csswg-drafts/css3-align/Overview.src.html at 09c5ec2861c1f9a6a9c4ac2fbf95653d70110a59 · xfq/csswg-drafts · GitHub

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<!DOCTYPE html>
<html lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Box Alignment Module Level 3</title>
  <link rel=contents href="#contents">
  <link rel=index href="#index">
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link rel="stylesheet" type="text/css"
        href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
  <style type="text/css">
    body, html { max-width: 85em; }
    .issue th:first-child { text-align: left !important; }
    [rowspan] > img { max-height: 4em; float: right; }
    [rowspan] { vertical-align: middle; }
    small { display: block; }
  </style>
</head>

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

<h1>CSS Box Alignment Module Level 3</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>

  <dt>This version:
    <dd><a href="[VERSION]">http://www.w3.org/TR/[YEAR]/WD-css3-align-[CDATE]/</a>
  <dt>Latest version:
    <dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>
  <dt>Editor's draft:
    <dd><a href="http://dev.w3.org/csswg/css3-align/">http://dev.w3.org/csswg/css3-align/</a>
  <dt>Previous version:
    <dd>&mdash;
<!--
  <dt>Issue Tracking:</dt>
    <dd><a href="http://www.w3.org/Style/CSS/Tracker/products/FIXME">http://www.w3.org/Style/CSS/Tracker/products/FIXME</a>
-->
  <dt>Feedback:</dt>
    <dd><a href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a> with subject line &ldquo;<kbd>[css3-align] <var>&hellip; message topic &hellip;</var></kbd>&rdquo;

  <dt>Editors:
    <dd class="h-card vcard">
      <a class="p-name fn u-url url" rel="author"
         href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>,
      <span class="p-org org">Mozilla</span>
</dl>

<!--copyright-->

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

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

  <p>This module contains the features of <a href="http://www.w3.org/TR/CSS/">CSS</a> relating to
  the alignment of boxes within their containers
  in the various CSS box layout models:
  block layout, table layout, flex layout, and grid layout.
  (The alignment of text and inline-level content is defined in [[CSS3TEXT]]
  and [[CSS3LINEBOX]].)
  CSS is a language for describing the rendering of structured documents
  (such as HTML and XML) on screen, on paper, in speech, etc.

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

<!--status-->

<p>The following features are at risk: &hellip;

<h2 class="no-num no-toc" id="contents">
Table of contents</h2>

<!--toc-->

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

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

  <p>CSS Levels 1 and 2 allowed for the alignment of text via 'text-align' and
  the alignment of blocks by balancing ''auto'' margins. However, except in
  table cells, vertical alignment was not possible. As CSS3 adds further
  capabilities, the ability to align boxes in various dimensions becomes more
  critical. This module attempts to create a cohesive and common box alignment
  model to share among all of CSS.

  <p class=note>The alignment of text and inline-level content is defined in [[CSS3TEXT]]
  and [[CSS3LINEBOX]].

  <p>Inspiration for this document:
  <ul>
    <li><a href="http://wiki.csswg.org/ideas/centering#alignment-property">summary of a discussion for implementing &lt;CENTER&gt;</a>
    <li><a href="https://lists.w3.org/Archives/Member/w3c-css-wg/2008AprJun/0015.html">Minutes from March 2008 F2F</a>
    <li><a href="http://lists.w3.org/Archives/Public/www-style/2012Feb/0743.html">fantasai's attempt to merge all alignment properties</a>
  </ul>

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

  <p>This module adds some new alignment capabilities to the block layout
  model described in [[!CSS21]] chapters 9 and 10 and defines the interaction
  of these properties with the alignment of table cell content using
  'vertical-align', as defined in [[!CSS21]] chapter 17.
  The interaction of these properties with
  Grid Layout [[!CSS3-GRID-LAYOUT]]
  and Flexible Box Layout [[!CSS3-FLEXBOX]]
  is defined in their respective modules.

  <p>No properties in this module apply to the <code>::first-line</code> or
  <code>::first-letter</code> pseudo-elements.

<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 [[CSS3VALUES]], when combined with this module, adds the
  ''initial'' keyword as a possible property value.

  <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="overview">
Overview of Alignment Properties</h2>

  <p>The alignment properties in CSS can be described along two axes:
  <ul>
    <li>which dimension they apply to (inline vs. stacking), and
    <li>whether they control the position of the box within its parent,
  or the box's content within itself.
  </ul>

  <p class=note>This proposal uses the terms 'justify' and 'align' to distinguish
  between alignment in the inline and stacking dimensions, respectively.
  The choice is somewhat arbitrary, but having the two terms allows for
  a consistent naming scheme that works across all of CSS's layout models.

  <p>The following table summarizes the proposed alignment properties
  and the display types they can apply to.

  <table class=data>
  <colgroup span=2></colgroup>
  <colgroup span=1></colgroup>
  <thead>
    <tr><th>Common
        <th>Axis
        <th>Aligns
        <th>Applies to
  </thead>
  <tbody>
    <tr><th>'justify-self'
        <td>inline
        <td rowspan=2><img src="self-target.jpg" alt=""> element within parent <small>(effectively adjusts margins)</small>
        <td>block-level elements and grid items
    <tr><th>'align-self'
        <td>stacking
        <td>flex items and grid items
  </tbody>
  <tbody>
    <tr><th>'justify-content'
        <td>inline
        <td rowspan=2><img src="content-target.jpg" alt="">content within element <small>(effectively adjusts padding)</small>
        <td>block containers and flex containers
    <tr><th>'align-content'
        <td>stacking
        <td>block containers and flex containers
  </tbody>
  <tbody>
    <tr><th>'justify-items'
        <td>inline
        <td rowspan=2><img src="child-target.jpg" alt=""> items inside element <small>(controls child items&rsquo; ''align/justify-self: auto'')</small>
        <td>grid containers
    <tr><th>'align-items'
        <td>stacking
        <td>flex containers and grid containers
  </tbody>
  </table>

  <p class=issue>
  The exact behavior of these properties on layout models other than Flexbox is still
  <strong>to be determined</strong>. This is a <strong>First Public Working Draft</strong>
  and is <strong>NOT STABLE</strong>.

  <p class=note>
  The '-items' values don't affect the element itself.
  When set on a flex container or grid container,
  they specify the interpretation of any ''align/justify-self: auto'' used on the items in the container element.

  <p class=issue>True alignment vs. safe alignment.
  Maybe make safe by default, true if ''true'' is specified?
  Or safe/true depending on layout model (e.g. safe for blocks, true for flexbox)?
  Current draft introduces a ''true'' keyword, but other options should be considered.

<h2 id="self-alignment">
Self-Alignment: Aligning the Box within its Parent</h2>

  <p>The 'justify-self' and 'align-self' properties control alignment
  of the box within its containing block.

  <div class=figure>
    <p><img src=self-target.jpg
            alt="Diagram showing that the alignment of the element within its containing block is affected.">
  </div>

<h3 id="justify-self">
  Inline/Main-Axis Alignment: the 'justify-self' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>justify-self</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>auto | [ start | end | center ] && true?
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
      <td>block-level elements and grid items
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <p>Justifies the box within its parent along the inline/row axis:
  the box's outer edges are aligned within its containing block as described below.
  <dl>
    <dt><dfn>''auto''</dfn></dt>
      <dd>For block-level elements, use alignment specified by the 'justify-content'
      property of the containing block.
      For grid items, indicates that the element should stretch to fill its slot.
    <dt><dfn>''start''</dfn></dt>
      <dd>Start-aligns the box within its containing block.
    <dt><dfn>''center''</dfn></dt>
      <dd>Centers the box within its containing block.
    <dt><dfn>''end''</dfn></dt>
      <dd>End-aligns the box within its containing block.
    <dt><dfn>''true''</dfn></dt>
      <dd>If specified, alignment is "true", and may cause the box to overflow
      the start edge of the containing block.
      Otherwise, the box is forced to start-align
      when its outer measure is wider than the containing block.
  </dl>

  <p>In terms of CSS2.1 block-level formatting,
  the rules for "over-constrained" computations in section 10.3.3 are used
  only when the element's margin box overflows the containing block
  <em>and</em> the ''true'' keyword is not specified.
  Otherwise those rules are ignored in favor of alignment as specified above,
  and the used value of the end margin is not adjusted to correct for the over-constraint.
  <div class="example">
    <p>The effect of these rules is that an auto-sized block-level table,
    for example, can be aligned while still having side margins.
    If the table's max-content size is narrower than its containing block,
    then it is shrink-wrapped to that size and aligned as specified.
    If the table's max-content size is wider, then it fills its containing block,
    and the margins provide appropriate spacing from the containing block edges.
  </div>

  <p>For block-level elements that establish a block formatting context
  and are placed next to a float, alignment is with respect to the
  available space (subtracting out from the containing block measure the space taken up by the float),
  not with respect to the containing block.
  (Note: This is the legacy behavior of HTML <code>align</code>.)

  <p class=note>This property will replace 'grid-column-align'.

<h3 id="align-self">
  Stacking/Cross-Axis Alignment: the 'align-self' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>align-self</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>auto | [ baseline | head | foot | center | stretch ] && true?
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
      <td>flex items and grid items
    <tr>
      <th>Inherited:
      <td>no
    <tr>
      <th>Percentages:
      <td>N/A
    <tr>
      <th>Media:
      <td>visual
    <tr>
      <th>Computed&nbsp;value:
      <td>specified value
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <p>Aligns the box within its parent along the block/column/cross axis:
  the box's outer edges are aligned within its containing block as described below.
  <dl>
    <dt><dfn>''auto''</dfn></dt>
      <dd>For flex items, computes to alignment specified by the 'align-items'
      property of the flex container.
      For grid items, computes to ''stretch''.
    <dt><dfn>''baseline''</dfn></dt>
      <dd>Shifts the box such that the baseline of its <i>first formatted line</i>
        aligns with the corresponding baselines of other ''baseline''-aligned siblings in the same row/line.
    <dt><dfn>''head''</dfn></dt>
      <dd>Head-aligns the box within its containing block.
    <dt><dfn>''foot''</dfn></dt>
      <dd>Foot-aligns the box within its containing block.
    <dt><dfn>''center''</dfn></dt>
      <dd>Centers the box within its containing block.
    <dt><dfn>''stretch''</dfn></dt>
      <dd>Sizes the box to exactly fit its containing block.
    <dt><dfn>''true''</dfn></dt>
      <dd>If specified, alignment is "true", and may cause the box to overflow
      the head edge of the containing block.
      Otherwise, the box is forced to head-align
      if its outer extent is longer than the containing block.
  </dl>

  <p>For flex items, if the "head" and "foot" are not in the alignment
    dimension, the "start" and "end" sides are used instead.

<h2 id="content-alignment">
Content-Alignment: Aligning the Content within the Box</h2>

  <p>The 'justify-content' and 'align-content' properties control alignment
  of the box's content within the box.

  <div class=figure>
    <p><img src=content-target.jpg
            alt="Diagram showing that the alignment of the content within the element is affected.">
  </div>

<h3 id="justify-content">
  Inline/Main-Axis Alignment: the 'justify-content' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>justify-content</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>auto | [ start | end | left | right | flex-start | flex-end | center | space-between | space-around ] && true?
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
      <td>block containers and flex 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
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <p>Gives the default alignment of the box's content along the inline/row axis.
  For block containers, this simply changes the meaning of ''justify-self: auto'' on its children.
  
831E
For flex containers, this aligns the contents of each flex line within its flex line box.
  See [[!CSS3FLEXBOX]] for exact details.

  <dl>
    <dt><dfn>''auto''</dfn></dt>
      <dd>If the element has ''display: block'' and ''justify-self: auto'',
      computes to the value inherited from the parent.
      Otherwise computes to ''start''.
    <dt><dfn>''start''</dfn></dt>
      <dd>Start-aligns the content within its box.
    <dt><dfn>''end''</dfn></dt>
      <dd>End-aligns the content within its box.
    <dt><dfn>''flex-start''</dfn></dt>
      <dd>For flex containers, aligns the items to the main-start edge.
      Otherwise equivalent to ''start''.
    <dt><dfn>''end''</dfn></dt>
      <dd>For flex containers, aligns the items to the main-end edge.
      Otherwise equivalent to ''end''.
    <dt><dfn>''left''</dfn></dt>
      <dd>Aligns content to the line-left edge.
      On a flex container, this is treated as ''auto'' if the line-left direction is in the wrong dimension.
    <dt><dfn>''right''</dfn></dt>
      <dd>Aligns content to the line-right edge.
      On a flex container, this is treated as ''auto'' if the line-right direction is in the wrong dimension.
    <dt><dfn>''center''</dfn></dt>
      <dd>Centers the content within its box
    <dt><dfn>''space-between''</dfn></dt>
      <dd>Distributes the content evenly throughout the line by adjusting inter-child spacing.
      If there is only one child (or if the element is a block container), this is equivalent to centering.
    <dt><dfn>''space-around''</dfn></dt>
      <dd>Distributes the content evenly throughout the line by adjusting inter-child spacing
      and applying half of that spacing between the first/last child and the box edges.
      If there is only one child (or if the element is a block container), this is equivalent to centering.
    <dt><dfn>''true''</dfn></dt>
      <dd>If specified, alignment is "true", and may cause the contents to overflow
      the start edge of the box.
      Otherwise, the content is forced to start-align
      if its total outer measure (before ''space-between'' or ''space-around'' is applied)
      is wider than the containing block.
  </dl>

  <p>For flex containers, if the "start" and "end" are not in the alignment
    dimension, the "head" and "foot" sides are used instead, respectively.

  <p class=note>This would, in conjunction with 'text-align', implement the HTML <code>align</code> attribute and <code>&lt;center&gt;</code>.
  The weird behavior of ''auto'' is for that and because ''display: block'' elements are essentially transparent, layout-wise,
  so passing through the alignment unchanged is useful and probably intuitive to authors (though it's really weird for implementers).

<h3 id="align-content">
  Stacking/Cross-Axis Alignment: the 'align-content' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>align-content</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>auto | [ head | foot | flex-start | flex-end | baseline | center | space-between | space-around ] && true?
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
      <td>block containers and flex 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
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <p>Aligns the box's content within the box along the block/column/cross axis.
  Values have the following meanings:
  <dl>
    <dt><dfn>''auto''</dfn></dt>
      <dd>For block containers, equivalent to ''head'',
      except that it does not cause the block to establish a block formatting context.
      For table cells, defers alignment to 'vertical-align' as specified in [[!CSS21]] chapter 17.
      For flex containers, it causes any positive free space to be distributed equally across the lines. [[!CSS3-FLEXBOX]]
    <dt><dfn>''head''</dfn></dt>
      <dd>Head-aligns the box's content within its content box:
      the content is laid out beginning at the box's head content edge.
    <dt><dfn>''center''</dfn></dt>
      <dd>Centers the box's content within its content box:
      the content is centered between the box's head and foot content edges.
    <dt><dfn>''foot''</dfn></dt>
      <dd>Foot-aligns the box's content within its content box:
      the content is laid out so that it finishes at the foot content edge.
    <dt><dfn>''flex-start''</dfn></dt>
      <dd>For flex containers, aligns the items to the cross-start edge.
      Otherwise equivalent to ''head''.
    <dt><dfn>''end''</dfn></dt>
      <dd>For flex containers, aligns the items to the cross-end edge.
      Otherwise equivalent to ''foot''.
    <dt><dfn>''baseline''</dfn></dt>
      <dd>For table cells, grid items, and flex items, aligns this elements's
      <i>first formatted line</i>&apos;s baseline, if any,
      with the same baseline in other ''baseline''-aligned cells/items in the same row.
      This is equivalent to the behavior of ''vertical-align: baseline'' on table cells;
      see [[!CSS21]] chapter 17 for details.
      If size constraints prevent the baselines from fully aligning,
      alignment is as close as possible.
      If the content's position is not fully determined by baseline alignment,
      the content is head-aligned insofar as possible while preserving the baseline alignment.
      (Content that has no first-line baseline is thus also head-aligned.)
    <dt><dfn>''space-between''</dfn></dt>
      <dd>For flex containers, distributes lines evenly by adjusting inter-line spacing.
      If there is only one line (or if the element is container of line boxes), this is equivalent to centering.
      UAs may also apply ''space-between'' to lines in a block container.
    <dt><dfn>''space-around''</dfn></dt>
      <dd>For flex containers, distributes lines evenly by adjusting inter-line spacing
      and applying half of that spacing between the first/last child and the box edges.
      If there is only one line (or if the element is not a container of line boxes), this is equivalent to centering.
      UAs may also apply ''space-between'' to lines in a block container.
    <dt><dfn>''true''</dfn></dt>
      <dd>If specified, alignment is "true", and may cause the content to overflow
      the head edge of the containing block.
      Otherwise, the content is forced to head-align
      if is longer than the box's content area,
      and will thus overflow only the foot edge.
  </dl>

  <p>All values other than ''auto'' cause a block box to establish a block formatting context
  such that its margins do not collapse with its children and floats do not intrude into its content box.

  <p>For flex containers, if the "head" and "foot" are not in the alignment
    dimension, the "start" and "end" sides are used instead, respectively.

<h2 id="default-alignment">
  Default Alignment of Child Items</h2>

  <p>The 'align-items' and 'justify-items' properties, when set on a
  flex or grid container, set the default 'align-self' and 'justify-self'
  behavior of the items contained by the element.

  <div class=figure>
    <p><img src=child-target.jpg
            alt="Diagram showing that the alignment of grid items within the element is affected.">
  </div>

<h3 id="justify-items">
  Inline/Main-Axis Alignment: the 'justify-items' property</h3>

  <p class=issue>This could be applied to grid elements and set
  the default alignment of the grid items. It mainly exists because Flexbox
  wants 'align-items'; see below.

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>justify-items</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>auto | [ start | center | end ] && true?
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
      <td>grid 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
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <p>Sets the default 'justify-self' of the grid container's items.

<h3 id="align-items">
  Stacking/Cross-Axis Alignment: the 'align-items' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>align-items</dfn>
    <tr>
      <th><a href="#values">Value</a>:
      <td>auto | [ head | foot | flex-start | flex-end | center | baseline | stretch ] && true?
    <tr>
      <th>Initial:
      <td>auto
    <tr>
      <th>Applies to:
      <td>flexbox and grid 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
    <tr>
      <th>Animatable:
      <td>no
    <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr>
  </table>

  <p>Sets the default 'align-self' of the flex container's children or the
  grid container's items.
  The ''auto'' value computes to ''stretch'' on flex and grid containers.

<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. [[!RFC211
103B4
9]]</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 CSS Box Alignment Module Level 3
  is defined for three conformance classes:
  <dl>
    <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
      style sheet</a>.
    <dt><dfn>renderer</dfn></dt>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
      that interprets the semantics of a style sheet and renders
      documents that use them.
    <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
      that writes a style sheet.
  </dl>

  <p>A style sheet is conformant to CSS Box Alignment Module Level 3
  if all of its statements that use syntax defined in this module are valid
  according to the generic CSS grammar and the individual grammars of each
  feature defined in this module.

  <p>A renderer is conformant to CSS Box Alignment Module Level 3
  if, in addition to interpreting the style sheet as defined by the
  appropriate specifications, it supports all the features defined
  by CSS Box Alignment Module Level 3 by parsing them correctly
  and rendering the document accordingly. However, the inability of a
  UA to correctly render a document due to limitations of the device
  does not make the UA non-conformant. (For example, a UA is not
  required to render color on a monochrome monitor.)

  <p>An authoring tool is conformant to CSS Box Alignment Module Level 3
  if it writes style sheets that are syntactically correct according to the
  generic CSS grammar and the individual grammars of each feature in
  this module, and meet all other conformance requirements of style sheets
  as described in this module.

<h3 id="partial">
Partial implementations</h3>

  <p>So that authors can exploit the forward-compatible parsing rules to
  assign fallback values, CSS renderers <strong>must</strong>
  treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
  as appropriate</a>) any at-rules, properties, property values, keywords,
  and other syntactic constructs for which they have no usable level of
  support. In particular, user agents <strong>must not</strong> selectively
  ignore unsupported component values and honor supported values in a single
  multi-value property declaration: if any value is considered invalid
  (as unsupported values must be), CSS requires that the entire declaration
  be ignored.</p>

<h3 id="experimental">
Experimental implementations</h3>

  <p>To avoid clashes with future CSS features, the CSS2.1 specification
  reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
  syntax</a> for proprietary and experimental extensions to CSS.

  <p>Prior to a specification reaching the Candidate Recommendation stage
  in the W3C process, all implementations of a CSS feature are considered
  experimental. The CSS Working Group recommends that implementations
  use a vendor-prefixed syntax for such features, including those in
  W3C Working Drafts. This avoids incompatibilities with future changes
  in the draft.
  </p>

<h3 id="testing">
Non-experimental implementations</h3>

  <p>Once a specification reaches the Candidate Recommendation stage,
  non-experimental implementations are possible, and implementors should
  release an unprefixed implementation of any CR-level feature they
  can demonstrate to be correctly implemented according to spec.

  <p>To establish and maintain the interoperability of CSS across
  implementations, the CSS Working Group requests that non-experimental
  CSS renderers submit an implementation report (and, if necessary, the
  testcases used for that implementation report) to the W3C before
  releasing an unprefixed implementation of any CSS features. Testcases
  submitted to W3C are subject to review and correction by the CSS
  Working Group.

  <p>Further information on submitting testcases and implementation reports
  can be found from on the CSS Working Group's website at
  <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
  Questions should be directed to the
  <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
  mailing list.

<h3 id="cr-exit-criteria">
CR exit criteria</h3>

  <p class=issue>[Change or remove the following CR exit criteria if
  the spec is not a module, but, e.g., a Note or a profile. This text was <a
  href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>

  <p>
  For this specification to be advanced to Proposed Recommendation,
  there must be at least two independent, interoperable implementations
  of each feature. Each feature may be implemented by a different set of
  products, there is no requirement that all features be implemented by
  a single product. For the purposes of this criterion, we define the
  following terms:

  <dl>
    <dt>independent <dd>each implementation must be developed by a
    different party and cannot share, reuse, or derive from code
    used by another qualifying implementation. Sections of code that
    have no bearing on the implementation of this specification are
    exempt from this requirement.

    <dt>interoperable <dd>passing the respective test case(s) in the
    official CSS test suite, or, if the implementation is not a Web
    browser, an equivalent test. Every relevant test in the test
    suite should have an equivalent test created if such a user
    agent (UA) is to be used to claim interoperability. In addition
    if such a UA is to be used to claim interoperability, then there
    must one or more additional UAs which can also pass those
    equivalent tests in the same way for the purpose of
    interoperability. The equivalent tests must be made publicly
    available for the purposes of peer review.

    <dt>implementation <dd>a user agent which:

    <ol class=inline>
      <li>implements the specification.

      <li>is available to the general public. The implementation may
      be a shipping product or other publicly available version
      (i.e., beta version, preview release, or “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>Special thanks goes to Markus Mielke, Alex Mogilevsky, and the
  participants in the CSSWG's March 2008 F2F alignment discussions.

<h2 class=no-num id="references">
References</h2>

<h3 class="no-num" id="normative-references">
Normative references</h3>
<!--normative-->

<h3 class="no-num" id="other-references">
Other references</h3>
<!--informative-->

<h2 class="no-num" id="index">
Index</h2>
<!--index-->

<h2 class="no-num" id="property-index">
Property index</h2>
<!-- properties -->

</body>
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