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  <title>CSS Image Values and Replaced Content Module Level 3</title>
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   <h1>CSS Image Values and Replaced Content Module Level 3</h1>

   <h2 class="no-num no-toc" id=longstatus-date>W3C Working Draft 12 January
    2012</h2>

   <dl>
    <dt>This Version:</dt>
    <!--<dd><a href="http://dev.w3.org/csswg/css3-images/">http://dev.w3.org/csswg/css3-images/</a>-->

    <dd><a
     href="http://www.w3.org/TR/2012/WD-css3-images-20120112/">http://www.w3.org/TR/2012/WD-css3-images-20120112/</a>


    <dt>Latest Version:

    <dd><a
     href="http://www.w3.org/TR/css3-images/">http://www.w3.org/TR/css3-images/</a>

    <dt>Editor's Draft:

    <dd><a
     href="http://dev.w3.org/csswg/css3-images/">http://dev.w3.org/csswg/css3-images/</a>


    <dt>Previous Version:

    <dd><a
     href="http://www.w3.org/TR/2011/WD-css3-images-20111206/">http://www.w3.org/TR/2011/WD-css3-images-20111206/</a>

    <dd><a
     href="http://www.w3.org/TR/2011/WD-css3-images-20110908/">http://www.w3.org/TR/2011/WD-css3-images-20110908/</a>

    <dd><a
     href="http://www.w3.org/TR/2011/WD-css3-images-20110712/">http://www.w3.org/TR/2011/WD-css3-images-20110712/</a>

    <dd><a
     href="http://www.w3.org/TR/2011/WD-css3-images-20110217/">http://www.w3.org/TR/2011/WD-css3-images-20110217/</a>

    <dd><a
     href="http://www.w3.org/TR/2009/WD-css3-images-20090723/">http://www.w3.org/TR/2009/WD-css3-images-20090723/</a>

    <dt>Issues List:

    <dd><a
     href="http://www.w3.org/Style/CSS/Tracker/products/27">http://www.w3.org/Style/CSS/Tracker/products/27</a>


    <dt>Discussion:

    <dd><a
     href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
     with subject line &ldquo;<kbd>[css3-images] <var>&hellip; message topic
     &hellip;</var></kbd>&rdquo;

    <dt>Editors:

    <dd class=vcard> <a class="url fn"
     href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (<span
     class=org>Mozilla</span>)

    <dd class=vcard> <a class="url fn"
     href="http://www.xanthir.com/contact/">Tab Atkins Jr.</a> (<span
     class=org>Google</span>)
   </dl>
   <!--begin-copyright-->
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    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2012 <a
    href="http://www.w3.org/"><acronym
    title="World Wide Web Consortium">W3C</acronym></a><sup>&reg;</sup> (<a
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    title="Massachusetts Institute of Technology">MIT</acronym></a>, <a
    href="http://www.ercim.eu/"><acronym
    title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>,
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  </div>

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

  <p>CSS is a language for describing the rendering of structured documents
   (such as HTML and XML) on screen, on paper, in speech, etc. This module
   contains the features of CSS level&nbsp;3 relating to the &lt;image> type
   and replaced elements. It includes and extends the functionality of CSS
   level&nbsp;2 <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS
   level&nbsp;1 <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>.
   The main extensions compared to level&nbsp;2 are the generalization of the
   &lt;url> type to the &lt;image> type, several additions to the &lsquo;<a
   href="#ltimage"><code class=css>&lt;image></code></a>&rsquo; type, a
   generic sizing algorithm for images and other replaced content in CSS, and
   several properties controlling the interaction of replaced elements and
   CSS's layout models.

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

  <p><em>This section describes the status of this document at the time of
   its publication. Other documents may supersede this document. A list of
   current W3C publications and the latest revision of this technical report
   can be found in the <a href="http://www.w3.org/TR/">W3C technical reports
   index at http://www.w3.org/TR/.</a></em>

  <p>Publication as a Working Draft does not imply endorsement by the W3C
   Membership. This is a draft document and may be updated, replaced or
   obsoleted by other documents at any time. It is inappropriate to cite this
   document as other than 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">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 &#8220;css3-images&#8221; in the subject, preferably like this:
   &#8220;[<!---->css3-images<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

  <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>.</p>
  <!--end-status-->

  <p>&#8203;&#8203;&#8203;&#8203;&#8203;This specification is a <strong>Last
   Call Working Draft</strong>. All persons are encouraged to review this
   document and <strong>send comments to the <a
   href="http://lists.w3.org/Archives/Public/www-style/">www-style</a>
   mailing list</strong> as described above. The <strong>deadline for
   comments</strong> is <strong>7 February 2012</strong>.

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

  <ol>
   <li>The <a href="#image-notation">image() function</a>

   <li>The <a href="#element-reference">element() function</a>

   <li>The "none" and "scale-down" values for &lsquo;<a
    href="#object-fit0"><code class=property>object-fit</code></a>&rsquo;

   <li>The &lsquo;<a href="#image-orientation0"><code
    class=property>image-orientation</code></a>&rsquo; property

   <li>The "snap" keyword for &lsquo;<a href="#image-resolution0"><code
    class=property>image-resolution</code></a>&rsquo;
  </ol>

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

  <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="#resolution-units"><span class=secno>2. </span> Resolution
    Units: the &lt;resolution> type</a>

   <li><a href="#image"><span class=secno>3. </span> Image Values: the
    &lt;image> type</a>
    <ul class=toc>
     <li><a href="#url"><span class=secno>3.1. </span> Image References and
      Image Slices: the &lsquo;<code class=css>url()</code>&rsquo;
      notation</a>

     <li><a href="#image-notation"><span class=secno>3.2. </span> Image
      Fallbacks and Annotations: the &lsquo;<code
      class=css>image()</code>&rsquo; notation</a>

     <li><a href="#element-reference"><span class=secno>3.3. </span> Using
      Elements as Images: the &lsquo;<code class=css>element()</code>&rsquo;
      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 &lsquo;<code class=css>linear-gradient()</code>&rsquo;
      notation</a>

     <li><a href="#radial-gradients"><span class=secno>4.2. </span> Radial
      Gradients: the &lsquo;<code class=css>radial-gradient()</code>&rsquo;
      notation</a>

     <li><a href="#repeating-gradients"><span class=secno>4.3. </span>
      Repeating Gradients: the &lsquo;<code
      class=css>repeating-linear-gradient()</code>&rsquo; and &lsquo;<code
      class=css>repeating-radial-gradient()</code>&rsquo; notations</a>

     <li><a href="#color-stop-syntax"><span class=secno>4.4. </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&#x21CB;Object Negotiation</a>

     <li><a href="#default-sizing"><span class=secno>5.3. </span> Default
      Concrete Object Size Resolution</a>

     <li><a href="#object-fit"><span class=secno>5.4. </span> Sizing Objects:
      the &lsquo;<code class=property>object-fit</code>&rsquo; property</a>

     <li><a href="#object-position"><span class=secno>5.5. </span>
      Positioning Objects: the &lsquo;<code
      class=property>object-position</code>&rsquo; property</a>
    </ul>

   <li><a href="#image-processing"><span class=secno>6. </span> Image
    Processing</a>
    <ul class=toc>
     <li><a href="#image-resolution"><span class=secno>6.1. </span>
      Overriding Image Resolutions: the &lsquo;<code
      class=property>image-resolution</code>&rsquo; property</a>

     <li><a href="#image-orientation"><span class=secno>6.2. </span>
      Orienting an Image on the Page: the &lsquo;<code
      class=property>image-orientation</code>&rsquo; property</a>
    </ul>

   <li><a href="#conformance"><span class=secno>7. </span> Conformance</a>
    <ul class=toc>
     <li><a href="#conventions"><span class=secno>7.1. </span> Document
      Conventions</a>

     <li><a href="#conformance-classes"><span class=secno>7.2. </span>
      Conformance Classes</a>

     <li><a href="#partial"><span class=secno>7.3. </span> Partial
      Implementations</a>

     <li><a href="#experimental"><span class=secno>7.4. </span> Experimental
      Implementations</a>

     <li><a href="#testing"><span class=secno>7.5. </span>Non-Experimental
      Implementations</a>

     <li><a href="#cr-exit-criteria"><span class=secno>7.6. </span> CR Exit
      Criteria</a>
    </ul>

   <li class=no-num><a href="#acknowledgments">Acknowledgments</a>

   <li class=no-num><a href="#references">References</a>
    <ul class=toc>
     <li class=no-num><a href="#normative-references">Normative
      references</a>

     <li class=no-num><a href="#other-references">Other references</a>
    </ul>

   <li class=no-num><a href="#index">Index</a>

   <li class=no-num><a href="#property-index">Property index</a>
  </ul>
  <!--end-toc-->

  <h2 id=intro><span class=secno>1. </span> Introduction</h2>

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

  <p>In CSS Levels 1 and 2, image values, such as those used in the
   &lsquo;<code class=property>background-image</code>&rsquo; 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-reference">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 id=placement><span class=secno>1.1. </span> Module Interactions</h3>

  <p>This module defines and extends the &lsquo;<a href="#ltimage"><code
   class=css>&lt;image></code></a>&rsquo; value type defined in <a
   href="#CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>.
   Furthermore it replaces the &lsquo;<code class=css>&lt;url></code>&rsquo;
   type in the &lsquo;<code class=property>background-image</code>&rsquo; and
   &lsquo;<code class=property>list-style-image</code>&rsquo; definitions in
   CSS1 and CSS2 and adds &lsquo;<a href="#ltimage"><code
   class=css>&lt;image></code></a>&rsquo; as an alternative to &lsquo;<code
   class=css>&lt;url></code>&rsquo; in the &lsquo;<code
   class=property>content</code>&rsquo; property's value. It is presumed that
   CSS specifications beyond CSS2.1 will use the &lsquo;<a
   href="#ltimage"><code class=css>&lt;image></code></a>&rsquo; notation in
   place of &lsquo;<code class=css>&lt;url></code>&rsquo; where 2D images are
   expected. (See e.g. <a href="#CSS3BG"
   rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a>.)

  <h3 id=values><span class=secno>1.2. </span> Values</h3>

  <p>This specification follows the <a
   href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
   definition conventions</a> from <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
   this specification are defined in CSS Level 2 Revision 1 <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
   modules may expand the definitions of these value types: for example <a
   href="#CSS3COLOR" rel=biblioentry>[CSS3COLOR]<!--{{CSS3COLOR}}--></a>,
   when combined with this module, expands the definition of the &lt;color>
   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/CSS21/cascade.html#value-def-inherit">inherit</a>
   keyword as their property value. For readability it has not been repeated
   explicitly.

  <h2 id=resolution-units><span class=secno>2. </span> Resolution Units: the
   &lt;resolution> type</h2>

  <p>This specification defines the following units as part of the <dfn
   id=ltresolution>&lt;resolution></dfn> value type:

  <dl>
   <dt><dfn id=dpi>&lsquo;<code class=css>dpi</code>&rsquo;</dfn>

   <dd>dots per inch

   <dt><dfn id=dpcm>&lsquo;<code class=css>dpcm</code>&rsquo;</dfn>

   <dd>dots per centimeter

   <dt><dfn id=dppx>&lsquo;<code class=css>dppx</code>&rsquo;</dfn>

   <dd>dots per &lsquo;<code class=css>px</code>&rsquo; unit
  </dl>

  <p>The &lt;resolution> unit represents the size of a single "dot" in a
   graphical representation by indicating how many of these dots fit in a CSS
   &lsquo;<code class=property>in</code>&rsquo;, &lsquo;<code
   class=property>cm</code>&rsquo;, or &lsquo;<code
   class=property>px</code>&rsquo;. For uses, see e.g. the &lsquo;<code
   class=css>resolution</code>&rsquo; media query in <a href="#MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{MEDIAQ}}--></a> or the &lsquo;<a
   href="#image-resolution0"><code
   class=property>image-resolution</code></a>&rsquo; property defined below.

  <p class=note>Note that due to the 1:96 fixed ratio of CSS &lsquo;<code
   class=css>in</code>&rsquo; to CSS &lsquo;<code class=css>px</code>&rsquo;,
   &lsquo;<code class=css>1dppx</code>&rsquo; is equivalent to &lsquo;<code
   class=css>96dpi</code>&rsquo;. This corresponds to the default resolution
   of images displayed in CSS: see &lsquo;<code
   class=property>image-reslution</code>&rsquo;.
   <!-- ====================================================================== -->


  <h2 id=image><span class=secno>3. </span> Image Values: the &lt;image> type</h2>

  <p>The &lt;image> value type denotes a 2D image. It represents either a <a
   href="#url">url reference</a>, <a href="#image-notation">image
   notation</a>, <a href="#element-reference">element reference</a>, or <a
   href="#gradients">gradient notation</a>. Syntactically it is defined as

  <pre class=prod><dfn id=ltimage>&lt;image></dfn> = <i>&lt;url></i> | <a
   href="#ltimage-list"><i>&lt;image-list></i></a> | <a
   href="#ltelement-reference"><i>&lt;element-reference></i></a>  | <a
   href="#ltgradient"><i>&lt;gradient></i></a></pre>

  <p>Image values can be used in many CSS properties, including the
   &lsquo;<code class=property>background-image</code>&rsquo;, &lsquo;<code
   class=property>list-style-image</code>&rsquo;, &lsquo;<code
   class=property>cursor</code>&rsquo; properties <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
   <!-- ====================================================================== -->


  <h3 id=url><span class=secno>3.1. </span> Image References and Image
   Slices: the &lsquo;<code class=css>url()</code>&rsquo; notation</h3>

  <p>The simplest way to indicate an image is to reference an image file by
   URL. This is done with the <a
   href="http://www.w3.org/TR/CSS21/syndata.html#uri">&lsquo;<code
   class=css>url()</code>&rsquo; notation</a>, defined in <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.

  <div class=example>
   <p>In the example below, a background image is specified with &lsquo;<code
    class=css>url()</code>&rsquo; syntax:</p>

   <pre>background-image: url(wavy.png);</pre>
  </div>

  <p>A portion of an image may be referenced (clipped out and used as a
   standalone image) by use of <a
   href="http://www.w3.org/TR/media-frags/#naming-space">media fragment
   identifiers</a>. <a href="#MEDIA-FRAGS"
   rel=biblioentry>[MEDIA-FRAGS]<!--{{!MEDIA-FRAGS}}--></a>

  <div class=example>
   <p>For example, given the following image* and CSS:</p>
   <a href=sprites.svg> <img alt="[9 circles, with 0 to 8 eighths filled in]"
   height=20 src=sprites.svg width=180> </a>
   <pre>background-image: url('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>

   <p><small>* SVG-in-&lt;img> support required. Click the picture to view
    the SVG directly.</small></p>
  </div>

  <p class=note>Note that a legacy UA that doesn't understand the media
   fragments notation will ignore the fragment and simply display the
   entirety of an image specified with &lsquo;<code
   class=css>url</code>&rsquo;. However, since URLs with media fragment
   identifiers can also be used in the &lsquo;<code
   class=css>image()</code>&rsquo; notation defined below, authors can take
   advantage of CSS's forward-compatible parsing rules to provide a fallback
   when using an image fragment URL:

  <div class=example>
   <p>In the example below, the &lsquo;<code class=css>image()</code>&rsquo;
    notation is used together with the media fragment syntax, so that UAs
    that don't support media fragments fail to parse the second declaration
    and use the first.</p>

   <pre>
<!-- -->background-image: url('swirl.png'); /* old UAs */
<!-- -->background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */</pre>
  </div>
  <!-- ====================================================================== -->

  <h3 id=image-notation><span class=secno>3.2. </span> Image Fallbacks and
   Annotations: the &lsquo;<code class=css>image()</code>&rsquo; notation</h3>

  <p>The &lsquo;<code class=css>image()</code>&rsquo; function allows an
   author to specify an image with fallback images to be used if the original
   image can't be decoded or is a type that the browser doesn't recognize.
   Additionally, the author can specify a color as an ultimate fallback to be
   used when none of the images can be.

  <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 &lsquo;<code class=css>image()</code>&rsquo; notation <em>must</em>
   support the <code>xywh=#,#,#,#</code> form of media fragment identifiers
   for images. <a href="#MEDIA-FRAGS"
   rel=biblioentry>[MEDIA-FRAGS]<!--{{!MEDIA-FRAGS}}--></a>

  <p>The &lsquo;<code class=css>image()</code>&rsquo; notation is defined as:


  <pre class=prod><dfn
   id=ltimage-list>&lt;image-list></dfn> = image( [ &lt;image-decl> , ]* [ &lt;image-decl> | &lt;color> ] )
<dfn
   id=ltimage-decl>&lt;image-decl></dfn> = &lt;string> [ ltr | rtl ]?</pre>

  <p>Each <code>&lt;string></code> must represent a <a
   href="http://dev.w3.org/csswg/css3-values/#urls">URL</a>.

  <p>Multiple arguments can be given separated by commas, in which case the
   function represents the first &lt;string> representing an image that the
   browser can successfully load and display. The final argument can specify
   a &lt;color> to serve as an ultimate fallback; this can be used, e.g. for
   &lsquo;<code class=property>background-image</code>&rsquo;, to ensure
   adequate contrast if none of the preceding &lt;image-decl>s can be used.
   If the final argument is a &lt;color>, it represents a solid-color image
   of the given color with no <a href="#intrinsic-dimensions"><i>intrinsic
   dimensions</i></a>.

  <div class=example>
   <p>The rule below would tell the UA to load &lsquo;<code
    class=css>wavy.svg</code>&rsquo; if it can; failing that to load
    &lsquo;<code class=css>wavy.png</code>&rsquo;; failing that to display
    &lsquo;<code class=css>wavy.gif</code>&rsquo;. 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.</p>

   <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:</p>

   <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 &lsquo;<code class=css>image()</code>&rsquo; 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>

  <div class=example>
   <p>At times, one may need a solid-color image for a property or function
    that does not accept the &lt;color> type directly. The &lsquo;<code
    class=css>image()</code>&rsquo; 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>

  <p>Along with each URL, the author may specify a directionality, similar to
   adding a <code>dir</code> attribute to an element in HTML. The image
   represented by the function takes on the directionality of the used URL.
   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).

  <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:</p>

   <pre>
&lt;ul style="list-style-image: image("arrow.png" ltr);">
	&lt;li dir='ltr'>My bullet is on the left!&lt;/li>
	&lt;li dir='rtl'>MY BULLET IS ON THE RIGHT!&lt;/li>
&lt;/ul></pre>

   <p>This should render something like:</p>

   <pre>
&#8658; My bullet is on the left!
  !THGIR EHT NO SI TELLUB YM &#8656;</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.</p>
  </div>
  <!-- ====================================================================== -->

  <h3 id=element-reference><span class=secno>3.3. </span> Using Elements as
   Images: the &lsquo;<code class=css>element()</code>&rsquo; notation</h3>

  <p>The &lsquo;<code class=css>element()</code>&rsquo; 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. The syntax for &lsquo;<code
   class=css>element()</code>&rsquo; is:

  <pre class=prod><dfn
   id=ltelement-reference>&lt;element-reference></dfn> = element( [&lt;id-selector> | &lt;identifier> ] )</pre>

  <p>where &lt;id-selector> is an ID selector <a href="#SELECT"
   rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a>, and &lt;identifier> is an
   identifer <a href="#CSS3VAL"
   rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>.

  <p>If the argument to the &lsquo;<code class=css>element()</code>&rsquo;
   function is an ID selector, the function references the element matched by
   the selector. If it's an identifier, the function references the element
   whose <dfn id=css-element-reference-identifier>CSS element reference
   identifier</dfn> is the given identifier. (CSS does not define how an
   element acquires a <a href="#css-element-reference-identifier"><i>CSS
   element reference identifier</i></a>; that is determined by the host
   language.)

  <div class=example>
   <p>The &lsquo;<code class=css>element()</code>&rsquo; 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:</p>

   <pre>
&lt;!DOCTYPE html>
&lt;script>
function navigateSlides() {
	var currentSlide = ...;
	var prevSlide = currentSlide.previousElementSibling;
	var nextSlide = currentSlide.nextElementSibling;
	document.CSSElementMap['prev-slide'] = prevSlide;
	document.CSSElementMap['next-slide'] = nextSlide;
}
&lt;/script>
&lt;style>
#prev-preview, #next-preview {
	position: fixed;
	...
}
#prev-preview { background: element(prev-slide); }
#next-preview { background: element(next-slide); }
&lt;/style>
&lt;div id='prev-preview'>&lt;/div>
&lt;div id='next-preview'>&lt;/div>
&lt;section class='slide'>...&lt;/section>
&lt;section class='slide current-slide'>...&lt;/section>
...</pre>

   <p>In this example, the <code>navigateSlides</code> function updates
    HTML's <code>CSSElementMap</code> to always point to 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 &lsquo;<code class=css>element()</code>&rsquo; function (it's just an
    image), you could even use <code>click</code> handlers on the preview
    boxes to help navigate through the page.

   <p class=note>Note: A future version of this specification may allow more
    than just ID selectors to be passed to &lsquo;<code
    class=css>element()</code>&rsquo;, allowing an example like this to be
    done with even less javascript - something like &lsquo;<code
    class=css>background: element(.current-slide + .slide);</code>&rsquo;.</p>
  </div>

  <p>The image represented by the &lsquo;<code
   class=css>element()</code>&rsquo; function can vary based on a number of
   factors. The function must represent the image described by the first set
   of conditions, following, that are true:

  <dl>
   <dt>The function doesn't reference an element

   <dd>The function represents a solid-color transparent-black image with no
    intrinsic dimensions.

   <dt>The function references an element that is not rendered, but which <a
    href="#paint-source"><i title=paint-source>provides a paint
    source</i></a>

   <dd>
    <p>The function represents an image with the dimensions and appearance of
     the <a href="#paint-source"><i title=paint-source>paint source</i></a>.
     The host language defines the dimensions and appearance of paint
     sources.

    <div class=example>
     <p>For example, the &lsquo;<code class=css>element()</code>&rsquo;
      function can reference an SVG &lt;pattern> element in an HTML document:</p>

     <pre>
&lt;!DOCTYPE html>
&lt;svg>
	&lt;defs>
		&lt;pattern id='pattern1'>
			&lt;path d='...'>
		&lt;/pattern>
	&lt;/defs>
&lt;/svg>
&lt;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!
&lt;/p></pre>

     <p>HTML also defines that a handful of elements, such as &lt;canvas>,
      &lt;img>, and &lt;video>, provide a paint source. This means that CSS
      can, for example, reference a canvas that's being drawn into, but not
      included in the page:</p>

     <pre>
&lt;!DOCTYPE html>
&lt;script>
	var canvas = document.createElement('canvas');
	canvas.width = 20; canvas.height = 20;
	document.CSSElementMap.foo = canvas;
	drawAnimation(canvas);
&lt;/script>
&lt;ul style="list-style-image: element(foo);">
	&lt;li>I'm using the canvas as a bullet!&lt;/li>
	&lt;li>So am I!&lt;/li>
	&lt;li>As the canvas is changed over time with Javascript,
	    we'll all update our bullet image with it!&lt;/li>
&lt;/ul></pre>
    </div>

   <dt>The function references an element that is not rendered

   <dt>The function references an element that is a descendant of a replaced
    element

   <dd>
    <p>The function represents a solid-color transparent-black image with no
     intrinsic dimensions.</p>

    <div class=example>
     <p>For example, all of the following &lsquo;<code
      class=css>element()</code>&rsquo; uses will result in a transparent
      background:</p>

     <pre>
&lt;!DOCTYPE html>
&lt;script>
var p = document.createElement('p');
p.textContent = "one";
document.CSSElementMap.one = p;
&lt;/script>
&lt;p id='two' style="display:none;">two&lt;/p>
&lt;iframe src="http://example.com">
	&lt;p id='three'>I'm fallback content!&lt;/p>
&lt;/iframe>
&lt;ul>
	&lt;li style="background: element(one);">
	  A P element doesn't provide a paint source,
	  and it's not rendered unless it's in a document.
	&lt;/li>
	&lt;li style="background: element(#two);">
	  Similarly, a display:none element isn't rendered, and
	  P still doesn't provide a paint source.
	&lt;/li>
	&lt;li style="background: element(#three);">
	  The descendants of a replaced element like an IFRAME
	  can't be used in element() either.
	&lt;/li>
&lt;/ul></pre>
    </div>

   <dt>Otherwise

   <dd>
    <p>The function represents an image with width and height equal to the
     bounding box of the referenced element. The image must be constructed by
     rendering the referenced element and its descendants at the same size
     that the element would be in its document, over an infinite
     transparent-black canvas, positioned so that the edges of the referenced
     element's bounding box is flush with the edges of the image. <span
     class=note>Note: Because images clip anything outside their bounds by
     default, this means that decorations that extend outside the bounding
     box, like box-shadows or some border-images, may be clipped.</span></p>

    <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. If the referenced element is broken across pages, the element
     must be displayed as if the page content areas were joined flush in the
     pagination direction (elements broken across lines or columns just
     render with their bounding box, as normal, which may have unintended
     visual effects).</p>

    <div class=example>
     <p>As a somewhat silly example, a &lt;p> element can be reused as a
      background elsewhere in the document:</p>

     <pre>
&lt;style>
#src { color: white; background: lime; width: 300px; height: 40px; }
#dst { color: black; background: element(#src); padding: 20px; margin: 20px 0; }
&lt;/style>
&lt;p id='src'>I'm an ordinary element!&lt;/p>
&lt;p id='dst'>I'm using the previous element as my background!&lt;/p></pre>
     <img alt="" src=element-function.png></div>
  </dl>

  <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.

  <p>Host languages may define that some elements provide a <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 not being
   rendered. Examples of elements that provide paint sources are the
   &lt;linearGradient>, &lt;radialGradient>, and &lt;pattern> elements in
   SVG, or the &lt;img>, &lt;video>, and &lt;canvas> elements in HTML.</p>
  <!-- ====================================================================== -->

  <h4 class="no-num no-toc" id=element-cycles> Detecting and Resolving
   Circular Relationships Introduced by &lsquo;<code
   class=css>element()</code>&rsquo;</h4>

  <p>The &lsquo;<code class=css>element()</code>&rsquo; 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>Populate the dependency graph initially by having every element depend
   on each of its children. Then, whenever a property on an element A uses
   the &lsquo;<code class=css>element()</code>&rsquo; function to refer to an
   element B, add an edge to the graph by having A depend on B. If a
   dependency cycle is detected, any &lsquo;<code
   class=css>element()</code>&rsquo; functions that produced a dependency in
   the cycle must represent a fully transparent image with no intrinsic
   dimensions.

  <p class=issue>Someone else needs to review this and make sure that I'm not
   missing any cycles.

  <h2 id=gradients><span class=secno>4. </span> Gradients</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 two functions 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 href="#ltgradient"><i>&lt;gradient></i></a> is:

  <pre class=prod><dfn
   id=ltgradient>&lt;gradient></dfn> = [ &lt;linear-gradient> | &lt;radial-gradient> | &lt;repeating-linear-gradient> | &lt;repeating-radial-gradient> ]</pre>

  <p>where <a href="#ltlinear-gradient"><i>&lt;linear-gradient></i></a>,
   <i>&lt;radial-gradient></i>, <i>&lt;repeating-linear-gradient></i>, and
   <i>&lt;repeating-radial-gradient></i> are defined in their applicable
   sections below.

  <p>Gradients are a type of image, and can be used anywhere an image can,
   such as in the &lsquo;<code class=property>background-image</code>&rsquo;
   or &lsquo;<code class=property>list-style-image</code>&rsquo; properties.

  <div class=example>
   <p>As with the other <a href="#ltimage"><i>&lt;image></i></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
   href="#concrete-object-size"><i>concrete object size</i></a>, referred to
   as the <dfn id=gradient-box>gradient box</dfn>. However, the gradient
   itself has no <a href="#intrinsic-dimensions"><i>intrinsic
   dimensions</i></a>.

  <div class=example>
   <p>For example, if you use a gradient as a background, by default the
    gradient will draw into a <a href="#gradient-box"><i>gradient box</i></a>
    the size of the element's padding box. If &lsquo;<code
    class=property>background-size</code>&rsquo; is explicitly set to a value
    such as &lsquo;<code class=css>100px 200px</code>&rsquo;, then the <a
    href="#gradient-box"><i>gradient box</i></a> will be 100px wide and 200px
    tall. Similarly, for a gradient used as a &lsquo;<code
    class=property>list-style-image</code>&rsquo;, the box would be a 1em
    square, which is the <a href="#default-object-size"><i>default object
    size</i></a> for that property.</p>
  </div>
  <!-- ====================================================================== -->

  <h3 id=linear-gradients><span class=secno>4.1. </span> Linear Gradients:
   the &lsquo;<code class=css>linear-gradient()</code>&rsquo; notation</h3>

  <p>A linear gradient is created by specifying a gradient-line 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 id=linear-gradient-syntax><span class=secno>4.1.1. </span>
   linear-gradient() syntax</h4>

  <pre class=prod><code>
<dfn
   id=ltlinear-gradient>&lt;linear-gradient></dfn> = linear-gradient(
	[ [ &lt;angle> | to &lt;side-or-corner> ] ,]?
	&lt;color-stop>[, &lt;color-stop>]+
)

<dfn
   id=side-or-corner>&lt;side-or-corner></dfn> = [left | right] || [top | bottom]</code></pre>

  <p>The first argument to the function specifies the <dfn
   id=gradient-line>gradient-line</dfn>, which gives the gradient a direction
   and determines how color-stops are positioned. It may be omitted; if so,
   it defaults to "to bottom".

  <p>The <a href="#gradient-line"><i>gradient-line</i></a> may be specified
   in two different ways. The first is by specifying the angle the <a
   href="#gradient-line"><i>gradient-line</i></a> should assume; for the
   purposes of this argument, 0deg points upwards, 90deg points toward the
   right, and positive angles go clockwise. The starting-point and
   ending-point of the <a href="#gradient-line"><i>gradient-line</i></a> are
   determined by extending a line in both directions from the center of the
   <a href="#gradient-box"><i>gradient box</i></a> at the angle specified. In
   the direction of the angle, the ending-point is the point on the <a
   href="#gradient-line"><i>gradient-line</i></a> where a line drawn
   perpendicular to the <a href="#gradient-line"><i>gradient-line</i></a>
   would intersect the corner of the <a href="#gradient-box"><i>gradient
   box</i></a> in that direction. The starting-point is determined
   identically, except in the opposite direction of the angle.

  <p>Alternately, the direction may be specified with keywords that denote
   the direction. If the argument is &lsquo;<code class=css>to
   top</code>&rsquo;, &lsquo;<code class=css>to right</code>&rsquo;,
   &lsquo;<code class=css>to bottom</code>&rsquo;, or &lsquo;<code
   class=css>to left</code>&rsquo;, the gradient must be rendered identically
   to &lsquo;<code class=css>0deg</code>&rsquo;, &lsquo;<code
   class=css>90deg</code>&rsquo;, &lsquo;<code
   class=css>180deg</code>&rsquo;, or &lsquo;<code
   class=css>270deg</code>&rsquo;, respectively. If the argument specifies a
   corner to angle towards, the gradient must be rendered identically to an
   angle-based gradient with an angle chosen such that the endpoint of the
   gradient is in the same quadrant as the indicated corner, and a line drawn
   perpendicular to the gradient-line through the center of the <a
   href="#gradient-box"><i>gradient box</i></a> intersects the two
   neighboring corners.

  <p class=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=gradient-diagram.png style="float: right; margin-left: 1em;">
    <p>This example illustrates visually how to calculate the <a
     href="#gradient-line"><i>gradient-line</i></a> from the rules above.
     This shows the starting and ending-point of the <a
     href="#gradient-line"><i>gradient-line</i></a>, along with the actual
     gradient, produced by an element with &lsquo;<code class=css>background:
     linear-gradient(45deg, white, black);</code>&rsquo;.</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>

  <p>The gradient's color stops are typically placed between the
   starting-point and ending-point on the <a
   href="#gradient-line"><i>gradient-line</i></a>, but this isn't required -
   the <a href="#gradient-line"><i>gradient-line</i></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>
  <!-- ====================================================================== -->

  <h4 class=no-toc id=linear-gradient-examples><span class=secno>4.1.2.
   </span> Linear Gradient Examples</h4>

  <p>All of the following &lsquo;<code
   class=css>linear-gradient()</code>&rsquo; examples are presumed to be
   backgrounds applied to a box that is 200px wide and 100px tall.

  <div class=example>
   <p>Below are various ways of specifying a basic vertical gradient:</p>

   <pre><code>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%);</code></pre>

   <p><img alt="" src=linear1.png></p>
  </div>

  <div class=example>
   <p>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 href="#gradient-line"><i>gradient-line</i></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.</p>

   <pre><code>linear-gradient(135deg, yellow, blue);
linear-gradient(-45deg, blue, yellow);</code></pre>

   <p><img alt="" src=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><code>linear-gradient(yellow, blue 20%, #0f0);</code></pre>

   <p><img alt="" src=linear4.png></p>
  </div>

  <div class=example>
   <p>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.</p>

   <pre><code>linear-gradient(to top right, red, white, blue)</code></pre>

   <p><object data=gradient1.svg height=100 width=200>(Image requires
    SVG)</object></p>

   <p><small>* SVG-in-HTML support required to view the image.</small></p>
  </div>
  <!-- ====================================================================== -->

  <h3 id=radial-gradients><span class=secno>4.2. </span> Radial Gradients:
   the &lsquo;<code class=css>radial-gradient()</code>&rsquo; notation</h3>

  <p>In a radial gradient, rather than colors smoothly fading from one side
   of the <a href="#gradient-box"><i>gradient box</i></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>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
   id=ending-shape>ending shape</dfn> (the 100% ellipse). Color stops are
   given as a list, just as for &lsquo;<code
   class=css>linear-gradient()</code>&rsquo;. Starting from the <i>center</i>
   and progressing towards (and potentially beyond) the <a
   href="#ending-shape"><i>ending shape</i></a> concentric ellipses are drawn
   and colored according to the specified color stops.</p>
  <!-- ====================================================================== -->

  <h4 class=no-toc id=radial-gradient-syntax><span class=secno>4.2.1. </span>
   radial-gradient() Syntax</h4>

  <p>The radial gradient syntax is defined as follows:

  <pre>
&lt;radial-gradient> = radial-gradient(
  [ [ &lt;shape> || &lt;size> ] [ at &lt;position> ]? , |
    at &lt;position>,
  ]?
  &lt;color-stop> [ , &lt;color-stop> ]+
)</pre>

  <div class=example>
   <p>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>The arguments are defined as follows:

  <dl>
   <dt id=radial-position><dfn id=ltposition>&lt;position></dfn>

   <dd>Determines the center of the gradient. The
    <!-- FIXME a href="//www.w3.org/TR/css3-values/#position"--><a
    href="#ltposition"><i>&lt;position></i></a><!--/a--> value type (which is
    also used for &lsquo;<code
    class=property>background-position</code>&rsquo;) is defined in <a
    href="#CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>, and is
    resolved using the center-point as the object area and the <a
    href="#gradient-box"><i>gradient box</i></a> as the positioning area. If
    this argument is omitted, it defaults to &lsquo;<code
    class=css>center</code>&rsquo;.

   <dt id=radial-shape><dfn id=ltshape>&lt;shape></dfn>

   <dd>Can be either &lsquo;<code class=css>circle</code>&rsquo; or
    &lsquo;<code class=css>ellipse</code>&rsquo;; determines whether the
    gradient's <a href="#ending-shape"><i>ending shape</i></a> is a circle or
    an ellipse, respectively. If <a href="#ltshape"><i>&lt;shape></i></a> is
    omitted, the <a href="#ending-shape"><i>ending shape</i></a> defaults to
    a circle if the <a href="#ltsize"><i>&lt;size></i></a> is a single
    &lt;length>, and to an ellipse otherwise.

   <dt id=radial-size><dfn id=ltsize>&lt;size></dfn>

   <dd>
    <p>Determines the size of the gradient's <a
     href="#ending-shape"><i>ending shape</i></a>. If omitted it defaults to
     &lsquo;<code class=css>farthest-corner</code>&rsquo;. It can be given
     explicitly or by keyword. For the purpose of the keyword definitions,
     consider the <a href="#gradient-box"><i>gradient box</i></a> edges as
     extending infinitely in both directions, rather than being finite line
     segments.

    <p>Both &lsquo;<code class=css>circle</code>&rsquo; and &lsquo;<code
     class=css>ellipse</code>&rsquo; gradients accept the following keywords
     as their <a href="#ltsize"><i>&lt;size></i></a>:

    <dl>
     <dt><dfn id=radial-closest-side>&lsquo;<code
      class=css>closest-side</code>&rsquo;</dfn>

     <dd>The <a href="#ending-shape"><i>ending shape</i></a> is sized so that
      that it exactly meets the side of the <a
      href="#gradient-box"><i>gradient box</i></a> closest to the gradient's
      center. If the shape is an ellipse, it exactly meets the closest side
      in each dimension.

     <dt><dfn id=radial-farthest-side>&lsquo;<code
      class=css>farthest-side</code>&rsquo;</dfn>

     <dd>Same as &lsquo;<code class=css>closest-side</code>&rsquo;, except
      the <a href="#ending-shape"><i>ending shape</i></a> is sized based on
      the farthest side(s).

     <dt><dfn id=radial-closest-corner>&lsquo;<code
      class=css>closest-corner</code>&rsquo;</dfn>

     <dd>The <a href="#ending-shape"><i>ending shape</i></a> is sized so that
      that it passes through the corner of the <a
      href="#gradient-box"><i>gradient box</i></a> closest to the gradient's
      center. If the shape is an ellipse, the <a
      href="#ending-shape"><i>ending shape</i></a> is given the same
      aspect-ratio it would have if &lsquo;<code
      class=css>closest-side</code>&rsquo; were specified.

     <dt><dfn id=radial-farthest-corner>&lsquo;<code
      class=css>farthest-corner</code>&rsquo;</dfn>

     <dd>Same as &lsquo;<code class=css>closest-corner</code>&rsquo;, except
      the <a href="#ending-shape"><i>ending shape</i></a> is sized based on
      the farthest corner. If the shape is an ellipse, the <a
      href="#ending-shape"><i>ending shape</i></a> is given the same aspect
      ratio it would have if &lsquo;<code
      class=css>farthest-side</code>&rsquo; were specified.
    </dl>

    <p>If <a href="#ltshape"><i>&lt;shape></i></a> is specified as
     &lsquo;<code class=css>circle</code>&rsquo; or is omitted, the <a
     href="#ltsize"><i>&lt;size></i></a> may be given explicitly as:

    <dl>
     <dt><dfn id=radial-size-circle>&lt;length></dfn>

     <dd>
      <p>Gives the radius of the circle explicitly. Negative values are
       invalid.</p>

      <p class=note>Note that 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 href="#ltshape"><i>&lt;shape></i></a> is specified as
     &lsquo;<code class=css>ellipse</code>&rsquo; or is omitted,
     <i>&lt;extent></i> may instead be given explicitly as:

    <dl>
     <dt><dfn id=radial-size-ellipse>[&lt;length> | &lt;percentage>]{2}</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
      href="#gradient-box"><i>gradient box</i></a>. Negative values are
      invalid.
    </dl>
  </dl>

  <div class=note>
   <p>Expanded with the above definitions, the grammar becomes:

   <pre>
&lt;radial-gradient> = radial-gradient(
  [ [ circle               || &lt;length> ]                          [ at &lt;position> ]? , |
    [ ellipse              || [ &lt;length> | &lt;percentage> ]{2} ]    [ at &lt;position> ]? , |
    [ [ circle | ellipse ] || &lt;extent-keyword> ]                  [ at &lt;position> ]? , |
    at &lt;position> ,
  ]?
  &lt;color-stop> [ , &lt;color-stop> ]+
)
&lt;extent-keyword> = closest-corner | closest-side | farthest-corner | farthest-side</pre>
  </div>

  <h4 class=no-toc id=radial-color-stops><span class=secno>4.2.2. </span>
   Placing Color Stops</h4>

  <p>Color-stops are placed on a <dfn id=gradient-ray>gradient-ray</dfn>,
   similar to the <a href="#gradient-line"><i>gradient-line</i></a> of linear
   gradients. The <a href="#gradient-ray"><i>gradient-ray</i></a> is anchored
   at the center of the gradient and extends toward the right. The 0%
   location is at the start of the <a
   href="#gradient-ray"><i>gradient-ray</i></a>, and the 100% location is on
   the point where the <a href="#gradient-ray"><i>gradient-ray</i></a>
   intersects the <a href="#ending-shape"><i>ending shape</i></a>. Negative
   locations can be specified; though negative locations are never directly
   consulted for rendering, they can affect the color of non-negative
   locations on the <a href="#gradient-ray"><i>gradient-ray</i></a> through
   interpolation. For example, &lsquo;<code class=css>radial-gradient(red
   -50px, yellow 100px)</code>&rsquo; 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>When drawing the concentric ellipses of the gradient, the color of each
   ellipse is the color of the <a
   href="#gradient-ray"><i>gradient-ray</i></a> at the point where the
   ellipse intersects the ray.

  <h4 class=no-toc id=degenerate-radials><span class=secno>4.2.3. </span>
   Degenerate Radial Gradients</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 href="#gradient-box"><i>gradient box</i></a> edge and &lsquo;<code
   class=css>closest-side</code>&rsquo; or &lsquo;<code
   class=css>closest-corner</code>&rsquo; 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:

  <dl>
   <dt>If the <a href="#ending-shape"><i>ending shape</i></a> is a circle
    with zero width or height:

   <dd>Render as if the <a href="#ending-shape"><i>ending shape</i></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 href="#ending-shape"><i>ending shape</i></a> has zero width
    (regardless of the height):

   <dd>Render as if the <a href="#ending-shape"><i>ending shape</i></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 &lsquo;<code class=css>0px</code>&rsquo;.</span>

   <dt>Otherwise, if the <a href="#ending-shape"><i>ending shape</i></a> has
    zero height:

   <dd>Render as if the <a href="#ending-shape"><i>ending shape</i></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 id=radial-gradient-examples><span class=secno>4.2.4.
   </span> Radial Gradient Examples</h4>

  <p>All of the following examples are applied to a box that is 200px wide
   and 100px tall.

  <div class=example>
   <p>These examples demonstrate different ways to write the basic syntax for
    radial gradients:</p>

   <pre><code>radial-gradient(yellow, green);
radial-gradient(ellipse at center, yellow 0%, green 100%);
radial-gradient(farthest-corner at 50% 50%, yellow, green);</code></pre>

   <p><img alt="" src=radial1.png></p>

   <pre><code>radial-gradient(circle, yellow, green);</code></pre>

   <p><img alt="" src=radial2.png></p>

   <pre><code>radial-gradient(red, yellow, green);</code></pre>

   <p><img alt="" src=radial3.png></p>
  </div>

  <div class=example>
   <p>This image shows a gradient originating from somewhere other than the
    center of the box:</p>

   <pre><code>radial-gradient(farthest-side at left bottom, red, yellow 50px, green);</code></pre>

   <p><img alt="" src=radial4.png></p>
  </div>

  <div class=example>
   <p>Here we illustrate a &lsquo;<code
    class=property>closest-side</code>&rsquo; gradient.</p>

   <pre><code>radial-gradient(closest-side at 20px 30px, red, yellow, green);
radial-gradient(20px 30px at 20px 30px, red, yellow, green);</code></pre>

   <p><img alt="" src=radial6.png></p>

   <pre><code>radial-gradient(closest-side circle at 20px 30px, red, yellow, green);
radial-gradient(20px 20px at 20px 30px, red, yellow, green);</code></pre>

   <p><img alt="" src=radial7.png></p>
  </div>
  <!-- ====================================================================== -->

  <h3 id=repeating-gradients><span class=secno>4.3. </span> Repeating
   Gradients: the &lsquo;<code
   class=css>repeating-linear-gradient()</code>&rsquo; and &lsquo;<code
   class=css>repeating-radial-gradient()</code>&rsquo; notations</h3>

  <p>In addition to the &lsquo;<code
   class=css>linear-gradient()</code>&rsquo; and &lsquo;<code
   class=css>radial-gradient()</code>&rsquo; functions, this specification
   defines &lsquo;<code class=css>repeating-linear-gradient()</code>&rsquo;
   and &lsquo;<code class=css>repeating-radial-gradient()</code>&rsquo;
   functions. These two functions take the same values and are interpreted
   the same as their respective non-repeating siblings defined previously.

  <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, &lsquo;<code
   class=css>repeating-linear-gradient(red 10px, blue 50px)</code>&rsquo; is
   equivalent to &lsquo;<code class=css>linear-gradient(..., red -30px, blue
   10px, red 10px, blue 50px, red 50px, blue 90px, ...)</code>&rsquo;. 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.

  <div class=example>
   <p>Repeating gradient syntax is basically identical to that of
    non-repeating gradients:</p>

   <pre><code>repeating-linear-gradient(red, blue 20px, red 40px)</code></pre>

   <p><img alt="" src=repeating1.png></p>

   <pre><code>repeating-radial-gradient(red, blue 20px, red 40px)</code></pre>

   <p><img alt="" src=repeating2.png></p>

   <pre><code>repeating-radial-gradient(circle closest-side at 20px 30px, red, yellow, green 100%, yellow 150%, red 200%)</code></pre>

   <p><img alt="" src=repeating3.png></p>
  </div>

  <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 href="#find-the-average-color-of-a-gradient"><i
   title=gradient-average-color>find the average color of the
   gradient</i></a> and render the gradient as a solid-color image equal to
   the average color.

  <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 href="#find-the-average-color-of-a-gradient"><i
   title=gradient-average-color>find the average color</i></a> of a gradient
   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>If the height of a repeating radial gradient 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 href="#find-the-average-color-of-a-gradient"><i
   title=gradient-average-color>find the average color of the
   gradient</i></a> and render the gradient as a solid-color image equal to
   the average color.

  <p>To <dfn id=find-the-average-color-of-a-gradient
   title=gradient-average-color>find the average color of a gradient</dfn>,
   run these steps:

  <ol>
   <li>Define <var>list</var> as an initially-empty list of premultiplied
    RGBA colors, and <var>total-length</var> as the distance between first
    and last color stops.

   <li>For each adjacent pair of color-stops, define <var>weight</var> as
    half the distance between the two color-stops, divided by
    <var>total-length</var>. Add two entries to <var>list</var>, the first
    obtained by representing the color of the first color-stop in
    premultiplied sRGBA and scaling all of the components by
    <var>weight</var>, and the second obtained in the same way with the
    second color-stop.

   <li>Sum the entries of <var>list</var> component-wise to produce the
    average color, and return it.
  </ol>

  <p class=note>As usual, implementations may use whatever algorithm they
   wish, so long as it produces the same result as the above.

  <div class=example>
   <p>For example, the following gradient is rendered as a solid light-purple
    image (equal to <code>rgb(75%,50%,75%)</code>):</p>

   <pre class=css>repeating-linear-gradient(red 0px, white 0px, blue 0px);</pre>

   <p>The following gradient would render the same as the previous under
    normal circumstances (because desktop monitors can't faithfully render
    color-stops 1/10th of a pixel apart), but would render as a normal
    repeating gradient if, for example, the author applied "zoom:100;" to the
    element on which the gradient appears:</p>

   <pre class=css>repeating-linear-gradient(red 0px, white .1px, blue .2px);</pre>
  </div>
  <!-- ====================================================================== -->

  <h3 id=color-stop-syntax><span class=secno>4.4. </span> Gradient
   Color-Stops</h3>

  <pre class=prod><code><dfn
   id=ltcolor-stop>&lt;color-stop></dfn> = &lt;color> [ &lt;percentage> | &lt;length> ]?</code></pre>

  <p>Color-stops are points placed along the line defined by the <a
   href="#gradient-line"><i>gradient-line</i></a> at the beginning of the
   rule. Color-stops must be specified in order. Percentages refer to the
   length of the gradient-line, with 0% being at the starting point and 100%
   being at the ending point. Lengths are measured from the starting-point in
   the direction of the ending-point. Color-stops are usually placed between
   the starting-point and ending-point, but that's not required; the
   gradient-line extends infinitely in both directions, and a color-stop can
   be placed at any position on the line.

  <p>At each color-stop, the line is the color of the color-stop. Between two
   color-stops, the line's color is linearly interpolated between the colors
   of the two color-stops, with the interpolation taking place in
   premultiplied RGBA space. Before the first color-stop, the line is the
   color of the first color-stop. After the last color-stop, the line is the
   color of the last color-stop.

  <p>The following steps must be applied <em>in order</em> to process the
   list of color-stops. After applying these rules, all color-stops will have
   a definite position and they will be in ascending order:

  <ol>
   <li>If the first color-stop does not have a position, set its position to
    0%. If the last color-stop does not have a position, set its position to
    100%.

   <li>If a color-stop has a position that is less than the specified
    position of any color-stop before it in the list, set its position to be
    equal to the largest specified position of any color-stop before it.

   <li>If any color-stop still does not have a position, then, for each run
    of adjacent color-stops without positions, set their positions so that
    they are evenly spaced between the preceding and following color-stops
    with positions.
  </ol>

  <p>If multiple color-stops have the same position, they produce an
   infinitesimal transition from the one specified first in the rule to the
   one specified last. In effect, the color suddenly changes at that position
   rather than smoothly transitioning.

  <div class=example>
   <p>Below are several pairs of gradients. The latter of each pair is a
    manually "fixed-up" version of the former, obtained by applying the above
    rules. For each pair, both gradients will render identically. <span
    class=note>The numbers in each arrow specify which fixup steps are
    invoked in the transformation.</span></p>

   <pre><code>
1. linear-gradient(red, white 20%, blue)
   =1=>
   linear-gradient(red 0%, white 20%, blue 100%)

2. linear-gradient(red 40%, white, black, blue)
   =13=>
   linear-gradient(red 40%, white 60%, black 80%, blue 100%)

3. linear-gradient(red -50%, white, blue)
   =13=>
   linear-gradient(red -50%, white 25%, blue 100%)

4. linear-gradient(red -50px, white, blue)
   =13=>
   linear-gradient(red -50px, white calc(-25px + 50%), blue 100%)

5. linear-gradient(red 20px, white 0px, blue 40px)
   =2=>
   linear-gradient(red 20px, white 20px, blue 40px)

6. linear-gradient(red, white -50%, black 150%, blue)
   =12=>
   linear-gradient(red 0%, white 0%, black 150%, blue 150%)

7. linear-gradient(red 80px, white 0px, black, blue 100px)
   =23=>
   linear-gradient(red 80px, white 80px, black 90px, blue 100px)</code></pre>
  </div>

  <div class=example>
   <p>The following example illustrates* the difference between a gradient
    transitioning in pre-multiplied sRGBA and one transitioning (incorrectly)
    in non-premultiplied. In both of these example, the gradient is drawn
    over a white background. Both gradients could be written with the
    following value:</p>

   <pre><code>linear-gradient(90deg, red, transparent, blue)</code></pre>

   <p>In premultiplied space, transitions to or from "transparent" always
    look nice:</p>

   <p><object data=gradient2.svg height=100 width=200>(Image requires
    SVG)</object></p>

   <p>On the other hand, if a gradient were to incorrectly transition in
    non-premultiplied space, the colors near "transparent" would noticeably
    darken to a grayish color, because "transparent" is actually a shorthand
    for &lsquo;<code class=css>rgba(0,0,0,0)</code>&rsquo;, or transparent
    black:</p>

   <p><object data=gradient3.svg height=100 width=200>(Image requires
    SVG)</object></p>

   <p><small>* SVG-in-HTML support required to view the images.</small></p>
  </div>

  <p class=note>Note: It is recommended that authors not mix different types
   of units, such as px, em, or %, in a single rule, as this can cause a
   color-stop to unintentionally try to move before an earlier one. For
   example, the rule &lsquo;<code class=css>background-image:
   linear-gradient(yellow 100px, blue 50%)</code>&rsquo; wouldn't require any
   fix-up as long as the background area is at least 200px tall. If it was
   150px tall, however, the blue color-stop's position would be equivalent to
   "75px", which precedes the yellow color-stop, and would be corrected to a
   position of 100px.

  <p class=note>Note: The definition and implications of "premultiplied"
   color spaces are given elsewhere in the technical literature, but a quick
   primer is given here to illuminate the process. Given a color expressed as
   an rgba() 4-tuple, one can convert this to a premultiplied representation
   by multiplying the red, green, and blue components by the alpha component.
   For example, a partially-transparent blue may be given as
   rgba(0,0,255,.5), which would then be expressed as [0, 0, 127.5, .5] in
   its premultiplied representation. Interpolating colors using the
   premultiplied representations rather than the plain rgba representations
   tends to produce more attractive transitions, particularly when
   transitioning from a fully opaque color to fully transparent. Note that
   transitions where either the transparency or the color are held constant
   (for example, transitioning between rgba(255,0,0,100%) and
   rgba(0,0,255,100%) or rgba(255,0,0,100%) and rgba(255,0,0,0%)) have
   identical results whether the color interpolation is done in premultiplied
   or non-premultiplied color-space. Differences only arise when both the
   color and transparency differ between the two endpoints.</p>
  <!-- ====================================================================== -->

  <h2 id=sizing><span class=secno>5. </span> Sizing Images and Objects in CSS</h2>

  <p>Images used in CSS may come from a number of sources, from binary image
   formats (such as gif, jpeg, etc), dedicated markup formats (such as SVG),
   and CSS-specific formats (such as the linear-gradient() value type defined
   in this specification). As well, a document may contain many other types
   of objects, such as video, plugins, or nested documents. These images and
   objects (just <dfn id=objects>objects</dfn> hereafter) may offer many
   types of sizing information to CSS, or none at all. This section defines
   generically the size negotiation model between the object and the CSS
   layout algorithms.</p>
  <!-- ====================================================================== -->

  <h3 id=sizing-terms><span class=secno>5.1. </span> Object-Sizing
   Terminology</h3>

  <p>In order to define this handling, we define a few terms, to make it
   easier to refer to various concepts:

  <dl>
   <dt><dfn id=intrinsic-dimensions>intrinsic dimensions</dfn>

   <dd>
    <p>An object's intrinsic dimensions are its preferred, natural width,
     height, and aspect ratio, if they exist. There can be an <dfn
     id=intrinsic-height>intrinsic height</dfn> and <dfn
     id=intrinsic-width>intrinsic width</dfn>, defining a definite rectangle
     and an <a href="#intrinsic-aspect-ratio"><i>intrinsic aspect
     ratio</i></a>. (Most bitmap images fall into this category.) There can
     be only an <dfn id=intrinsic-aspect-ratio>intrinsic aspect ratio</dfn>
     defining the relation of the width to the height, but no definite size.
     (SVG images designed to scale may fall into this category.) There can be
     just an intrinsic height or width. Or there can be no intrinsic
     dimensions at all, implying that the object has no preferred size or
     aspect ratio. (Embedded documents are often assumed to have no intrinsic
     size, as are CSS gradients, defined in this specification.)</p>

    <p>If an object (such as an icon) has multiple sizes, then the largest
     size is taken as its intrinsic size. If it has multiple aspect ratios at
     that size, or has multiple aspect ratios and no size, then the aspect
     ratio closest to the aspect ratio of the <a
     href="#default-object-size"><i>default object size</i></a> is used.</p>

   <dt><dfn id=specified-size>specified size</dfn>

   <dd>The specified size of an object is given by CSS, such as through the
    &lsquo;<code class=property>width</code>&rsquo; and &lsquo;<code
    class=property>height</code>&rsquo; or &lsquo;<code
    class=property>background-size</code>&rsquo; properties. The specified
    size can be a definite width and height, a set of constraints, or a
    combination thereof.

   <dt><dfn id=concrete-object-size>concrete object size</dfn>

   <dd>The <a href="#concrete-object-size"><i>concrete object size</i></a> is
    the result of transforming the <a
    href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a> into a
    concrete size, based on the <a href="#specified-size"><i>specified
    size</i></a> and the <a href="#default-object-size"><i>default object
    size</i></a>. A <a href="#concrete-object-size"><i>concrete object
    size</i></a> always has a definite height and width.

   <dt><dfn id=default-object-size>default object size</dfn>

   <dd>
    <p>The <a href="#default-object-size"><i>default object size</i></a> is a
     rectangle with a definite height and width used to determine the <a
     href="#concrete-object-size"><i>concrete object size</i></a> when both
     the <a href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a> and
     <a href="#specified-size"><i>specified size</i></a> are missing
     dimensions. It varies based on the context in which that the image is
     being laid out.</p>

    <p>The following list defines the <a
     href="#default-object-size"><i>default object size</i></a> of properties
     and contexts that appear in CSS 2.1, plus &lsquo;<code
     class=property>border-image</code>&rsquo; from CSS 3 Backgrounds &amp;
     Borders. Newer modules that accept an &lsquo;<a href="#ltimage"><code
     class=css>&lt;image></code></a>&rsquo; component value in a new context
     must define the <a href="#default-object-size"><i>default object
     size</i></a> in that context.</p>

    <dl>
     <dt>&lsquo;<code class=property>background-image</code>&rsquo;

     <dd>The <a href="#default-object-size"><i>default object size</i></a> is
      the size of the element's <a
      href="http://www.w3.org/TR/css3-background/#background-positioning-area">background
      positioning area</a>. <a href="#CSS3BG"
      rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a>

     <dt>&lsquo;<code class=property>list-style-image</code>&rsquo;

     <dd>The <a href="#default-object-size"><i>default object size</i></a> is
      a 1em square. <a href="#CSS21"
      rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>

     <dt>&lsquo;<code class=property>border-image</code>&rsquo;

     <dd>The <a href="#default-object-size"><i>default object size</i></a> is
      the size of the element's <a
      href="http://www.w3.org/TR/css3-background/#border-image-area">border
      image area</a>. <a href="#CSS3BG"
      rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a>

     <dt>&lsquo;<code class=property>cursor</code>&rsquo;

     <dd>The <a href="#default-object-size"><i>default object size</i></a> is
      a UA-defined size that should be based on the size of a typical cursor
      on the UA's operating system. <a href="#CSS21"
      rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>

     <dt>replaced elements

     <dd>The <a href="#default-object-size"><i>default object size</i></a> is
      a rectangle 300px wide and 150px tall. <a href="#CSS21"
      rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
    </dl>
  </dl>
  <!-- ====================================================================== -->

  <h3 id=object-negotiation><span class=secno>5.2. </span> CSS&#x21CB;Object
   Negotiation</h3>

  <p>Objects in CSS are sized and rendered as follows:

  <ol>
   <li>When an image or object is specified in a document, such as through a
    &lsquo;<code class=css>url()</code>&rsquo; value in a &lsquo;<code
    class=property>background-image</code>&rsquo; property or an
    <code>src</code> attribute on an <code>&lt;img></code> element, CSS
    queries the object for its <a href="#intrinsic-dimensions"><i>intrinsic
    dimensions</i></a>.

   <li>Using the <a href="#intrinsic-dimensions"><i>intrinsic
    dimensions</i></a>, the <a href="#specified-size"><i>specified
    size</i></a>, and the <a href="#default-object-size"><i>default object
    size</i></a> for the context the image or object is used in, CSS then
    computes a <a href="#concrete-object-size"><i>concrete object
    size</i></a>. (See the <a href="#default-sizing">following section</a>.)
    This defines the size and position of the region the object will render
    in.

   <li>CSS asks the object to render itself at the <a
    href="#concrete-object-size"><i>concrete object size</i></a>. CSS does
    not define how objects render when the <a
    href="#concrete-object-size"><i>concrete object size</i></a> is different
    from the object's <a href="#intrinsic-dimensions"><i>intrinsic
    dimensions</i></a>. The object may adjust itself to match the <a
    href="#concrete-object-size"><i>concrete object size</i></a> in some way,
    or even render itself larger or smaller than the <a
    href="#concrete-object-size"><i>concrete object size</i></a> to satisfy
    sizing constraints of its own.

   <li>Unless otherwise specified by CSS, the object is then clipped to the
    <a href="#concrete-object-size"><i>concrete object size</i></a>.
  </ol>
  <!-- ====================================================================== -->

  <h3 id=default-sizing><span class=secno>5.3. </span> Default Concrete
   Object Size Resolution</h3>

  <p>In the absence of more specific rules, an object's <a
   href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a> are resolved
   to a <a href="#concrete-object-size"><i>concrete object size</i></a> as
   follows:

  <ul>
   <li>If the <a href="#specified-size"><i>specified size</i></a> is a
    definite width and height, the <a
    href="#concrete-object-size"><i>concrete object size</i></a> is given
    that width and height.

   <li>If the <a href="#specified-size"><i>specified size</i></a> has only a
    width or height, but not both, then the <a
    href="#concrete-object-size"><i>concrete object size</i></a> is given
    that specified width or height. The other dimension is calculated as
    follows:
    <ol>
     <li>If the object has an <a href="#intrinsic-aspect-ratio"><i>intrinsic
      aspect ratio</i></a>, the missing dimension of the <a
      href="#concrete-object-size"><i>concrete object size</i></a> is
      calculated using the <i>intrinsic aspect-ratio</i> and the present
      dimension.

     <li>Otherwise, if the missing dimension is present in the object's <a
      href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a>, the
      missing dimension is taken from the object's <a
      href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a>.

     <li>Otherwise, the missing dimension of the <a
      href="#concrete-object-size"><i>concrete object size</i></a> is taken
      from the <a href="#default-object-size"><i>default object size</i></a>.
    </ol>

   <li>If the <a href="#specified-size"><i>specified size</i></a> has neither
    a definite width nor height, and has no additional contraints, the
    dimensions of the <a href="#concrete-object-size"><i>concrete object
    size</i></a> are calculated as follows:
    <ol>
     <li>If the object has only an <a
      href="#intrinsic-aspect-ratio"><i>intrinsic aspect ratio</i></a>, the
      <a href="#concrete-object-size"><i>concrete object size</i></a> must
      have that aspect ratio, and additionally be as large as possible
      without either its height or width exceeding the height or width of the
      <a href="#default-object-size"><i>default object size</i></a>.
      Otherwise, the width and height of the <a
      href="#concrete-object-size"><i>concrete object size</i></a> is the
      same as the object's <a href="#intrinsic-width"><i>intrinsic
      width</i></a> and <a href="#intrinsic-height"><i>intrinsic
      height</i></a>, if they exist.

     <li>If the <a href="#concrete-object-size"><i>concrete object
      size</i></a> is still missing a width or height, and the object has an
      <a href="#intrinsic-aspect-ratio"><i>intrinsic aspect ratio</i></a>,
      the missing dimension is calculated from the present dimension and the
      <a href="#intrinsic-aspect-ratio"><i>intrinsic aspect ratio</i></a>.
      Otherwise, the missing dimension is taken from the <a
      href="#default-object-size"><i>default object size</i></a>.
    </ol>
  </ul>

  <p>If the <a href="#specified-size"><i>specified size</i></a> has
   additional constraints, the <a href="#concrete-object-size"><i>concrete
   object size</i></a> must be sized to satisfy those constraints. For
   example, the &lsquo;<code class=property>min-width</code>&rsquo;,
   &lsquo;<code class=property>min-height</code>&rsquo;, &lsquo;<code
   class=property>max-width</code>&rsquo;, and &lsquo;<code
   class=property>max-height</code>&rsquo; properties specify slightly more
   complex handling for sizing replaced elements, and &lsquo;<code
   class=css>background-repeat: round</code>&rsquo; can further adjust the
   size calculated by &lsquo;<code
   class=property>background-size</code>&rsquo; so that the image fits a
   whole number of times into the background positioning area.</p>
  <!-- ====================================================================== -->

  <h3 id=object-fit><span class=secno>5.4. </span> Sizing Objects: the
   &lsquo;<a href="#object-fit0"><code
   class=property>object-fit</code></a>&rsquo; property</h3>

  <table class=propdef>
   <thead>
    <tr>
     <th>Name:

     <td><dfn id=object-fit0>object-fit</dfn>

   <tbody>
    <tr>
     <th>Value:

     <td>fill | contain | cover | none | scale-down

    <tr>
     <th>Initial:

     <td>fill

    <tr>
     <th>Applies to:

     <td>replaced elements

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed value:

     <td>specified value
  </table>

  <p>The &lsquo;<a href="#object-fit0"><code
   class=property>object-fit</code></a>&rsquo; property specifies how the
   contents of a replaced element should be scaled relative to the box
   established by its used height and width. It also enables scaling a
   replaced element up to a specified maximum size or down to a specified
   minimum size while preserving its aspect ratio.

  <p>Not all replaced elements can be scaled, but images typically can.

  <p>If the replaced element's content does not have an <a
   href="#intrinsic-aspect-ratio"><i>intrinsic aspect ratio</i></a>, all of
   the values for &lsquo;<a href="#object-fit0"><code
   class=property>object-fit</code></a>&rsquo; are treated as &lsquo;<code
   class=css>fill</code>&rsquo;. Otherwise, the object is scaled as follows:

  <dl>
   <dt>&lsquo;<code class=css>fill</code>&rsquo;

   <dd>
    <p>Set the object's size to the <a
     href="#concrete-object-size"><i>concrete object size</i></a> obtained by
     running the <a href="#default-sizing">object sizing algorithm</a> with a
     <a href="#specified-size"><i>specified size</i></a> and a <a
     href="#default-object-size"><i>default object size</i></a> equal to the
     replaced element's used width and height.</p>

    <p>This will make the contents exactly fill the replaced element's
     content box.</p>

   <dt>&lsquo;<code class=css>contain</code>&rsquo;

   <dd>
    <p>Determine the used &lsquo;<code class=property>height</code>&rsquo;
     and &lsquo;<code class=property>width</code>&rsquo; of the element as
     usual, except: If both &lsquo;<code class=property>height</code>&rsquo;
     and &lsquo;<code class=property>width</code>&rsquo; are &lsquo;<code
     class=css>auto</code>&rsquo;, and the used value of at least one of
     &lsquo;<code class=property>max-width</code>&rsquo; and &lsquo;<code
     class=property>max-height</code>&rsquo; is not &lsquo;<code
     class=css>none</code>&rsquo;, then compute the element's used width and
     used height as though the intrinsic dimensions of the contents were
     infinitely large numbers whose ratio is the actual intrinsic ratio of
     the contents. This will proportionally scale the used width and height
     up to the given maximum constraints.</p>

    <p>Set the <a href="#concrete-object-size"><i>concrete object
     size</i></a> to the largest width and height that has the same aspect
     ratio as the object's intrinsic aspect ratio, and additionally has
     neither width nor height larger than the replaced element's used width
     and height, respectively.</p>

   <dt>&lsquo;<code class=css>cover</code>&rsquo;

   <dd>
    <p>Determine the used &lsquo;<code class=property>height</code>&rsquo;
     and &lsquo;<code class=property>width</code>&rsquo; of the element as
     usual, except: If both &lsquo;<code class=property>height</code>&rsquo;
     and &lsquo;<code class=property>width</code>&rsquo; are &lsquo;<code
     class=css>auto</code>&rsquo;, and the used value of at least one of
     &lsquo;<code class=property>min-width</code>&rsquo; and &lsquo;<code
     class=property>min-height</code>&rsquo; is not &lsquo;<code
     class=css>none</code>&rsquo;, then compute the element's used width and
     used height as though the intrinsic dimensions of the contents were
     infinitely small numbers whose ratio is the actual intrinsic ratio of
     the contents. This will proportionally scale the used width and height
     down to the given minimum constraints.</p>

    <p>Set the <a href="#concrete-object-size"><i>concrete object
     size</i></a> to the smallest width and height that has the same aspect
     ratio as the object's intrinsic aspect ratio, and additionally has
     neither width nor height smaller than the replaced element's used width
     and height, respectively.</p>

   <dt>&lsquo;<code class=css>none</code>&rsquo;

   <dd>
    <p>Set the content's size to the <a
     href="#concrete-object-size"><i>concrete object size</i></a> obtained by
     running the <i title=default-sizing>object sizing algorithm</i> with no
     <a href="#specified-size"><i>specified size</i></a>, and a <a
     href="#default-object-size"><i>default object size</i></a> equal to the
     replaced element's used width and height.</p>

   <dt>&lsquo;<code class=css>scale-down</code>&rsquo;

   <dd>
    <p>Size the content as if &lsquo;<code class=css>none</code>&rsquo; or
     &lsquo;<code class=css>contain</code>&rsquo; were specified, whichever
     would result in a smaller <a href="#concrete-object-size"><i>concrete
     object size</i></a>.</p>

    <p class=note>Note that both &lsquo;<code class=css>none</code>&rsquo;
     and &lsquo;<code class=css>contain</code>&rsquo; respect the content's
     intrinsic aspect ratio, so the concept of "smaller" is well-defined.</p>
  </dl>

  <p>If the content does not completely fill the replaced element's content
   box, the unfilled space shows the replaced element's background. Since
   replaced elements always clip their contents to the content box, the
   content will never overflow. See the &lsquo;<a
   href="#object-position0"><code
   class=property>object-position</code></a>&rsquo; property for positioning
   the object with respect to the content box.

  <div class=figure>
   <p><img alt="" src="img_scale.png" style="border: thin solid black;"></p>

   <p class=caption>An example showing how four of the values of &lsquo;<a
    href="#object-fit0"><code class=property>object-fit</code></a>&rsquo;
    cause the replaced element (blue figure) to be scaled to fit its
    height/width box (shownwith a green background), using the initial value
    for &lsquo;<a href="#object-position0"><code
    class=property>object-position</code></a>&rsquo;. The fifth value,
    &lsquo;<code class=css><span class=css>scale-down</span>, in this case
    looks identical to </code>&rsquo;<span class=css>contain</span>.</p>
  </div>

  <p class=issue>Find some good use-cases and make examples out of them.

  <p class=note>Note: the &lsquo;<a href="#object-fit0"><code
   class=property>object-fit</code></a>&rsquo; property has similar semantics
   to the <code>fit</code> attribute in <a href="#SMIL10"
   rel=biblioentry>[SMIL10]<!--{{SMIL10}}--></a>.

  <p class=note>Note: Per the <i title=object-negotiation>CSS&#8651;Object
   Negotiation</i> algorithm, the <a href="#concrete-object-size"><i>concrete
   object size</i></a> (or, in this case, the size of the content) does not
   directly scale the object itself - it is merely passed to the object as
   information about the size of the visible canvas. How to then draw into
   that size is up to the image format. In particular, raster images always
   scale to the given size, while SVG uses the given size as the size of the
   "SVG Viewport" (a term defined by SVG) and then uses the values of several
   attributes on the root &lt;svg> element to determine how to draw itself.

  <p>User agents MAY accept &lsquo;<code
   class=property>image-fit</code>&rsquo; as an alias for &lsquo;<a
   href="#object-fit0"><code class=property>object-fit</code></a>&rsquo;, as
   a previous version of this specification used that name. Authors must not
   use &lsquo;<code class=property>image-fit</code>&rsquo; in their
   stylesheets.</p>
  <!-- ====================================================================== -->

  <h3 id=object-position><span class=secno>5.5. </span> Positioning Objects:
   the &lsquo;<a href="#object-position0"><code
   class=property>object-position</code></a>&rsquo; property</h3>

  <table class=propdef>
   <thead>
    <tr>
     <th>Name:

     <td><dfn id=object-position0>object-position</dfn>

   <tbody>
    <tr>
     <th>Value:

     <td>&lt;position>

    <tr>
     <th>Initial:

     <td>50% 50%

    <tr>
     <th>Applies to:

     <td>replaced elements

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Percentages:

     <td>refer to width and height of box itself

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed value:

     <td>specified value
  </table>

  <p>The &lsquo;<a href="#object-position0"><code
   class=property>object-position</code></a>&rsquo; property determines the
   alignment of the replaced element inside its box. The
   <!-- FIXME a href="//www.w3.org/TR/css3-values/#position"--><a
   href="#ltposition"><i>&lt;position></i></a><!--/a--> value type (which is
   also used for &lsquo;<code
   class=property>background-position</code>&rsquo;) is defined in <a
   href="#CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>, and is
   resolved using the <a href="#concrete-object-size"><i>concrete object
   size</i></a> as the object area and the content box as the positioning
   area.

  <p class=note>Note that areas of the box not covered by the replaced
   element will show the element's background.

  <p class=issue>Find some good use-cases and make examples out of them.

  <p>User agents MAY accept &lsquo;<code
   class=property>image-position</code>&rsquo; as an alias for &lsquo;<a
   href="#object-position0"><code
   class=property>object-position</code></a>&rsquo;, as a previous version of
   this specification used that name. Authors must not use &lsquo;<code
   class=property>image-position</code>&rsquo; in their stylesheets.</p>
  <!-- ====================================================================== -->

  <h2 id=image-processing><span class=secno>6. </span> Image Processing</h2>

  <h3 id=image-resolution><span class=secno>6.1. </span> Overriding Image
   Resolutions: the &lsquo;<a href="#image-resolution0"><code
   class=property>image-resolution</code></a>&rsquo; property</h3>

  <p>The <i>image resolution</i> is defined as the number of image pixels per
   unit length, e.g., pixels per inch. Some image formats can record
   information about the resolution of images. This information can be
   helpful when determining the actual size of the image in the formatting
   process. However, the information can also be wrong, in which case it
   should be ignored. By default, CSS assumes a resolution of one image pixel
   per CSS &lsquo;<code class=css>px</code>&rsquo; unit; however, the
   &lsquo;<a href="#image-resolution0"><code
   class=property>image-resolution</code></a>&rsquo; property allows using
   some other resolution.

  <table class=propdef>
   <thead>
    <tr>
     <th>Name:

     <td><dfn id=image-resolution0>image-resolution</dfn>

   <tbody>
    <tr>
     <th>Value:

     <td>[from-image || &lt;resolution>] && snap?

    <tr>
     <th>Initial:

     <td>1dppx

    <tr>
     <th>Applies to:

     <td>all elements

    <tr>
     <th>Inherited:

     <td>yes

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed value:

     <td>as specified, except with &lt;resolution> possibly altered by
      computing for &lsquo;<code class=css>snap</code>&rsquo; (see below)
  </table>

  <p>The &lsquo;<a href="#image-resolution0"><code
   class=property>image-resolution</code></a>&rsquo; property specifies the
   intrinsic resolution of all raster images used in or on the element. (As
   vector formats such as SVG do not have an intrinsic resolution, this
   property has no effect on vector images.) The <dfn
   id=intrinsic-resolution>intrinsic resolution</dfn> of an image is used to
   determine the image's <i>intrinsic size</i>. Affected images include
   images in the element's content (e.g. replaced elements and/or generated
   content), background images, list markers, etc. Values have the following
   meanings:

  <dl>
   <dt>&lsquo;<a href="#ltresolution"><code
    class=css>&lt;resolution></code></a>&rsquo;

   <dd>Specifies the intrinsic resolution explicitly.

   <dt>&lsquo;<code class=css>from-image</code>&rsquo;

   <dd>The image's intrinsic resolution is taken as that specified by the
    image format. If the image does not specify its own resolution, the
    explicitly specified resolution is used (if given), else it defaults to
    &lsquo;<code class=css>1ddpx</code>&rsquo;.

   <dt>&lsquo;<code class=css>snap</code>&rsquo;

   <dd>If the "snap" keyword is provided, the computed &lsquo;<a
    href="#ltresolution"><code class=css>&lt;resolution></code></a>&rsquo;
    (if any) is the specified resolution rounded to the nearest value that
    would map one image pixel to an integer number of device pixels. If the
    resolution is taken from the image, then the used intrinsic resolution is
    the image's native resolution similarly adjusted.
  </dl>

  <div class=example>
   <p>Printers tend to have substantially higher resolution than computer
    monitors; due to this, an image that looks fine on the screen may look
    pixellated when printed out. The &lsquo;<a
    href="#image-resolution0"><code
    class=property>image-resolution</code></a>&rsquo; property can be used to
    embed a high-resolution image into the document and maintain an
    appropriate size, ensuring attractive display both on screen and on
    paper:</p>

   <pre class=css><code>
img.high-res {
	image-resolution: 300dpi;
}</code></pre>

   <p>With this set, an image meant to be 5 inches wide at 300dpi will
    actually display as 5in wide; without this set, the image would display
    as approximately 15.6in wide since the image is 15000 image pixels
    across, and by default CSS displays 96 image pixels per inch.</p>
  </div>

  <div class=example>
   <p>Some image formats can encode the image resolution into the image data.
    This rule specifies that the UA should use the image resolution found in
    the image itself, falling back to 1 image pixel per CSS &lsquo;<code
    class=css>px</code>&rsquo; unit.</p>

   <pre class=css>img { image-resolution: from-image }</pre>

   <p>These rules both specify that the UA should use the image resolution
    found in the image itself, but if the image has no resolution, the
    resolution is set to 300dpi instead of the default &lsquo;<code
    class=css>1dppx</code>&rsquo;.</p>

   <pre>
img { image-resolution: from-image 300dpi }
img { image-resolution: 300dpi from-image }
		</pre>
  </div>

  <div class=example>
   <p>Using this rule, the image resolution is set to 300dpi. (The resolution
    in the image, if any, is ignored.)</p>

   <pre>img { image-resolution: 300dpi }</pre>

   <p>This rule, on the other hand, if used when the screen's resolution is
    96dpi, would instead render the image at 288dpi (so that 3 image pixels
    map to 1 device pixel):</p>

   <pre>img { image-resolution: 300dpi snap; }</pre>

   <p>The &lsquo;<code class=css>snap</code>&rsquo; keyword can also be used
    when the resolution is taken from the image. In this rule:

   <pre>img { image-resolution: snap from-image; }</pre>

   <p>An image declaring itself as 300dpi will, in the situation above,
    display at 288dpi (3 image pixels per device pixel) whereas an image
    declaring 72dpi will render at 96dpi (1 image pixel per device pixel).
  </div>
  <!-- ====================================================================== -->

  <h3 id=image-orientation><span class=secno>6.2. </span> Orienting an Image
   on the Page: the &lsquo;<a href="#image-orientation0"><code
   class=property>image-orientation</code></a>&rsquo; property</h3>

  <p>Sometimes images from camera phones, digital cameras or scanners are
   encoded sideways. For example, the first row of image data can represent
   the leftmost or rightmost column of image pixels. Furthermore, often such
   devices have limited resources, and do not have the capability to rotate
   the image into an upright orientation. However, this type of device may
   have internal knowledge or can accept input from its user as to the
   rotational correction to perform. The &lsquo;<a
   href="#image-orientation0"><code
   class=property>image-orientation</code></a>&rsquo; property provides a way
   to apply an &rdquo;out-of-band&ldquo; rotation to image source data to
   correctly orient an image.

  <p class=note>Note this facility is not intended to specify layout
   transformations such as arbitrary rotation or flipping the image in the
   horizontal or vertical direction. (See <a href="#CSS3-2D-TRANSFORMS"
   rel=biblioentry>[CSS3-2D-TRANSFORMS]<!--{{CSS3-2D-TRANSFORMS}}--></a> for
   a feature designed to do that.) It is also not needed to correctly orient
   an image when printing in landscape versus portrait orientation, as that
   rotation is done as part of layout. (See <a href="#CSS3PAGE"
   rel=biblioentry>[CSS3PAGE]<!--{{CSS3PAGE}}--></a>.) It should only be used
   to correct incorrectly-oriented images.

  <table class=propdef>
   <thead>
    <tr>
     <th>Name:

     <td><dfn id=image-orientation0>image-orientation</dfn>

   <tbody>
    <tr>
     <th>Value:

     <td>&lt;angle>

    <tr>
     <th>Initial:

     <td>0deg

    <tr>
     <th>Applies to:

     <td>images

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed value:

     <td>specified value, rounded and normalized (see text)
  </table>

  <p>This property specifies an orthogonal rotation to be applied to an image
   before it is laid out. CSS layout processing applies to the image
   <em>after</em> rotation. This implies, for example:

  <ul>
   <li>The intrinsic height and width are derived from the rotated rather
    than the original image dimensions.

   <li>The height (width) property applies to the vertical (horizontal)
    dimension of the image, <em>after</em> rotation.
  </ul>

  <p>Positive values cause the image to be rotated to the right (in a
   clockwise direction), while negative values cause a rotation to the left.
   The computed value of the property is calculated by rounding the specified
   angle to the nearest quarter-turn (90deg, 100grad, .25turn, etc.),
   rounding away from 0 (that is, 45deg is rounded to 90deg, while -45deg is
   rounded to -90deg), then moduloing the value by 1 turn (360deg, 400grad,
   etc.).

  <p>If the image itself is transformed in some way (for example, if the
   content of an element is provided by the &lsquo;<code
   class=css>image()</code>&rsquo; function with a directionality opposite
   the element's directionality), the image's transformation must be applied
   before &lsquo;<a href="#image-orientation0"><code
   class=property>image-orientation</code></a>&rsquo; is. As well, &lsquo;<a
   href="#image-orientation0"><code
   class=property>image-orientation</code></a>&rsquo; must be applied before
   any further transformation of the element, such as through CSS Transforms.

  <p class=note>Note that in CSS, orientation data encoded in the image (e.g.
   EXIF data) is ignored. <span class=issue>Is this an issue? What do
   printers do?</span>

  <div class=example>
   <p>The following example rotates the image 90 degrees clockwise:</p>

   <pre>
img.ninety     { image-orientation: 90deg }
...
&lt;img class="ninety" src=... />
		</pre>

   <p>The same effect could be achieved with, for example, an angle of
    -270deg or 450deg.</p>
  </div>

  <h2 id=conformance><span class=secno>7. </span> Conformance</h2>

  <h3 id=conventions><span class=secno>7.1. </span> Document Conventions</h3>

  <p>Conformance requirements are expressed with a combination of descriptive
   assertions and RFC 2119 terminology. The key words “MUST”, “MUST
   NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
   “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the
   normative parts of this document are to be interpreted as described in RFC
   2119. However, for readability, these words do not appear in all uppercase
   letters in this specification.

  <p>All of the text of this specification is normative except sections
   explicitly marked as non-normative, examples, and notes. <a
   href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>

  <p>Examples in this specification are introduced with the words “for
   example” or are set apart from the normative text with
   <code>class="example"</code>, like this:

  <div class=example>
   <p>This is an example of an informative example.</p>
  </div>

  <p>Informative notes begin with the word “Note” and are set apart from
   the normative text with <code>class="note"</code>, like this:

  <p class=note>Note, this is an informative note.

  <h3 id=conformance-classes><span class=secno>7.2. </span> Conformance
   Classes</h3>

  <p>Conformance to CSS Image Values and Replaced Content Module Level 3 is
   defined for three conformance classes:

  <dl>
   <dt><dfn id=style-sheet title="style sheet!!as conformance class">style
    sheet</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
    style sheet</a>.

   <dt><dfn id=renderer>renderer</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
    that interprets the semantics of a style sheet and renders documents that
    use them.

   <dt><dfn id=authoring-tool>authoring tool</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
    that writes a style sheet.
  </dl>

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

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

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

  <h3 id=partial><span class=secno>7.3. </span> Partial Implementations</h3>

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

  <h3 id=experimental><span class=secno>7.4. </span> Experimental
   Implementations</h3>

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

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

  <h3 id=testing><span class=secno>7.5. </span>Non-Experimental
   Implementations</h3>

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

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

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

  <h3 id=cr-exit-criteria><span class=secno>7.6. </span> CR Exit Criteria</h3>

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

  <dl>
   <dt>independent

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

   <dt>interoperable

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

   <dt>implementation

   <dd>a user agent which:
    <ol class=inline>
     <li>implements the specification.

     <li>is available to the general public. The implementation may be a
      shipping product or other publicly available version (i.e., beta
      version, preview release, or “nightly build”). Non-shipping product
      releases must have implemented the feature(s) for a period of at least
      one month in order to demonstrate stability.

     <li>is not experimental (i.e., a version specifically designed to pass
      the test suite and is not intended for normal usage going forward).
    </ol>
  </dl>

  <p>The specification will remain Candidate Recommendation for at least six
   months.

  <h2 class=no-num id=acknowledgments>Acknowledgments</h2>

  <p>Thanks to the Webkit team, Brad Kemper, Brian Manthos, and Alan Gresley
   for their contributions to the definition of gradients; to Melinda Grant
   for her work on &lsquo;<a href="#object-fit0"><code
   class=property>object-fit</code></a>&rsquo;, &lsquo;<a
   href="#object-position0"><code
   class=property>object-position</code></a>&rsquo;, and &lsquo;<a
   href="#image-orientation0"><code
   class=property>image-orientation</code></a>&rsquo;; to Robert O'Callahan
   for the definition of &lsquo;<code class=css>element()</code>&rsquo;; and
   to Michael Day, Håkon Lie, and Shinyu Murakami for &lsquo;<a
   href="#image-resolution0"><code
   class=css>image-resolution</code></a>&rsquo;.

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

  <h3 class=no-num id=normative-references>Normative references</h3>
  <!--begin-normative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS21>[CSS21]

   <dd>Bert Bos; et al. <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607"><cite>Cascading Style
    Sheets Level 2 Revision 1 (CSS&#160;2.1) Specification.</cite></a> 7 June
    2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607">http://www.w3.org/TR/2011/REC-CSS2-20110607</a>
    </dd>
   <!---->

   <dt id=CSS3VAL>[CSS3VAL]

   <dd>H&#229;kon Wium Lie; Tab Atkins; Elika J. Etemad. <a
    href="http://www.w3.org/TR/2011/WD-css3-values-20110906/"><cite>CSS
    Values and Units Module Level 3.</cite></a> 6 September 2011. W3C Working
    Draft. (Work in progress.) URL: <a
    href="http://www.w3.org/TR/2011/WD-css3-values-20110906/">http://www.w3.org/TR/2011/WD-css3-values-20110906/</a>
    </dd>
   <!---->

   <dt id=MEDIA-FRAGS>[MEDIA-FRAGS]

   <dd>Rapha&#235;l Troncy; et al. <a
    href="http://www.w3.org/TR/2011/CR-media-frags-20111201/"><cite>Media
    Fragments URI 1.0.</cite></a> 1 December 2011. W3C Candidate
    Recommendation. (Work in progress.) URL: <a
    href="http://www.w3.org/TR/2011/CR-media-frags-20111201/">http://www.w3.org/TR/2011/CR-media-frags-20111201/</a>
    </dd>
   <!---->

   <dt id=RFC2119>[RFC2119]

   <dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key
    words for use in RFCs to Indicate Requirement Levels.</cite></a> Internet
    RFC 2119. URL: <a
    href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
    </dd>
   <!---->

   <dt id=SELECT>[SELECT]

   <dd>Tantek &#199;elik; et al. <a
    href="http://www.w3.org/TR/2011/REC-css3-selectors-20110929/"><cite>Selectors
    Level 3.</cite></a> 29 September 2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-css3-selectors-20110929/">http://www.w3.org/TR/2011/REC-css3-selectors-20110929/</a>
    </dd>
   <!---->
  </dl>
  <!--end-normative-->

  <h3 class=no-num id=other-references>Other references</h3>
  <!--begin-informative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS1>[CSS1]

   <dd>H&#229;kon Wium Lie; Bert Bos. <a
    href="http://www.w3.org/TR/2008/REC-CSS1-20080411"><cite>Cascading Style
    Sheets (CSS1) Level 1 Specification.</cite></a> 11 April 2008. W3C
    Recommendation. URL: <a
    href="http://www.w3.org/TR/2008/REC-CSS1-20080411">http://www.w3.org/TR/2008/REC-CSS1-20080411</a>
    </dd>
   <!---->

   <dt id=CSS3-2D-TRANSFORMS>[CSS3-2D-TRANSFORMS]

   <dd>Dean Jackson; David Hyatt; Chris Marrin. <a
    href="http://www.w3.org/TR/2009/WD-css3-2d-transforms-20091201"><cite>CSS
    2D Transforms Module Level 3.</cite></a> 1 December 2009. W3C Working
    Draft. (Work in progress.) URL: <a
    href="http://www.w3.org/TR/2009/WD-css3-2d-transforms-20091201">http://www.w3.org/TR/2009/WD-css3-2d-transforms-20091201</a>
    </dd>
   <!---->

   <dt id=CSS3BG>[CSS3BG]

   <dd>Bert Bos; Elika J. Etemad; Brad Kemper. <a
    href="http://www.w3.org/TR/2011/CR-css3-background-20110215"><cite>CSS
    Backgrounds and Borders Module Level 3.</cite></a> 15 February 2011. W3C
    Candidate Recommendation. (Work in progress.) URL: <a
    href="http://www.w3.org/TR/2011/CR-css3-background-20110215">http://www.w3.org/TR/2011/CR-css3-background-20110215</a>
    </dd>
   <!---->

   <dt id=CSS3COLOR>[CSS3COLOR]

   <dd>Tantek &#199;elik; Chris Lilley; L. David Baron. <a
    href="http://www.w3.org/TR/2011/REC-css3-color-20110607"><cite>CSS Color
    Module Level 3.</cite></a> 7 June 2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-css3-color-20110607">http://www.w3.org/TR/2011/REC-css3-color-20110607</a>
    </dd>
   <!---->

   <dt id=CSS3PAGE>[CSS3PAGE]

   <dd>H&#229;kon Wium Lie; Melinda Grant. <a
    href="http://www.w3.org/TR/2006/WD-css3-page-20061010"><cite>CSS3 Module:
    Paged Media.</cite></a> 10 October 2006. W3C Working Draft. (Work in
    progress.) URL: <a
    href="http://www.w3.org/TR/2006/WD-css3-page-20061010">http://www.w3.org/TR/2006/WD-css3-page-20061010</a>
    </dd>
   <!---->

   <dt id=MEDIAQ>[MEDIAQ]

   <dd>H&#229;kon Wium Lie; et al. <a
    href="http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/"><cite>Media
    Queries.</cite></a> 27 July 2010. W3C Candidate Recommendation. (Work in
    progress.) URL: <a
    href="http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/">http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/</a>
    </dd>
   <!---->

   <dt id=SMIL10>[SMIL10]

   <dd>Philipp Hoschka. <a
    href="http://www.w3.org/TR/1998/REC-smil-19980615"><cite>Synchronized
    Multimedia Integration Language (SMIL) 1.0 Specification.</cite></a> 15
    June 1998. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/1998/REC-smil-19980615">http://www.w3.org/TR/1998/REC-smil-19980615</a>
    </dd>
   <!---->
  </dl>
  <!--end-informative-->

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

  <ul class=indexlist>
   <li>authoring tool, <a href="#authoring-tool"
    title="authoring tool"><strong>7.2.</strong></a>

   <li>&lsquo;<code class=css>closest-corner</code>&rsquo;, <a
    href="#radial-closest-corner"
    title="''closest-corner''"><strong>4.2.1.</strong></a>

   <li>&lsquo;<code class=css>closest-side</code>&rsquo;, <a
    href="#radial-closest-side"
    title="''closest-side''"><strong>4.2.1.</strong></a>

   <li>&lt;color-stop>, <a href="#ltcolor-stop"
    title="&lt;color-stop>"><strong>4.4.</strong></a>

   <li>concrete object size, <a href="#concrete-object-size"
    title="concrete object size"><strong>5.1.</strong></a>

   <li>CSS element reference identifier, <a
    href="#css-element-reference-identifier"
    title="CSS element reference identifier"><strong>3.3.</strong></a>

   <li>default object size, <a href="#default-object-size"
    title="default object size"><strong>5.1.</strong></a>

   <li>&lsquo;<code class=css>dpcm</code>&rsquo;, <a href="#dpcm"
    title="''dpcm''"><strong>2.</strong></a>

   <li>&lsquo;<code class=css>dpi</code>&rsquo;, <a href="#dpi"
    title="''dpi''"><strong>2.</strong></a>

   <li>&lsquo;<code class=css>dppx</code>&rsquo;, <a href="#dppx"
    title="''dppx''"><strong>2.</strong></a>

   <li>&lt;element-reference>, <a href="#ltelement-reference"
    title="&lt;element-reference>"><strong>3.3.</strong></a>

   <li>ending shape, <a href="#ending-shape"
    title="ending shape"><strong>4.2.</strong></a>

   <li>&lsquo;<code class=css>farthest-corner</code>&rsquo;, <a
    href="#radial-farthest-corner"
    title="''farthest-corner''"><strong>4.2.1.</strong></a>

   <li>&lsquo;<code class=css>farthest-side</code>&rsquo;, <a
    href="#radial-farthest-side"
    title="''farthest-side''"><strong>4.2.1.</strong></a>

   <li>&lt;gradient>, <a href="#ltgradient"
    title="&lt;gradient>"><strong>4.</strong></a>

   <li>gradient-average-color, <a
    href="#find-the-average-color-of-a-gradient"
    title=gradient-average-color><strong>4.3.</strong></a>

   <li>gradient box, <a href="#gradient-box"
    title="gradient box"><strong>4.</strong></a>

   <li>gradient-line, <a href="#gradient-line"
    title=gradient-line><strong>4.1.1.</strong></a>

   <li>gradient-ray, <a href="#gradient-ray"
    title=gradient-ray><strong>4.2.2.</strong></a>

   <li>&lt;image>, <a href="#ltimage"
    title="&lt;image>"><strong>3.</strong></a>

   <li>&lt;image-decl>, <a href="#ltimage-decl"
    title="&lt;image-decl>"><strong>3.2.</strong></a>

   <li>&lt;image-list>, <a href="#ltimage-list"
    title="&lt;image-list>"><strong>3.2.</strong></a>

   <li>image-orientation, <a href="#image-orientation0"
    title=image-orientation><strong>6.2.</strong></a>

   <li>image-resolution, <a href="#image-resolution0"
    title=image-resolution><strong>6.1.</strong></a>

   <li>intrinsic aspect ratio, <a href="#intrinsic-aspect-ratio"
    title="intrinsic aspect ratio"><strong>5.1.</strong></a>

   <li>intrinsic dimensions, <a href="#intrinsic-dimensions"
    title="intrinsic dimensions"><strong>5.1.</strong></a>

   <li>intrinsic height, <a href="#intrinsic-height"
    title="intrinsic height"><strong>5.1.</strong></a>

   <li>intrinsic resolution, <a href="#intrinsic-resolution"
    title="intrinsic resolution"><strong>6.1.</strong></a>

   <li>intrinsic width, <a href="#intrinsic-width"
    title="intrinsic width"><strong>5.1.</strong></a>

   <li>&lt;length>, <a href="#radial-size-circle"
    title="&lt;length>"><strong>4.2.1.</strong></a>

   <li>[&lt;length> | &lt;percentage>]{2}, <a href="#radial-size-ellipse"
    title="[&lt;length> | &lt;percentage>]{2}"><strong>4.2.1.</strong></a>

   <li>&lt;linear-gradient>, <a href="#ltlinear-gradient"
    title="&lt;linear-gradient>"><strong>4.1.1.</strong></a>

   <li>object-fit, <a href="#object-fit0"
    title=object-fit><strong>5.4.</strong></a>

   <li>object-position, <a href="#object-position0"
    title=object-position><strong>5.5.</strong></a>

   <li>objects, <a href="#objects" title=objects><strong>5.</strong></a>

   <li>paint-source, <a href="#paint-source"
    title=paint-source><strong>3.3.</strong></a>

   <li>&lt;position>, <a href="#ltposition"
    title="&lt;position>"><strong>4.2.1.</strong></a>

   <li>renderer, <a href="#renderer" title=renderer><strong>7.2.</strong></a>


   <li>&lt;resolution>, <a href="#ltresolution"
    title="&lt;resolution>"><strong>2.</strong></a>

   <li>&lt;shape>, <a href="#ltshape"
    title="&lt;shape>"><strong>4.2.1.</strong></a>

   <li>&lt;side-or-corner>, <a href="#side-or-corner"
    title="&lt;side-or-corner>"><strong>4.1.1.</strong></a>

   <li>&lt;size>, <a href="#ltsize"
    title="&lt;size>"><strong>4.2.1.</strong></a>

   <li>specified size, <a href="#specified-size"
    title="specified size"><strong>5.1.</strong></a>

   <li>style sheet
    <ul>
     <li>as conformance class, <a href="#style-sheet"
      title="style sheet, as conformance class"><strong>7.2.</strong></a>
    </ul>
  </ul>
  <!--end-index-->

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

  <table class=proptable>
   <thead>
    <tr>
     <th>Property

     <th>Values

     <th>Initial

     <th>Applies&nbsp;to

     <th>Inh.

     <th>Percentages

     <th>Media

   <tbody>
    <tr>
     <th><span class=property>Name:</span>

     <td>Value:

     <td>Initial:

     <td>Applies to:

     <td>Inherited:

     <td>Media:

     <td>Computed value:

    <tr>
     <th><span class=property>Name:</span>

     <td>Value:

     <td>Initial:

     <td>Applies to:

     <td>Inherited:

     <td>Media:

     <td>Computed value:

    <tr>
     <th><span class=property>Name:</span>

     <td>Value:

     <td>Initial:

     <td>Applies to:

     <td>Inherited:

     <td>Percentages:

     <td>Media:

    <tr>
     <th><span class=property>Name:</span>

     <td>Value:

     <td>Initial:

     <td>Applies to:

     <td>Inherited:

     <td>Percentages:

     <td>Media:
  </table>
  <!--end-properties-->
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