csswg-drafts/css-line-grid/Overview.src.html at 5ffb903d6ee02b24af56829e50a75ab2903579db · 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 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><a href="http://fantasai.inkedblade.net/contact" class="fn url">Elika Etemad</a>,
      <span class=org>Invited Expert</span>

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

    <br style="clear:both;"/>
   </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>This module extends the line box model defined in [[!CSS21]] sections
  9.4.2 and 10.8.

<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 id=line-grid>
Defining a Line Grid: the 'line-grid' property</h2>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>line-grid</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>&lt;identifier&gt;
    <tr>
      <th>Initial:
      <td>root-grid
    <tr>
      <th>Applies to:
      <td>block containers
    <tr>
      <th>Inherited:
      <td>yes
    <tr>
      <th>Animatable:
      <td>no
    <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>

  <p>Specifies which line grid the line boxes of this element (if any)
  will align.

  <p>If no ancestor has the same value,
  then this property also creates a line grid using the font and line layout
  settings of this element. The line grid consists of a series of horizontal
  lines corresponding to all the baselines (alphabetic, text-top, text-bottom,
  mathematic, central, hanging, etc.) and to the line-over and line-under
  edges, positioned where they would fall if the contents of this element
  consisted entirely of line boxes filled with text (no sub-elements) using
  the first available font.
  If the element is paginated, the line grid is restarted on each page;
  since line boxes cannot be paginated, no page begins with the bottom part
  of a line's grid.

  <p class="note">As a side effect of the initial value and this definition,
  the root element always sets up a line grid, so that 'line-snap' can be used
  even if the author has not explicitly set up a line grid.

  <p class="issue">An alternative proposal is to have a non-inherited property with two values,
  "establish a new grid" or "use same grid as my parent".
  (Let's call it ''line-grid: create | match-parent''.)
  The grid of the nearest ancestor with ''line-grid: create'' is used
  (or one is established anyway, if this is the root element).

  <p class="issue">There might need to be an offset for more complicated designs.
  How to set this offset is problematic:
  usually it's not a fixed length, but the distance to clear some header content.

<h2>Snapping to a Grid</h2>

<h3 id=line-snap>
Snapping Line Boxes: the 'line-snap' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>line-snap</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>none | baseline | contain
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>yes
    <tr>
      <th>Animatable:
      <td>no
    <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>

  <p>This property applies to all the line boxes directly contained by
  the element, and, when not ''none'', causes each line box to shift
  downward (possibly by zero) until it snaps to the line grid specified
  by 'line-grid'. (The unshifted position is the position that would be
  determined by normal line stacking rules, with consideration of any
  new controls defined by other modules such as [[CSS3LINE]].) Values
  have the following meanings:

  <dl>
  <dt><dfn>''none''</dfn>
    <dd>Line boxes do not snap to the grid; they stack normally.
  <dt><dfn>''baseline''</dfn>
    <dd>The dominant baseline snaps with the matching baseline on the
    line grid applying to the element.
  <dt><dfn>''contain''</dfn>
    <dd>Two baselines are used to align the line box: the line box is
    snapped so that its ''central'' baseline (halfway between the
    ''text-over'' and ''text-under'' baselines) is centered between
    one of the line grid's ''text-over'' baselines and a subsequent
    (but not necessarily consecutive) ''text-under'' baseline.
  </dl>

  <p>In some cases lines of equal line height will not align perfectly
  to a baseline grid: this happens, for example, when fonts (of the same
  size) with different baseline tables are mixed on a line.
  For this reason, if shifting the line by the largest difference between
  the smallest ascent and largest ascent of a single size used on the line
  would result in a smaller shift, then the contents of the line box are
  shifted up within the line box so as to allow the line to snap without
  jumping downward to the next grid line.

<h3 id=line-slack>
Line Snap Allowance: the 'line-slack' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>line-slack</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>none | &lt;integer&gt;
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>all elements
    <tr>
      <th>Inherited:
      <td>yes
    <tr>
      <th>Animatable:
      <td>no
    <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>

  <p>This property specifies the slack in snapping lines to a grid.

  <dl>
  <dt><dfn>''none''</dfn>
    <dd>No slack: each line box is snapped to the grid.
  <dt><dfn>&lt;integer&gt;</dfn>
    <dd>Nominally, each <var>n</var>th line box is snapped to the grid <em>or</em>
    each <var>n</var>th grid line has a line box snapped to it, whichever
    results in smaller shifts, for the specified integer as <var>n</var>.
    A value of ''1'' allows, for example, lines of half the grid height
    to fit twice as densely, while a value of ''2'' also allows lines
    1.5 times the grid height to match the grid 2:3.
    A value less than ''1'' is invalid.
  </dl>

  <p>When 'line-snap' is given as a &lt;baseline&gt;, the 'line-slack'
  property interpolates a grid against the main line grid and snaps
  the element's line boxes against this grid. The interpolated grid is
  drawn by creating a new line grid using the baseline table and
  'line-height' of the current element and increasing the inter-line
  spacing of this new grid to snap according to the element's 'line-snap'
  and 'line-slack' values. The first snapped line is chosen to minimize
  the shift of the first line box of the element.
  The interpolated grid is only used for this block's line boxes;
  descendant boxes interpolate their own grid against the main grid.)

  <p class="issue">Define something sane for ''line-snap: contain''.

<!--
  <p>When 'line-snap' is ''contain'', then 'line-slack' not ''none''
  is handled by line-snapping using the ''central'' baseline any time
  the interpolated grid is narrower than the main grid. It has no effect
  when the interpolated grid is wider than the main grid.

  <p>When 'line-snap' is ''contain'', the 'line-slack' property creates
  an interpolated grid as above, but instead of snapping according to
  a fixed ratio, it increases the spacing of the interpolated grid so
  that its first ''text-over'' baseline aligns to one of the main grid's
  ''text-over'' baselines and its last ''text-under'' baseline aligns
  to one of the main grid's ''text-under'' baseline. In this case, the
  integer represents the maximum number of line boxes in the block
  before slack is ignored. The slack is
-->

  <p>User Agents are not required to support values greater than 4;
  unsupported used values higher than 4 must be clamped to 4.

<p class="issue">
Can it be extended to handle vertical justification?

<h3 id=box-snap>
Snapping Block Boxes: the 'box-snap' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>box-snap</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>[ none | margin-box | border-box | half-border ]? ]{1,4} [ / &lt;identifier&gt;{1,4} ]
    <tr>
      <th>Initial:
      <td>none
    <tr>
      <th>Applies to:
      <td>block-level boxes and internal table elements except table cells
    <tr>
      <th>Inherited:
      <td>yes
    <tr>
      <th>Animatable:
      <td>no
    <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>

  <p>Specifies which edges, if any, are snapped to the line box grid.
  If four values are specified, they apply to the before, end, after, and
  start edges of the box, respectively. If the start edge is omitted, it is the
  same as the end. If the after edge is omitted, it is the same as the
  before. If the end edge is omitted, it is assigned ''none''.
  Values have the following meanings:

  <dl>
  <dt><dfn>none</dfn>
    <dd>The edge is not snapped to any grid.</dd>
  <dt><dfn>margin-box</dfn>
    <dd>The margin edge is snapped to the nearest grid line.
  <dt><dfn>border-box</dfn>
    <dd>The border edge is snapped to the nearest grid line.
  <dt><dfn>half-border</dfn>
    <dd>The edge halfway between the border and padding edges is snapped to
    the nearest grid line.
  <dt><dfn>&lt;identifier&gt;</dfn>
    <dd>Names the grid to snap to. Defaults to the effective 'line-grid'.
    If the named grid does not exist (or has no lines in the appropriate
    dimension), then the affected edge is not snapped.
    <p class="note">This allows the author to specify a grid that is not
    a line grid.
  </dl>

  <p>When snapping to baselines on a line grid, either the <i>text-over</i>
  or <i>text-under</i> baseline is chosen: whichever one is on the matching
  side of the central baseline. For example, when snapping the before edge
  in horizontal writing mode, the <i>over</i> edge is chosen. In some
  cases the <i>under</i> edge might be used instead for the before edge:
  for example, when the writing mode of the line grid doesn't match that of
  the affected element, or when due to the 'text-orientation' settings the
  <i>under</i> side corresponds to the <i>after</i> edge.

  <p>To snap a before, start, or end edge of a block-level element or its
  after margin-box edge to a grid line, the effective margin is increased
  at that edge. To snap an after border-box or half-border edge to a grid
  line, the effective content-box height is increased. If, however, the
  box is an empty block that could be
  <a href="http://www.w3.org/TR/CSS21/box.html">collapsed through</a>,
  then this property has no effect. [[!CSS21]]

  <p>When applied to table row group and table row boxes, 'box-snap' only
  affects the before and after edges, and only if those edges are not at
  the beginning or end of the table, respectively. To snap a before edge
  on a table row or row group, the preceding row's height is increased.
  To snap an after edge on a table row or row group, the affected row's
  height is increased.

  <p>When applied to table column group and table column boxes, 'box-snap'
  only affects the start and end edges, and only if those edges are not at
  the start or end of the table, respectively. How the space is redistributed
  among columns to satisfy snapping constraints is not defined, however:
  <ul>
    <li>In an auto-sized table no column may be smaller than its minimum
       content width.
    <li>The resulting table must not exceed its original measure if it
      had a non-''auto'' measure.
    <li>The adjusted widths must not cause the table to overflow its
      containing block any more than it would with ''box-snap: none''.
  </ul>
  <p>To satisfy these constraints, some column edges may remain unsnapped.
<!--
  Line snapping for columnsis processed from start to end: column widths
  are increased or decreased in order to snap the column edges.
  By preference, the widths are increased.
  However if the table has an explicit size or is auto-sized and the resulting
  table overflows its containing block, then columns whose widths were
  increased must, one by one, switch their snap to decreasing the column
  width, until the table reaches its specified size or
-->
  <p>For internal table elements in separated borders tables, ''margin-box''
  represents the edge at the midpoint of the border spacing. For collapsed
  tables, ''margin-box'' and ''border-box'' are treated as ''half-border''.
  When two coinciding edges have conflicting snap values, the later value
  wins.

<h2>Layout in Grid Units</h2>

<p class="issue">This section is mostly unrevised brainstorming.</p>

<h3 id="grid-length-unit">
Grid Length Units: the ''gd'' 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 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>''
  </h3>

  <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>

  <p>Similarly, the ''roundup(<em>length</em>)'' computes to ''auto'',
    and then the result will be rounded up to
    the smallest multiple of the specified unit
<
1337
div>    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>

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

<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 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 “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="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>
  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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