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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: none; }
.issue th:first-child { text-align: left !important; }
</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]/ED-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><a href="http://fantasai.inkedblade.net/style/specs/css3-align/02">http://fantasai.inkedblade.net/style/specs/css3-align/02</a>
<!--
<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 “<kbd>[css3-align] <var>… message topic …</var></kbd>”
<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, flexbox 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: …
<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 <CENTER></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 Flexbox 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:
which dimension they apply to,
and whether they control the position of the box within its parent,
or the box's content within itself.
<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 class=issue>True alignment vs. safe alignment. Maybe make safe by default, true if ''true'' is specified?
(Using that in the spec for now.) Or safe/true depending on layout model (e.g. safe for blocks, true for flexbox)?
<p>The following table summarizes the proposed alignment properties
and their relation to the alignment properties existing in other modules.
<table class=data>
<colgroup span=1></colgroup>
<colgroup span=2></colgroup>
<colgroup span=4></colgroup>
<thead>
<tr><th>Common
<th>Axis
<th>Aligns
<th>Flexbox
<th>Grid
<th>Block
<th>Cell
</thead>
<tbody>
<tr><th>'justify-outside'
<td>inline
<td rowspan=2>element within parent
<td>✘
<td>'grid-row-align'
<td>✔
<td>✘
<tr><th>'align-outside'
<td>stacking
<td>'flex-item-align'
<td>'grid-column-align'
<td>✘
<td>✘
</tbody>
<tbody>
<tr><th>'justify-inside'
<td>inline
<td rowspan=2>content within element
<td>'flex-pack'
<td>✔
<td>✔
<td>✔
<tr><th>'align-inside'
<td>stacking
<td>'flex-line-pack'
<td>✔
<td>✔
<td>✔
</tbody>
<tbody>
<tr><th>'justify-items'
<td>inline
<td rowspan=2>items inside element (controls items with ''align/justify-outside: auto'')
<td>✘
<td>✔
<td>✘
<td>✘
<tr><th>'align-items'
<td>stacking
<td>'flex-align'
<td>✔
<td>✘
<td>✘
</tbody>
<tfoot>
<tr><td colspan=0><small>✔ indicates the property can be defined to be useful in that layout model<br>
✘ indicates the property doesn't apply and is not useful in that layout model</small>
</table>
<p class=note>
The '-items' values exist due to 'flex-align'.
They don't affect the element itself.
When set on a flex container or grid container,
they specify the interpretation of any ''align/justify-outside: auto'' used on the items in the container element.
<div class=issue>
<p><strong>How to consistently and intuitively name alignment targets?</strong>
<ul>
<li>Using 'box-' for self-alignment, 'content-' for contents-alignment
<li>Using 'self-' for self-alignment, 'content-' for contents-alignment
<li>Using 'child-', 'items-', or 'default-' for setting flex-item childrens' defaults
<li>Using '-inside'/'-outside' pair, like in 'display-inside'/'display-outside', for self vs. contents
</ul>
<table class=data>
<caption>Reference: Flexbox Alignment</caption>
<col align=char charoff="|">
<col align=char charoff="-" span=2>
<thead>
<tr><th>thing|reference
<th scope=col>inline <th scope=col>stacking
</thead>
<tbody>
<tr><th scope=row>box|container
<td>— <td>flex-item-align
<tr><th scope=row>contents|box
<td>flex-pack <td>flex-line-pack
<tr><th scope=row>items|box
<td>— <td>flex-align
</tbody>
</table>
<table class=data>
<caption>Box/Content/Default</caption>
<col align=char charoff="|">
<col align=char charoff="-" span=2>
<thead>
<tr><th>thing|reference
<th scope=col>inline <th scope=col>stacking
</thead>
<tbody>
<tr><th scope=row>box|container
<td>box-justify <td>box-align
<tr><th scope=row>contents|box
<td>content-justify <td>content-align
<tr><th scope=row>items|box
<td>default-justify <td>default-align
</tbody>
</table>
<table class=data>
<caption>Self/Content/Item</caption>
<col align=char charoff="|">
<col align=char charoff="-" span=2>
<thead>
<tr><th>thing|reference
<th scope=col>inline <th scope=col>stacking
</thead>
<tbody>
<tr><th scope=row>box|container
<td>self-justify <td>self-align
<tr><th scope=row>contents|box
<td>content-justify <td>content-align
<tr><th scope=row>items|box
<td>item-justify <td>item-align
</tbody>
</table>
<table class=data>
<caption>Outside/Inside/Items</caption>
<col align=char charoff="|">
<col align=char charoff="-" span=2>
<thead>
<tr><th>thing|reference
<th scope=col>inline <th scope=col>stacking
</thead>
<tbody>
<tr><th scope=row>box|container
<td>justify-outside <td>align-inside
<tr><th scope=row>contents|box
<td>justify-inside <td>align-outside
<tr><th scope=row>items|box
<td>justify-items <td>align-items
</tbody>
</table>
</div>
<h2 id="self-alignment">
Self-Alignment: Aligning the Box within its Parent</h2>
<p>The 'justify-outside' and 'align-outside' properties control alignment
of the box within its parent.
<div class=figure>
<p><img src=self-target.jpg>
</div>
<h3 id="box-justify">
Inline/Main-Axis Alignment: the 'box-justify' property</h3>
<table class=propdef>
<tr>
<th>Name:
<td><dfn>justify-outside</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>auto | [ start | center | end ] && 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 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-inside'
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 would replace 'grid-column-align'.
<h3 id="box-align">
Stacking/Cross-Axis Alignment: the 'align-outside' property</h3>
<table class=propdef>
<tr>
<th>Name:
<td><dfn>align-outside</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>auto | [ baseline | before | after | 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 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 flexbox items, computes to alignment specified by the 'child-align'
property of the flexbox 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>''before''</dfn></dt>
<dd>Before-aligns the box within its containing block.
<dt><dfn>''after''</dfn></dt>
<dd>After-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 before edge of the containing block.
Otherwise, the box is forced to before-align
if its outer extent is longer than the containing block.
</dl>
<p>For flex items "before" and "after" are interpreted relative to the
flex flow, not the writing mode. (Is this actually what we want?
Even in the overflow-correction case?)
<p class=note>This replaces 'flex-item-align'.
<h2 id="content-alignment">
Content-Alignment: Aligning the Content within the Box</h2>
<p>The 'justify-inside' and 'align-inside' properties control alignment
of the box's content within the box.
<div class=figure>
<p><img src=content-target.jpg>
</div>
<h3 id="box-justify">
Inline/Main-Axis Alignment: the 'justify-inside' property</h3>
<table class=propdef>
<tr>
<th>Name:
<td><dfn>justify-inside</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>auto | [ start | end | left | right | center | distribute | space ] && true?
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>block containers and flexbox containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed 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 ''box-justify: auto'' on its children.
For flexbox containers, this aligns the contents of each flexbox line within its flexbox line box.
See [[!CSS3FLEXBOX]] for exact details.
<dl>
<dt><dfn>''auto''</dfn></dt>
<dd>If the element has ''display: block'' and ''box-justify: 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>''left''</dfn></dt>
<dd>Aligns content to the line-left edge.
On a flexbox 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 flexbox 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>''distribute''</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''</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 ''distribute'' or ''space'' is applied)
is wider than the containing block.
</dl>
<p class=note>This property would replace 'flex-pack'
and would, in conjunction with 'text-align', implement the HTML <code>align</code> attribute and <code><center></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).</span>
<h3 id="box-align">
Stacking/Cross-Axis Alignment: the 'align-inside' property</h3>
<table class=propdef>
<tr>
<th>Name:
<td><dfn>align-inside</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>auto | [ before | after | baseline | center | distribute | space ] && true?
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>block containers and flexbox containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed 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 ''before'',
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 flexbox containers, it causes any positive free space to be distributed equally across the lines. [[!CSS3-FLEXBOX]]
<dt><dfn>''before''</dfn></dt>
<dd>Before-aligns the box's content within its content box:
the content is laid out beginning at the box's before 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 before and after content edges.
<dt><dfn>''after''</dfn></dt>
<dd>After-aligns the box's content within its content box:
the content is laid out so that it finishes at the after content edge.
<dt><dfn>''baseline''</dfn></dt>
<dd>For table cells, grid items, and flexbox items, aligns this elements's
<i>first formatted line</i>'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 before-aligned insofar as possible while preserving the baseline alignment.
(Content that has no first-line baseline is thus also before-aligned.)
<dt><dfn>''distribute''</dfn></dt>
<dd>For flexbox 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 ''distribute'' to lines in a block container.
<dt><dfn>''space''</dfn></dt>
<dd>For flexbox 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 ''distribute'' 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 before edge of the containing block.
Otherwise, the content is forced to before-align
if is longer than the box's content area,
and will thus overflow only the after 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 flexbox containers "before" and "after" are interpreted relative to the
flex flow, not the writing mode. <span class=issue>Is this actually what we want?
Even in the overflow-correction case?</span>
<p class=note>This would replace 'flex-line-pack'.
<h2 id="default-alignment">
Default Alignment of Child Items</h2>
<p>The 'align-items' and 'justify-items' properties, when set on a
flexbox or grid container, set the default 'box-align' and 'box-justify'
behavior of the items contained by the element.
<div class=figure>
<p><img src=child-target.jpg>
</div>
<h3 id="child-justify">
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
requires 'align-items' (i.e. 'flex-align'); 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 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-outside' of the grid container's items.
<h3 id="child-align">
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 | [ before | after | 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 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-outside' of the flexbox container's children or the
grid container's items.
The ''auto'' value computes to ''stretch'' on flexbox and grid containers.
<p class=note>This property replaces 'flex-align', which sets the default 'flex-item-align' of its children.
<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 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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