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<!DOCTYPE html>
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>CSS Intrinsic & Extrinsic Sizing Module Level 3</title>
<link rel=contents href="#contents">
<link rel=index href="#index">
<link rel="stylesheet" type="text/css" href="../default.css">
<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
</head>
<div class="head">
<!--logo-->
<h1>CSS Intrinsic & Extrinsic Sizing 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-sizing-[CDATE]/</a>
-->
<dt>Latest version:
<dd><a href="[LATEST]">http://www.w3.org/TR/css3-sizing/</a>
<dt>Editor's draft:
<dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
<dt>Previous Versions:
<dd><a href="http://www.w3.org/TR/2012/WD-css3-sizing-20120927/">http://www.w3.org/TR/2012/WD-css3-sizing-20120927/</a>
<dt>Issue Tracking:</dt>
<dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&component=Sizing&resolution=---">W3C Bugzilla</a>
<dt>Feedback:
<dd><a href="mailto:www-style@w3.org?subject=%5Bcss-sizing%5D%20feedback">www-style@w3.org</a> with subject line “<kbd>[css-sizing] <var>… message topic …</var></kbd>” (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/">archives</a>)
<dt>Editors:
<dd class="hcard"><a href="http://www.xanthir.com/contact" class="fn url">Tab Atkins Jr.</a>, <span class="org">Google Inc.</span>
<dd class="hcard"><a href="http://fantasai.inkedblade.net/contact" class="fn url">Elika J. Etemad</a>, <span class="org">Mozilla</span>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id="abstract">Abstract</h2>
<p>This module extends the CSS sizing properties with keywords
that represent content-based "intrinsic" sizes
and context-based "extrinsic" sizes,
allowing CSS to more easily describe boxes that fit their content
or fit into a particular layout context.
<a href="http://www.w3.org/TR/CSS/">CSS</a> 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 layout has several different concepts of automatic sizing that are used in various layout calculations.
This section defines some more precise terminology
to help connect the layout behaviors of this spec to the calculations used in other modules,
and some new keywords for the 'width' and 'height' properties
to allow authors to assign elements the dimensions resulting from these size calculations.
<h3 id="placement">
Module interactions</h3>
<p>This module extends the 'width', 'height', 'min-width', 'min-height', 'max-width', 'max-height', and 'column-width'
features defined in [[!CSS21]] chapter 10 and in [[!CSS3COL]]
<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 <color> 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="terms">
Terminology</h2>
<dl>
<dt><dfn>size</dfn>
<dd>
A one- or two-dimensional measurement:
a <a href="http://www.w3.org/TR/css3-writing-modes/#measure"><i>measure</i></a>
and/or <a href="http://www.w3.org/TR/css3-writing-modes/#extent"><i>extent</i></a>;
alternatively a width and/or height.
<dt><dfn id="definite" title="definite|definite size">definite size</dfn>
<dd>
A size that can be determined without measuring content;
that is, a <var><length></var>,
a size of the <i>initial containing block</i>,
or a <var><percentage></var> that is resolved against a <i>definite</i> size.
Additionally, the size of the <i>containing block</i> of an absolutely positioned element is always <i>definite</i>
with respect to that element.
<dt><dfn id="indefinite" title="indefinite|indefinite size">indefinite size</dfn>
<dd>
A size that is not <i>definite</i>.
An <i>indefinite</i> <i>available size</i> is essentially infinite.
<dt><dfn id="available" title="available|available size|available measure|available extent|available space">available size</dfn>
<dd>
The space into which a box is laid out,
typically either a measurement of its <i>containing block</i>
or an infinite size.
An <i>available size</i> can alternatively be either a
<dfn>min-content constraint</dfn> or
a <dfn>max-content constraint</dfn>.
<!--
<p>
The space <i>available</i> to a box is determined by the formatting context in which it participates.
In block, table, and inline formatting contexts [[!CSS21]],
the <i>available size</i> is infinite in the block-axis dimension.
<span class="issue">What, exactly, is the available size in the inline axis??? What about other formatting models?</span>
-->
<dt><dfn>fill-available fit</dfn>
<dd>
The <i>fill-available fit</i> into a given size
is that size,
minus the element's computed margins (not collapsed, treating ''auto'' as zero),
border, and padding in the given dimension.
<p class='note'>
Note: This is the formula used to calculate the ''auto'' widths
of non-replaced blocks in normal flow in <a href="http://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
<dt><dfn title="fallback|fallback size">fallback size</dfn>
<dd>
Some sizing algorithms do not work well with an infinite size.
In these cases, the <i>fallback size</i> is used instead.
Unless otherwise specified,
this is the size of the <i>initial containing block</i>.
<dt><dfn>fill-available measure</dfn>
<dd>
Roughly, the <i>measure<i> a box would take
if it filled its <i>available measure</i>.
(See <a href="http://dev.w3.org/csswg/css3-sizing/#extrinsic-sizing">Extrinsic Size Determination</a>.)
<p class='note'>
Note: This is called the <i>available width</i> in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and computed by the rules in <a href="http://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
<dt><dfn>max-content measure</dfn>
<dd>
Roughly, the narrowest <i>measure</i> a box could take while fitting around its contents
if <em>none</em> of the soft wrap opportunities within the box were taken.
(See <a href="http://dev.w3.org/csswg/css3-sizing/#intrinsic-sizing">Intrinsic Size Determination</a>.)
<p class='note'>
Note: This is called the <i>preferred width</i> in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and the <i>maximum cell width</i> in <a href="http://www.w3.org/TR/CSS21/tables.html#auto-table-layout">CSS2.1§17.5.2.2</a>.
<dt><dfn>min-content measure</dfn>
<dd>
The narrowest <i>measure</i> a box could take
that doesn't lead to inline-dimension overflow
that could be avoided by choosing a larger <i>measure</i>.
Roughly, the <i>measure</i> that would fit around its contents
if <em>all</em> soft wrap opportunities within the box were taken.
(See <a href="http://dev.w3.org/csswg/css3-sizing/#intrinsic-sizing">Intrinsic Size Determination</a>.)
<p class='note'>
Note: This is called the <i>preferred minimum width</i> in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and the <i>minimum content width</i> in <a href="http://www.w3.org/TR/CSS21/tables.html#auto-table-layout">CSS2.1§17.5.2.2</a>.
<dt><dfn>fit-content measure</dfn>
<dd>
If the <i>available</i> measure is finite,
equal to <code>min(<i>max-content</i>, max(<i>min-content</i>, <i>fill-available</i>))</code>.
Otherwise, equal to the <i>max-content measure</i>.
<p class='note'>
Note: This is called the <i>shrink-to-fit</i> width in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and <a href="http://www.w3.org/TR/css3-multicol/#pseudo-algorithm">CSS Multi-column Layout § 3.4</a>.
<dt><dfn>fill-available extent</dfn>
<dd>
Roughly, the <i>extent</i> a box would take
if it filled its <i>available extent</i>.
(See <a href="http://dev.w3.org/csswg/css3-sizing/#intrinsic-sizing">Intrinsic Size Determination</a>.)
<dt><dfn>max-content extent</dfn>
<dd>
Roughly, the <i>extent</i> of the content after layout.
<dt><dfn>min-content extent</dfn>
<dd>
Equivalent to the <i>max-content extent</i>.
<span class="issue">Or should this be the minimum between allowed break points? It might make sense in multi-col contexts to have min-content and max-content extents be different, even if they are the same elsewhere.</span>
<dt><dfn>fit-content extent</dfn>
<dd>
Analogous to the <i>fit-content measure</i>.
<dt><dfn>max-size contribution</dfn>
<dd>
The size that a box contributes to its <i>containing block</i>'s <i>max-content size</i>.
<dt><dfn>min-size contribution</dfn>
<dd>
The size that a box contributes to its <i>containing block</i>'s <i>min-content size</i>.
<dt><dfn>max-size constraint</dfn>
<dd>
A sizing constraint imposed by the box's <i>containing block</i>
that causes it to produce its <i>max-size contribution</i>.
<dt><dfn>min-size constraint</dfn>
<dd>
A sizing constraint imposed by the box's <i>containing block</i>
that causes it to produce its <i>min-size contribution</i>.
</dl>
<h2 id='size-keywords'>
New Sizing Keywords</h2>
<h3 id='width-height-keywords'>
New Keywords for 'width' and 'height'</h3>
<table class="propdef">
<tr>
<th>Properties:
<td>'width', 'min-width', 'max-width', 'height', 'min-height', 'max-height'
<tr>
<th>New Values:
<td>''fill-available'' | ''max-content'' | ''min-content'' | ''fit-content''
<tr>
<th>Initial:
<td>as defined in [[!CSS21]] and [[!CSS3-FLEXBOX]]
<tr>
<th>Applies to:
<td>as defined in [[!CSS21]]
<tr>
<th>Inherited:
<td>as defined in [[!CSS21]]
<tr>
<th>Percentages:
<td>as defined in [[!CSS21]]
<tr>
<th>Media:
<td>as defined in [[!CSS21]]
<tr>
<th>Computed value:
<td>specified value if keyword specified, else as defined in [[!CSS21]]
</table>
<p>There are four types of automatically-determined sizes in CSS (which
are represented in the width and height properties by the keywords
defined above):
<dl>
<dt><dfn>''fill-available''</dfn>
<dd>
Use the <i>fill-available measure</i> or <i>fill-available extent</i>,
as appropriate to the writing mode.
<dt><dfn>''max-content''</dfn>
<dd>
Use the <i>max-content measure</i> or <i>max-content extent</i>,
as appropriate to the writing mode.
<dt><dfn>''min-content''</dfn>
<dd>
Use the <i>min-content measure</i> or <i>min-content extent</i>,
as appropriate to the writing mode.
<dt><dfn>''fit-content''</dfn>
<dd>
Use the <i>fit-content measure</i> or <i>fit-content extent</i>,
as appropriate to the writing mode.
<dt><dfn>''repudiate-floats''</dfn> <span class='issue'>Less stupid name?</span>
<dd>
In the inline axis,
use the larger of the min-content measure,
and the fill-available measure minus the size of any non-descendant floats in the same formatting context.
(This is similar to the behavior of a box that establishes a formatting context next to a float.)
In the block axis,
this is identical to ''auto''.
<p class='issue'>
Does this value work?
Is it needed?
</dl>
<p class="issue">
Right now all of these except ''fill-available'' mean the same thing
for extents. This may or may not be ideal.
<p class='issue'>
If the measure is ''auto'',
we could have min-content extent imply a max-content measure,
and vice versa.
<p class='note'>
Note that percentages resolved against the intrinsic sizes
(''max-content'', ''min-content'', ''fit-content'', ''repudiate-floats'')
will compute to ''auto'',
<a href="http://www.w3.org/TR/CSS21/visudet.html#the-height-property">as defined by CSS 2.1</a>. [[!CSS21]]
<h3 id='the-contain-floats-value'>
Containing Floats</h3>
<table class="propdef">
<tr>
<th>Properties:
<td>'min-width', 'min-height'
<tr>
<th>New Value:
<td>''contain-floats''
<tr>
<th>Initial:
<td>as defined in [[!CSS21]] and [[!CSS3-FLEXBOX]]
<tr>
<th>Applies to:
<td>as defined in [[!CSS21]]
<tr>
<th>Inherited:
<td>as defined in [[!CSS21]]
<tr>
<th>Percentages:
<td>as defined in [[!CSS21]]
<tr>
<th>Media:
<td>as defined in [[!CSS21]]
<tr>
<th>Computed value:
<td>specified value if keyword specified, else as defined in [[!CSS21]]
</table>
<dl>
<dt><dfn>''contain-floats''</dfn>
<dd>
Equivalent to ''min-content''
except that when applied to the extent of a block box
it forces the inner extent to be large enough to contain
the margin boxes of any floats that originate inside the block
and that participate in the same block formatting context as the block's immediate contents.
</dl>
<h3 id='column-sizing'>
Column Sizing Keywords</h3>
<table class="propdef">
<tbody>
<tr>
<th>Property:
<td>'column-width'
</tr>
<tr>
<th>New Values:
<td>''min-content'' | ''max-content'' | ''fill-available'' | ''fit-content''
</tr>
<tr>
<th>Initial:
<td>as defined in [[!CSS3COL]]
</tr>
<tr>
<th>Applies to:
<td>as defined in [[!CSS3COL]]
</tr>
<tr>
<th>Inherited:
<td>as defined in [[!CSS3COL]]
</tr>
<tr>
<th>Percentages:
<td>as defined in [[!CSS3COL]]
</tr>
<tr>
<th>Media:
<td>as defined in [[!CSS3COL]]
</tr>
<tr>
<th>Computed value:
<td>specified value if keyword specified, else as defined in [[!CSS3COL]]
</tr>
</tbody>
</table>
<p>When used as values for <a href="http://www.w3.org/TR/css3-multicol/#cw">'column-width'</a>,
the new keywords specify the optimal column width:
<dl>
<dt>''min-content''
<dd>Specifies the optimal column width as the <i>min-content measure</i>
of the multi-column element's contents.
<dt>''max-content''
<dd>Specifies the optimal column width as the <i>max-content measure</i>
of the multi-column element's contents.
<dt>''fill-available''
<dd>Specifies the optimal column width as the <i>fill-available measure</i>
of the multi-column element.
<dt>''fit-content''
<dd>Specifies the optimal column width as
<code>min(<i>max-content</i>, max(<i>min-content</i>, <i>fill-available</i>))</code>.
</dl>
<h2 id='intrinsic-sizing'>
Intrinsic Size Determination</h2>
<p>
<dfn>Intrinsic sizing</dfn> determines sizes based on the contents of an element,
without regard for its context.
<h3 id='replaced-intrinsic'>
Intrinsic Sizes of Replaced Elements</h3>
<p>
For replaced elements,
the <i>min-content</i> and <i>max-content</i> sizes are equivalent
and correspond to the appropriate dimension of the concrete object size returned by
the <a href="http://www.w3.org/TR/css3-images/#default-sizing">default sizing algorithm</a> [[!CSS3-IMAGES]]
of the element,
calculated with an unconstrained specified size.
<h3 id='inline-intrinsic'>
Intrinsic Sizes of Non-Replaced Inlines</h3>
<p>
The <i>min-content measure</i> of an inline box
is the length of the largest unbreakable sequence of inline content.
The <i>min-measure contribution</i> of an inline box
is its <i>min-content measure</i>,
plus any inline-axis margin, border, and padding
adjacent to that sequence.
<p>
The <i>max-content measure</i> of an inline box
is the length of the largest sequence of inline content
on a single line
when only forced line breaks are taken.
The <i>max-measure contribution</i> of an inline box
is its <i>max-content measure</i>,
plus any inline-axis margin, border, and padding
adjacent to that sequence.
<p>
The <i>min-content extent</i>, <i>max-content extent</i>, <i>min-extent contribution</i>, and <i>max-content contribution</i> of an inline box
are the distance from the head edge of the first line box
to the foot edge of the last line box
on which the inline appears.
<h3 id='block-intrinsic'>
Intrinsic Sizes of Non-Replaced Blocks</h3>
<p>
The <i>min-content measure</i> of a <i>block container box</i>
is the largest <i>min-measure contribution</i>
of its in-flow or floated children.
<p>
The <i>max-content measure</i> of a <i>block container box</i>
is the measure of the box after layout,
if all children are sized under a <i>max-size constraint</i>.
<p>
If the computed measure of a <i>block-level box</i>
is ''min-content'', ''max-content'', or a <i>definite size</i>,
its <i>min-measure contribution</i>
is that size
plus any inline-axis margin, border, and padding.
Otherwise, if the computed measure of the block is ''fit-content'', ''auto'', or ''fill-available'',
its <i>min-measure contribution</i>
is its <i>min-content measure</i>
plus any inline-axis margin, border, and padding.
<p>
If the computed measure of a <i>block-level box</i>
is ''min-content'', ''max-content'', or a <i>definite size</i>,
its <i>max-measure contribution</i>
is that size
plus any inline-axis margin, border, and padding.
Otherwise, if the computed measure of the block is ''fit-content'', ''auto'', or ''fill-available'',
its <i>max-measure contribution</i>
is its <i>max-content measure</i>
plus any inline-axis margin, border, and padding.
<p>
The <i>min-content extent</i> and <i>max-content extent</i> of a <i>block container box</i>
is the content extent as defined
(for horizontal writing modes)
in <a href="http://www.w3.org/TR/CSS21/visudet.html#normal-block">CSS2.1§10.6.3</a>
and <a href="http://www.w3.org/TR/CSS21/tables.html#height-layout">CSS2.1§17.5.3</a>
for elements with ''height: auto'',
and analogously for vertical writing modes.
<p>
The <i>min-extent contribution</i> and <i>max-extent contribution</i> of a <i>block-level box</i>
is the extent of the block after layout,
plus any block-axis margin, border, and padding.
<h3 id='table-intrinsic'>
Intrinsic Sizes in Table Layout</h3>
<p class='issue'>
???
<h3 id="multicol-intrinsic">
Intrinsic Sizes in Multi-column Layout</h3>
<p>
The <i>min-content measure</i> of a multi-column element
with a computed 'column-width' not ''auto''
is the smaller of
its 'column-width'
and the largest <i>min-measure contribution</i> of its contents.
<p>
The <i>min-content measure</i> of a multi-column element
with a computed 'column-width' of ''auto''
is the largest <i>min-measure contribution</i> of its contents
multiplied by its 'column-count'
(treating ''auto'' as ''1'').
<p>
The <i>max-content measure</i> of a multi-column element
with unrestrained column heights
and a computed 'column-count' not ''auto''
is its 'column-count'
multiplied by the larger of
its 'column-width' (treating ''auto'' as 0)
and the largest <i>max-measure contribution</i> of its contents.
<p>
The <i>max-content measure</i> of a multi-column element
with unrestrained column heights
and a computed 'column-count' of ''auto''
is its 'column-width'
multiplied by the number of columns obtained by taking all allowed column breaks [[CSS3-BREAK]].
<!--
The above was decided based on dholbert's example in http://lists.w3.org/Archives/Public/www-style/2012Oct/0017.html .
These definitions make multicols work well in flexbox,
both in 'stretch' and non-'stretch' cases,
without wasted space or needless overflow.
-->
<p>
The <i>max-content measure</i> of a multi-column element
with restrained-height columns (i.e. a specified 'extent' or 'max-extent')
is the <i>measure</i> that would exactly include all of its columns.
It may be approximated by:
<ul>
<li>
Laying out the element with column-spanning elements given 'display: none',
and taking a measure that includes all the columns.
<li>
Laying out all of the column-spanning elements into that resulting measure,
and taking the resulting extent.
<li>
Subtracting that resulting extent from the specified restraint,
laying out the element without column-spanning elements again into this adjusted extent,
and taking the measure of all its columns as its <i>max-content measure</i>.
</ul>
<p>
or by some more accurate method.
<p class='note'>
This approximation can result in some slack,
but avoids overflow in the most common cases,
where the balanced height of the columns above spanning elements are approximately equal.
<p class='note'>
In the common case of no column-spanning elements,
this approximation collapses to simply doing a layout,
and measuring the resulting columns.
<h2 id='extrinsic-sizing'>
Extrinsic Size Determination</h2>
<p>
<dfn>Extrinsic sizing</dfn> determines sizes based on the context of an element,
without regard for its contents.
<p>
The inner <i>fill-available measure</i> of a box is…
<ul>
<li>
If the box is the root or is absolutely-positioned, the measure of its containing block, else
<li>
<code>max(<i>min-measure</i>|0, min(<i>max-measure</i>|infinity, <i>measure</i>|<i>fill-available</i>))</code>
where the sizes are inner measures of the element establishing the box's <i>containing block</i>,
and where the first value is used if it is <i>definite</i> and the second value otherwise.
</ul>
<p>
…less the box's inline-axis margins
(after any <a href="http://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin collapsing</a>, and treating ''auto'' margins as zero),
borders, and padding.
<p>
The <i>fill-available extent</i> of a box is defined analogously,
but in the other dimension.
<p class='issue'>
This definition might end up skipping further up the ancestor chain than we'd like in some cases.
<a href="http://lists.w3.org/Archives/Public/www-style/2012Oct/0204.html">Example.</a>
Maybe it should stop at each formatting root, or something similar?
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TTTTTTTTTTT hhhhhhh hhhhhhh eeeeeeeeeeeeee EEEEEEEEEEEEEEEEEEEEEE nnnnnn nnnnnn ddddddddd ddddd
-->
<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 Intrinsic & Extrinsic Sizing 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 Intrinsic & Extrinsic Sizing 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 Intrinsic & Extrinsic Sizing Level 3
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by CSS Intrinsic & Extrinsic Sizing 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 Intrinsic & Extrinsic Sizing 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>
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 go to Aaron Gustafson, L. David Baron
for their contributions to this module.
<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>
</html>
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