h1 {
        color: crimson;
        display: run-in;
        border: none;
        padding: 0px;
    }
<strong>}</strong>
&lt;/style&gt;
</pre>
    </div>

    <p>The '@region' rule for region 1 limits its selectors to content flowing
    into region 1. The following figure shows how the rendering changes if we
    do not increase the font size nor make it bold for content flowing into
    region 1. As more content can be fitted, more content is subject to the
    contextual selectors, resulting in more dark blue text into region 1.</p>

    <p>The ''::region-lines'' pseudo-element is described later in this
    specification.</p>

    <div class="figure">
        <img src="images/region-styling.png" width="450" alt=
        "Illustrate how changing region styling affects the flow of content.">

        <p class="caption">Different rendering with a different region
        styling</p>
    </div>

    <h2 id="css-regions-model">CSS Regions Model</h2>

    <h3 id="named-flows">Named flows</h3>

    <p>In the <a href="http://www.w3.org/TR/CSS21/visuren.html">CSS formatting
    model</a>, elements can be in the <em>normal flow</em> or out of the
    <em>(normal) flow</em>. Boxes generated by elements in the normal flow are
    subject to their container box's normal layout scheme. Boxes generated by
    elements out of the normal flow are subject to a different layout scheme.
    For example, absolutely positioned elements are subject to absolute
    positioning into their <a href=
    "http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">containing
    block</a>. This can be described by saying that the absolutely positioned
    elements are part of a special flow (called <span title=
    "positioned-flow">positioned flow</span>) which is subject to a special
    layout by its container box (i.e., its container box positions it into the
    containing block's box).</p>

    <p>&nbsp;</p>

    <p>In both cases, there is a notion of flow containing a sequence of
    elements and there is a notion of <a href=
    "http://www.w3.org/Style/Group/css2-src/visuren.html#block-boxes">(block)
    container box</a> into which the elements flow.</p>

    <p>&nbsp;</p>

    <p>The CSS Regions module generalizes the concept of flow by adding the
    concept of a <em>named flow</em>. This module lets authors explicitly place
    elements into a <em>named flow</em>.</p>

    <p>&nbsp;</p>

    <p>With this model, all elements are moved to a flow as part of the visual
    formatting. That flow may be the normal flow, a named flow or a positioned
    flow, for example. A flow gets formatted visually when associated with one
    or several elements' container box(es). When an flow is associated with
    container boxes, the boxes generated by the flow's elements are laid out
    according to the container box's layout scheme and the flow is subject to
    the <a href='#flow-breaking-rules'>'flow breaking rules'</a> described
    below.</p>

    <p>A container's layout scheme is the strategy used by a container to
    position the boxes generated by its children and itself. Examples are the
    normal layout (block and inline formatting) ([[!CSS21]]), table layout
    ([[!CSS21]]), the multi-column layout ([[!CSS3COL]]) or the grid layout
    ([[CSS3-GRID-LAYOUT]]).</p>

    <h3 id="regions">Regions</h3>

    <p class="index" title="region" id="region">A region is an element that
    generates a <a href=
    "http://www.w3.org/TR/CSS21/visuren.html#block-boxes">block container
    box</a> and has an associated <em>named flow</em> (see the 'content'
    property).</p>

    <p>A <span title="region">region</span>'s intrinsic width is zero.</p>

    <h3 id="flow-breaking-rules">Flow breaking rules</h3>

    <p>Breaking a named flow across multiple regions is similar to breaking a
    document's content across multiple pages. One important difference is that
    page boxes are generated based on the available content whereas regions are
    a predefined set of recipients for the named flow content.</p>

    <h4 id="breaking-a-named-flow-and-determining-the-current-flow-region">
    Breaking a named flow and determining the current flow region</h4>

    <p>The 'content-order' property defines how <span title=
    "region">regions</span> are organized in to a <span title=
    "region-chain">region chain</span>.</p>

    <p>Each region in turn consumes content from its associated <span title=
    "named-flow">named flow</span>. This means that the named flow content is
    positioned in the <dfn title="current-region">current region</dfn> until a
    region break occurs, at which point the <span title=
    "current-region">current region</span> becomes the next one in the
    <span title="region-chain">region chain</span>. If there is no more
    <span>region</span> in the region chain and there is still content in the
    flow, the positioning of the remaining content is controlled by the
    'region-overflow' property on the last <span>region</span> in the
    chain.</p>

    <p>The following sections define where region breaks may happen when
    positioning a named flow across <span title="region">regions</span> in a
    <span title="region-chain">region chain</span>.</p>

    <div class="issue">
        <p>The sections on "Allowed region breaks", "Forced region breaks" and
        ""Best" region breaks" are adapted from the [[!CSS21]] specification.
        There is an updated version of these paragraphs in [[CSS3PAGE]] as
        well.</p>

        <p>In addition, the CSS Multi-column Layout Module [[!CSS3COL]] defines
        unified breaking properties (which this specification extends).</p>

        <p>It seems that we should:</p>

        <ol>
            <li>Define a common set of rules for breaking content that accounts
            for pages, columns and regions.</li>

            <li>Define the properties for breaking content in a single
            specification as well.</li>

            <li>Specify how column, page and region breaks interact.</li>
        </ol>
    </div>

    <h4 id="allowed-region-breaks">Allowed region breaks</h4>

    <p>In a named flow, region breaks can occur at the following places:</p>

    <ol>
        <li>In the vertical margin between block-level boxes. When an unforced
        region break occurs here, the <a href="/TR/CSS2/cascade.html#used-value">used
        values</a> of the relevant <a href="/TR/CSS2/box.html#propdef-margin-top" class=
        "noxref"><span class="propinst-margin-top">'margin-top'</span></a> and
        <a href="/TR/CSS2/box.html#propdef-margin-bottom" class="noxref"><span class=
        "propinst-margin-bottom">'margin-bottom'</span></a> properties are set
        to '0'. When a forced region break occurs here, the used value of the
        relevant <a href="/TR/CSS2/box.html#propdef-margin-bottom" class=
        "noxref"><span class=
        "propinst-margin-bottom">'margin-bottom'</span></a> property is set to
        '0'; the relevant <a href="/TR/CSS2/box.html#propdef-margin-top" class=
        "noxref"><span class="propinst-margin-top">'margin-top'</span></a> used
        value may either be set to '0' or retained.</li>

        <li>Between <a href="/TR/CSS2/visuren.html#line-box">line boxes</a> inside a
        <a href="/TR/CSS2/visuren.html#block-boxes">block container</a> box.</li>

        <li>Between the content edge of a block container box and the outer
        edges of its child content (margin edges of block-level children or
        line box edges for inline-level children) if there is a (non-zero) gap
        between them.</li>
    </ol>

    <p>These breaks are subject to the following rules:</p>

    <ul>
        <li><strong>Rule A:</strong> Breaking at (1) is allowed only if the
        'break-after' 'break-before' properties of all the elements generating
        boxes that meet at this margin allow it, which is when at least one of
        them has the value 'always', 'left', or 'right', or when all of them
        are 'auto'.</li>

        <li><strong>Rule B:</strong> However, if all of them are 'auto' and a
        common ancestor of all the elements has a 'break-inside' value of
        'avoid-region', then breaking here is not allowed.</li>

        <li><strong>Rule C:</strong> Breaking at (2) is allowed only if the
        number of <a href="/TR/CSS2/visuren.html#line-box">line boxes</a> between the
        break and the start of the enclosing block box is the value of
        'orphans' or more, and the number of line boxes between the break and
        the end of the box is the value of 'widows' or more.</li>

        <li><strong>Rule D:</strong> In addition, breaking at (2) or (3) is
        allowed only if the 'break-inside' property of the element and all its
        ancestors is 'auto'.</li>
    </ul>

    <p>If the above does not provide enough break points to keep content from
    overflowing the region boxes, then rules A, B and D are dropped in order to
    find additional breakpoints.</p>

    <p>If that still does not lead to sufficient break points, rule C is
    dropped as well, to find still more break points.</p>

    <h4 id="forced-region-breaks">Forced region breaks</h4>

    <p>A region break <em>must</em> occur at (1) if, among the 'break-after'
    and 'break-before' properties of all the elements generating boxes that
    meet at this margin, there is at least one with the value 'always', 'left',
    or 'right'.</p>

    <h4 id="best-region-breaks">"Best" region breaks</h4>

    <p>CSS&nbsp;2.1 does <em>not</em> define which of a set of allowed region
    breaks must be used; CSS&nbsp;2.1 does not forbid a user agent from
    breaking at every possible break point, or not to break at all. But
    CSS&nbsp;2.1 does recommend that user agents observe the following
    heuristics (while recognizing that they are sometimes contradictory):</p>

    <ul>
        <li>Break as few times as possible.</li>

        <li>Avoid breaking inside a replaced element.</li>
    </ul>

    <div class="example">
        <p style="display:none">Example(s):</p>

        <p>Suppose, for example, that the style sheet contains 'orphans: 4',
        'widows: 2', and there are 20 lines (<a href=
        "/TR/CSS2/visuren.html#line-box">line boxes</a>) available at the bottom of the
        current region:</p>

        <ul>
            <li>If a paragraph at the end of the current region contains 20
            lines or fewer, it should be placed on the current region.</li>

            <li>If the paragraph contains 21 or 22 lines, the second part of
            the paragraph must not violate the <a href=
            "/TR/CSS2/page.html#propdef-widows" class="noxref"><span class=
            "propinst-widows">'widows'</span></a> constraint, and so the second
            part must contain exactly two lines</li>

            <li>If the paragraph contains 23 lines or more, the first part
            should contain 20 lines and the second part the remaining
            lines.</li>
        </ul>

        <p>Now suppose that <a href="/TR/CSS2/page.html#propdef-orphans" class=
        "noxref"><span class="propinst-orphans">'orphans'</span></a> is '10',
        <a href="/TR/CSS2/page.html#propdef-w
6777
idows" class="noxref"><span class=
        "propinst-widows">'widows'</span></a> is '20', and there are 8 lines
        available at the bottom of the current region:</p>

        <ul>
            <li>If a paragraph at the end of the current region contains 8
            lines or fewer, it should be placed on the current region.</li>

            <li>If the paragraph contains 9 lines or more, it cannot be split
            (that would violate the orphan constraint), so it should move as a
            block to the next region.</li>
        </ul>
    </div>

    <h3 id="the-visual-formatting-model-and-flows">The Visual Formatting Model
    and Flows</h3>

    <p>This section describes the flows to which elements formatted according
    to the CSS <a href="http://www.w3.org/TR/CSS21/visuren.html">Visual
    Formatting Model</a> and other layout modules (such as Multi-Column Layout
    [[!CSS3COL]]) belong to.</p>

    <p>In particular, this section describes the 'generated flow' where
    generated content of pseudo-elements is placed and several <dfn id=
    "auto-flows" title="auto flows">'auto flows'</dfn> where children of content
    elements are automatically placed.</p>

    <h4 id="normal-flow">Normal flow</h4>

    <p>In the CSS formatting model, elements are by default placed in the
    <dfn id="normal-flow-def">normal flow</dfn> of their container. Also by
    default, a container element gets its content from its <em>normal
    flow</em>. This means that by default, a container element will visually
    format its children elements and will be the only container associated with
    its normal flow.</p>

    <p>Note that floats and relatively positioned elements, in this model, are
    part of the same flow of content and flow into the same container but are
    positioned in different ways.</p>

    <p>The <em>normal flow</em> is one of the <em>auto flows</em>.</p>

    <h4 id="positioned-flow">Positioned flow</h4>

    <p>An absolutely positioned element is placed into the <dfn title=
    "positioned-flow">positioned flow</dfn> of its container. The container
    positions this element into its <em>containing block</em>.</p>

    <p>If a container has children in the <em>normal flow</em> and in the
    <em>positioned flow</em>, it applies different positioning schemes to each
    flow.</p>

    <p>The <em>positioned flow</em> is one of the <em>auto flows</em>.</p>

    <h4 id="column-flow">Column flow</h4>

    <p>Children of the multi-column element are placed in the element's
    <dfn id="column-flow-def">column flow</dfn>. The column boxes are then
    implicitly associated with the element's 'column flow'. In this model,
    multiple regions (column boxes) get their content from a single flow (the
    element's 'column flow', which is the content of the element).</p>

    <p>The <em>column flow</em> is one of the <em>auto flows</em>.</p>

    <div class="issue">
        Need to add a <dfn id="grid-cell-flow">grid cell flow</dfn> section.
    </div>

    <div class="issue">
        Need to add examples for the different flow types.
    </div>

    <h4 id="generated-flow">Generated flow</h4>

    <p>In the CSS formatting model, when the ''::before'' or ''::after''
    pseudo-elements have their content property set to one of
    ''&lt;string&gt;'', ''&lt;uri&gt;'', ''&lt;counter&gt;'',
    ''attr(&lt;identifier&gt;)'', ''open-quote'', ''close-quote'',
    ''no-open-quote'' or ''no-close-quote'', they create a <dfn id=
    "generated-flow-def">generated flow</dfn> which they format visually.</p>

    <h3 id="relation-to-document-events">Relation to document events</h3>

    <p>The CSS Region module does not alter in any way the normal processing of
    events in the document tree. In particular, if an event occurs on an
    element that is part of a named flow, the <a href=
    "http://www.w3.org/TR/DOM-Level-3-Events/#dom-event-architecture">event's
    bubble and capture phases</a> happen following the document tree order.</p>

    <div class="issue">
        <p>Should we consider having something to be able to add event
        listeners to a region to detect events happening in the range of the
        flow it positions?</p>
        <pre class="code">
&lt;script&gt;
var region = ...; // get DOM r
8096
eference to region

region.addEventListener('click', function (evt) {
    alert('I was able to capture an event from the flow content!');
}, true /* useCapture */);

region.addEventListener('click', function (evt) {
    alert('I was able to see an event bubble from the flow content!');
}, false /* useCapture */);

</pre>

        <p>Note that this is a complex proposition since it modifies the normal
        bubble and capture event propagation model. However, it seems that it
        would be a worthwhile feature.</p>
    </div>

    <h2 id="properties-and-rules">Properties and Rules</h2>

    <p>The main CSS Regions module properties are the ‘flow’ and 'content'
    properties. The ‘flow’ property places an element into a specific named
    flow or lets the <a href="#auto-flows">automatic flow assignment</a> take
    place. The ‘content’ property binds a region with a flow. When multiple
    regions are bound to the same flow, the 'content-order' property determines
    the order in which content flows into the sequence of regions. This
    sequences of regions is called a <dfn>region chain</dfn>.</p>

    <p>&nbsp;</p>

    <p>The way in which the content is broken into segments that fit in a
    region can be controlled by the “break” properties. Finally, styling that
    is unique to the region can be specified by ‘region-style’ rules.</p>

    <h3 id="the-flow-property">The 'flow' property</h3>

    <p>The ‘flow’ property places an element into a named flow or automatically
    places it in the appropriate flow (see the Visual Formatting Model and Flows
    section). Elements that belong to the same flow are laid out in the regions
    associated with that flow.</p>

    <table class="propdef" summary="flow property definition">
        <tr>
            <th>Name:</th>

            <td><dfn id="flow2">flow</dfn></td>
        </tr>

        <tr>
            <th>Value:</th>

            <td>&lt;string&gt;</td>
        </tr>

        <tr>
            <th>Initial:</th>

            <td>auto</td>
        </tr>

        <tr>
            <th>Applies to:</th>

            <td>any element</td>
        </tr>

        <tr>
            <th>Inherited:</th>

            <td>no</td>
        </tr>

        <tr>
            <th>Percentages:</th>

            <td>N/A</td>
        </tr>

        <tr>
            <th>Media:</th>

            <td>visual</td>
        </tr>

        <tr>
            <th>Computed&nbsp;value:</th>

            <td>as specified</td>
        </tr>
    </table>

    <dl>
        <dt>auto</dt>

        <dd>The element is automatically moved to the appropriate flow
        according to the Visual
        Formatting Model and Flows definitions.</dd>

        <dt>&lt;string&gt;</dt>

        <dd>The element is placed (appended) to the flow with that name. The
        element is said to have a <dfn id="specified-flow">specified
        flow</dfn>.</dd>
    </dl>

    <div class="issue">
        Should the 'flow' property be an identifier and not a string?
    </div>

    <p>A named flow needs to be associated with one or more regions for its
    elements to be visually formatted. If no region is associated with a given
    named flow, the elements in the named flow are not rendered: they do not
    generate boxes and are not displayed.</p>

    <p>The children of an element with a specified flow may themselves have a
    specified flow.</p>

    <p>If an element has a the same specified flow value as one of its
    ancestors, it is moved out of its parent's normal flow and becomes a
    sibling of it's ancestor for the purpose of layout in the regions taking
    content from this flow.</p>

    <p>The 'flow' property does not affect the CSS cascade and inheritance for
    the elements on which it is specified. The flow property affects the visual
    formatting of elements associated to a named flow and of regions getting
    their content from a named flow.</p>

    <p>The containing block for absolutely positioned descendants of an element
    with a specified flow is the region into which the element is rendered.</p>

    <p>All the elements participating inside a named flow are rendered as
    children of an anonymous block that spans across all the regions associated
    with the specified named flow. The elements flowed inside the same named
    flow are taken in document order.</p>

    <p><span title="region">Regions</span> create a new stacking context, but
    inherit the floats that are already defined by the parent context.</p>

    <div class="example">
        <p>In the following example, the inline content coming from the
        <code class="html">content</code> <span title="named-flow">named
        flow</span> wraps around the <code class="html">#float</code> box.</p>
        <pre>
&lt;style&gt;
#float {
    float: left;
    width: 100px;
    height: 300px;
}

#region1, #region2 {
    width: 200px;
    height: 200px;
    content: from-flow(body_text);
}

#content {
    flow: body_text;
}
&lt;/style&gt;

&lt;div id="float"&gt;&lt;/div&gt;

&lt;div id="region1"&gt;&lt;/div&gt;
&lt;div id="region2"&gt;&lt;/div&gt;

&lt;div id="content"&gt;&lt;/div&gt;
</pre>
    </div>

    <h3 id="the-content-property">The 'content' property</h3>

    <p>This specification extends the definition of the 'content' property and
    makes it applicable to all block elements and pseudo-elements.</p>

    <table class="propdef" summary="content property definition">
        <tr>
            <th>Name:</th>

            <td><dfn id="content-from">content</dfn></td>
        </tr>

        <tr>
            <th>Value:</th>

            <td>normal | none | from-flow(&lt;string&gt;)| [ &lt;string&gt; |
            &lt;uri&gt; | &lt;counter&gt; | attr(&lt;identifier&gt;) |
            open-quote | close-quote | no-open-quote | no-close-quote ]+ |
            inherit</td>
        </tr>

        <tr>
            <th>Initial:</th>

            <td>normal</td>
        </tr>

        <tr>
            <th>Applies to:</th>

            <td>non-replaced block elements and the ::before and ::after
            pseudo-elements</td>
        </tr>

        <tr>
            <th>Inherited:</th>

            <td>no</td>
        </tr>

        <tr>
            <th>Percentages:</th>

            <td>N/A</td>
        </tr>

        <tr>
            <th>Media:</th>

            <td>visual</td>
        </tr>

        <tr>
            <th>Computed&nbsp;value:</th>

            <td>as specified</td>
        </tr>
    </table>

    <dl>
        <dt><a name="x9" id="x9"><span class="index-inst" title=
        "none"><strong>none</strong></span></a></dt>

        <dd>For ''::before'' and ''::after'' pseudo-elements, the
        pseudo-element is not generated. For other elements, the element will
        not get any content for its visual formatting. If the children of the
        element are not directed to a flow referenced by another region, then
        they are not visually formatted. An element with its value set to
        'none' is said to be <dfn>disconnected</dfn></dd>

        <dt><strong>normal</strong></dt>

        <dd>Computes to 'none' for the ''::before'' and ''::after''
        pseudo-elements. For other elements, the element uses the flow assigned
        to its children following the <a href="#auto-flows">automatic flow
        assignment for the Visual Formatting Model..</a></dd>

        <dt><strong>&lt;string&gt;</strong></dt>

        <dd>Text content (see the section on <a href=
        "/TR/CSS2/syndata.html#strings">strings</a>).</dd>

        <dt><strong>&lt;uri&gt;</strong></dt>

        <dd>The value is a URI that designates an external resource (such as an
        image). If the user agent cannot display the resource it must either
        leave it out as if it were not specified or display some indication
        that the resource cannot be displayed.</dd>

        <dt><strong>&lt;counter&gt;</strong></dt>

        <dd><a href="/TR/CSS2/syndata.html#counter">Counters</a> may be specified with
        two different functions: 'counter()' or 'counters()'. The former has
        two forms: 'counter(<var>name</var>)' or 'counter(<var>name</var>,
        <var>style</var>)'. The generated text is the value of the innermost
        counter of the given name in scope at this pseudo-element; it is
        formatted in the indicated style
        ('decimal' by default). The latter function also has two forms:
        'counters(<var>name</var>, <var>string</var>)' or
        'counters(<var>name</var>, <var>string</var>, <var>style</var>)'. The
        generated text is the value of all counters with the given name in
        scope at this pseudo-element, from outermost to innermost separated by
        the specified string. The counters are rendered in the indicated
        style ('decimal' by default). See the
        section on <a href="#counters">automatic counters and numbering</a> for
        more information. The name must not be 'none', 'inherit' or 'initial'.
        Such a name causes the declaration to be ignored.</dd>

        <dt><strong>open-quote</strong> and <strong>close-quote</strong></dt>

        <dd>These values are replaced by the appropriate string from the
        <a href="http://www.w3.org/TR/CSS21/generate.html#propdef-quotes"
        class="noxref"><span class=
        "propinst-quotes"><strong>'quotes'</strong></span></a> property.</dd>

        <dt><strong>no-open-quote</strong> and
        <strong>no-close-quote</strong></dt>

        <dd>Introduces no content, but increments (decrements) the level of
        nesting for quotes.</dd>

        <dt><strong>attr(X)</strong></dt>

        <dd>This function returns as a string the value of attribute X for the
        subject of the selector. The string is not parsed by the CSS processor.
        If the subject of the selector does not have an attribute X, an empty
        string is returned. The case-sensitivity of attribute names depends on
        the document language.</dd>

        <dt><dfn id="from-flow-name">from-flow(&lt;string&gt;)</dfn></dt>

        <dd>The element will visually format the available elements in the
        named flow with name &lt;string&gt;. If there is no flow with name
        &lt;string&gt;, then the element is disconnected and does not format
        any children visually. Multiple elements may be associated with the
        same named flow. See the 'content-order' property discussion to
        understand how content segments are associated with different
        regions.</dd>
    </dl>

    <div class="note">
        <p>The 'content' property applies to block-level elements and the
        '::before' and '::after' pseudo elements. User agents may apply the
        content property to inline elements. The behavior is undefined in that
        case, but is generally expected to be similar to that of inline
        '::before' and '::after' pseudo-elements.</p>

        <p>In particular, note that the behavior of a '::before' or '::after'
        pseudo-element with a 'display' value that makes it an inline element
        combined with a 'content' value set to 'from-flow(&lt;name&gt;)' is
        undefined.</p>
    </div>

    <div class="issue">
        <p>Should we consider adding the ability to do the following:</p>

        <ul>
            <li>Support repeatable content. For example, have a repeatable
            heading at the start of each region where content flows.</li>

            <li>Support flow breaking marks. It might be very useful to support
            a <em>mark</em> (such as "<code class="html">...</code>") at the
            region breaking points to indicate that content has been broken out
            and is continuing on a different region.</li><!--
            <li>Support "atomic pull-outs". For example, we could have a named
            flow made of a set of atomic quotes that should be placed, one by
            one, in discrete regions. In that use case, we would need a
            property to specify that content should be taken atomically from
            the flow. May be something like the following.
                <pre class="code">
&lt;div id="quote_1" class="quote"&gt;...&lt;/div&gt;
...
&lt;div id="quote_n" class="quote"&gt;...&lt;/div&gt;

&lt;div id="pull-out-1" class="pull-out"&gt;&lt;/div&gt;
...
&lt;div id="pull-out-n" class="pull-out"&gt;&lt;/div&gt;

&lt;style&gt;
.quote {
    flow: "quotes";
}

.pull-out {
    content: from-flow(quotes);
    content-consume: atomic;
}
&lt;/style&gt;

</pre>
            </li>
            -->
        </ul>
    </div>

    <div class="issue">
        <p><a href=
        "http://lists.w3.org/Archives/Public/www-style/2011May/0521.html">Mailing
        list comment</a></p>

        <p>''content: from-flow(&lt;name&gt;)'' moves an element to a named
        flow. Should we allow the content to be copied to a flow instead of
        moved to a flow. For example:</p>

        <div class="code">
            #quote_A { flow: "quotes"; content: contents; }
        </div>

        <p>would keep ''#quote_A'' in the normal flow but also copy its content
        to the ''quotes'' flow. This essentially clones the node and raises
        questions regarding DOM access, CSSOM View and the computed style.</p>
    </div>

    <h3 id="the-conte
23B3
nt-order-property">The 'content-order' property</h3>

    <p>Defines the ordering of the chain of regions into which content flows.
    If region A and region B are in the same chain of regions and the
    ‘content-order’ of region A is lower than that of region B, then region A
    will precede region B in that chain: content will flow into region A before
    it flows into region B.</p>

    <table class="propdef" summary="content-order property definition">
        <tr>
            <th>Name:</th>

            <td><dfn id="content-order">content-order</dfn></td>
        </tr>

        <tr>
            <th>Value:</th>

            <td>&lt;float&gt;</td>