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<!DOCTYPE html><html lang=en><head>
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Image Values and Replaced Content Module Level 4</title>
<link href=../default.css rel=stylesheet type=text/css>
<link href=../csslogo.ico rel="shortcut icon" type=image/x-icon>
<link href=https://www.w3.org/StyleSheets/TR/W3C-ED rel=stylesheet type=text/css>
</head>
<body class=h-entry>
<div class=head>
<p data-fill-with=logo><a class=logo href=http://www.w3.org/>
<img alt=W3C height=48 src=http://www.w3.org/Icons/w3c_home width=72>
</a></p>
<h1 class="p-name no-ref" id=title>CSS Image Values and Replaced Content Module Level 4</h1>
<h2 class="no-num no-toc no-ref" id=subtitle><span class=content>Editor's Draft,
<span class=dt-updated><span class=value-title title=20130809>9 August 2013</span></span></span></h2>
<div data-fill-with=spec-metadata><dl><dt>This version:<dd><a class=u-url href=http://dev.w3.org/csswg/css-images-4/>http://dev.w3.org/csswg/css-images-4/</a><dt>Latest version:<dd><a href=http://www.w3.org/TR/css4-images/>http://www.w3.org/TR/css4-images/</a><dt>Editor's Draft:<dd><a href=http://dev.w3.org/csswg/css-images-4/>http://dev.w3.org/csswg/css-images-4/</a><dt>Previous Versions:<dd><a href=http://www.w3.org/TR/2012/WD-css4-images-20120911/ rel=previous>http://www.w3.org/TR/2012/WD-css4-images-20120911/</a>
<dt>Feedback:</dt>
<dd><a href="mailto:www-style@w3.org?subject=%5Bcss-images%5D%20feedback">www-style@w3.org</a>
with subject line
“<kbd>[css-images] <var>… message topic …</var></kbd>”
(<a href=http://lists.w3.org/Archives/Public/www-style/ rel=discussion>archives</a>)<dt>Test Suite:<dd>None Yet<dt>Editors:
<dd class="p-author h-card vcard"><a class="p-name fn u-url url" href=http://xanthir.com/contact/>Tab Atkins Jr.</a> (<span class="p-org org">Google</span>)<dd class="p-author h-card vcard"><a class="p-name fn u-url url" href=http://fantasai.inkedblade.net/contact>fantasai</a> (<span class="p-org org">Mozilla</span>)<dt>Issue Tracking:<dd><a href=http://www.w3.org/Style/CSS/Tracker/products/27>http://www.w3.org/Style/CSS/Tracker/products/27</a></dl></div>
<div data-fill-with=warning><details class=obsolete open>
<summary>Not Ready For Implementation</summary>
<p>
This spec is not yet ready for implementation.
It exists in this repository to record the ideas and promote discussion.
<p>
Before attempting to implement this spec,
please contact the CSSWG at www-style@w3.org.
</details></div>
<p class=copyright data-fill-with=copyright><a href=http://www.w3.org/Consortium/Legal/ipr-notice#Copyright>Copyright</a> © 2013 <a href=http://www.w3.org/><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href=http://www.csail.mit.edu/><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href=http://www.ercim.eu/><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href=http://www.keio.ac.jp/>Keio</a>, <a href=http://ev.buaa.edu.cn/>Beihang</a>), All Rights Reserved. W3C <a href=http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer>liability</a>, <a href=http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks>trademark</a> and <a href=http://www.w3.org/Consortium/Legal/copyright-documents>document use</a> rules apply.</p>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref" id=abstract><span class=content>Abstract</span></h2>
<p class=p-summary data-fill-with=abstract>This module contains the features of CSS level 4 relating to the <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> type and replaced elements. It includes and extends the functionality of CSS Image Values and Replaced Content Module Level 3. The main extensions compared to level 3 are several additions to the <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> type, additions to the <a class=production data-link-type=type href=#gradient-type><var><gradient></var></a> type, extentions to the <a class=css data-link-type=maybe href=#funcdef-image>image()</a> function, definitions for interpolating several <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> types, and several properties controlling the interaction of replaced elements and CSS's layout models.
<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.</p>
<h2 class="no-num no-toc no-ref" id=status><span class=content>Status of this document</span></h2>
<div data-fill-with=status><p>
This is a public copy of the editors' draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don't cite this document other than as work in progress.
<p>
The (<a href=http://lists.w3.org/Archives/Public/www-style/>archived</a>) public mailing list
<a href="mailto:www-style@w3.org?Subject=%5Bcss-images%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>
(see <a href=http://www.w3.org/Mail/Request>instructions</a>)
is preferred for discussion of this specification.
When sending e-mail,
please put the text “css-images” in the subject,
preferably like this:
“[css-images] <em>…summary of comment…</em>”
<p>
This document was produced by the <a href=/Style/CSS/members>CSS Working Group</a>
(part of the <a href=/Style/>Style Activity</a>).
<p>
This document was produced by a group operating under
the <a href=/Consortium/Patent-Policy-20040205/>5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href=/2004/01/pp-impl/32061/status rel=disclosure>public list of any patent disclosures</a>
made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes contains <a href=/Consortium/Patent-Policy-20040205/#def-essential>Essential Claim(s)</a>
must disclose the information in accordance with <a href=/Consortium/Patent-Policy-20040205/#sec-Disclosure>section 6 of the W3C Patent Policy</a>.</div>
<div data-fill-with=at-risk></div>
<h2 class="no-num no-toc no-ref" id=contents><span class=content>Table of contents</span></h2>
<div data-fill-with=table-of-contents><ul class=toc><li><a href=#intro><span class=secno>1</span>
Introduction</a><ul class=toc><li><a href=#placement><span class=secno>1.1</span>
Module Interactions</a><li><a href=#values><span class=secno>1.2</span>
Values</a></ul><li><a href=#planned-features><span class=secno>2</span>Planned Features</a><li><a href=#image-values><span class=secno>3</span>
Image Values: the <span class=production data-link-type=type><var><image></var></span> type</a><ul class=toc><li><a href=#url-notation><span class=secno>3.1</span>
Image References and Image Slices: the <span class=production data-link-type=type><var><url></var></span> type and <span class=css data-link-type=maybe>url()</span> notation</a><li><a href=#image-notation><span class=secno>3.2</span>
Image Fallbacks and Annotations: the <span class=css data-link-type=maybe>image()</span> notation</a><ul class=toc><li><a href=#image-fragments><span class=secno>3.2.1</span>
Image Fragments</a><li><a href=#image-fallbacks><span class=secno>3.2.2</span>
Image Fallbacks</a><li><a href=#color-images><span class=secno>3.2.3</span>
Solid-color Images</a><li><a href=#bidi-images><span class=secno>3.2.4</span>
Bidi-sensitive Images</a></ul><li><a href=#image-set-notation><span class=secno>3.3</span>
Resolution Negotiation: the <span class=css data-link-type=maybe>image-set()</span> notation</a><li><a href=#element-notation><span class=secno>3.4</span>
Using Elements as Images: the <span class=css data-link-type=maybe>element()</span> notation</a><ul class=toc><li><a href=#paint-sources><span class=secno>3.4.1</span>
Paint Sources</a><li><a href=#element-cycles><span class=secno>3.4.2</span>
Cycle Detection</a></ul><li><a href=#cross-fade-function><span class=secno>3.5</span>
Combining images: the <span class=css data-link-type=maybe>cross-fade()</span> notation</a></ul><li><a href=#gradients><span class=secno>4</span>
Gradients</a><ul class=toc><li><a href=#linear-gradients><span class=secno>4.1</span>
Linear Gradients: the <span class=css data-link-type=maybe>linear-gradient()</span> notation</a><li><a href=#radial-gradients><span class=secno>4.2</span>
Radial Gradients: the <span class=css data-link-type=maybe>radial-gradient()</span> notation</a><li><a href=#conic-gradients><span class=secno>4.3</span>
Conic Gradients: the <span class=css data-link-type=maybe>conic-gradient()</span> notation</a><li><a href=#repeating-gradients><span class=secno>4.4</span>
Repeating Gradients: the <span class=css data-link-type=maybe>repeating-linear-gradient()</span>, <span class=css data-link-type=maybe>repeating-radial-gradient()</span>, and <span class=css data-link-type=maybe>repeating-conic-gradient()</span> notations</a><li><a href=#color-stop-syntax><span class=secno>4.5</span>
Gradient Color-Stops</a></ul><li><a href=#sizing><span class=secno>5</span>
Sizing Images and Objects in CSS</a><ul class=toc><li><a href=#sizing-terms><span class=secno>5.1</span>
Object-Sizing Terminology</a><li><a href=#object-negotiation><span class=secno>5.2</span>
CSS⇋Object Negotiation</a><li><a href=#concrete-size-resolution><span class=secno>5.3</span>
Concrete Object Size Resolution</a><ul class=toc><li><a href=#default-sizing><span class=secno>5.3.1</span>
Default Sizing Algorithm</a><li><a href=#cover-contain><span class=secno>5.3.2</span>
Cover and Contain Constraint Sizing</a></ul><li><a href=#object-sizing-examples><span class=secno>5.4</span>
Examples of CSS Object Sizing</a><li><a href=#the-object-fit><span class=secno>5.5</span>
Sizing Objects: the <span class=property data-link-type=propdesc title=object-fit>object-fit</span> property</a><li><a href=#the-object-position><span class=secno>5.6</span>
Positioning Objects: the <span class=property data-link-type=propdesc title=object-position>object-position</span> property</a></ul><li><a href=#image-processing><span class=secno>6</span>
Image Processing</a><ul class=toc><li><a href=#the-image-resolution><span class=secno>6.1</span>
Overriding Image Resolutions: the <span class=property data-link-type=propdesc title=image-resolution>image-resolution</span> property</a><li><a href=#the-image-orientation><span class=secno>6.2</span>
Orienting an Image on the Page: the <span class=property data-link-type=propdesc title=image-orientation>image-orientation</span> property</a><li><a href=#the-image-rendering><span class=secno>6.3</span>
Determing How To Scale an Image: the <span class=property data-link-type=propdesc title=image-rendering>image-rendering</span> property</a></ul><li><a href=#interpolation><span class=secno>7</span>
Interpolation</a><ul class=toc><li><a href=#interpolating-images><span class=secno>7.1</span>
Interpolating <span class=production data-link-type=type><var><image></var></span></a><li><a href=#interpolating-image-combinations><span class=secno>7.2</span>
Interpolating cross-fade()</a><li><a href=#interpolating-gradients><span class=secno>7.3</span>
Interpolating <span class=production data-link-type=type><var><gradient></var></span></a></ul><li><a href=#serialization><span class=secno>8</span>
Serialization</a><li><a href=#changes><span class=secno>9</span>
Changes since Images 3</a><li><a href=#conformance><span class=secno></span>
Conformance</a><ul class=toc><li><a href=#conventions><span class=secno></span>
Document conventions</a><li><a href=#conformance-classes><span class=secno></span>
Conformance classes</a><li><a href=#partial><span class=secno></span>
Partial implementations</a><li><a href=#experimental><span class=secno></span>
Experimental implementations</a><li><a href=#testing><span class=secno></span>
Non-experimental implementations</a></ul><li><a href=#references><span class=secno></span>
References</a><ul class=toc><li><a href=#normative><span class=secno></span>
Normative References</a><li><a href=#informative><span class=secno></span>
Informative References</a></ul><li><a href=#index><span class=secno></span>
Index</a><li><a href=#property-index><span class=secno></span>
Property index</a></ul></div>
<h2 data-level=1 id=intro><span class=secno>1 </span><span class=content>
Introduction</span><a class=section-link href=#intro>§</a></h2>
<p> <em>This section is not normative.</em>
<p> In CSS Levels 1 and 2,
image values, such as those used in the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-4/#background-image title=background-image>background-image</a> property,
could only be given by a single URL value.
This module introduces additional ways of representing 2D images,
for example as <a href=#image-notation>a list of URIs denoting fallbacks</a>,
as <a href=#element-image-type>a reference to an element</a>,
or as <a href=#gradients>a gradient</a>.
<p> This module also defines several properties for <a href=#image-processing>manipulating raster images</a>
and for <a href=#object-fit>sizing</a>
or <a href=#object-position>positioning</a>
replaced elements such as images within the box determined by the CSS layout algorithms.
It also defines in a generic way CSS's <a href=#sizing>sizing algorithm</a> for images and other replaced elements.
<h3 data-level=1.1 id=placement><span class=secno>1.1 </span><span class=content>
Module Interactions</span><a class=section-link href=#placement>§</a></h3>
<p> This module defines and extends the <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> value type defined in <a data-biblio-type=normative data-link-type=biblio href=#css3val title=CSS3VAL>[CSS3VAL]</a>.
Furthermore it replaces the <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> type
in the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-4/#background-image title=background-image>background-image</a> and <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-lists-3/#list-style-image title=list-style-image>list-style-image</a> definitions in CSS1 and CSS2
and adds <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> as an alternative to <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> in the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-ui-3/#content title=content>content</a> property's value.
It is presumed that CSS specifications beyond CSS2.1 will use the <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> notation
in place of <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> where 2D images are expected. (See e.g. <a data-biblio-type=informative data-link-type=biblio href=#css3bg title=CSS3BG>[CSS3BG]</a>.)
<p> Of the properties defined in this module,
only <a class=property data-link-type=propdesc href=#propdef-image-resolution title=image-resolution>image-resolution</a>
applies to <code>::first-line</code> and <code>::first-letter</code>.
<h3 data-level=1.2 id=values><span class=secno>1.2 </span><span class=content>
Values</span><a class=section-link href=#values>§</a></h3>
<p> This specification follows the <a href=http://www.w3.org/TR/CSS21/about.html#property-defs>CSS property definition conventions</a> from <a data-biblio-type=normative data-link-type=biblio href=#css21 title=CSS21>[CSS21]</a>.
Value types not defined in this specification
are defined in CSS Level 2 Revision 1 <a data-biblio-type=normative data-link-type=biblio href=#css21 title=CSS21>[CSS21]</a>.
Other CSS modules may expand the definitions of these value types:
for example <a data-biblio-type=informative data-link-type=biblio href=#css3color title=CSS3COLOR>[CSS3COLOR]</a>,
when combined with this module,
expands the definition of the <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a> value type as used in this specification.
<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/css3-values/>global keywords</a> as their property value.
For readability they has not been repeated explicitly.
<h2 data-level=2 id=planned-features><span class=secno>2 </span><span class=content>Planned Features</span><a class=section-link href=#planned-features>§</a></h2>
<ol>
<li>Resolution tagging in image()
<li>Logical keywords for specifying linear gradient direction.
<li>More interpolation modes for gradients: <a href=http://lists.w3.org/Archives/Public/www-style/2012Jan/0635.html>http://lists.w3.org/Archives/Public/www-style/2012Jan/0635.html</a>
<li>Ability to reference an SVG paint server via url (maybe as element(<a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#font-language-override-string-value><var><string></var></a>)?)
</ol>
<!-- ====================================================================== -->
<h2 data-level=3 id=image-values><span class=secno>3 </span><span class=content>
Image Values: the <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> type</span><a class=section-link href=#image-values>§</a></h2>
<p> The <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> value type denotes a 2D image. It can be a
<a href=#url-notation>url reference</a>,
<a href=#image-notation>image notation</a>,
<!-- <a href="#element-image">element reference</a>, -->
or <a href=#gradients>gradient notation</a>.
Its syntax is:
<pre class=prod><dfn data-dfn-type=type id=typedef-image><image></dfn> = <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> | image() | image-set() | element() | cross-fade() | <a class=production data-link-type=type href=#gradient-type><var><gradient></var></a></pre>
<p> An <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> can be used in many CSS properties,
including the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-4/#background-image title=background-image>background-image</a>, <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-lists-3/#list-style-image title=list-style-image>list-style-image</a>, <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-ui-3/#cursor0 title=cursor>cursor</a> properties <a data-biblio-type=normative data-link-type=biblio href=#css21 title=CSS21>[CSS21]</a>
(where it replaces the <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> component in the property's value).
<p> In some cases, an image is invalid,
such as a <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> pointing to a resource that is not a valid image format.
An <dfn data-dfn-type=dfn id=invalid-image>invalid image</dfn> is rendered as a solid-color <span class=css data-link-type=maybe>transparent</span> image with no intrinsic dimensions.
However, <a href=#invalid-image>invalid images</a> have special behavior in some contexts,
such as the <a class=css data-link-type=maybe href=#funcdef-image>image()</a> notation.
<!--
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U:::::D D:::::U R::::RRRRRR:::::R L:::::L
U:::::D D:::::U R::::R R:::::R L:::::L
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U::::::U U::::::U R::::R R:::::R L:::::L LLLLLL
U:::::::UUU:::::::U RR:::::R R:::::RLL:::::::LLLLLLLLL:::::L
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UUUUUUUUU RRRRRRRR RRRRRRRLLLLLLLLLLLLLLLLLLLLLLLL
-->
<h3 data-level=3.1 id=url-notation><span class=secno>3.1 </span><span class=content>
Image References and Image Slices: the <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> type and <span class=css data-link-type=maybe>url()</span> notation</span><a class=section-link href=#url-notation>§</a></h3>
<p> The simplest way to indicate an image is to reference an image file by URL.
This can be done with the <a href=http://www.w3.org/TR/CSS21/syndata.html#uri>>url()'' notation</a>, defined in <a data-biblio-type=normative data-link-type=biblio href=#css21 title=CSS21>[CSS21]</a>.
<div class=example>
<p> In the example below, a background image is specified with <span class=css data-link-type=maybe>url()</span>syntax:
<pre>background-image: url(wavy.png);</pre> </div>
<p> If the UA cannot download, parse, or otherwise successfully display the contents at the URL as an image,
it must be treated as an <a href=#invalid-image>invalid image</a>.
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<h3 data-level=3.2 id=image-notation><span class=secno>3.2 </span><span class=content>
Image Fallbacks and Annotations: the <a class=css data-link-type=maybe href=#funcdef-image>image()</a> notation</span><a class=section-link href=#image-notation>§</a></h3>
<p> The <a class=css data-link-type=maybe href=#funcdef-image>image()</a> function allows an author to:
<p> <ul>
<li>
use <a href=http://www.w3.org/TR/media-frags/>media fragments</a> to clip out a portion of an image
<li>
specify fallback images in case the preferred image can't be decoded or is a type that the browser doesn't recognize
<li>
use a solid color as an image
</ul>
<p> The <a class=css data-link-type=maybe href=#funcdef-image>image()</a> notation is defined as:
<pre class=prod> <dfn data-dfn-type=function id=funcdef-image>image()</dfn> = image( <a class=production data-link-type=type href=#typedef-image-tags><var><image-tags></var></a>? [ <a class=production data-link-type=type href=#typedef-image-src><var><image-src></var></a> , ]* [ <a class=production data-link-type=type href=#typedef-image-src><var><image-src></var></a> | <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a> ] )
<dfn data-dfn-type=type id=typedef-image-tags><image-tags></dfn> = [ ltr | rtl ]
<dfn data-dfn-type=type id=typedef-image-src><image-src></dfn> = [ <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> | <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#font-language-override-string-value><var><string></var></a> ]
</pre>
<p> Each <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#font-language-override-string-value><var><string></var></a> or <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> inside <a class=css data-link-type=maybe href=#funcdef-image>image()</a> represents an image,
just as if the <a href=#url-notation>>url()'' notation</a> had been used.
As usual for URLs in CSS,
relative URLs are resolved to an absolute URL
(as described in Values & Units <a data-biblio-type=normative data-link-type=biblio href=#css3val title=CSS3VAL>[CSS3VAL]</a>)
when a specified <a class=css data-link-type=maybe href=#funcdef-image>image()</a> value is computed.
<h4 data-level=3.2.1 id=image-fragments><span class=secno>3.2.1 </span><span class=content>
Image Fragments</span><a class=section-link href=#image-fragments>§</a></h4>
<p> When a URL specified in <a class=css data-link-type=maybe href=#funcdef-image>image()</a> represents a portion of a resource
(e.g. by the use of <a href=http://www.w3.org/TR/media-frags/#naming-space>media fragment identifiers</a>)
that portion is clipped out of its context and used as a standalone image.
<div class=example>
<p> For example, given the following image and CSS:
<p> <a href=images/sprites.svg>
<img alt="[9 circles, with 0 to 8 eighths filled in]" height=20 src=images/sprites.svg width=180>
</a>
<pre>background-image: image('sprites.svg#xywh=40,0,20,20')</pre>
<p> ...the background of the element will be the portion of the image that starts at (40px,0px) and is 20px wide and tall,
which is just the circle with a quarter filled in.
</div>
<p> So that authors can take advantage of CSS's forwards-compatible parsing rules to provide a fallback for image slices,
implementations that support the <a class=css data-link-type=maybe href=#funcdef-image>image()</a> notation
<em>must</em> support the <code>xywh=#,#,#,#</code> form of media fragment identifiers
for images specified via <a class=css data-link-type=maybe href=#funcdef-image>image()</a>. <a data-biblio-type=normative data-link-type=biblio href=#media-frags title=MEDIA-FRAGS>[MEDIA-FRAGS]</a>
<div class=example>
Note that image fragments can also be used with the <span class=css data-link-type=maybe>url()</span> notation.
However, a legacy UA that doesn't understand the media fragments notation
will ignore the fragment and simply display the entirety of the image.
<p> Since the <a class=css data-link-type=maybe href=#funcdef-image>image()</a> notation requires UAs to support media fragments,
authors can take advantage of CSS's forward-compatible parsing rules
to provide a fallback when using an image fragment URL:
<pre> background-image: url('swirl.png'); /* old UAs */
background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */
</pre> </div>
<h4 data-level=3.2.2 id=image-fallbacks><span class=secno>3.2.2 </span><span class=content>
Image Fallbacks</span><a class=section-link href=#image-fallbacks>§</a></h4>
<p> Multiple <a class=production data-link-type=type href=#typedef-image-src><var><image-src></var></a>s can be given separated by commas,
in which case the function represents the first image that's not an <a href=#invalid-image>invalid image</a>.
The final argument can specify a <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a> to serve as an ultimate fallback;
this can be used, e.g. for <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-4/#background-image title=background-image>background-image</a>,
to ensure adequate contrast if none of the preceding <a class=production data-link-type=type href=#typedef-image-src><var><image-src></var></a>s can be used.
If the final argument is a <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a>,
it represents a solid-color image of the given color with no <a href=#intrinsic-dimensions>intrinsic dimensions</a>.
If all of the provided <a class=production data-link-type=type href=#typedef-image-src><var><image-src></var></a>s are <a href=#invalid-image>invalid images</a>
and a fallback color was not provided as the last argument,
the entire <a class=css data-link-type=maybe href=#funcdef-image>image()</a> function must be treated as an <a href=#invalid-image>invalid image</a>.
<div class=example>
<p> The rule below would tell the UA to load "wavy.svg" if it can;
failing that to load "wavy.png";
and failing that to display "wavy.gif".
For example, the browser might not understand how to render SVG images,
and the PNG may be temporarily 404
(returning an HTML 404 page, which the browser can't decode as an image)
due to a server move,
so the GIF is used until one of the previous problems corrects itself.
<pre>background-image: image("wavy.svg", "wavy.png" , "wavy.gif");</pre> </div>
<div class=example>
<p> The fallback color can be used to ensure that text is still readable
even when the image fails to load.
For example, the following code works fine if the image is rectangular and has no transparency:
<pre> body { color: black; background: white; }
p.special { color: white; background: url("dark.png") black; }
</pre>
<p> When the image doesn't load,
the background color is still there to ensure that the white text is readable.
However, if the image has some transparency,
the black will be visible behind it,
which is probably not desired.
The <a class=css data-link-type=maybe href=#funcdef-image>image()</a> function addresses this:
<pre> body { color: black; background: white; }
p.special { color: white; background: image("dark.png", black); }
</pre>
<p> Now, the black won't show at all if the image loads,
but if for whatever reason the image fails,
it'll pop in and prevent the white text from being set against a white background.
</div>
<p> If a URL uses a fragment identifier syntax that the implementation does not understand,
or does not consider valid for that type of image,
the URL must be treated as representing an <a href=#invalid-image>invalid image</a>.
<span class=note>This error-handling is limited to image(),
and not in the definition of URL,
for legacy compat reasons.</span>
<div class=example>
<p> For example, if a future specification defined a way to refer to a specific frame of an animated GIF with a fragment identifier,
an author could write the following to get newer browsers to use the GIF's frame,
and older browsers to instead download the fallback image:
<pre>background-image: image("cat_meme.gif#frame=5", "lolcat.png");</pre> </div>
<!-- XXX SVG fragment example with spriting / fallbacks (once SVG can provide this) -->
<h4 data-level=3.2.3 id=color-images><span class=secno>3.2.3 </span><span class=content>
Solid-color Images</span><a class=section-link href=#color-images>§</a></h4>
<div class=example>
<p> At times, one may need a solid-color image for a property or function that does not accept the <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a> type directly.
The <a class=css data-link-type=maybe href=#funcdef-image>image()</a> function can be used for this:
by specifying <em>only</em> a color without any URLs,
the function immediately falls back to representing a solid-color image of the chosen color.
<pre>background-image: image(rgba(0,0,255,.5)), url("bg-image.png");</pre>
<p> In the above, the background is the image "bg-image.png",
overlaid with partially-transparent blue.
</div>
<h4 data-level=3.2.4 id=bidi-images><span class=secno>3.2.4 </span><span class=content>
Bidi-sensitive Images</span><a class=section-link href=#bidi-images>§</a></h4>
<p> Before listing any <code><image-src>s</code>,
the author may specify a directionality for the image,
similar to adding a <code>dir</code> attribute to an element in HTML.
If a directional image is used on or in an element with opposite <a href=http://www.w3.org/TR/CSS21/visuren.html#propdef-direction>direction</a>,
the image must be flipped in the inline direction
(as if it was transformed by, e.g., <code>scaleX(-1)</code>, if the inline direction is the X axis).
<p class=note> Note: Absent this declaration,
images default to no directionality at all,
and thus don't care about the directionality of the surrounding element.
<div class=example>
<p> A list may use an arrow for a bullet that points into the content.
If the list can contain both LTR and RTL text,
though, the bullet may be on the left or the right,
and an image designed to point into the text on one side will point out of the text on the other side.
This can be fixed with code like:
<pre> <ul style="list-style-image: image(ltr 'arrow.png');">
<li dir='ltr'>My bullet is on the left!</li>
<li dir='rtl'>MY BULLET IS ON THE RIGHT!</li>
</ul>
</pre>
<p> This should render something like:
<pre> ⇒ My bullet is on the left!
!THGIR EHT NO SI TELLUB YM ⇐
</pre>
<p> In LTR list items, the image will be used as-is.
In the RTL list items, however,
it will be flipped in the inline direction,
so it still points into the content.
</div>
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i::::i m:::::mmm::::::mmm:::::m aaaaaaa:::::ag:::::g g:::::g e:::::::eeeee::::::e s:::::s ssssss e:::::::eeeee::::::e t:::::t
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i::::i m::::m m::::m m::::ma::::a a:::::ag::::::g g:::::g e:::::::e ssssss s:::::s e:::::::e t:::::t tttttt
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i::::::im::::m m::::m m::::m a::::::::::aa:::a gg::::::::::::::g ee:::::::::::::e s:::::::::::ss ee:::::::::::::e tt:::::::::::tt
iiiiiiiimmmmmm mmmmmm mmmmmm aaaaaaaaaa aaaa gggggggg::::::g eeeeeeeeeeeeee sssssssssss eeeeeeeeeeeeee ttttttttttt
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<h3 data-level=3.3 id=image-set-notation><span class=secno>3.3 </span><span class=content>
Resolution Negotiation: the <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> notation</span><a class=section-link href=#image-set-notation>§</a></h3>
<p> Delivering the most appropriate image resolution for a user's device can be a difficult task.
Ideally, images should be in the same resolution as the device they're being viewed in,
which can vary between users.
However, other factors can factor into the decision of which image to send;
for example, if the user is on a slow mobile connection,
they may prefer to receive lower-res images
rather than waiting for a large proper-res image to load.
The <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> function allows an author to ignore most of these issues,
simply providing multiple resolutions of an image
and letting the UA decide which is most appropriate in a given situation.
<p class=issue>
This solution assumes that resolution is a proxy for filesize,
and therefore doesn't appropriately handle multi-resolution sets of vector images,
or mixing vector images with raster ones (e.g. for icons).
For example, use a vector for high-res,
pixel-optimized bitmap for low-res,
and same vector again for low-bandwidth (because it's much smaller, even though it's higher resolution).
<p> The syntax for <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> is:
<pre class=prod><dfn data-dfn-type=function id=funcdef-image-set>image-set()</dfn> = image-set( [ <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a>, ]* [ <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a> | <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a>] )
<dfn data-dfn-type=type id=typedef-image-set-decl><image-set-decl></dfn> = [ <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> | <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#font-language-override-string-value><var><string></var></a> ] <a class=production data-link-type=type href=#typedef-resolution><var><resolution></var></a></pre>
<p> The <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> function can not be nested inside of itself,
either directly
or indirectly
(as an argument to another <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> type).
<p class=issue>
WebKit's experimental implementation has an <span class=css data-link-type=maybe>x</span> unit
which is identical to <span class=css data-link-type=maybe>dppx</span>.
Should we add it?
<p> Each <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#font-language-override-string-value><var><string></var></a> or <a class=production data-link-type=type href=http://dev.w3.org/fxtf/masking/#lturlgt><var><url></var></a> inside <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> represents an image,
just like in <a class=css data-link-type=maybe href=#funcdef-image>image()</a>.
<p> Every <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a> in a given <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> must have a different <a class=production data-link-type=type href=#typedef-resolution><var><resolution></var></a>,
or else the function is invalid.
<p> The value that a given <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> represents is determined by
the UA first ordering the set of <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a>s
in a UA-determined order
(which should be based on the <a class=production data-link-type=type href=#typedef-resolution><var><resolution></var></a> of each <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a>,
but may include other factors that the UA deems relevant,
such as connection speed).
The function then represents the first such image that is not an <a href=#invalid-image>invalid image</a>.
<p> If a fallback color was provided,
the UA may decide at any point that the remaining <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a>s are unsuitable,
and choose to use the fallback color instead,
in which case the function represents a solid-color image of the given color with no <a href=#intrinsic-dimensions>intrinsic dimensions</a>.
<p> If <em>all</em> of the <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a>s represent <a href=#invalid-image>invalid images</a>,
then if a fallback color was provided,
the function represents a solid-color image of the given color with no <a href=#intrinsic-dimensions>intrinsic dimensions</a>;
otherwise, the function represents an <a href=#invalid-image>invalid image</a>.
<p class=issue>
HTML currently doesnt' do fallback if the first-chosen image doesn't load.
We should harmonize with HTML,
either by convincing them to do fallback,
or by removing our own fallback.
<p> If an <a class=production data-link-type=type href=#typedef-image-set-decl><var><image-set-decl></var></a> is chosen and successfully loaded,
the associated <a class=production data-link-type=type href=#typedef-resolution><var><resolution></var></a> is the image's <a href=#intrinsic-resolution>intrinsic resolution</a>.
<p class=issue>
It's been suggested that we don't need a full <a class=production data-link-type=type href=#typedef-resolution><var><resolution></var></a> here,
and should instead just use the <span class=css data-link-type=maybe>x</span> unit by itself.
This seems silly to me,
since the <span class=css data-link-type=maybe>x</span> unit is just an easier synonym for <span class=css data-link-type=maybe>dppx</span>,
but thoughts?
Beyond theoretical purity considerations,
it seems that <span class=css data-link-type=maybe>dpi</span> can be useful for use-cases involving printing,
where dpi is actually used to talk about images
and having an integer correlation between dots and pixels
is either unnecessary or strictly non-sensical
(as the screening resolution can depend on the colors being used).
<div class=example>
This example shows how to use <a class=css data-link-type=maybe href=#funcdef-image-set>image-set()</a> to provide an image in three versions:
a "normal" version,
a "high-res" version,
and an extra-high resolution version for use in high-quality printing
(as printers can have <em>extremely</em> high resolution):
<pre> background-image: image-set( "foo.png" 1x,
"foo-2x.png" 2x,
"foo-print.png" 600dpi );
</pre> </div>
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<h3 data-level=3.4 id=element-notation><span class=secno>3.4 </span><span class=content>
Using Elements as Images: the <a class=css data-link-type=maybe href=#funcdef-element>element()</a> notation</span><a class=section-link href=#element-notation>§</a></h3>
<p> The <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function allows an author to use an element in the document as an image.
As the referenced element changes appearance,
the image changes as well.
This can be used, for example,
to create live previews of the next/previous slide in a slideshow,
or to reference a canvas element for a fancy generated gradient or even an animated background.
<p class=note> Note: The <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function only reproduces the <em>appearance</em> of the referenced element,
not the actual content and its structure.
Authors should only use this for decorative purposes,
and must not use <a class=css data-link-type=maybe href=#funcdef-element>element()</a> to reproduce an element with significant content across the page.
Instead, just insert multiple copies of the element into the document.
<p> The syntax for <a class=css data-link-type=maybe href=#funcdef-element>element()</a> is:
<pre class=prod><dfn data-dfn-type=function id=funcdef-element>element()</dfn> = element( <a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#ltid-selector><var><id-selector></var></a> )</pre>
<p> where <a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#ltid-selector><var><id-selector></var></a> is an ID selector <a data-biblio-type=normative data-link-type=biblio href=#select title=SELECT>[SELECT]</a>.
<p class=issue>
Need to add some syntax + API for referring to an element that's just in script,
not in the document,
so you can, say, make an HTML <code><canvas></code> and just animate it directly.
<p class=issue>
Do we need to be able to refer to elements in external documents
(such as SVG paint servers)?
Or is it enough to just use url() for this?
<p class=issue>
This name conflicts with a somewhat similar function in GCPM.
This needs to be resolved somehow.
<p class=issue>
Want the ability to do "reflections" of an element,
either as a background-image on the element or in a pseudo-element.
This needs to be specially-handled to avoid triggering the cycle-detection.
<p class=issue>
When we have overflow:paged,
how can we address a single page in the view?
<p> The <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function references the element matched by its argument.
If multiple elements are matched,
the function references the first such element.
<p> The image represented by the <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function can vary based on whether the element is visible in the document:
<dl>
<dt>an <a href=#element-not-rendered title=element-not-rendered>element that is rendered</a> and is not a descendant of a replaced element
<dd>
<p> The function represents an image with its intrinsic size equal to the <dfn data-dfn-type=dfn id=decorated-bounding-box>decorated bounding box</dfn> of the referenced element:
<p> <ul>
<li>
for an element rendered using a CSS rendering model,
the <a href=#decorated-bounding-box>decorated bounding box</a> is the smallest axis-aligned rectangle
that contains the <a href=http://www.w3.org/TR/2011/CR-css3-background-20110215/#border-image-area>border image areas</a> of all the fragments of the principal box
<li>
for an element rendered using the SVG rendering model,
<a href=http://www.w3.org/TR/SVGTiny12/intro.html#TermDecoratedBoundingBox>the decorated bounding box is defined by SVG</a>
</ul>
<p class=note> Note: Because images clip anything outside their bounds by default,
this means that decorations that extend outside the <a href=#decorated-bounding-box>decorated bounding box</a>,
like box shadows,
may be clipped.
<p> The image is constructed by rendering the referenced element and its descendants
(at the same size that they would be in the document)
over an infinite <span class=css data-link-type=maybe>transparent</span> canvas,
positioned so that the edges of the <a href=#decorated-bounding-box>decorated bounding box</a> are flush with the edges of the image.
<p class=issue>
Requiring some degree of stacking context on the element appears to be required for an efficient implementation.
Do we need a full stacking context, or just a pseudo-stacking context?
Should it need to be a stacking context normally,
or can we just render it as a stacking context when rendering it to element()?
<p> If the referenced element has a transform applied to it or an ancestor,
the transform must be ignored when rendering the element as an image. <a data-biblio-type=normative data-link-type=biblio href=#css3-transforms title=CSS3-TRANSFORMS>[CSS3-TRANSFORMS]</a>
<p> If the referenced element is broken across pages,
the element is displayed as if the page content areas were joined flush in the pagination direction,
with pages' edges corresponding to the initial containing block's start edge aligned.
<span class=note>Elements broken across lines or columns are just rendered with their <a href=#decorated-bounding-box>decorated bounding box</a>.</span>
<p> Implementations may either re-use existing bitmap data generated for the referenced element
or regenerate the display of the element to maximize quality at the image's size
(for example, if the implementation detects that the referenced element is an SVG fragment);
in the latter case, the layout of the referenced element in the image must not be changed by the regeneration process.
That is, the image must look identical to the referenced element,
modulo rasterization quality.
<div class=example>
<p> As a somewhat silly example, a <code><p></code> element can be reused as a background elsewhere in the document:
<pre> <style>
#src { color: white; background: lime; width: 300px; height: 40px; }
#dst { color: black; background: element(#src); padding: 20px; margin: 20px 0; }
</style>
<p id='src'>I'm an ordinary element!</p>
<p id='dst'>I'm using the previous element as my background!</p>
</pre>
<p> <img alt="" src=images/element-function.png>
</div>
<dt>an <a href=#element-not-rendered title=element-not-rendered>element that is not rendered</a>, but which <a href=#paint-source title=paint-source>provides a paint source</a>
<dd>
The function represents an image with the intrinsic size and appearance of the <a href=#paint-source title=paint-source>paint source</a>.
The host language defines the size and appearance of paint sources.
<div class=example>
For example, the <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function can reference an SVG <code><pattern></code> element in an HTML document:
<pre> <!DOCTYPE html>
<svg>
<defs>
<pattern id='pattern1'>
<path d='...'>
</pattern>
</defs>
</svg>
<p style="background: element(#pattern1)">
I'm using the pattern as a background!
If the pattern is changed or animated,
my background will be updated too!
</p>
</pre>
<p> HTML also defines that a handful of elements,
such as <code><canvas></code>, <code><img></code>, and <code><video></code>,
provide a paint source.
This means that CSS can, for example,
reference a canvas that's being drawn into,
but not displayed in the page:
<pre> <!DOCTYPE html>
<script>
var canvas = document.querySelector('#animated-bullet');
canvas.width = 20; canvas.height = 20;
drawAnimation(canvas);
</script>
<canvas id='animated-bullet' style='display:none'></canvas>
<ul style="list-style-image: element(#animated-bullet);">
<li>I'm using the canvas as a bullet!</li>
<li>So am I!</li>
<li>As the canvas is changed over time with Javascript,
we'll all update our bullet image with it!</li>
</ul>
</pre> </div>
<dt>anything else
<dd>
<p> The function represents an <a href=#invalid-image>invalid image</a>.
<div class=example>
<p> For example, all of the following <a class=css data-link-type=maybe href=#funcdef-element>element()</a> uses will result in a transparent background:
<pre> <!DOCTYPE html>
<p id='one' style="display:none;">one</p>
<iframe src="http://example.com">
<p id='two'>I'm fallback content!</p>
</iframe>
<ul>
<li style="background: element(#one);">
A display:none element isn't rendered, and a P element
doesn't provide a paint source.
</li>
<li style="background: element(#two);">
The descendants of a replaced element like an IFRAME
can't be used in element() either.
</li>
<li style="background: element(#three);">
There's no element with an id of "three", so this also
gets rendered as a transparent image.
</li>
</ul>
</pre> </div>
</dl>
<p> An element is <dfn data-dfn-type=dfn id=element-not-rendered title=element-not-rendered>not rendered</dfn> if it does not have an associated box.
This can happen, for example,
if the element or an ancestor is <span class=css data-link-type=maybe>display:none</span>.
Host languages may define additional ways in which an element can be considered not rendered;
for example, in SVG,
any descendant of a <code><defs></code> element is considered to be not rendered.
<div class=example>
<p> The <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function can be put to many uses.
For example, it can be used to show a preview of the previous or next slide in a slideshow:
<pre> <!DOCTYPE html>
<script>
function navigateSlides() {
var currentSlide = ...;
document.querySelector('#prev-slide').id = '';
document.querySelector('#next-slide').id = '';
currentSlide.previousElementSibling.id = 'prev-slide';
currentSlide.nextElementSibling.id = 'next-slide';
}
</script>
<style>
#prev-preview, #next-preview {
position: fixed;
...
}
#prev-preview { background: element(#prev-slide); }
#next-preview { background: element(#next-slide); }
</style>
<a id='prev-preview'>Previous Slide</a>
<a id='next-preview'>Next Slide</a>
<section class='slide'>...</section>
<section class='slide current-slide'>...</section>
...
</pre>
<p> In this example, the <code>navigateSlides</code> function updates the ids of the next and previous slides,
which are then displayed in small floating boxes alongside the slides.
Since you can't interact with the slides through the <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function (it's just an image),
you could even use <code>click</code> handlers on the preview boxes to help navigate through the page.
</div>
<h4 data-level=3.4.1 id=paint-sources><span class=secno>3.4.1 </span><span class=content>
Paint Sources</span><a class=section-link href=#paint-sources>§</a></h4>
<p> Host languages may define that some elements provide a <dfn data-dfn-type=dfn id=paint-source title=paint-source>paint source</dfn>.
Paint sources have an intrinsic width, height, and appearance,
separate from the process of rendering,
and so may be used as images even when they're <a href=#element-not-rendered title=element-not-rendered>not rendered</a>.
<p> In HTML, the <code><img></code>, <code><video></code>, and <code><canvas></code> elements provide paint sources
(defined in each element's section in <a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/>HTML5</a>).
<p> In SVG, any element that provides a <a href=http://www.w3.org/TR/SVG/pservers.html>paint server</a> provides a paint source.
<span class=note>Note: In SVG1.1,
the <code><linearGradient></code>,
<code><radialGradient></code>,
and <code><pattern></code> elements
provide paint sources.</span>
They are drawn as described in the spec,
with the coordinate systems defined as follows:
<dl>
<dt>objectBoundingBox
<dd>
The coordinate system has its origin at the top left corner of the rectangle defined by the <a href=#concrete-object-size>concrete object size</a> that it's being drawn into,
and the same width and height as the <a href=#concrete-object-size>concrete object size</a>.
A single <a href=http://www.w3.org/TR/SVG/coords.html#Units>user coordinate</a> is the width and height of the <a href=#concrete-object-size>concrete object size</a>.
<dt>userSpaceOnUse
<dd>
The coordinate system has its origin at the top left corner of the rectangle defined by the <a href=#concrete-object-size>concrete object size</a> that it's being drawn into,
and the same width and height as the <a href=#concrete-object-size>concrete object size</a>.
<a href=http://www.w3.org/TR/SVG/coords.html#Units>User coordinates</a> are sized equivalently to the CSS <span class=css data-link-type=maybe>px</span> unit.
</dl>
<p class=note> Note: It is expected that a future version of this module will define ways to refer to paint sources in external documents,
or ones that are created solely by script and never inserted into a document at all.
<h4 data-level=3.4.2 id=element-cycles><span class=secno>3.4.2 </span><span class=content>
Cycle Detection</span><a class=section-link href=#element-cycles>§</a></h4>
<p> The <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function can produce nonsensical circular relationships,
such as an element using itself as its own background.
These relationships can be easily and reliably detected and resolved, however,
by keeping track of a dependency graph and using common cycle-detection algorithms.
<p> The dependency graph consists of edges such that:
<p> <ul>
<li>
every element depends on its children
<li>
for any element A with a property using the <a class=css data-link-type=maybe href=#funcdef-element>element()</a> function pointing to an element B,
A depends on B
<li>
if a host language defines a way for elements to refer to the rendering of other elements,
the referencing element depends on the referenced element.
For example, in SVG,
a <code><use></code> element depends on the element it referenced.
</ul>
<p> If the graph contains a cycle,
any <a class=css data-link-type=maybe href=#funcdef-element>element()</a> functions participating in the cycle are <a href=#invalid-image>invalid images</a>.
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-->
<h3 data-level=3.5 id=cross-fade-function><span class=secno>3.5 </span><span class=content>
Combining images: the <a class=css data-link-type=maybe href=#funcdef-cross-fade>cross-fade()</a> notation</span><a class=section-link href=#cross-fade-function>§</a></h3>
<p> When transitioning between images,
CSS requires a way to explicitly refer to the intermediate image
that is a combination of the start and end images.
This is accomplished with the <a class=css data-link-type=maybe href=#funcdef-cross-fade>cross-fade()</a> function,
which indicates the two images to be combined
and how far along in the transition the combination is.
<p class=note> Note: Authors can also use the <a class=css data-link-type=maybe href=#funcdef-cross-fade>cross-fade()</a> function for many simple image manipulations,
such as tinting an image with a solid color
or highlighting a particular area of the page by combining an image with a radial gradient.
<p> The syntax for <a class=css data-link-type=maybe href=#funcdef-cross-fade>cross-fade()</a> is defined as:
<pre class=prod><dfn data-dfn-type=function id=funcdef-cross-fade>cross-fade()</dfn> = cross-fade( <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#percentage-size-value><var><percentage></var></a>? <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> [, <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-images-3/#image-type><var><image></var></a> | <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a> ]? )</pre>
<p> The function represents an image generated by
combining two images.
<p> The <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#percentage-size-value><var><percentage></var></a> represents how much of the first image is retained
when it is blended with the second image.
The <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-fonts-3/#percentage-size-value><var><percentage></var></a> must be between <span class=css data-link-type=maybe>0%</span> and <span class=css data-link-type=maybe>100%</span> inclusive;
any other value is invalid.
If omitted,
it defaults to the value <span class=css data-link-type=maybe>50%</span>.
<p> If the last argument is a <a class=production data-link-type=type href=http://dev.w3.org/fxtf/filters/#ltcolorgt><var><color></var></a>,
it represents a solid-color image with the same intrinsic dimensions as the first image.
it defaults to the color <span class=css data-link-type=maybe>transparent</span>.
<p> More precisely,
given <span class=css data-link-type=maybe>cross-fade(<var>p</var> <var>A</var>, <var>B</var>)</span>,
where <var>A</var> and <var>B</var> are images
and <var>p</var> is a percentage between 0% and 100%,
the function represents an image
with width equal to <code>width<sub>A</sub> × <var>p</var> + width<sub>B</sub> × (1-<var>p</var>)</code>
and height equal to <code>height<sub>A</sub> × <var>p</var> + height<sub>B</sub> × (1-<var>p</var>)</code>.
The contents of the image must be constructed by
first scaling <var>A</var> and <var>B</var> to the size of the generated image,
then applying <code>dissolve(<var>A</var>,<var>p</var>) plus dissolve(<var>B</var>,1-<var>p</var>)</code>.
The "dissolve()" function and "plus" compositing operator are defined in the literature by Porter-Duff. <a data-biblio-type=informative data-link-type=biblio href=#porterduff title=PORTERDUFF>[PORTERDUFF]</a>
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<h2 data-level=4 id=gradients><span class=secno>4 </span><span class=content>
Gradients</span><a class=section-link href=#gradients>§</a></h2>
<p> A gradient is an image that smoothly fades from one color to another.
These are commonly used for subtle shading in background images, buttons, and many other things.
The gradient notations described in this section allow an author to specify such an image in a terse syntax,