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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang=en>
<head><meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Image Values and Replaced Content Module Level 3</title>
<link href="../default.css" rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED" rel=stylesheet
type="text/css">
<style>
p { margin: .5em 0; }
</style>
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS Image Values and Replaced Content Module Level 3</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 15 December
2010</h2>
<dl>
<dt>This version:</dt>
<!-- <dd><a href="http://www.w3.org/TR/2010/CR-css3-images-20101215/">http://www.w3.org/TR/2010/CR-css3-images-20101215/</a></dd> -->
<dd><a
href="http://dev.w3.org/csswg/css3-images/">http://dev.w3.org/csswg/css3-images/</a>
<!-- <dt>Latest version:</dt>
<dd><a href="http://www.w3.org/TR/css3-images/">http://www.w3.org/TR/css3-images/</a></dd>
<dt>Previous version:</dt>
<dd><a href="http://www.w3.org/TR/2008/CR-css3-namespace-20080523/">http://www.w3.org/TR/2008/CR-css3-namespace-20080523/</a></dd> -->
<dt>Editor:
<dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>
<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2010 <a
href="http://www.w3.org/"><acronym title="World Wide Web
Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym 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>, <a
href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>This CSS Image Values and Replaced Content module has two parts: First,
it defines the syntax for <image> values in CSS. <image>
values can be a single URI to an image, a list of URIs denoting a series
of fallbacks, a reference to an element in the document, or gradients.
Second, it defines properties used to control the interaction of replaced
content and the CSS layout algorithms. These properties can affect the
used image resolution for bitmaps, the replaced object's orientation, and
whether and how to preserve the object's aspect ratio.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss3-images%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“css3-images” in the subject, preferably like this:
“[<!---->css3-images<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<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>
<li><a href="#conformance"><span class=secno>2. </span>Conformance</a>
<li><a href="#resolution-units-the-ltresolutiongt-valu"><span
class=secno>3. </span>Resolution Units: the <resolution> value
type</a>
<li><a href="#image"><span class=secno>4. </span>Image Values: the
<image> value type</a>
<ul class=toc>
<li><a href="#url"><span class=secno>4.1. </span>Image References and
Image Slices: the ‘<code class=css>url()</code>’
notation</a>
<li><a href="#image-notation"><span class=secno>4.2. </span>Image
Fallbacks: the ‘<code class=css>image()</code>’
notation</a>
<li><a href="#element-reference"><span class=secno>4.3. </span>Using
Elements as Images: The ‘<code class=css>element()</code>’
notation</a>
<li><a href="#cross-fade-function"><span class=secno>4.4.
</span>Combining images: The ‘<code
class=css>cross-fade()</code>’ notation</a>
</ul>
<li><a href="#gradients"><span class=secno>5. </span>Gradients</a>
<ul class=toc>
<li><a href="#linear-gradients"><span class=secno>5.1. </span>Linear
Gradients</a>
<li><a href="#radial-gradients"><span class=secno>5.2. </span>Radial
Gradients</a>
<li><a href="#repeating-gradients"><span class=secno>5.3.
</span>Repeating Gradients</a>
<li><a href="#color-stop-syntax"><span class=secno>5.4.
</span><code>color-stop</code> Syntax</a>
</ul>
<li><a href="#sizing"><span class=secno>6. </span>Sizing Images and
Objects in CSS</a>
<ul class=toc>
<li><a href="#image-resolution"><span class=secno>6.1. </span>Overriding
Image Resolutions: the ‘<code
class=property>image-resolution</code>’ property</a>
<li><a href="#image-orientation"><span class=secno>6.2. </span>Orienting
an Image on the Page: the ‘<code
class=property>image-orientation</code>’ property</a>
<li><a href="#image-rendering"><span class=secno>6.3. </span>Determining
How to Scale an Image: The ‘<code
class=property>image-rendering</code>’ Property</a>
<li><a href="#object-fit"><span class=secno>6.4. </span>Sizing Replaced
Elements: The ‘<code class=property>object-fit</code>’
Property</a>
<li><a href="#object-position"><span class=secno>6.5. </span>Positioning
Replaced Images: The ‘<code
class=property>object-position</code>’ Property</a>
</ul>
<li><a href="#serialization"><span class=secno>7. </span>Serialization</a>
<ul class=toc>
<li><a href="#serializing-resolution"><span class=secno>7.1.
</span>Serializing a <resolution></a>
<li><a href="#serializing-url-notation"><span class=secno>7.2.
</span>Serializing the ‘<code class=css>url()</code>’
notation</a>
<li><a href="#serializing-image-notation"><span class=secno>7.3.
</span>Serializing the ‘<code class=css>image()</code>’ /
‘<code class=css><image-list></code>’ notation</a>
<li><a href="#serializing-element-notation"><span class=secno>7.4.
</span>Serializing the ‘<code class=css>element()</code>’ /
‘<code class=css><element-reference></code>’
notation</a>
<li><a href="#serializing-cross-fade"><span class=secno>7.5.
</span>Serializing the ‘<code
class=css>cross-fade()</code>’ / ‘<code
class=css><image-combination></code>’ notation</a>
<li><a href="#serializing-gradients"><span class=secno>7.6.
</span>Serializing Gradients</a>
<li><a href="#serializing-properties"><span class=secno>7.7.
</span>Serializing new properties</a>
</ul>
<li><a href="#interpolation"><span class=secno>8. </span>Interpolation</a>
<ul class=toc>
<li><a href="#interpolating-ltimage"><span class=secno>8.1.
</span>Interpolating ‘<code
class=css><image></code>’</a>
<li><a href="#interpolating-gradients"><span class=secno>8.2.
</span>Interpolating ‘<code
class=css><gradient></code>’</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="#informative-references">Informative
references</a>
</ul>
</ul>
<!--end-toc-->
<div>
<h2 id=intro><span class=secno>1. </span>Introduction</h2>
<p><em>This section is non-normative.</em></p>
<p>In CSS Levels 1 and 2, image values, such as those used in the
‘<code class=property>background-image</code>’ property,
could only be given by a single URI value. This module introduces
additional notations that allow a 2D image to be given as a list of URIs
denoting fallbacks, as a reference to an element in the document, and as
a gradient.</p>
</div>
<div>
<h2 id=conformance><span class=secno>2. </span>Conformance</h2>
<p>A document or implementation cannot conform to CSS Image Values &
Replaced Content Level 3 alone, but can claim conformance to CSS Image
Values & Replaced Content Level 3 if it satisfies the conformance
requirements in this specification when implementing CSS or another host
language that normatively references this specification.</p>
<p>Conformance to CSS Image Values &
1D74
Replaced Content Level 3 is defined
for three classes:
<dl>
<dt><dfn id=minimal>minimal</dfn>
<dd>A device that does not implement CSS Transforms, CSS Animations, nor
CSSOM may ignore the chapters on Serializing and Interpolating values
for the purpose of claiming conformance.
<dt><dfn id=transition-capable>transition-capable</dfn>
<dd>A device that implements CSS Transitions or CSS Animations must
conform to the ‘<a href="#minimal"><code
class=css>minimal</code></a>’ class, and additionally must
implement the chapter on Interpolation.
<dt><dfn id=cssom-capable>CSSOM-capable</dfn>
<dd>A device that implements CSSOM must conform to the ‘<a
href="#minimal"><code class=css>minimal</code></a>’ class, and
additionally must implement the chapter on Serialization.
</dl>
<p>The 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. 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>
<p>Examples in this specification are introduced with the words "for
example" or are set apart from the normative text with
<code>class="example"</code>, like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word "Note" and are set apart from the
normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.</p>
</div>
<div>
<h2 id=resolution-units-the-ltresolutiongt-valu><span class=secno>3.
</span>Resolution Units: the <resolution> value type</h2>
<p>This specification defines the following units as part of the <dfn
id=ltresolutiongt><resolution></dfn> value type:
<dl>
<dt>dpi
<dd>dots per inch
<dt>dpcm
<dd>dots per centimeter
<dt>dppx
<dd>dots per ‘<code class=css>px</code>’ unit
</dl>
<p class=note>The default resolution of raster images in CSS is
‘<code class=css>1dppx</code>’.</p>
</div>
<div>
<h2 id=image><span class=secno>4. </span>Image Values: the <image>
value type</h2>
<p>The <image> value type denotes a 2D image. It is defined as
<pre
class=prod><dfn id=ltimagegt><image></dfn> = <url> | <image-list> | <element-reference> | <image-combination> | <gradient></pre>
<p>Image values can be used in many CSS properties, including the
‘<code class=property>background-image</code>’, ‘<code
class=property>list-style-image</code>’, ‘<code
class=property>cursor</code>’ properties <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<div>
<h3 id=url><span class=secno>4.1. </span>Image References and Image
Slices: the ‘<code class=css>url()</code>’ notation</h3>
<p>The simplest way to indicate an image is to reference an image file by
URI. This is done with the <a
href="http://www.w3.org/TR/CSS21/syndata.html#uri">‘<code
class=css>url()</code>’ 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
‘<code class=css>url()</code>’ 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 fragment identifiers. <span class=issue>Need
a spec to reference here. Expecting to get one from <a
href="http://www.w3.org/2008/WebVideo/Fragments/">Media Fragments
WG</a>.</span>
<div class=example>
<p>For example,</p>
<pre>background-image: url('logos.png#xywh=10,30,60,20')</pre>
<p>uses the 60 pixel by 20 pixel rectangle of <code>logos.png</code>
beginning at the point 10 pixels in from the left, 30 pixels down from
the top.
<p class=note>Note that quotation marks are required here, because
unquoted commas are not allowed in ‘<code
class=css>url()</code>’ syntax.</p>
</div>
</div>
<div>
<h3 id=image-notation><span class=secno>4.2. </span>Image Fallbacks: the
‘<code class=css>image()</code>’ notation</h3>
<p>The ‘<code class=css>image()</code>’ notation allows an
author to specify multiple images, each one a fallback for the previous.
The UA must use only the first image that it can load and display. The
syntax for ‘<code class=css>image()</code>’ is defined as
<pre class=prod><dfn id=ltimage-list><image-list></dfn> = <!--
-->image( [ <image-decl> , ]* [ <image-decl> | <color> | <element-reference> | <image-combination> | <gradient> ] )</pre>
<p>where <image-decl> is given by
<pre class=prod><dfn id=ltimage-decl><image-decl></dfn> = <!--
-->[ <string> | <url-token> ] [ snap? && <resolution> ]?</pre>
<p><url-token> is given as
<code>[!#$%&*-~]|{nonascii}|{escape}</code> (i.e. the contents of
‘<code class=css>url()</code>’) using the productions in the
<a href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">CSS2.1
tokenization</a>. <strong>The <url-token> must not contain
unescaped brackets, commas, white space characters, single quotes (') or
double quotes ("); if it does the ‘<code
class=css>image()</code>’ containing it is invalid.</strong>
<p>Each string or url-token represents the URI of an image. If a
resolution is given, then the image must be rendered at the specified
resolution. If the ‘<code class=css>snap</code>’ keyword is
also specified, and the image is a raster image, then the image must be
rendered at the resolution closest to the specified resolution that
would result in no pixel rounding. <span class=issue>I don't think "no
pixel rounding" is the right terminology here... basically we want to
avoid blurry images.</span></p>
<p>The optional color at the end of the list is the <dfn
id=fallback-color>fallback color</dfn>. It is used if, and only if, none
of the images can be loaded and displayed (e.g. because they are in
unsupported formats, or cannot be found, or are corrupted in some way,
or because image loading is disabled in the UA). It is treated as a
single-color image with no intrinsic dimensions.
<div class=example>
<p>For example, the rule below would tell the UA to load ‘<code
class=css>wavy.svg</code>’ if it can; failing that to load
‘<code class=css>wavy.png</code>’ and display it at 150dpi;
failing that to display ‘<code class=css>wavy.gif</code>’;
and finally, if none of the images can be loaded and displayed, to use
the color ‘<code class=css>blue</code>’ to create a
dimensionless background image.
<pre>background-image: image(wavy.svg, 'wavy.png' 150dpi, "wavy.gif", blue);</pre>
<p>The ‘<code class=property>background-image</code>’
property specifies that dimensionless images must stretch to cover the
entire background positioning area <a href="#CSS3BG"
rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a>, so if none of the
specified images can be displayed the background will be painted blue.
As with any image, this fallback will be painted over the ‘<code
class=property>background-color</code>’ (if any).
</div>
<p>If the URL for the image ends in a file extension (a period followed
by letters) of four ascii letters ([a-zA-Z]) or less, then the UA
<em>may</em> skip trying to load images whose file extension matches an
image format it knows it does not support. For example, a UA that knows
it supports JPEG and GIF, but expects to encounter many PNG and SVG
images may choose to skip over images with filenames ending in
‘<code class=css>.png</code>’ or ‘<code
class=css>.svg</code>’ without loading them to check their MIME
type. However it must not skip over images with filenames ending in
extensions it doesn't recognize.
</div>
<div>
<h3 id=element-reference><span class=secno>4.3. </span>Using Elements as
Images: The ‘<code class=css>element()</code>’ notation</h3>
<p>The ‘<code class=css>element()</code>’ function allows an
author to reference an element in the document that should be used as an
image. As the referenced element changes, for example, by the user
typing into a <textarea> element or a script drawing into a
<canvas> element in HTML, the image produced by the ‘<code
class=css>element()</code>’ function stays in sync, allowing
dynamic effects such as script-animated background images or previews of
the next slide in a slideshow. The syntax for ‘<code
class=css>element()</code>’ is defined as:</p>
<pre
class=prod><dfn id=ltelement-reference><element-reference></dfn> = element( [<id-selector> | <identifier> ] )</pre>
<p>where <id-selector> is an ID selector <a href="#SELECT"
rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a>, and <identifier> is
an identifer <a href="#CSS3VAL"
rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>.</p>
<p>If the argument to the ‘<code class=css>element()</code>’
function is an ID selector, the function references the element matched
by the selector. If it's an identifier, the function references the
element who's <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"><code class=css>CSS element
reference identifier</code></a>’; that is determined by the host
language.) If no element in the document matches the selector, or no
element has the identifier as its ‘<a
href="#css-element-reference-identifier"><code class=css>CSS element
reference identifier</code></a>’, the function represents a fully
transparent image with no intrinsic dimensions, equivalent to
<code>image(transparent)</code>. If the document changes so that which
element is matched, or whether an element is matched at all, changes,
the image represented by the function must change accordingly.</p>
<p>If the ‘<code class=css>element()</code>’ function refers
to an element, then it represents an image with width and height equal
to the width and height of the margin 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 background, positioned so that the
edges of the margin box of the element is flush with the edges of th
1D74
e
image. <span class=note>If the element has decorations or descendants
that extend outside the margin box, these will be clipped to the margin
box in the generated image by default. ‘<code
class=css>background-repeat:extend</code>’ may allow the author to
override this behavior so that decorations and descendants outside the
margin box are still painted.</span> If the referenced element or an
ancestor of the referenced element has a transform applied to it, the
transform must be ignored for the purpose of constructing this image
(transforms on descendants must be unaffected).</p>
<p>If the argument passed to ‘<code
class=css>element()</code>’ isn't an ID selector or an ident, it
is a syntax error.</p>
<div class=example> TODO: copy an example from the MozHacks article</div>
<div>
<h4 class="no-num no-toc" id=element-cycles>Detecting and Resolving
Circular Relationships Introduced by ‘<code
class=css>element()</code>’</h4>
<p>The ‘<code class=css>element()</code>’ 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>
<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 ‘<code class=css>element()</code>’ 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 ‘<code
class=css>element()</code>’ functions that produced a dependency
in the cycle represent a fully transparent image with no intrinsic
dimensions.</p>
<p class=issue>Someone else needs to review this and make sure that I'm
not missing any cycles.</p>
</div>
</div>
<div>
<h3 id=cross-fade-function><span class=secno>4.4. </span>Combining
images: The ‘<code class=css>cross-fade()</code>’ notation</h3>
<p>When transitioning between images, CSS requires a way to explicitly
refer to the intermediate image that is a combination of the start and
end images. This is accomplished with the ‘<code
class=css>cross-fade()</code>’ function, which indicates the two
images to be combined and how far along in the transition the
combination is. Authors may also use the ‘<code
class=css>cross-fade()</code>’ function for many simple image
manipulations, such as tinting an image with a solid color or
highlighting a particular area of the page by combining an image with a
radial gradient. The syntax for ‘<code
class=css>cross-fade()</code>’ is defined as:</p>
<pre
class=prod><dfn id=ltimage-combination><image-combination></dfn> = cross-fade( <image>, <image>, <percentage> )</pre>
<p>The function represents an image generated by combining the first and
second image (referred to in this section as the "start" and "end"
images, respectively). The percentage represents how far along the
transformation is, with 0% representing the start image, 100%
representing the end image, and percentages between that representing
corresponding combinations of the two images. The <percentage> must
be between 0% and 100% inclusive; any other value is a syntax error.</p>
<p>Given the <percentage> p, the combined image represented by the
‘<code class=css>cross-fade()</code>’ function has a width
equal to <code>start image width * (1-p) + end image width * p</code>
and a height equal to <code>start image height * (1-p) + end image
height * p</code>. The image itself is generated by first scaling both
the start and end images to the size of the combined image. Then, the
start image has a global alpha applied to it equal to (1-p), the end
image has a global alpha applied to it equal to p, and the end image is
then composited over the start image with the source-over operation.
[[PORTERDUFF]]</p>
</div>
</div>
<div>
<h2 id=gradients><span class=secno>5. </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. Gradients are a
type of image, and can be used anywhere an image can, such as in the
<code>background-image</code> or <code>list-style-image</code>
properties. For official purposes, the syntax of a <a
href="#ltgradient"><code><gradient></code></a> is:</p>
<pre
class=prod><dfn id=ltgradient><gradient></dfn> = [ <linear-gradient> | <radial-gradient> | <repeating-linear-gradient> | <repeating-radial-gradient> ]</pre>
<p>where <a
href="#ltlinear-gradient"><code><linear-gradient></code></a> and <a
href="#ltradial-gradient"><code><radial-gradient></code></a> are
defined in their applicable sections below.</p>
<div class=example>
<p>As with the other <image> 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 class=note>In many places this section references a box, such as "the
box's top-left corner" or "the box's right side". In all of these
circumstances, the "box" refers to the ‘<a
href="#css-view-box"><code class=css>CSS View Box</code></a>’ (see
the "Sizing Images and Objects in CSS" section of this spec for
clarification). A gradient has no intrinsic dimensions. This means that,
for example, if you use a gradient in a background-image, the "box" will
simply be the size of the background area. If you use a gradient in a
list-style-image, the "box" will be a 1em square.</p>
<div>
<h3 id=linear-gradients><span class=secno>5.1. </span>Linear Gradients</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 perdendicular 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>
<div>
<h4 class=no-toc id=linear-gradient-syntax><span class=secno>5.1.1.
</span><code>linear-gradient()</code> syntax</h4>
<pre
class=prod><code><dfn id=ltlinear-gradient><linear-gradient></dfn> = linear-gradient(
[
[ [top | bottom] || [left | right] ]
||
<angle>
,]?
<color-stop>[, <color-stop>]+
);</code></pre>
<p>The first argument to the function specifies the
<strong>gradient-line</strong>, which gives the gradient a direction
and determines how color-stops are positioned. It may be omitted; if
so, it defaults to "<code>top</code>".</p>
<p>The gradient-line may be specified in two different ways. The first
is by specifying the angle the gradient-line should assume; this uses
the standard algebraic notation for angles where 0deg points to the
right, 90deg points up, and positive angles go counterclockwise. The
starting-point and ending-point of the gradient-line are determined by
extending a line in both direction from the center of the box at the
angle specified. In the direction of the angle, the ending-point is the
point on the gradient-line line where a line drawn perpendicular to the
gradient-line would intersect the corner of the box in that direction.
The starting-point is determined identically, except in the opposite
direction of the angle.</p>
<p>The second way is to simply provide a side or corner of the box that
the gradient should start at; the gradient will then automatically
angle itself to extend from the specified side or corner to the
opposite side or corner in a straight line. To be precise, the gradient
is converted to the angle form described in the previous paragraph at
used-value time. If a <code>left</code>, <code>bottom</code>,
<code>right</code>, or <code>top</code> is given, the used value of the
gradient is 0deg, 90deg, 180deg, or 270 deg, respectively. If a corner
is given, the used value of the gradient is the angle necessary to
place the starting-point of the gradient in that corner of the box.</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
gradient-line from the rules above. This shows the starting and
ending-point of the gradient-line, along with the actual gradient,
produced by an element with ‘<code class=css>background:
linear-gradient(45deg, white, black);</code>’.</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 gradient-line, but this isn't
required - the gradient-line extends infinitely in both directions. The
starting-point and ending-point are merely arbitrary distance 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>
</div>
<h4 class=no-toc id=linear-gradient-examples><span class=secno>5.1.2.
</span>Linear Gradient Examples</h4>
<p>All of the following <code>linear-gradient()</code> examples are
presumed to be backgrounds applied to a box that is 200px wide and 100px
tall.</p>
<div class=example>
<p>Below are various ways of specifying a basic vertical gradient:</p>
<pre><code>linear-gradient(yellow, blue);
linear-gradient(top, yellow, blue);
linear-gradient(bottom, blue, yellow);
linear-gradient(-90deg, yellow, blue);
linear-gradient(270deg, yellow, blue);
linear-gradient(top, yellow 0%, blue 100%);</code></pre>
<p><img alt="" src=linear1.png></p>
</div>
<div class=example>
<p>This gradient goes from the upper-left to the lower-right corner.</p>
<pre><code>linear-gradient(top left, yellow, blue);</code></pre>
<p><img alt="" src=linear2.png></p>
</div>
<div class=example>
<p>This demonstrates the use of an angle in the gradient. Compare this
image with the previous example. In both gradients, the top-left of the
box is pure yellow, and the bottom-right of the box is pure blue. The
difference is in the angle that the gradient follows.</p>
<pre><code>linear-gradient(-45deg, yellow, blue);
linear-gradient(315deg, yellow, blue);</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>
<div>
<h3 id=radial-gradients><span class=secno>5.2. </span>Radial Gradients</h3>
<p>In a radial gradient, rather than colors smoothly fading from one side
of the box to the other as with linear gradients, they instead emerge
from a single point and smoothly spread outward in a circular or
elliptical shape.</p>
<p>A radial gradient is specified by first pinpointing the center of the
gradient, where the 0% ellipse will be, then specifying the size and
shape of the 100% ellipse, ending with a list of color-stops just like a
linear-gradient. Between the center and the ending-ellipse, and past the
ending-ellipse, concentric ellipses are drawn and colored according to
the specified color-stops.</p>
<h4 class=no-toc id=radial-gradient-syntax><span class=secno>5.2.1.
</span><code>radial-gradient()</code> Syntax</h4>
<pre
class=prod><code><dfn id=ltradial-gradient><radial-gradient></dfn> = radial-gradient(
[<bg-position>,]?
[
[<shape> || <size>]
|
[<length> | <percentage>]{2}
,]?
<color-stop>[, <color-stop>]+
)</code></pre>
<p>The first argument to the function specifies the center of the
ellipse. <code><bg-position></code> is taken from the Backgrounds
and Borders Module, and has the same definition. It specifies the center
of the gradient. If omitted, it defaults to <code>center</code>.
Color-stop positions are measured along an imaginary line extending from
the center of the gradient to the right.</p>
<p>The second argument to the function specifies the size and shape of
the ending-ellipse. This can be specified in two ways, with different
characteristics:</p>
<dl>
<dt>Implicitly
<dd>
<p>The size and shape of the ending-ellipse can be defined
<em>implicitly</em> with a size and shape keyword. The <a
href="#ltshape"><code><shape></code></a> is defined as</p>
<pre><code><dfn id=ltshape><shape></dfn> = [ circle | ellipse ]</code></pre>
<p><code>circle</code> indicates that the ending-ellipse will be a
circle with a constant radius. <code>ellipse</code> indicates that the
gradient-shape will be an axis-aligned ellipse (that is, its major and
minor radiuses will be horizontal and vertical, not necessarily in
that order).</p>
<p>The <a href="#ltsize"><code><size></code></a>keyword is defined
as</p>
<pre><code><dfn id=ltsize><size></dfn> = [ closest-side | closest-corner | farthest-side | farthest-corner | contain | cover ]</code></pre>
<p>If <code><shape></code> is <code>circle</code> and
<code><size></code> is <code>closest-side</code>, the
ending-shape is a circle sized so that it exactly meets the side of
the box closest to its center. For example, if the box was 100px wide
and 200px tall, and the starting-point of the gradient-line was
<code>'10% 10%'</code>, then the closest side is the left side of the
box (it is 10px from the starting-point, while the top is 20px from
it, and the right and bottom sides are much further). The
gradient-shape would be a circle with a radius of 10px. If
<code><shape></code> is <code>ellipse</code> and
<code><size></code> is <code>closest-side</code>, the
gradient-shape is an ellipse sized so that it exactly meets the
vertical and horizontal sides of the box closest to its center. Using
the same box and starting-point as the previous example, the
gradient-shape would be an ellipse with a 20px vertical radius and a
10px horizontal radius. (If necessary, such as if the starting-point
is outside of the box, extend the sides of the box so that there is a
line the ellipse can meet.)</p>
<p><code>farthest-side</code> is identical to
<code>closest-side</code>, except that the gradient-shape is sized to
meet the side of the box that is farthest from its center (or the
farthest vertical and horizontal sides, in the case of
<code>ellipse</code>). <code>closest-corner</code> and
<code>farthest-corner</code> size the gradient-shape so that it
exactly meets the closest or farthest corner of the box from its
center, respectively. If <code><shape></code> is
<code>ellipse</code>, the gradient-shape has the same ratio of width
to height that it would if <code>closest-side</code> or
<code>farthest-side</code> were specified, as appropriate.
<code>contain</code> is a synonym for <code>closest-side</code>, and
<code>cover</code> is a synonym for <code>farthest-corner</code>.</p>
<p>If this implicit form is used, then it is converted to an equivalent
explicit form (described below) at used-value time.</p>
<dt>Explicitly
<dd>
<p>Alternately, the ending-shape's size and shape can be defined
explicitly, by providing two lengths or percentages. These measure the
length of the horizontal and vertical axes of the ellipse,
respectively. (The axis length is the length from the center of the
ellipse to the edge, similar to the radius of a circle, not the
diameter.)</p>
<p>Percentages used in the first value are relative to the width of the
box, while percentages used in the second value are relative to the
height of the box.</p>
<p>Both of the values must be positive - specifying either as zero or
negative is a syntax error.</p>
</dl>
<p>If this argument is omitted, it defaults to <code>ellipse
cover</code>.</p>
<p>In certain circumstances the given parameters may define a degenerate
shape - a circle or ellipse with a radius of 0. In these instances the
gradient image is just a solid color equal to the color of the last
color-stop in the rule. The following combinations of values will
trigger this: <code>closest-side</code> if the starting-point is on a
box edge, <code>closest-corner</code> if the starting-point is on a box
corner, and <code>ellipse</code> and <code>closest-corner</code> if the
starting-point is on a box edge.</p>
<p>The ending-point of the gradient-line is determined by extending a
line from the starting-point in the direction specified by the
<code><angle></code> in the first argument. The ending-point is
where this line intersects the gradient-shape.</p>
<p><a href="#ltcolor-stopgt"><code><color-stop></code></a> is
defined identically to its treatment in <code>linear-gradient()</code>.
Color-stops are placed on an imaginary line extending from the center of
the gradient, with the 0% point at the center of the gradient, and 100%
at the point where the line intersects the ending-ellipse. The color of
each ellipse is equal to the color of the line where the ellipse
intersects it. Distances past 100% can be specified, and simply indicate
a color-stop placed on the line a corresponding distance from the
center. Negative distances are allowed in a radial gradient and work the
same as in linear gradients with respect to setting the color of the
gradient-line, but colors before the starting-point of the gradient-line
are not displayed. For example, <code>radial-gradient(red -50px, yellow
100px)</code> would produce an elliptical gradient which starts with a
reddish-orange color in the center (the color 1/3 between red and
yellow) and transitions to yellow at 100px wide.</p>
<h4 class=no-toc id=radial-gradient-examples><span class=secno>5.2.2.
</span>Radial Gradient Examples</h4>
<p>All of the following examples are applied to a box that is 200px wide
and 100px tall.</p>
<div class=example>
<p>These examples demonstrate the basic syntax for radial gradients:</p>
<pre><code>radial-gradient(yellow, green);
radial-gradient(center, ellipse cover, yellow 0%, green 100%);
radial-gradient(50% 50%, farthest-corner, 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(bottom left, farthest-side, red, yellow 50px, green);</code></pre>
<p><img alt="" src=radial4.png></p>
</div>
<div class=example>
<p>Here we illustrate a ‘<code
class=property>contain</code>’ gradient.</p>
<pre><code>radial-gradient(20px 30px, contain, red, yellow, green);
radial-gradient(20px 30px, 20px 30px, red, yellow, green);</code></pre>
<p><img alt="" src=radial6.png></p>
<pre><code>radial-gradient(20px 30px, circle contain, red, yellow, green);
radial-gradient(20px 30px, 20px 20px, red, yellow, green);</code></pre>
<p><img alt="" src=radial7.png></p>
</div>
</div>
<div>
<h3 id=repeating-gradients><span class=secno>5.3. </span>Repeating
Gradients</h3>
<p>In addition to the ‘<code
class=css>linear-gradient()</code>’ and ‘<code
class=css>radial-gradient()</code>’ functions, this specification
defines ‘<code class=css>repeating-linear-gradient()</code>’
and ‘<code class=css>repeating-radial-gradient()</code>’
functions. These two functions take the same values and are interpreted
the same as their respective non-repeating siblings defined previously:</p>
<pre
class=prod><code><dfn id=ltrepeating-linear-gradient><repeating-linear-gradient></dfn> = repeating-linear-gradient(
[
[ [top | bottom] || [left | right] ]
||
<angle>
,]?
<color-stop>[, <color-stop>]+
)
<dfn id=ltrepeating-radial-gradient><repeating-radial-gradient></dfn> = repeating-radial-gradient(
[<bg-position>,]?
[
[<shape> || <size>]
|
[<length> | <percentage>]{2}
,]?
<color-stop>[, <color-stop>]+
)</code></pre>
<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, ‘<code
class=css>repeating-linear-gradient(red 10px, blue 50px)</code>’
is equivalent to ‘<code class=css>linear-gradient(..., red -30px,
blue 10px, red 10px, blue 50px, red 50px, blue 90px, ...)</code>’.
Note that the last color-stop and first color-stop will always coincide