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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">
<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 7 March 2011</h2>
<dl>
<dt>Latest Version:
<dd><a
href="http://dev.w3.org/csswg/css3-images/">http://dev.w3.org/csswg/css3-images/</a>
<dt>Latest Published Version:
<dd><a
href="http://www.w3.org/TR/css3-images/">http://www.w3.org/TR/css3-images/</a>
<dt>Previous Version:
<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>Editor:
<dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>
(Invited Expert)
<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a> (Google)
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2011 <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 <a href="#ltimage"><i><image></i></a> values
in CSS. <a href="#ltimage"><i><image></i></a> 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.</p>
<!-- ====================================================================== -->
<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"><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> Gradient
Color-Stops</a>
</ul>
<li><a href="#sizing"><span class=secno>6. </span> Sizing Images and
Objects in CSS</a>
<ul class=toc>
<li><a href="#sizing-terms"><span class=secno>6.1. </span> Object-Sizing
Terminology</a>
<li><a href="#object-negotiation"><span class=secno>6.2. </span>
CSS⇋Object Negotiation</a>
<li><a href="#default-sizing"><span class=secno>6.3. </span> Default
Intrinsic-Size Resolution</a>
<li><a href="#object-fit"><span class=secno>6.4. </span> Sizing Objects:
The ‘<code class=property>object-fit</code>’ Property</a>
<li><a href="#object-position"><span class=secno>6.5. </span>
Positioning Objects: The ‘<code
class=property>object-position</code>’ Property</a>
</ul>
<li><a href="#image-resolution"><span class=secno>7. </span> Overriding
Image Resolutions: the ‘<code
class=property>image-resolution</code>’ property</a>
<li><a href="#image-orientation"><span class=secno>8. </span> Orienting an
Image on the Page: the ‘<code
class=property>image-orientation</code>’ property</a>
<li><a href="#image-rendering"><span class=secno>9. </span> Determining
How to Scale an Image: The ‘<code
class=property>image-rendering</code>’ Property</a>
<li><a href="#serialization"><span class=secno>10. </span>
Serialization</a>
<ul class=toc>
<li><a href="#serializing-resolution"><span class=secno>10.1. </span>
Serializing a <resolution></a>
<li><a href="#serializing-url-notation"><span class=secno>10.2. </span>
Serializing the ‘<code class=css>url()</code>’ notation</a>
<li><a href="#serializing-image-notation"><span class=secno>10.3.
</span> Serializing the ‘<code class=css>image()</code>’ /
<i><image-list></i> notation</a>
<li><a href="#serializing-element-notation"><span class=secno>10.4.
</span> Serializing the ‘<code class=css>element()</code>’
/ <i><element-reference></i> notation</a>
<li><a href="#serializing-cross-fade"><span class=secno>10.5. </span>
Serializing the ‘<code class=css>cross-fade()</code>’ /
<i><image-combination></i> notation</a>
<li><a href="#serializing-gradients"><span class=secno>10.6. </span>
Serializing Gradients</a>
<li><a href="#serializing-properties"><span class=secno>10.7. </span>
Serializing new properties</a>
</ul>
<li><a href="#interpolation"><span class=secno>11. </span>
Interpolation</a>
<ul class=toc>
<li><a href="#interpolating-images"><span class=secno>11.1. </span>
Interpolating <i><image></i></a>
<li><a href="#interpolating-gradients"><span class=secno>11.2. </span>
Interpolating <i><gradient></i></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-->
<!-- ====================================================================== -->
<h2 id=intro><span class=secno>1. </span> Introduction</h2>
<p><em>This section is non-normative.</em>
<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>
<!-- ====================================================================== -->
<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>Conformance to CSS Image Values & Replaced Content Level 3 is defined
for three classes:
<dl>
<dt><dfn id=minimal>minimal</dfn>
<dd>A device that does not implement CSS Transitions, 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"><i>minimal</i></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"><i>minimal</i></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>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>
<!-- ====================================================================== -->
<h2 id=resolution-units><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=ltresolution><resolution></dfn> value type:
<dl>
<dt><dfn id=dpi>dpi</dfn>
<dd>dots per inch
<dt><dfn id=dpcm>dpcm</dfn>
<dd>dots per centimeter
<dt><dfn id=dppx>dppx</dfn>
<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>
<!-- ====================================================================== -->
<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=ltimage><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>.
<!-- ====================================================================== -->
<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>
<!-- ====================================================================== -->
<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>The final argument may be a color or generated image, to serve as an
ultimate fallback if none of the preceeding <a
href="#ltimage-decl"><i><image-decl></i></a>s can be used. If it is a
color, the <a href="#ltimage-list"><i><image-list></i></a> must
represent a single-color image of that color with no <a
href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a>.
<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.</p>
<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).</p>
</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:
<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>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"><i>CSS
element reference identifier</i></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"><i>CSS element reference
identifier</i></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>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 the
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>If the argument passed to ‘<code class=css>element()</code>’
isn't an ID selector or an ident, it is a syntax error.
<div class=example> TODO: copy an example from the MozHacks article</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>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 class=issue>Someone else needs to review this and make sure that I'm not
missing any cycles.</p>
<!-- ====================================================================== -->
<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:
<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>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>
<!-- ====================================================================== -->
<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 class=property>background-image</code>’ or ‘<code
class=property>list-style-image</code>’ properties. Gradients have
no <a href="#intrinsic-dimensions"><i>intrinsic dimensions</i></a>. The
syntax of a <a href="#ltgradient"><i><gradient></i></a> is:
<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"><i><linear-gradient></i></a>, <a
href="#ltradial-gradient"><i><radial-gradient></i></a>, <a
href="#ltrepeating-linear-gradient"><i><repeating-linear-gradient></i></a>,
and <a
href="#ltrepeating-radial-gradient"><i><repeating-radial-gradient></i></a>
are defined in their applicable sections below.
<div class=example>
<p>As with the other <a href="#ltimage"><i><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 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 ‘<code class=css>CSS View
Box</code>’ (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 ‘<code
class=property>background-image</code>’, the "box" will simply be
the size of the background sizing area. If you use a gradient in a
list-style-image, the "box" will be a 1em square.
<p class=issue>It has been suggested that several of the controls offered
by gradients are unnecessary. Repeating gradients could potentially be
done by hooking into ‘<code
class=property>background-repeat</code>’, sizing and positioning
radial gradients could be done by hooking into ‘<code
class=property>background-size</code>’ and ‘<code
class=property>background-position</code>’, etc.
<p class=issue>Angles in gradients denote directions and match the behavior
of polar coordinates, where 0deg is East, 90deg is North, and in general a
larger angle corresponds to an angle further CCW. Other CSS properties
that use angles to denote rotations use the convention that larger angles
are further CW. It has been suggested that gradients be changed so that
larger angles are more CW, and 0deg either remain East (matching a polar
coordinate system with the Y axis flipped) or changed to North (matching
bearings).</p>
<!-- ====================================================================== -->
<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>
<!-- ====================================================================== -->
<h4 class=no-toc id=linear-gradient-syntax><span class=secno>5.1.1. </span>
‘<code class=css>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 <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 ‘<code class=css>top</code>’.
<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; 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 <a
href="#gradient-line"><i>gradient-line</i></a> 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 <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 box in that direction. The
starting-point is determined identically, except in the opposite direction
of the angle.
<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 class=css>left</code>’, ‘<code
class=css>bottom</code>’, ‘<code
class=css>right</code>’, or ‘<code class=css>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.
<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 ‘<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 <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 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>
<!-- ====================================================================== -->
<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
class=css>linear-gradient()</code>’ 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(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>
<!-- ====================================================================== -->
<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>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.
<i><bg-position></i> 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 class=css>center</code>’.
Color-stop positions are measured along an imaginary line extending from
the center of the gradient to the right.
<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:
<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"><i><shape></i></a> is defined as</p>
<pre><code><dfn id=ltshape><shape></dfn> = [ circle | ellipse ]</code></pre>
<p>‘<code class=css>circle</code>’ indicates that the
ending-ellipse will be a circle with a constant radius. ‘<code
class=css>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"><i><size></i></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 <a href="#ltshape"><i><shape></i></a> is ‘<code
class=css>circle</code>’ and <a
href="#ltsize"><i><size></i></a> is ‘<code
class=css>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 center of the
gradient was ‘<code class=css>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 thus be a circle with
a radius of 10px. If <a href="#ltshape"><i><shape></i></a> is
‘<code class=css>ellipse</code>’ and <a
href="#ltsize"><i><size></i></a> is ‘<code
class=css>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 class=css>farthest-side</code>’ is identical to
‘<code class=css>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, if the
shape is ‘<code class=css>ellipse</code>’). ‘<code
class=css>closest-corner</code>’ and ‘<code
class=css>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 <a href="#ltshape"><i><shape></i></a> is
‘<code class=css>ellipse</code>’, the gradient-shape has the
same ratio of width to height that it would if ‘<code
class=css>closest-side</code>’ or ‘<code
class=css>farthest-side</code>’ were specified, as appropriate.
‘<code class=css>contain</code>’ is a synonym for
‘<code class=css>closest-side</code>’, and ‘<code
class=css>cover</code>’ is a synonym for ‘<code
class=css>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
class=css>ellipse cover</code>’.
<p>If only one argument is provided before the color-stops, and it could be
interpreted as either a position or an explicit size (for example, in
‘<code class=css>radial-gradient(10% 10%, red, blue)</code>’),
it must be interpreted as a position.
<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 class=css>closest-side</code>’ if the
starting-point is on a box edge, ‘<code
class=css>closest-corner</code>’ if the starting-point is on a box
corner, and ‘<code class=css>ellipse closest-corner</code>’ if
the starting-point is on a box edge.
<p>Color-stops are placed on an imaginary line extending from the center of
the gradient toward the right, 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 <a
href="#gradient-line"><i>gradient-line</i></a>, but colors before the
starting-point of the <a href="#gradient-line"><i>gradient-line</i></a>
are not displayed. For example, ‘<code class=css>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.
<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>
<!-- ====================================================================== -->
<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:
<pre
class=prod><code><dfn id=ltrepeating-linear-gradient><repeating-linear-gradient></dfn> = repeating-linear-gradient(
[[
[ [top | bottom] || [left | right] ]
||
<angle>
],]?