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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>
<style>
body {
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
background-attachment: fixed;
color: black;
font-family: sans-serif;
margin: 0 auto;
max-width: 50em;
padding: 2em 1em 2em 70px;
}
:link { color: #00C; background: transparent }
:visited { color: #609; background: transparent }
a[href]:active { color: #C00; background: transparent }
a[href]:hover { background: #ffa }
a[href] img { border-style: none }
h1, h2, h3, h4, h5, h6 { text-align: left }
h1, h2, h3 { color: #005A9C; }
h1 { font: 170% sans-serif }
h2 { font: 140% sans-serif }
h3 { font: 120% sans-serif }
h4 { font: bold 100% sans-serif }
h5 { font: italic 100% sans-serif }
h6 { font: small-caps 100% sans-serif }
.hide { display: none }
div.head { margin-bottom: 1em }
div.head h1 { margin-top: 2em; clear: both }
div.head table { margin-left: 2em; margin-top: 2em }
p.copyright { font-size: small }
p.copyright small { font-size: small }
pre { margin-left: 2em }
dt { font-weight: bold }
ul.toc, ol.toc {
list-style: none;
}
</style>
</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=https://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 heading settled" id=subtitle><span class=content>Editor’s Draft,
<span class=dt-updated><span class=value-title title=20140923>23 September 2014</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-3/>http://dev.w3.org/csswg/css-images-3/</a><dt>Latest version:<dd><a href=http://www.w3.org/TR/css-images/>http://www.w3.org/TR/css-images/</a><dt>Previous Versions:<dd><a href=http://www.w3.org/TR/2012/CR-css3-images-20120417/ rel=previous>http://www.w3.org/TR/2012/CR-css3-images-20120417/</a><dd><a href=http://www.w3.org/TR/2012/WD-css3-images-20120112/ rel=previous>http://www.w3.org/TR/2012/WD-css3-images-20120112/</a><dt>Feedback:<dd><span><a href="mailto:www-style@w3.org?subject=[css-images] feedback">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>)</span><dt>Editors:<dd><div 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>)</div><dd><div 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>)</div><dt>Issue Tracking:<dd><span><a href=http://www.w3.org/Style/CSS/Tracker/products/27>http://www.w3.org/Style/CSS/Tracker/products/27</a></span></dl></div>
<div data-fill-with=warning></div>
<p class=copyright data-fill-with=copyright><a href=http://www.w3.org/Consortium/Legal/ipr-notice#Copyright>Copyright</a> © 2014 <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 heading settled" id=abstract><span class=content>Abstract</span></h2>
<div class=p-summary data-fill-with=abstract><p>This module contains the features of CSS level 3 relating to the <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></a> type and replaced elements. It includes and extends the functionality of CSS level 2 <a data-biblio-type=informative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>. The main extensions compared to CSS2.1 are the generalization of the <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#url-value title="<url>"><url></a> type to the <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></a> type, several additions to the <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></a> type, a generic sizing algorithm for images and other replaced content in CSS, definitions for interpolating several <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></a> types, and several properties controlling the interaction of replaced elements and CSS’s layout models.</p>
<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.</div>
<h2 class="no-num no-toc no-ref heading settled" 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=http://www.w3.org/Style/CSS/members>CSS Working Group</a>
(part of the <a href=http://www.w3.org/Style/>Style Activity</a>).
<p>
This document was produced by a group operating under
the <a href=http://www.w3.org/Consortium/Patent-Policy-20040205/>5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href=http://www.w3.org/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=http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential>Essential Claim(s)</a>
must disclose the information in accordance with <a href=http://www.w3.org/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 heading settled" 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> <span class=content>
Introduction</span></a><li><a href=#image-values><span class=secno>2</span> <span class=content>
Image Values: the <span class=production data-link-type=type title="<image>"><image></span> type</span></a><ul class=toc><li><a href=#url-notation><span class=secno>2.1</span> <span class=content>
Image References and Image Slices: the <span class=production data-link-type=type title="<url>"><url></span> type and <span class=css>url()</span> notation</span></a><li><a href=#image-notation><span class=secno>2.2</span> <span class=content>
Image Fallbacks and Annotations: the <span class=css data-link-type=maybe title=image()>image()</span> notation</span></a><ul class=toc><li><a href=#image-fallbacks><span class=secno>2.2.1</span> <span class=content>
Image Fallbacks</span></a><li><a href=#image-fragments><span class=secno>2.2.2</span> <span class=content>
Image Fragments</span></a><li><a href=#color-images><span class=secno>2.2.3</span> <span class=content>
Solid-color Images</span></a></ul><li><a href=#image-set-notation><span class=secno>2.3</span> <span class=content>
Resolution Negotiation: the <span class=css data-link-type=maybe title=image-set()>image-set()</span> notation</span></a><li><a href=#cross-fade-function><span class=secno>2.4</span> <span class=content>
Combining images: the <span class=css data-link-type=maybe title=cross-fade()>cross-fade()</span> notation</span></a></ul><li><a href=#gradients><span class=secno>3</span> <span class=content>
Gradients</span></a><ul class=toc><li><a href=#linear-gradients><span class=secno>3.1</span> <span class=content>
Linear Gradients: the <span class=css data-link-type=maybe title=linear-gradient()>linear-gradient()</span> notation</span></a><li><a href=#radial-gradients><span class=secno>3.2</span> <span class=content>
Radial Gradients: the <span class=css data-link-type=maybe title=radial-gradient()>radial-gradient()</span> notation</span></a><li><a href=#repeating-gradients><span class=secno>3.3</span> <span class=content>
Repeating Gradients: the <span class=css data-link-type=maybe title=repeating-linear-gradient()>repeating-linear-gradient()</span>, <span class=css data-link-type=maybe title=repeating-radial-gradient()>repeating-radial-gradient()</span>, and <span class=css>repeating-conic-gradient()</span> notations</span></a><li><a href=#color-stop-syntax><span class=secno>3.4</span> <span class=content>
Gradient Color-Stops</span></a></ul><li><a href=#sizing><span class=secno>4</span> <span class=content>
Sizing Images and Objects in CSS</span></a><ul class=toc><li><a href=#sizing-terms><span class=secno>4.1</span> <span class=content>
Object-Sizing Terminology</span></a><li><a href=#object-negotiation><span class=secno>4.2</span> <span class=content>
CSS⇋Object Negotiation</span></a><li><a href=#concrete-size-resolution><span class=secno>4.3</span> <span class=content>
Concrete Object Size Resolution</span></a><ul class=toc><li><a href=#default-sizing><span class=secno>4.3.1</span> <span class=content>
Default Sizing Algorithm</span></a><li><a href=#cover-contain><span class=secno>4.3.2</span> <span class=content>
Cover and Contain Constraint Sizing</span></a></ul><li><a href=#object-sizing-examples><span class=secno>4.4</span> <span class=content>
Examples of CSS Object Sizing</span></a><li><a href=#the-object-fit><span class=secno>4.5</span> <span class=content>
Sizing Objects: the <span class=property data-link-type=propdesc title=object-fit>object-fit</span> property</span></a><li><a href=#the-object-position><span class=secno>4.6</span> <span class=content>
Positioning Objects: the <span class=property data-link-type=propdesc title=object-position>object-position</span> property</span></a></ul><li><a href=#image-processing><span class=secno>5</span> <span class=content>
Image Processing</span></a><ul class=toc><li><a href=#the-image-resolution><span class=secno>5.1</span> <span class=content>
Overriding Image Resolutions: the <span class=property data-link-type=propdesc title=image-resolution>image-resolution</span> property</span></a><li><a href=#the-image-orientation><span class=secno>5.2</span> <span class=content>
Orienting an Image on the Page: the <span class=property data-link-type=propdesc title=image-orientation>image-orientation</span> property</span></a><li><a href=#the-image-rendering><span class=secno>5.3</span> <span class=content>
Determing How To Scale an Image: the <span class=property data-link-type=propdesc title=image-rendering>image-rendering</span> property</span></a></ul><li><a href=#interpolation><span class=secno>6</span> <span class=content>
Interpolation</span></a><ul class=toc><li><a href=#interpolating-images><span class=secno>6.1</span> <span class=content>
Interpolating <span class=production data-link-type=type title="<image>"><image></span></span></a><li><a href=#interpolating-image-combinations><span class=secno>6.2</span> <span class=content>
Interpolating cross-fade()</span></a><li><a href=#interpolating-gradients><span class=secno>6.3</span> <span class=content>
Interpolating <span class=production data-link-type=type title="<gradient>"><gradient></span></span></a></ul><li><a href=#serialization><span class=secno>7</span> <span class=content>
Serialization</span></a><li><a href=#changes><span class=secno>8</span> <span class=content>
Changes since Images 3</span></a><li><a href=#conformance><span class=secno></span> <span class=content>
Conformance</span></a><ul class=toc><li><a href=#conventions><span class=secno></span> <span class=content>
Document conventions</span></a><li><a href=#conformance-classes><span class=secno></span> <span class=content>
Conformance classes</span></a><li><a href=#partial><span class=secno></span> <span class=content>
Partial implementations</span></a><li><a href=#experimental><span class=secno></span> <span class=content>
Experimental implementations</span></a><li><a href=#testing><span class=secno></span> <span class=content>
Non-experimental implementations</span></a></ul><li><a href=#references><span class=secno></span> <span class=content>
References</span></a><ul class=toc><li><a href=#normative><span class=secno></span> <span class=content>
Normative References</span></a><li><a href=#informative><span class=secno></span> <span class=content>
Informative References</span></a></ul><li><a href=#index><span class=secno></span> <span class=content>
Index</span></a><li><a href=#property-index><span class=secno></span> <span class=content>
Property index</span></a><li><a href=#issues-index><span class=secno></span> <span class=content>Issues Index</span></a></ul></div>
<h2 class="heading settled" data-level=1 id=intro><span class=secno>1 </span><span class=content>
Introduction</span><a class=self-link href=#intro></a></h2>
<p><em>This section is not normative.</em></p>
<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-3/#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 data-section="" href=#image-notation>a URL with color fallback</a>
or as <a data-section="" href=#gradients>a gradient</a>.</p>
<p>This module also defines several properties for <a data-section="" href=#image-processing>manipulating raster images</a>
and for <a data-section="" href=#the-object-fit>sizing</a>
or <a data-section="" href=#the-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 data-section="" href=#sizing>sizing algorithm</a> for images and other replaced elements.</p>
<h2 class="heading settled" data-level=2 id=image-values><span class=secno>2 </span><span class=content>
Image Values: the <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></a> type</span><a class=self-link href=#image-values></a></h2>
<p>The <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></a> value type denotes a 2D image. It can be a
<a data-section="" href=#url-notation>url reference</a>,
<a data-section="" href=#image-notation>image notation</a>,
or <a data-section="" href=#gradients>gradient notation</a>.
Its syntax is:</p>
<pre class=prod><dfn data-dfn-type=type data-export="" id=typedef-image><image><a class=self-link href=#typedef-image></a></dfn> = <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#url-value title="<url>"><url></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=function href=#funcdef-image title=image()><image()></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=function href=#funcdef-image-set title=image-set()><image-set()></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=function href=http://dev.w3.org/csswg/css-gcpm-3/#funcdef-element title=element()><element()></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=function href=#funcdef-cross-fade title=cross-fade()><cross-fade()></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=type href=#typedef-gradient title="<gradient>"><gradient></a></pre>
<p>An <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></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-3/#background-image title=background-image>background-image</a>, <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-lists-3/#propdef-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=#biblio-css21 title=CSS21>[CSS21]</a>
(where it replaces the <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#url-value title="<url>"><url></a> component in the property’s value).</p>
<p>In some cases, an image is invalid,
such as a <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#url-value title="<url>"><url></a> pointing to a resource that is not a valid image format.
An <dfn data-dfn-type=dfn data-export="" id=invalid-image>invalid image<a class=self-link href=#invalid-image></a></dfn> is rendered as a solid-color <span class=css>transparent</span> image with no intrinsic dimensions.
However, <a data-link-type=dfn href=#invalid-image title="invalid images">invalid images</a> have special behavior in some contexts,
such as the <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> notation.</p>
<h3 class="heading settled" data-level=2.1 id=url-notation><span class=secno>2.1 </span><span class=content>
Image References and Image Slices: the <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#url-value title="<url>"><url></a> type and <span class=css>url()</span> notation</span><a class=self-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 <span class=css>url()</span> notation, defined in <a data-biblio-type=informative data-link-type=biblio href=#biblio-css3val title=css3val>[css3val]</a>.</p>
<div class=example>
In the example below, a background image is specified with <span class=css>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 data-link-type=dfn href=#invalid-image title="invalid image">invalid image</a>.</p>
<h3 class="heading settled" data-level=2.2 id=image-notation><span class=secno>2.2 </span><span class=content>
Image Fallbacks and Annotations: the <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> notation</span><a class=self-link href=#image-notation></a></h3>
<p>The <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> function allows an author to:</p>
<ul><li data-md=""><p>use <a href=http://www.w3.org/TR/media-frags/>media fragments</a> to clip out a portion of an image</p>
<li data-md=""><p>use a solid color as an image</p>
<li data-md=""><p>fallback to a solid-color image, when the image at the specified url can’t be downloaded or decoded</p>
<li data-md=""><p>automatically respect the image orientation specified in the image’s metadata</p>
</ul>
<p>The <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> notation is defined as:</p>
<pre class=prod><dfn data-dfn-type=function data-export="" id=funcdef-image>image()<a class=self-link href=#funcdef-image></a></dfn> = image( [ [ <a class=production data-link-type=type href=#typedef-image title="<image>"><image></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#string-value title="<string>"><string></a> ]<a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-opt title=?>?</a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-comma title=,>,</a> <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-color-3/#valuea-def-color title="<color>"><color></a><a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-opt title=?>?</a> ]<a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-req title=!>!</a> )
</pre>
<p>A <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#string-value title="<string>"><string></a> used in <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> represents a <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#url-value title="<url>"><url></a>.
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=#biblio-css3val title=CSS3VAL>[CSS3VAL]</a>)
when a specified <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> value is computed.</p>
<p>If the image has an orientation specified in its metadata,
such as EXIF,
the UA must rotate or flip the image to correctly orient it
as the metadata specifies.</p>
<h4 class="heading settled" data-level=2.2.1 id=image-fallbacks><span class=secno>2.2.1 </span><span class=content>
Image Fallbacks</span><a class=self-link href=#image-fallbacks></a></h4>
<p>If both a URL and a <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-color-3/#valuea-def-color title="<color>"><color></a> are specified in <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a>,
then whenever the URL represents an <a data-link-type=dfn href=#invalid-image title="invalid image">invalid image</a>,
the <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> function renders as if the URL were not specified at all;
it generates a solid-color image as specified in <a data-section="" href=#color-images>§2.2.3
Solid-color Images</a>.</p>
<div class=example>
The fallback color can be used to ensure that text is still readable
even when the image fails to load.
For example, the following legacy 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 title=image()>image()</a> function addresses this:</p>
<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.</p>
</div>
<h4 class="heading settled" data-level=2.2.2 id=image-fragments><span class=secno>2.2.2 </span><span class=content>
Image Fragments</span><a class=self-link href=#image-fragments></a></h4>
<p>When a URL specified in <a class=css data-link-type=maybe href=#funcdef-image title=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.</p>
<div class=example>
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></p>
<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.</p>
</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 title=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 title=image()>image()</a>. <a data-biblio-type=normative data-link-type=biblio href=#biblio-media-frags title=MEDIA-FRAGS>[MEDIA-FRAGS]</a></p>
<div class=example>
Note that image fragments can also be used with the <span class=css>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 title=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:</p>
<pre>background-image: url('swirl.png'); /* old UAs */
background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */
</pre>
</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 data-link-type=dfn href=#invalid-image title="invalid image">invalid image</a>.</p>
<p class=note>Note: This error-handling is limited to <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a>,
and not in the definition of URL,
for legacy compat reasons.</p>
<h4 class="heading settled" data-level=2.2.3 id=color-images><span class=secno>2.2.3 </span><span class=content>
Solid-color Images</span><a class=self-link href=#color-images></a></h4>
<p>If the <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> function is specified with only a <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-color-3/#valuea-def-color title="<color>"><color></a> argument (no URL),
it represents a solid-color image of the specified color with no intrinsic dimensions.</p>
<div class=example>
For example,
one can use this as a simple way to "tint" a background image,
by overlaying a partially-transparent color over the top of the other image:
<pre>background-image: image(rgba(0,0,255,.5)), url("bg-image.png");</pre>
<p><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-color title=background-color>background-color</a> does not work for this,
as the solid color it generates always lies <em>beneath</em> all the background images.</p>
</div>
<h3 class="heading settled" data-level=2.3 id=image-set-notation><span class=secno>2.3 </span><span class=content>
Resolution Negotiation: the <a class=css data-link-type=maybe href=#funcdef-image-set title=image-set()>image-set()</a> notation</span><a class=self-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 title=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>
<p class=issue id=issue-34671d1c><a class=self-link href=#issue-34671d1c></a>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>
<p>The syntax for <a class=css data-link-type=maybe href=#funcdef-image-set title=image-set()>image-set()</a> is:</p>
<pre class=prod><dfn data-dfn-type=function data-export="" id=funcdef-image-set>image-set()<a class=self-link href=#funcdef-image-set></a></dfn> = image-set( <a class=production data-link-type=type href=#typedef-image-set-option title="<image-set-option>"><image-set-option></a><a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-comma title=#>#</a> )
<dfn data-dfn-type=type data-export="" id=typedef-image-set-option><image-set-option><a class=self-link href=#typedef-image-set-option></a></dfn> = [ <a class=production data-link-type=type href=#typedef-image title="<image>"><image></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#string-value title="<string>"><string></a> ] <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#resolution-value title="<resolution>"><resolution></a>
</pre>
<p>The <a class=css data-link-type=maybe href=#funcdef-image-set title=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 css" data-link-type=type href=#typedef-image title="<image>"><image></a> type).</p>
<p class=issue id=issue-92756fb8><a class=self-link href=#issue-92756fb8></a>Is this restriction needed?</p>
<p>Each <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#string-value title="<string>"><string></a> inside <a class=css data-link-type=maybe href=#funcdef-image-set title=image-set()>image-set()</a> represents a <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#url-value title="<url>"><url></a>,
just like in <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a>.</p>
<p>Every <a class="production css" data-link-type=type href=#typedef-image-set-option title="<image-set-option>"><image-set-option></a> in a given <a class=css data-link-type=maybe href=#funcdef-image-set title=image-set()>image-set()</a> must have a different <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#resolution-value title="<resolution>"><resolution></a>,
or else the function is invalid.</p>
<p>UAs must make a UA-specific choice of which <a class="production css" data-link-type=type href=#typedef-image-set-option title="<image-set-option>"><image-set-option></a> to load,
based on whatever criteria they find relevant
(such as the resolution of the display,
connection speed,
etc).
The <a class=css data-link-type=maybe href=#funcdef-image-set title=image-set()>image-set()</a> then represents the image associated with the URL of that choice.
The image’s <a data-link-type=dfn href=#intrinsic-resolution title="intrinsic resolution">intrinsic resolution</a> is the resolution associated with that choice.
UAs <strong>may</strong> change which <a class="production css" data-link-type=type href=#typedef-image-set-option title="<image-set-option>"><image-set-option></a> they wish to use for a given <a class=css data-link-type=maybe href=#funcdef-image-set title=image-set()>image-set()</a>
over the lifetime of the page,
if the criteria used to determine which option to choose change significantly enough to make it worthwhile in the UA’s estimation.</p>
<div class=example>
This example shows how to use <a class=css data-link-type=maybe href=#funcdef-image-set title=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>
<h3 class="heading settled" data-level=2.4 id=cross-fade-function><span class=secno>2.4 </span><span class=content>
Combining images: the <a class=css data-link-type=maybe href=#funcdef-cross-fade title=cross-fade()>cross-fade()</a> notation</span><a class=self-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 title=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>
<p class=note>Note: Authors can also use the <a class=css data-link-type=maybe href=#funcdef-cross-fade title=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>
<p>The syntax for <a class=css data-link-type=maybe href=#funcdef-cross-fade title=cross-fade()>cross-fade()</a> is defined as:</p>
<pre class=prod><dfn data-dfn-type=function data-export="" id=funcdef-cross-fade>cross-fade()<a class=self-link href=#funcdef-cross-fade></a></dfn> = cross-fade( <a class=production data-link-type=type href=#typedef-cf-mixing-image title="<cf-mixing-image>"><cf-mixing-image></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-comma title=,>,</a> <a class=production data-link-type=type href=#typedef-cf-final-image title="<cf-final-image>"><cf-final-image></a><a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-opt title=?>?</a> )
<dfn data-dfn-type=type data-export="" id=typedef-cf-mixing-image><cf-mixing-image><a class=self-link href=#typedef-cf-mixing-image></a></dfn> = <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#percentage-value title="<percentage>"><percentage></a><a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-opt title=?>?</a> && <a class=production data-link-type=type href=#typedef-image title="<image>"><image></a>
<dfn data-dfn-type=type data-export="" id=typedef-cf-final-image><cf-final-image><a class=self-link href=#typedef-cf-final-image></a></dfn> = <a class=production data-link-type=type href=#typedef-image title="<image>"><image></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-color-3/#valuea-def-color title="<color>"><color></a>
</pre>
<p>The function represents an image generated by
combining two images.</p>
<p>The <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#percentage-value title="<percentage>"><percentage></a> represents how much of the first image is retained
when it is blended with the second image.
The <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#percentage-value title="<percentage>"><percentage></a> must be between <span class=css>0%</span> and <span class=css>100%</span> inclusive;
any other value is invalid.
If omitted,
it defaults to the value <span class=css>50%</span>.</p>
<p>If the last argument is a <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-color-3/#valuea-def-color title="<color>"><color></a>,
it represents a solid-color image with the same intrinsic dimensions as the first image
(as if it were an <a class=css data-link-type=maybe href=#funcdef-image title=image()>image()</a> function with the color as its sole argument).
If omitted,
it defaults to the color <span class=css>transparent</span>.</p>
<p>More precisely,
given <span class=css>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=#biblio-porterduff title=PORTERDUFF>[PORTERDUFF]</a></p>
<h2 class="heading settled" data-level=3 id=gradients><span class=secno>3 </span><span class=content>
Gradients</span><a class=self-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,
so that the UA can generate the image automatically when rendering the page.
The syntax of a <a class="production css" data-link-type=type href=#typedef-gradient title="<gradient>"><gradient></a> is:</p>
<pre class=prod><dfn data-dfn-type=type data-export="" id=typedef-gradient><gradient><a class=self-link href=#typedef-gradient></a></dfn> =
<a class=production data-link-type=function href=#funcdef-linear-gradient title=linear-gradient()><linear-gradient()></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=function href=#funcdef-repeating-linear-gradient title=repeating-linear-gradient()><repeating-linear-gradient()></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a>
<a class=production data-link-type=function href=#funcdef-radial-gradient title=radial-gradient()><radial-gradient()></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> <a class=production data-link-type=function href=#funcdef-repeating-radial-gradient title=repeating-radial-gradient()><repeating-radial-gradient()></a>
</pre>
<div class=example>
<p>As with the other <a class="production css" data-link-type=type href=#typedef-image title="<image>"><image></a> types defined in this specification,
gradients can be used in any property that accepts images.
For example:</p>
<ul>
<li><code>background: linear-gradient(white, gray);</code>
<li><code>list-style-image: radial-gradient(circle, #006, #00a 90%, #0000af 100%, white 100%)</code>
</ul>
</div>
<p>A gradient is drawn into a box with the dimensions of the <a data-link-type=dfn href=#concrete-object-size title="concrete object size">concrete object size</a>,
referred to as the <dfn data-dfn-type=dfn data-export="" id=gradient-box>gradient box<a class=self-link href=#gradient-box></a></dfn>.
However, the gradient itself has no <a data-link-type=dfn href=#intrinsic-dimensions title="intrinsic dimensions">intrinsic dimensions</a>.</p>
<div class=example>
For example, if you use a gradient as a background,
by default the gradient will draw into a <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> the size of the element’s padding box.
If <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-size title=background-size>background-size</a> is explicitly set to a value such as <span class=css>100px 200px</span>,
then the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> will be 100px wide and 200px tall.
Similarly, for a gradient used as a <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-lists-3/#propdef-list-style-image title=list-style-image>list-style-image</a>,
the box would be a 1em square,
which is the <a data-link-type=dfn href=#default-object-size title="default object size">default object size</a> for that property.
</div>
<p>Gradients are specified by defining the <dfn data-dfn-type=dfn data-noexport="" id=starting-point>starting point<a class=self-link href=#starting-point></a></dfn> and <dfn data-dfn-type=dfn data-noexport="" id=ending-point>ending point<a class=self-link href=#ending-point></a></dfn>
of a <dfn data-dfn-type=dfn data-export="" id=gradient-line>gradient line<a class=self-link href=#gradient-line></a></dfn>
(which, depending on the type of gradient,
may be technically a line, or a ray, or a spiral),
and then specifying colors at points along this line.
The colors are smoothly blended to fill in the rest of the line,
and then each type of gradient defines how to use the color of the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> to produce the actual gradient.</p>
<h3 class="heading settled" data-level=3.1 id=linear-gradients><span class=secno>3.1 </span><span class=content>
Linear Gradients: the <a class=css data-link-type=maybe href=#funcdef-linear-gradient title=linear-gradient()>linear-gradient()</a> notation</span><a class=self-link href=#linear-gradients></a></h3>
<p>A linear gradient is created by specifying a straight <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>,
and then several colors placed along that line.
The image is constructed by creating an infinite canvas
and painting it with lines perpendicular to the gradient line,
with the color of the painted line being the color of the gradient line where the two intersect.
This produces a smooth fade from each color to the next,
progressing in the specified direction.</p>
<h4 class="no-toc heading settled" data-level=3.1.1 id=linear-gradient-syntax><span class=secno>3.1.1 </span><span class=content>
linear-gradient() syntax</span><a class=self-link href=#linear-gradient-syntax></a></h4>
<p>The linear gradient syntax is:</p>
<pre class=prod><dfn data-dfn-type=function data-export="" id=funcdef-linear-gradient>linear-gradient()<a class=self-link href=#funcdef-linear-gradient></a></dfn> = linear-gradient(
[ <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#angle-value title="<angle>"><angle></a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> to <a class=production data-link-type=type href=#typedef-side-or-corner title="<side-or-corner>"><side-or-corner></a> ]<a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#mult-opt title=?>?</a> <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-comma title=,>,</a>
<a class=production data-link-type=type href=#typedef-color-stop-list title="<color-stop-list>"><color-stop-list></a>
)
<dfn data-dfn-type=type data-export="" id=typedef-side-or-corner><side-or-corner><a class=self-link href=#typedef-side-or-corner></a></dfn> = [left <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> right] <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-any title=||>||</a> [top <a class=prod-punc data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one title=|>|</a> bottom]
</pre>
<p>The first argument to the function specifies the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>,
which gives the gradient a direction and determines how color-stops are positioned.
It may be omitted;
if so, it defaults to <span class=css>to bottom</span>.</p>
<p>The <a data-link-type=dfn href=#gradient-line title="gradient line’s">gradient line’s</a> direction may be specified in two ways:</p>
<dl>
<dt>using angles
<dd>
For the purpose of this argument,
<span class=css>0deg</span> points upward,
and positive angles represent clockwise rotation,
so <span class=css>90deg</span> point toward the right.
<dt>using keywords
<dd>
If the argument is <span class=css>to top</span>, <span class=css>to right</span>, <span class=css>to bottom</span>, or <span class=css>to left</span>,
the angle of the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> is <span class=css>0deg</span>, <span class=css>90deg</span>, <span class=css>180deg</span>, or <span class=css>270deg</span>, respectively.
<p>If the argument instead specifies a corner of the box such as <span class=css>to top left</span>,
the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> must be angled such that it points into the same quadrant as the specified corner,
and is perpendicular to a line intersecting the two neighboring corners of the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a>.
<span class=note>This causes a color-stop at 50% to intersect the two neighboring corners (see <a href=#corner-gradient-example>example</a>).</span></p>
</dl>
<p>Starting from the center of the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a>,
extend a line at the specified angle in both directions.
The ending point is the point on the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>
where a line drawn perpendicular to the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>
would intersect the corner of the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> in the specified direction.
The starting point is determined identically, but in the opposite direction.</p>
<div class=note>
Given:
<ul>
<li><var>A</var> the angle (in any quadrant) defining the gradient line’s direction such that 0 degrees points upwards and positive angles represent clockwise rotation,
<li><var>W</var> the width of the gradient box,
<li><var>H</var> the height of the gradient box,
</ul>
<p>The length of the gradient line (between the <a data-link-type=dfn href=#starting-point title="starting point">starting point</a> and <a data-link-type=dfn href=#ending-point title="ending point">ending point</a>) is:</p>
<p><code>abs(<var>W</var> * sin(<var>A</var>)) + abs(<var>H</var> * cos(<var>A</var>))</code></p>
</div>
<p class=note>Note: It is expected that the next level of this module will provide the ability to define the gradient’s direction relative to the current text direction and writing-mode.</p>
<div class=example>
<div style="overflow: hidden">
<img alt="[An image showing a box with a background shading gradually from white in the bottom-left corner to black in the top-right corner. There is a line, illustrating the gradient line, angled at 45 degrees and passing through the center of the box. The starting point and ending point of the gradient line are indicated by the intersection of the gradient line with two additional lines that pass through the bottom-left and top-right corners of the box.]" src=images/gradient-diagram.png style="float: right; margin-left: 1em;">
<p>This example illustrates visually how to calculate the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> from the rules above.
This shows the starting and ending point of the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>,
long with the actual gradient,
produced by an element with <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background title=background>background: linear-gradient(45deg, white, black);</a>.</p>
<p>Notice how, though the starting point and ending point are outside of the box,
they’re positioned precisely right so that the gradient is pure white <em>exactly</em> at the corner,
and pure black <em>exactly</em> at the opposite corner.
That’s intentional, and will always be true for linear gradients.</p>
</div>
</div>
<div class=note>
Given:
<ul>
<li>
<var>A</var> the angle defining the gradient line’s direction
such that 0 degrees points upwards and positive angles represent clockwise rotation,
<li><var>W</var> the width of the gradient box,
<li><var>H</var> the height of the gradient box,
</ul>
<p>The length of the gradient line (between the starting and ending point) is:
<code>abs(W * sin(A)) + abs(H * cos(A))</code></p>
</div>
<p>The gradient’s color stops are typically placed between the starting point and ending point on the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>,
but this isn’t required -
the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> extends infinitely in both directions.
The starting point and ending point are merely arbitrary location markers -
the starting point defines where 0%, 0px, etc are located when specifying color-stops,
and the ending point defines where 100% is located.
Color-stops are allowed to have positions before 0% or after 100%.</p>
<p>The color of the gradient at any point is determined
by finding the unique line passing through that point that is perpendicular to the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>.
The point’s color is the color of the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a>
at the point where this line intersects it.</p>
<h4 class="no-toc heading settled" data-level=3.1.2 id=linear-gradient-examples><span class=secno>3.1.2 </span><span class=content>
Linear Gradient Examples</span><a class=self-link href=#linear-gradient-examples></a></h4>
<p>All of the following <a class=css data-link-type=maybe href=#funcdef-linear-gradient title=linear-gradient()>linear-gradient()</a> examples are presumed to be backgrounds applied to a box that is 200px wide and 100px tall.</p>
<div class=example>
Below are various ways of specifying a basic vertical gradient:
<pre>linear-gradient(yellow, blue);
linear-gradient(to bottom, yellow, blue);
linear-gradient(180deg, yellow, blue);
linear-gradient(to top, blue, yellow);
linear-gradient(to bottom, yellow 0%, blue 100%);
</pre>
<p><img alt="" src=images/linear1.png></p>
</div>
<div class=example>
This demonstrates the use of an angle in the gradient.
Note that, though the angle is not exactly the same as the angle between the corners,
the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> is still sized so as to make the gradient yellow exactly at the upper-left corner,
and blue exactly at the lower-right corner.
<pre>linear-gradient(135deg, yellow, blue);
linear-gradient(-45deg, blue, yellow);
</pre>
<p><img alt="" src=images/linear3.png></p>
</div>
<div class=example>
<p>This demonstrates a 3-color gradient, and how to specify the location of a stop explicitly:</p>
<pre>linear-gradient(yellow, blue 20%, #0f0);</pre>
<p><img alt="" src=images/linear4.png></p>
</div>
<div class=example id=corner-gradient-example><a class=self-link href=#corner-gradient-example></a>
This demonstrates a corner-to-corner gradient specified with keywords.
Note how the gradient is red and blue exactly in the bottom-left and top-right corners, respectively,
exactly like the second example.
Additionally, the angle of the gradient is automatically computed so that the color at 50% (in this case, white)
stretches across the top-left and bottom-right corners.
<pre>linear-gradient(to top right, red, white, blue)</pre>
<object data=images/gradient1.svg height=100 width=200>(Image requires SVG)</object>
</div>
<h3 class="heading settled" data-level=3.2 id=radial-gradients><span class=secno>3.2 </span><span class=content>
Radial Gradients: the <a class=css data-link-type=maybe href=#funcdef-radial-gradient title=radial-gradient()>radial-gradient()</a> notation</span><a class=self-link href=#radial-gradients></a></h3>
<p>In a radial gradient,
rather than colors smoothly fading from one side of the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> to the other
as with linear gradients,
they instead emerge from a single point
and smoothly spread outward in a circular or elliptical shape.</p>
<p>A radial gradient is specified by indicating the center of the gradient
(where the 0% ellipse will be)
and the size and shape of the <dfn data-dfn-type=dfn data-noexport="" id=ending-shape>ending shape<a class=self-link href=#ending-shape></a></dfn>
(the 100% ellipse).
Color stops are given as a list,
just as for <a class=css data-link-type=maybe href=#funcdef-linear-gradient title=linear-gradient()>linear-gradient()</a>.
Starting from the <a data-link-type=dfn href=#gradient-center title="gradient center">gradient center</a>
and progressing towards (and potentially beyond) the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a>
uniformly-scaled concentric ellipses are drawn and colored
according to the specified color stops.</p>
<h4 class="no-toc heading settled" data-level=3.2.1 id=radial-gradient-syntax><span class=secno>3.2.1 </span><span class=content>
radial-gradient() Syntax</span><a class=self-link href=#radial-gradient-syntax></a></h4>
<p>The radial gradient syntax is:</p>
<pre><dfn data-dfn-type=function data-export="" id=funcdef-radial-gradient>radial-gradient()<a class=self-link href=#funcdef-radial-gradient></a></dfn> = radial-gradient(
[ <a class=production data-link-type=type title="<ending-shape>"><ending-shape></a> || <a class=production data-link-type=type href=#typedef-size title="<size>"><size></a> ]? [ at <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a> ]? ,
<a class=production data-link-type=type href=#typedef-color-stop-list title="<color-stop-list>"><color-stop-list></a>
)
</pre>
<div class=example>
Here is an example of a circular radial gradient 5em wide
and positioned with its center in the top left corner:
<pre>radial-gradient(5em circle at top left, yellow, blue)</pre>
</div>
<p class=issue id=issue-403592eb><a class=self-link href=#issue-403592eb></a>
We should add the ability to move the focus of the gradient,
as in the original -webkit-gradient() function.
See proposal in <a href=http://lists.w3.org/Archives/Public/www-style/2011Nov/0210.html>http://lists.w3.org/Archives/Public/www-style/2011Nov/0210.html</a>
for "from <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a>" and "from offset <a class="production css" data-link-type=type title="<offset>"><offset></a>".
<p>The arguments are defined as follows:</p>
<dl data-dfn-for="radial-gradient() repeating-radial-gradient()" data-dfn-type=value>
<dt><dfn class=css data-dfn-for="radial-gradient() repeating-radial-gradient()" data-dfn-type=value data-export="" id=valdef-radial-gradient-repeating-radial-gradient-position><a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a><a class=self-link href=#valdef-radial-gradient-repeating-radial-gradient-position></a></dfn>
<dd>
Determines the <dfn data-dfn-type=dfn data-noexport="" id=gradient-center>gradient center<a class=self-link href=#gradient-center></a></dfn> of the gradient.
The <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a> value type
(which is also used for <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#background-position title=background-position>background-position</a>)
is defined in <a data-biblio-type=normative data-link-type=biblio href=#biblio-css3val title=CSS3VAL>[CSS3VAL]</a>,
and is resolved using the center-point as the object area
and the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> as the positioning area.
If this argument is omitted,
it defaults to <span class=css>center</span>.
<dt><dfn class=css data-dfn-for="radial-gradient() repeating-radial-gradient()" data-dfn-type=value data-export="" id=valdef-radial-gradient-repeating-radial-gradient-ending-shape><a class="production css" data-link-type=type title="<ending-shape>"><ending-shape></a><a class=self-link href=#valdef-radial-gradient-repeating-radial-gradient-ending-shape></a></dfn>
<dd>
Can be either <span class=css>circle</span> or <span class=css>ellipse</span>;
determines whether the gradient’s <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is a circle or an ellipse, respectively.
If <a class="production css" data-link-type=type title="<ending-shape>"><ending-shape></a> is omitted,
the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> defaults to a circle
if the <a class="production css" data-link-type=type href=#typedef-size title="<size>"><size></a> is a single <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#length-value title="<length>"><length></a>,
and to an ellipse otherwise.
<dt><dfn class=css data-dfn-for="radial-gradient() repeating-radial-gradient()" data-dfn-type=value data-export="" id=valdef-radial-gradient-repeating-radial-gradient-size><a class="production css" data-link-type=type href=#typedef-size title="<size>"><size></a><a class=self-link href=#valdef-radial-gradient-repeating-radial-gradient-size></a></dfn>
<dd>
Determines the size of the gradient’s <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a>.
If omitted it defaults to <a class=css data-link-type=maybe href=#valdef-size-farthest-corner title=farthest-corner>farthest-corner</a>.
It can be given explicitly or by keyword.
For the purpose of the keyword definitions,
consider the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> edges as extending infinitely in both directions,
rather than being finite line segments.
<p>If the ending-shape is an ellipse,
its axises are aligned with the horizontal and vertical axises.</p>
<p>Both <span class=css>circle</span> and <span class=css>ellipse</span> gradients accept the following keywords as their <dfn class=css data-dfn-for="" data-dfn-type=type data-export="" id=typedef-size><size><a class=self-link href=#typedef-size></a></dfn>:</p>
<dl data-dfn-for="<size>">
<dt><dfn class=css data-dfn-for="<size>" data-dfn-type=value data-export="" id=valdef-size-closest-side>closest-side<a class=self-link href=#valdef-size-closest-side></a></dfn>
<dd>
The <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is sized so that
it exactly meets the side of the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> closest to the gradient’s center.
If the shape is an ellipse,
it exactly meets the closest side in each dimension.
<dt><dfn class=css data-dfn-for="<size>" data-dfn-type=value data-export="" id=valdef-size-farthest-side>farthest-side<a class=self-link href=#valdef-size-farthest-side></a></dfn>
<dd>
Same as <a class=css data-link-type=maybe href=#valdef-size-closest-side title=closest-side>closest-side</a>,
except the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is sized based on the farthest side(s).
<dt><dfn class=css data-dfn-for="<size>" data-dfn-type=value data-export="" id=valdef-size-closest-corner>closest-corner<a class=self-link href=#valdef-size-closest-corner></a></dfn>
<dd>
The <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is sized so that
it passes through the corner of the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> closest to the gradient’s center.
If the shape is an ellipse,
the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is given the same aspect-ratio it would have if <a class=css data-link-type=maybe href=#valdef-size-closest-side title=closest-side>closest-side</a> were specified.
<dt><dfn class=css data-dfn-for="<size>" data-dfn-type=value data-export="" id=valdef-size-farthest-corner>farthest-corner<a class=self-link href=#valdef-size-farthest-corner></a></dfn>
<dd>
Same as <a class=css data-link-type=maybe href=#valdef-size-closest-corner title=closest-corner>closest-corner</a>,
except the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is sized based on the farthest corner.
If the shape is an ellipse,
the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is given the same aspect ratio it would have if <a class=css data-link-type=maybe href=#valdef-size-farthest-side title=farthest-side>farthest-side</a> were specified.
</dl>
<p>If <a class="production css" data-link-type=type title="<ending-shape>"><ending-shape></a> is specified as <span class=css>circle</span> or is omitted,
the <a class="production css" data-link-type=type href=#typedef-size title="<size>"><size></a> may be given explicitly as:</p>
<dl>
<dt><dfn class=css data-dfn-for="<size>" data-dfn-type=value data-export="" id=valdef-size-length><length><a class=self-link href=#valdef-size-length></a></dfn>
<dd>
Gives the radius of the circle explicitly.
Negative values are invalid.
<p class=note>Note: Percentages are <em>not</em> allowed here;
they can only be used to specify the size of an elliptical gradient,
not a circular one.
This restriction exists because there is are multiple reasonable answers as to which dimension the percentage should be relative to.
A future level of this module may provide the ability to size circles with percentages,
perhaps with more explicit controls over which dimension is used.</p>
</dl>
<p>If <a class="production css" data-link-type=type title="<ending-shape>"><ending-shape></a> is specified as <span class=css>ellipse</span> or is omitted,
<a class="production css" data-link-type=type href=#typedef-size title="<size>"><size></a> may instead be given explicitly as:</p>
<dl>
<dt><dfn class=css data-dfn-for="<size>" data-dfn-type=value data-export="" id=valdef-size--length>[ <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#length-value title="<length>"><length></a> | <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#percentage-value title="<percentage>"><percentage></a> ]{2}<a class=self-link href=#valdef-size--length></a></dfn>
<dd>
Gives the size of the ellipse explicitly.
The first value represents the horizontal radius,
the second the vertical radius.
Percentages values are relative to the corresponding dimension of the <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a>.
Negative values are invalid.
</dl>
</dl>
<div class=note>
Expanded with the above definitions,
the grammar becomes:
<pre>radial-gradient() = radial-gradient(
[ [ circle || <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#length-value title="<length>"><length></a> ] [ at <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a> ]? , |
[ ellipse || [ <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#length-value title="<length>"><length></a> | <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#percentage-value title="<percentage>"><percentage></a> ]{2} ] [ at <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a> ]? , |
[ [ circle | ellipse ] || <a class=production data-link-type=type href=#typedef-extent-keyword title="<extent-keyword>"><extent-keyword></a> ] [ at <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a> ]? , |
at <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-backgrounds-3/#position title="<position>"><position></a> ,
]?
<a class=production data-link-type=type href=#typedef-color-stop title="<color-stop>"><color-stop></a> [ , <a class=production data-link-type=type href=#typedef-color-stop title="<color-stop>"><color-stop></a> ]+
)
<dfn data-dfn-type=type data-noexport="" id=typedef-extent-keyword><extent-keyword><a class=self-link href=#typedef-extent-keyword></a></dfn> = closest-corner | closest-side | farthest-corner | farthest-side
</pre>
</div>
<h4 class="no-toc heading settled" data-level=3.2.2 id=radial-color-stops><span class=secno>3.2.2 </span><span class=content>
Placing Color Stops</span><a class=self-link href=#radial-color-stops></a></h4>
<p>Color-stops are placed on a <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> shaped like a ray
(a line that starts at one point, and extends infinitely in a one direction),
similar to the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> of linear gradients.
The <a data-link-type=dfn href=#gradient-line title="gradient line’s">gradient line’s</a> <a data-link-type=dfn href=#starting-point title="starting point">starting point</a> is at the center of the gradient,
and it extends toward the right,
with the <a data-link-type=dfn href=#ending-point title="ending point">ending point</a> on the point where the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> intersects the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a>.
A color-stop can be placed at a location before 0%;
though the negative region of the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> is never directly consulted for rendering,
color stops placed there can affect the color of non-negative locations on the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> through interpolation or repetition
(see <a data-section="" href=#repeating-gradients>repeating gradients</a>).
For example, <span class=css>radial-gradient(red -50px, yellow 100px)</span> produces an elliptical gradient
that starts with a reddish-orange color in the center
(specifically, #f50)
and transitions to yellow.
Locations greater than 100% simply specify a location a correspondingly greater distance from the center of the gradient.</p>
<p>The color of the gradient at any point is determined by
first finding the unique ellipse passing through that point
with the same center, orientation, and ratio between major and minor axises as the ending-shape.
The point’s color is then the color of the positive section of the <a data-link-type=dfn href=#gradient-line title="gradient line">gradient line</a> at the location where this ellipse intersects it.</p>
<h4 class="no-toc heading settled" data-level=3.2.3 id=degenerate-radials><span class=secno>3.2.3 </span><span class=content>
Degenerate Radial Gradients</span><a class=self-link href=#degenerate-radials></a></h4>
<p>Some combinations of position, size, and shape
will produce a circle or ellipse with a radius of 0.
This will occur, for example,
if the center is on a <a data-link-type=dfn href=#gradient-box title="gradient box">gradient box</a> edge and <a class=css data-link-type=maybe href=#valdef-size-closest-side title=closest-side>closest-side</a> or <a class=css data-link-type=maybe href=#valdef-size-closest-corner title=closest-corner>closest-corner</a> is specified
or if the size and shape are given explicitly and either of the radiuses is zero.
In these degenerate cases,
the gradient must be be rendered as follows:</p>
<dl>
<dt>If the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> is a circle with zero radius:
<dd>
Render as if the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> was
a circle whose radius was an arbitrary very small number greater than zero.
<span class=note>This will make the gradient continue to look like a circle.</span>
<dt>If the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> has zero width (regardless of the height):
<dd>
Render as if the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> was
an ellipse whose height was an arbitrary very large number
and whose width was an arbitrary very small number greater than zero.
<span class=note>This will make the gradient look similar to a horizontal linear gradient
that is mirrored across the center of the ellipse.
It also means that all color-stop positions specified with a percentage resolve to <span class=css>0px</span>.</span>
<dt>Otherwise, if the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> has zero height:
<dd>
Render as if the <a data-link-type=dfn href=#ending-shape title="ending shape">ending shape</a> was
an ellipse whose width was an arbitrary very large number
and whose height was an arbitrary very small number greater than zero.
<span class=note>This will make the gradient look like a solid-color image
equal to the color of the last color-stop,
or equal to the average color of the gradient if it’s repeating.</span>
</dl>
<h4 class="no-toc heading settled" data-level=3.2.4 id=radial-gradient-examples><span class=secno>3.2.4 </span><span class=content>
Radial Gradient Examples</span><a class=self-link href=#radial-gradient-examples></a></h4>
<p>All of the following examples are applied to a box that is 200px wide and 100px tall.</p>
<div class=example>
These examples demonstrate different ways to write the basic syntax for radial gradients:
<pre>radial-gradient(yellow, green);
radial-gradient(ellipse at center, yellow 0%, green 100%);
radial-gradient(farthest-corner at 50% 50%, yellow, green);
</pre>
<p><img alt="" src=images/radial1.png></p>
<pre>radial-gradient(circle, yellow, green);</pre>
<p><img alt="" src=images/radial2.png></p>
<pre>radial-gradient(red, yellow, green);</pre>
<p><img alt="" src=images/radial3.png></p>
</div>
<div class=example>
This image shows a gradient originating from somewhere other than the center of the box:
<pre>radial-gradient(farthest-side at left bottom, red, yellow 50px, green);</pre>
<p><img alt="" src=images/radial4.png></p>
</div>
<div class=example>
Here we illustrate a <a class=css data-link-type=maybe href=#valdef-size-closest-side title=closest-side>closest-side</a> gradient.
<pre>radial-gradient(closest-side at 20px 30px, red, yellow, green);
radial-gradient(20px 30px at 20px 30px, red, yellow, green);
</pre>
<p><img alt="" src=images/radial6.png></p>
<pre>radial-gradient(closest-side circle at 20px 30px, red, yellow, green);
radial-gradient(20px 20px at 20px 30px, red, yellow, green);
</pre>
<p><img alt="" src=images/radial7.png></p>
</div>
<h3 class="heading settled" data-level=3.3 id=repeating-gradients><span class=secno>3.3 </span><span class=content>
Repeating Gradients: the <a class=css data-link-type=maybe href=#funcdef-repeating-linear-gradient title=repeating-linear-gradient()>repeating-linear-gradient()</a>, <a class=css data-link-type=maybe href=#funcdef-repeating-radial-gradient title=repeating-radial-gradient()>repeating-radial-gradient()</a>, and <span class=css>repeating-conic-gradient()</span> notations</span><a class=self-link href=#repeating-gradients></a></h3>
<p>In addition to <a class=css data-link-type=maybe href=#funcdef-linear-gradient title=linear-gradient()>linear-gradient()</a> and <a class=css data-link-type=maybe href=#funcdef-radial-gradient title=radial-gradient()>radial-gradient()</a>,
this specification defines <dfn class=css data-dfn-type=function data-export="" id=funcdef-repeating-linear-gradient>repeating-linear-gradient()<a class=self-link href=#funcdef-repeating-linear-gradient></a></dfn>
and <dfn class=css data-dfn-type=function data-export="" id=funcdef-repeating-radial-gradient>repeating-radial-gradient()<a class=self-link href=#funcdef-repeating-radial-gradient></a></dfn> values.
These notations take the same values
and are interpreted the same
as their respective non-repeating siblings defined previously.</p>
<p>When rendered, however, the color-stops are repeated infinitely in both directions,
with their positions shifted by multiples of the difference between
the last specified color-stop’s position
and the first specified color-stop’s position.
For example, <span class=css>repeating-linear-gradient(red 10px, blue 50px)</span>
is equivalent to <span class=css>linear-gradient(..., red -30px, blue 10px, red 10px, blue 50px, red 50px, blue 90px, ...)</span>.
Note that the last color-stop and first color-stop will always coincide at the boundaries of each group,
which will produce sharp transitions if the gradient does not start and end with the same color.</p>
<div class=example>
Repeating gradient syntax is identical to that of non-repeating gradients:
<pre>repeating-linear-gradient(red, blue 20px, red 40px)</pre>
<p><img alt="" src=images/repeating1.png></p>
<pre>repeating-radial-gradient(red, blue 20px, red 40px)</pre>
<p><img alt="" src=images/repeating2.png></p>
<pre>repeating-radial-gradient(circle closest-side at 20px 30px, red, yellow, green 100%, yellow 150%, red 200%)</pre>
<p><img alt="" src=images/repeating3.png></p>
</div>
<p>If the gradient has only a single color-stop,
it must render as a solid-color image equal to the color of that color-stop.</p>
<p>If the distance between the first and last color-stops is non-zero,
but is small enough that the implementation knows that the physical resolution of the output device is insufficient to faithfully render the gradient,
the implementation must <a data-link-type=dfn href=#gradient-average-color title=gradient-average-color>find the average color of the gradient</a>
and render the gradient as a solid-color image equal to the average color.</p>
<p>If the distance between the first and last color-stops is zero
(or rounds to zero due to implementation limitations),
the implementation must <a data-link-type=dfn href=#gradient-average-color title=gradient-average-color>find the average color of a gradient</a>
with the same number and color of color-stops,
but with the first and last color-stop an arbitrary non-zero distance apart,
and the remaining color-stops equally spaced between them.
Then it must render the gradient as a solid-color image equal to that average color.</p>
<p>If the width of the ending shape of a repeating radial gradient is non-zero
and the height is zero,
or is close enough to zero that the implementation knows that the physical resolution of the output device is insufficient to faithfully render the gradient,
the implementation must <a data-link-type=dfn href=#gradient-average-color title=gradient-average-color>find the average color of the gradient</a>
and render the gradient as a solid-color image equal to the average color.</p>
<p class=note>Note: The <a data-section="" href=#degenerate-radials>Degenerate Radial Gradients</a> section
describes how the ending shape is adjusted when its width is zero.</p>
<p>To <dfn data-dfn-type=dfn data-noexport="" id=gradient-average-color title=gradient-average-color>find the average color of a gradient<a class=self-link href=#gradient-average-color></a></dfn>,
run these steps:</p>