csswg-drafts/css-line-grid/Overview.src.html at adf36bba941876e0a047bb5d33225d8f9a33b43c · xfq/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 lang="en">
<head profile="http://www.w3.org/2006/03/hcard">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Line Grid Module</title>
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link rel="stylesheet" type="text/css"
        href="http://www.w3.org/StyleSheets/TR/W3C-ED.css">
  <style type="text/css">
    .sidenote-block {
    	border: 1px solid gray;
    	margin: auto;
    	text-align: left;
    	padding: 4pt;
    	position: relative;
    	max-width: 400pt;
    }
    .sidenote-body {
    	margin-left: 120pt;
    	font-size: 12pt;
    	line-height: 18pt;
    }
    .sidenote-note {
    	font-size: 10pt;
    	line-height: 18pt;
    	position: absolute; left: 4pt; top: 4pt;
    	width: 110pt;
    }
  </style>
</head>

<div class="head">
<!--logo-->

<h1>CSS Line Grid Module</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
  <dt>This version:
    <dd><a href="[VERSION]">
    http://www.w3.org/TR/[YEAR]/[STATUS]-[SHORTNAME]-[CDATE]</a>

<!--
  <dt>Latest version:
    <dd><a href="http://www.w3.org/TR/css-line-grid">
    http://www.w3.org/TR/css-line-grid</a>

  <dt>Previous version:
    <dd><a href="http://www.w3.org/PreviousVersionURI">
    http://www.w3.org/PreviousVersionURI</a>
-->

  <dt>Editors:
    <dd class=vcard><span class=fn>Koji Ishii</span>,
      <span class=org>Invited Expert</span>,
      <span class=email><a href="mailto:kojiishi@gluesoft.co.jp">kojiishi@gluesoft.co.jp</a></span>
</dl>

<!--copyright-->

<hr title="Separator for header">
</div>

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

  <p>CSS 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 level&nbsp;3 relating to <var class="issue">DESCRIBE HERE</var>.
  It includes and extends the functionality of CSS level&nbsp;2 [[CSS21]],
  which builds on CSS level&nbsp;1 [[CSS1]].
  The main extensions compared to level&nbsp;2 are <var class="issue">SUMMARIZE HERE</var>.

<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>Introduction</h2>

<h3 id="context">Background</h3>

  <p><em>This section is not normative.</em></p>

  <p>This specification provides features to align lines and blocks
    to invisible grids in the document.</p>

  <p>Aligning lines and blocks to grids provides the following benefits:</p>

  <ul>
    <li>Vertical rhythm is kept for better readability.</li>
    <li>Lines are aligned between columns in multi-column documents.</li>
    <li>The top and the bottom margins of pictures are made equal,
      while keeping the vertical rhythm of text before and after the pictures.</li>
    <!--
    <li>Aligning to grids can sometimes be turned off for objects like tables,
      but then turned back on for the following text
      to the same grids as the one for the text before the objects.</li>
    -->
    <li>Layout lines are at the same position on every page in paged media.
      Keeping the position of the bottom line of a page has benefits for
      design and readability.
      This also improves the readability of duplex printing,
      two pages spreads,
      and displaying on slow display devices like e-ink.</li>
    <li>East Asian layouts require vertical rhythm
      more often than other scripts do,
      even in single column, non-paged media documents,
      as defined in [[JLREQ]].</li>
  </ul>

  <p>There are several types of objects in a document
    that can break the vertical rhythm.
    Examples include lines with different sizes of text, pictures, and tables.</p>

  <div class="figure">
    <img src="line-grid-multicol.png"
      width="480" height="246"
      alt="Vertical rhythm kept through pictures and different size of text in a multi-column document"
      />
    <p class="caption">Vertical rhythm kept through pictures and different size of text in a multi-column document.</p>
  </div>

  <div id="ex-sidenote" class="figure">
    <div class="sidenote-block">
      <div class="sidenote-body">
        Sidenotes (and footnotes for that matter) are often set
        at a smaller size than the basic text.
        This smaller text should still line up with the basic text.
        Authors can achieve this effect
        by calculating appropriate font-size, line-height,
        and margins*.
      </div>
      <div class="sidenote-note">
        Only if author controls everything.
        It can easily be broken by user stylesheet, for instance.
      </div>
    </div>
    <p class="caption">Sidenotes are set at a smaller size, but still line up with the basic text.</p>
  </div>

  <div id="ex-width" class="sidefigure">
    <img src="width-multiple-of-em.png"
      width="180" height="142"
      alt="East Asian layouts may require width be a multiple of em without fractions"
      />
    <p class="caption">East Asian layouts may require width
      be a multiple of <em>em</em> without fractions.</p>
  </div>

  <p>East Asian layouts may require grid-like features
    in inline progression direction as well.</p>
  <p>It is often desirable in East Asian layouts
    to make the line width
    a multiple of <em>em</em> without fractions.
    Because most East Asian characters have 1em advance
    and most East Asian documents are justified,
    this minimizes cases where justification needs to expand character spacing.</p>

  <p>This module provides the following capabilities:</p>
  <ul>
    <li>Defining grids in the line progression direction.</li>
    <li>Controling how lines and blocks align to the grids.</li>
    <li>Rounding lengths to multiples of units.</li>
  </ul>

  <p>It is important to control these capabilities independently,
    so that, for example, aligning to grids can be turned off for tables,
    but can then be turned back on for aligning the following text to the grids.
  </p>


<h3 id="placement">Module Interactions</h3>

  <p class="issue">Explain, normatively, how this module affects the definition of CSS.</p>

  <p>This module replaces and extends the <var class="issue">SUMMARIZE HERE</var>
  features defined in [[!CSS21]] sections <var class="issue">W.X and Y.Z</var>.

<h3 id="conventions">Document Conventions</h3>

  <p>Conformance requirements are expressed with a combination of
  descriptive assertions and RFC 2119 terminology. The key words “MUST”,
  “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
  “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
  document are to be interpreted as described in RFC 211
8000
9.
  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 “for example”
  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 “Note” 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="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 Level 2 Revision 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.

<h2>Defining Grids</h2>

<h3 id="layout-grid-line-prop">Define Line Grids: the 'layout-grid-line' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>layout-grid-line</dfn>
    <tr>
      <th>Value:
      <td>none
        | auto
        | &lt;length&gt;
        | &lt;percentage&gt;
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>block containers
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
  </table>

  <p>This property defines a line grid for the element and its descendent elements.
    Possible values are:</p>

  <p class="issue">An implementer said the grid's scope should be limited to the same BFC,
    as referring property values and position of parent elements could be difficult to implement
    if the element is in different BFC than the parent.
    This can mean a float cannot share the grids with its parent, which is not ideal.
    How can we solve this?</p>

  <dl>
    <dt><dfn title="layout-grid-line::none">''none''</dfn></dt>
    <dd>No line grid is specified.</dd>
    <dt><dfn title="layout-grid-line::auto">''auto''</dfn></dt>
    <dd>The grid line size is determined by the computed value of the 'line-height' property.</span></dd>
    <dt><dfn title="layout-grid-line::length">&lt;length&gt;</dfn></dt>
    <dd>Size of the line grid's unit space (a.k.a. "line pitch").</dd>
    <dt><dfn title="layout-grid-line::percent">&lt;percent&gt;</dfn></dt>
    <dd>Size of the line grid's unit space
      relative to the logical height of the parent element.
      If the parent has no explicit height, it is treated as ''auto''.
      <span class="issue">This was defined in
        <a href="http://www.w3.org/TR/2001/WD-css3-text-20010517/#layout-grid-line-prop">CSS3 module: text WD 20010517</a>,
        but not sure how much useful this is</span></dd>
  </dl>

  <p>The defined grid uses the position of the element as the origin of the grid.
    It uses the top of the element as the origin of the grid for horiozntal text flow.
    Similarly, it uses the right of the element as the origin of the grid
    for 'writing-mode' is ''vertical-rl'',
    and the left of the element for ''vertical-lr''.</p>

  <p class="issue">Define behavior for paged media/column break</p>
  <p class="issue">TBD:
    I might need to copy more text from
    <a href="http://www.w3.org/TR/2001/WD-css3-text-20010517/#layout-grid-line-prop">CSS3 module: text WD 20010517</a>.</p>

<h2>Using Grids</h2>

<h3 id="layout-grid-mode-prop">Grid Mode: the 'layout-grid-mode' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>layout-grid-mode</dfn>
    <tr>
      <th>Value:
      <td>none | baseline | block | line
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>block containers
    <tr>
      <th>Inherited:
      <td>yes
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
  </table>

  <p>This property controls how lines and blocks are aligned to grids.
    Values have the following meanings:</p>

  <dl>
    <dt><dfn title="layout-grid-mode::none">''none''</dfn></dt>
      <dd>Do not align any objects to the grids.</dd>
    <dt><dfn title="layout-grid-mode::baseline">''baseline''</dfn></dt>
      <dd>Align baselines to the line grids.
        <!--
        The line in this definition is a line box
        with before margin included if it is the first line of a block,
        and with after margin if last.
        -->
        The height is the height of the line box.
        Increasing heights to fit to grids is done by
        increasing the computed height of the line box.
        <span class="issue">How to make this value work well with images, headings, etc.
          needs further thoughts.</span>
      </dd>
    <dt><dfn title="layout-grid-mode::block">''block''</dfn></dt>
      <dd>Align blocks to the line grids.
        <!--
        The height of the block in this definition includes margins.
        -->
        The height includes paddings and borders, but not margins.
        Increasing heights to fit to grids is done by
        adding to computed margins.
      </dd>
    <dt><dfn title="layout-grid-mode::line">''line''</dfn></dt>
      <dd>Align line boxes to the line grids.
        <!--
        The line in this definition is a line box
        with before margin included if it is the first line of a block,
        and with after margin if last.
        -->
        The height is the height of the line box.
        Increasing heights to fit to grids is done by
        increasing the computed height of the line box.
      </dd>
  </dl>
  <p class="issue">Handling margins and borders needs review</p>

  <p>When this property is set to anything other than ''none'',
    the computed height of the object is increased to
    the smallest multiple of the grid height
    that is equal to or smaller than the original computed height.</p>

  <p>When the value of this property is ''baseline'',
    the line box is then moved so that its baseline is aligned
    to the next closest grid.</p>

  <p>When the value of this property is either ''block'' or ''line'',
    the object is vertically centered within the grid row.
    If the computed height is larger than the grid space,
    then the object is centered within the smallest number of grid rows
    necessary for the object to fit in.
    The starting alignment point is moved accordingly.
    This is illustrated below,
    where <em>a</em> represents the numerical 'layout-grid-line' value,
    and <em>b</em> and <em>c</em> are half the difference between the computed line-heights
    and either the layout-grid-line value or twice the value respectively.</p>

  <div class="figure">
    <img src="lgl.gif"
      alt="Layout of content within a line grid"
      />
    <p class="caption">Layout of content within a line grid,
      where <em>a</em> represents the layout-grid-line value,
      and the alignment baseline is at the bottom (after-edge).</p>
  </div>

  <p>As for regular height computation, the ruby box is treated specifically.
    In a similar fashion to the line-height determination process,
    only the ruby base element is considered for centering purposes,
    however the above and/or the after space should be sufficient
    to fit the ruby text above and/or after the base text within the grid-line space.
    If this is not the case,
    the grid-line size needs to be increased in multiple grid rows until the whole ruby box fits.
    <span class="issue">Is just ignoring ruby text enough? like rt { layout-grid-mode: ignore; }
      or nothing needed to be defined if line-stacking-ruby: auto does the work?</span>
  </p>

  <p>Note that in order for this property to have an effect,
    line grids must be defined by the 'layout-grid-line' property.</p>

  <p class="issue">This design does not allow authors to specify
    the exact top position of the first line.
    It is probably ok for web/epub where viewport is not fixed and therefore
    the UA has to absorb fractions using margins,
    but print folks may want more precise controls.</p>

  <p class="issue">Need to think more about use cases in tables.
    Turning off line grids for table cells is one easy way
    and probably is good enough when all cells are single line,
    but there probably are more use cases.</p>

  <p class="issue">Need to think more about use cases in forms.</p>

  <div class="example">
    <div class="sidefigure">
      <img src="line-grid-wrap.png"
        width="276" height="244"
        alt="Large text wraps within line grids"
        />
      <p class="caption">Large text wraps within line grids.</p>
    </div>

    <p>When a different size of text, such as a headings, wraps,
      it is usually aligned to grids as a block and
      the lines within the block do not align.</p>

    <p>This effect can be achieved by the following code:</p>
    <pre><code class="css">
<!-- -->body { line-grid-mode: line; }
<!-- -->h1 { line-grid-mode: block; }
<!-- --></code></pre>

    <br style="clear:both;"/>
   </div>

<h3>Grid Shorthand: the 'layout-grid' property</h3>

  <p class="issue">TBD</p>

<h3 id="grid-length-unit">The ''<span class="index" title="length unit::gd">gd</span>'' length unit</h3>

  <p>The existence of a grid in an element makes it possible
    and very useful to express various measurements in that
    element in terms of grid units.
    Grid units are used very frequently in East Asian typography,
    especially for the left, right, top and bottom element margins.</p>

  <p>Therefore, a new length unit is necessary,
    gd, to enable the author to specify the various measurements in terms of the grid.</p>

  <p>For example, consider the following style:</p>
  <pre><code class="css">P { layout-grid: strict both 20pt 15pt; margin: 1gd 3gd 1gd 2gd }</code></pre>
  <p>This way, all P elements would effectively acquire a 15pt top margin,
    a 60pt right margin, a 15pt bottom margin and a 40pt left margin.</p>

  <p>If no grid is specified, the gd unit should be treated the same as the em unit.</p>

  <p class="issue">Just copied from old WD, need to refresh</p>

  <div id="vert-rhythm-p-margin" class="example">
    <p>The most common addition of vertical space
      is that inserted between paragraphs.
      If the rhythm of the page is to be maintained,
      the spacing of paragraphs should be related to the basic leading.
      This is achieved by setting top- and bottom-margins equal to ''1gd''.
    <pre><code class="css">
<!-- -->p {
<!-- -->  layout-grid-line: 1.5;
<!-- -->  layout-grid-mode: baseline;
<!-- -->  margin-top: 1gd;
<!-- -->  margin-bottom: 1gd;
<!-- -->}
<!-- --></code></pre>
  </div>

  <div id="ex-gyou-dori" class="example">
    <p>East Asian layout often requires heights specified
      by the count of the line grids.
      In Japanese, this method is called Gyou-dori.
      Refer to [[JLREQ]] for more details about Gyou-dori.</p>
    <pre><code class="css">
<!-- -->h1 {
<!-- -->  line-grid-mode: block;
<!-- -->  min-height: 3gd; /* 3 Gyou-dori */
<!-- -->  margin-top: 1gd;
<!-- -->}
<!-- --></code></pre>
  </div>

  <p class="issue">''gem'' length unit is also requested</p>

<h3>Forcing Line Width to Multiple of <em>em</em></h3>
  <p class="issue">There are 3 ideas for this, and all ideas are listed here for disucssions.</p>

<h4>Character Grid: the 'layout-grid-char' property</h4>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>layout-grid-char</dfn>
    <tr>
      <th>Value:
      <td>none
        | auto
        | &lt;length&gt;
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>block containers
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
  </table>

  <p>This property defines a character grid for the element. Possible values are:</p>

  <p>When a character grid is defined,
    start and end edges of a line box snap to the nearest grid
    in the direction to shorten the line box.</p>

  <div class="figure">
    <object
      data="line-length.svg" type="image/svg+xml"
      width="340" height="158">
    </object>
    <p class="caption">The gray part of a line box is rounded down to snap to the grid.
      The second line box has a float on the left (green),
      and therefore its start edge snaps to the grid as well.</p>
  </div>

  <p class="issue">Should we add ''edges'' to the 'line-grid-mode' property?</p>

<h4 id="round-length">Rounding Lengths to Multiple of Units:
  ''<span class="index" title="length unit::rounddown()">rounddown()</span>''
  and ''<span class="index" title="length unit::rounddown()">roundup()</span>''
  </h4>

  <p>The 'width' and 'height' properties
    accept ''rounddown(<em>length</em>)''
    and ''roundup(<em>length</em>)'' values.</p>

  <p>The ''rounddown(<em>length</em>)'' computes to ''auto'',
    and then the result will be rounded down to
    the largest multiple of the specified unit
    that is equal to or less than the original computed value.</p>

    and then the result will be rounded up to
    the smallest multiple of the specified unit
    that is equal to or greater than the original computed value.</p>

  <p>The fraction is distributed evenly to the computed values of margins on each side.
    If it is used for 'height',
    half the fraction is added to the top and the bottom margins.
    If for 'width', to the left and the right margins.</p>

  <p>When the element is a float,
    the fraction is added to the opposite side of the float direction instead.
    For example, if the 'float' property is ''left'',
    the fraction is added to the right margins.</p>

  <p class="issue">Is this design appropriate to solve text with floats case?</p>
  <p class="issue">Are there requirements for table cell width?</p>
  <p class="issue">Should have an option to left/right align instead of center?
    Can author align running head and page numbers with body with this design?</p>
  <p class="issue">Use cases for non-East Asian layouts?</p>

  <div class="example">
    <p>It is often desirable for East Asian layouts
      to make the line width to be
      multiple of <em>em</em> without fractions.
      Because most East Asian characters have 1em advance
      and most East Asian documents are justified,
      this minimizes cases where justification is needed to expand lines.</p>
    <pre><code class="css">
<!-- -->body {
<!-- -->  margin: 3cm 3.5cm 3cm 3cm;
<!-- -->  text-align: justify;
<!-- -->  width: rounddown(1em);
<!-- -->}
<!-- -->.sidefigure {
<!-- -->  float: right;
<!-- -->  width: roundup(1em);
<!-- -->}
<!-- --></code></pre>
  </div>

<h4>Rounding Lengths to Multiple of Units: the 'layout-grid-measure' property</h4>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>layout-grid-measure</dfn>
    <tr>
      <th>Value:
      <td>none
        | auto
        | &lt;length&gt;
        || up | down
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>block containers
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
  </table>

<h2 id="conformance">
Conformance</h2>

<h3 id="conformance-classes">
Conformance Classes</h3>

  <p>Conformance to <var class="issue">CSS TEMPLATE Module</var>
  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 <var class="issue">CSS TEMPLATE Module</var>
  if all of its declarations that use properties defined in this module
  have values that are valid according to the generic CSS grammar and the
  individual grammars of each property as given in this module.

  <p>A renderer is conformant to <var class="issue">CSS TEMPLATE Module</var>
  if, in addition to interpreting the style sheet as defined by the
  appropriate specifications, it supports all the properties defined
  by <var class="issue">CSS TEMPLATE Module</var> 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 <var class="issue">CSS TEMPLATE Module</var>
  if it writes syntactically correct style sheets, according to the
  generic CSS grammar and the individual grammars of each property 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 CSS specifications
  reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
  syntax</a> for proprietary property and value extensions to CSS. The CSS
  Working Group recommends that experimental implementations of features in
  CSS Working Drafts also use vendor-prefixed property or value names. This
  avoids any incompatibilities with future changes in the draft. Once a
  specification reaches the Candidate Recommendation stage, implementors
  should implement the non-prefixed syntax for any feature they consider to
  be correctly implemented according to spec.</p>

<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="/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>

  <p id=cr-exit-criteria>
  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 “nightly build”).
      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><var class="issue">[acknowledgments]</var></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 -->
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