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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang=en>
<head><meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Values and Units Module Level 3</title>
<style type="text/css">
code, small { white-space: nowrap }
.say { color: gray; }
pre.value { font: inherit; white-space: pre-wrap; margin: 0; padding: 0; }
#propvalues td { text-align: right; }
#propvalues td + td { text-align: left; }
dt + dt::before { content: ", "; }
dt { display: inline; }
</style>
<link href="../default.css" rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" 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 Values and Units Module Level 3</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 7 February
2012</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2012/ED-css3-values-20120207/">http://www.w3.org/TR/2012/ED-css3-values-20120207/</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-values//">http://www.w3.org/TR/css3-values//</a>
<dt>Editor's draft:
<dd><a
href="http://dev.w3.org/csswg/css3-values/">http://dev.w3.org/csswg/css3-values/</a>
<dt>Previous version:
<dd><a
href="http://www.w3.org/TR/2011/WD-css3-values-20110906/">http://www.w3.org/TR/2011/WD-css3-values-20110906/</a>
<dt>Issues List:
<dd><a
href="http://www.w3.org/Style/CSS/Tracker/products/33">http://www.w3.org/Style/CSS/Tracker/products/33</a>
<dt>Discussion:
<dd><a
href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
with subject line “<kbd>[css3-values] <var>… message topic
…</var></kbd>”
<dt>Editors:
<dd><a href="mailto:howcome@opera.com">Håkon Wium Lie</a> (Opera
Software)
10BC8
<dd><a href="http://www.xanthir.com/contact">Tab Atkins</a> (Google)
<dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>
(Mozilla)
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2012 <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 CSS3 module describes the various values and units that CSS
properties accept. Also, it describes how values are computed from
"specified" through "computed" and "used" into "actual" values. The main
purpose of this module is to define common values and units in one
specification which can be referred to by other modules. As such, it does
not make sense to claim conformance with this module alone.
<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-values%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-values” in the subject, preferably like this:
“[<!---->css3-values<!---->] <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-->
<p>All features described in this specification that also exist in CSS 2.1
<a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> are intended
to be backwards compatible. In case of conflict between this draft and
CSS 2.1 <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, CSS 2.1 probably
represents the intention of the CSS WG better than this draft (other than
on values and units that are new to CSS3).
<p>The following features are at-risk and may be dropped during the CR
period: ‘<a href="#calc0"><code class=css>calc()</code></a>’,
‘<a href="#cycle-value"><code class=css>cycle()</code></a>’,
‘<a href="#attr-value"><code class=css>attr()</code></a>’.
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
<ul class=toc>
<li><a href="#placement"><span class=secno>1.1. </span> Module
Interactions</a>
</ul>
<li><a href="#value-defs"><span class=secno>2. </span> Value Definition
Syntax</a>
<ul class=toc>
<li><a href="#component-types"><span class=secno>2.1. </span> Component
value types</a>
<li><a href="#component-combinators"><span class=secno>2.2. </span>
Component value combinators</a>
<li><a href="#component-multipliers"><span class=secno>2.3. </span>
Component value multipliers</a>
<li><a href="#component-whitespace"><span class=secno>2.4. </span>
Component values and white space</a>
<li><a href="#value-examples"><span class=secno>2.5. </span> Property
value examples</a>
</ul>
<li><a href="#textual-values"><span class=secno>3. </span> Textual Data
Types</a>
<ul class=toc>
<li><a href="#keywords"><span class=secno>3.1. </span> Pre-defined
Keywords</a>
<ul class=toc>
<li><a href="#common-keywords"><span class=secno>3.1.1. </span>
CSS-wide keywords: ‘<code class=css>initial</code>’ and
‘<code class=css>inherit</code>’</a>
</ul>
<li><a href="#identifiers"><span class=secno>3.2. </span> User-defined
Identifiers: the ‘<code
class=css><identifier></code>’ type</a>
<li><a href="#strings"><span class=secno>3.3. </span> Quoted Strings:
the ‘<code class=css><string></code>’ type</a>
<li><a href="#urls"><span class=secno>3.4. </span> Resource Locators:
the ‘<code class=css><url></code>’ type</a>
</ul>
<li><a href="#numeric-types"><span class=secno>4. </span> Numeric Data
Types</a>
<ul class=toc>
<li><a href="#integers"><span class=secno>4.1. </span> Integers: the
‘<code class=css><integer></code>’ type</a>
<li><a href="#numbers"><span class=secno>4.2. </span> Numbers: the
‘<code class=css><number></code>’ type</a>
<li><a href="#percentages"><span class=secno>4.3. </span> Percentages:
the ‘<code class=css><percentage></code>’ type</a>
</ul>
<li><a href="#lengths"><span class=secno>5. </span> Distance Units: the
‘<code class=css><length></code>’ type</a>
<ul class=toc>
<li><a href="#relative-lengths"><span class=secno>5.1. </span> Relative
lengths</a>
<ul class=toc>
<li><a href="#font-relative-lengths"><span class=secno>5.1.1. </span>
Font-relative lengths: the ‘<code class=css>em</code>’,
‘<code class=css>ex</code>’, ‘<code
class=css>ch</code>’, ‘<code class=css>rem</code>’
units</a>
<li><a href="#viewport-relative-lengths"><span class=secno>5.1.2.
</span> Viewport-percentage lengths: the ‘<code
class=css>vw</code>’, ‘<code class=css>vh</code>’,
‘<code class=css>vmin</code>’ units</a>
</ul>
<li><a href="#absolute-lengths"><span class=secno>5.2. </span> Absolute
lengths: the ‘<code class=css>cm</code>’, ‘<code
class=css>mm</code>’, ‘<code class=css>in</code>’,
‘<code class=css>pt</code>’, ‘<code
class=css>pc</code>’, ‘<code class=css>px</code>’
units</a>
</ul>
<li><a href="#other-units"><span class=secno>6. </span> Other Units</a>
<ul class=toc>
<li><a href="#angles"><span class=secno>6.1. </span> Angles: the
‘<code class=css><angle></code>’ type and
‘<code class=css>deg</code>’, ‘<code
class=css>grad</code>’, ‘<code class=css>rad</code>’,
‘<code class=css>turn</code>’ units</a>
<li><a href="#time"><span class=secno>6.2. </span> Times: the
‘<code class=css><time></code>’ type and ‘<code
class=css>s</code>’, ‘<code class=css>ms</code>’
units</a>
<li><a href="#frequency"><span class=secno>6.3. </span> Frequencies: the
‘<code class=css><frequency></code>’ type and
‘<code class=css>Hz</code>’, ‘<code
class=css>kHz</code>’ units</a>
<li><a href="#resolution"><span class=secno>6.4. </span> Resolutions:
the ‘<code class=css>dpi</code>’, ‘<code
class=css>dpcm</code>’, and ‘<code
class=css>dppx</code>’ units</a>
</ul>
<li><a href="#defined-elsewhere"><span class=secno>7. </span> Data Types
Defined Elsewhere</a>
<ul class=toc>
<li><a href="#colors"><span class=secno>7.1. </span> Colors: the
‘<code class=css><color></code>’ type</a>
<li><a href="#images"><span class=secno>7.2. </span> Images: the
‘<code class=css><image></code>’ type</a>
<li><a href="#position"><span class=secno>7.3. </span> 2D Positioning:
the ‘<code class=css><position></code>’ type</a>
</ul>
<li><a href="#functional-notation"><span class=secno>8. </span> Functional
Notations</a>
<ul class=toc>
<li><a href="#calc"><span class=secno>8.1. </span> Mathematical
Expressions: ‘<code class=css>calc()</code>’</a>
<li><a href="#cycle"><span class=secno>8.2. </span> Cycling Values:
‘<code class=css>cycle()</code>’</a>
<li><a href="#attr"><span class=secno>8.3. </span> Attribute References:
‘<code class=css>attr()</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="#other-references">Other references</a>
</ul>
<li class=no-num><a href="#index">Index</a>
</ul>
<!--end-toc-->
<h2 id=introduction><span class=secno>1. </span>Introduction</h2>
<p>The value definition field of each CSS property can contain keywords,
data types (which appear between ‘<code class=css><</code>’
and ‘<code class=css>></code>’), and information on how they
can be combined. Generic data types (<a
href="#length-value"><code><length></code></a> being the most widely
used) that can be used by many properties are described in this
specification, while more specific data types (e.g.,
<code><spacing-limit></code>) are described in the corresponding
modules.
<h3 id=placement><span class=secno>1.1. </span> Module Interactions</h3>
<p>This module replaces and extends the data type definitions in <a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections <a
href="http://www.w3.org/TR/CSS21/about.html#value-defs">1.4.2.1</a>, <a
href="http://www.w3.org/TR/CSS21/syndata.html#values">4.3</a>, and <a
href="http://www.w3.org/TR/CSS21/aural.html#aural-intro">A.2</a>.
<h2 id=value-defs><span class=secno>2. </span> Value Definition Syntax</h2>
<p>The syntax described here is used to define the set of valid values for
CSS properties. A property value can have one or more components.
<h3 id=component-types><span class=secno>2.1. </span> Component value types</h3>
<p>Component value types are designated in several ways:
<ol>
<li><a href="#keywords">keyword</a> values (such as ‘<code
class=css>auto</code>’, ‘<code class=css>disc</code>’,
etc.), which appear literally, without quotes (e.g. <code>auto</code>)
<li>basic data types, which appear between ‘<code
class=css><</code>’ and ‘<code
class=css>></code>’ (e.g., <a
href="#length-value"><code><length></code></a>, <a
href="#percentage-value"><code><percentage></code></a>, etc.).
<li>types that have the same range of values as a property bearing the
same name (e.g., <code><'border-width'></code>
<code><'background-attachment'></code>, etc.). In this case, the
type name is the property name (complete with quotes) between the
brackets. Such a type does <em>not</em> include the value ‘<code
class=property>inherit</code>’.
<li>non-terminals that do not share the same name as a property. In this
case, the non-terminal name appears between ‘<code
class=css><</code>’ and ‘<code
class=css>></code>’, as in <code><spacing-limit></code>.
Notice the distinction between <code><border-width></code> and
<code><'border-width'></code>: the latter is defined as the value
of the ‘<code class=property>border-width</code>’ property,
the former requires an explicit expansion elsewhere. The definition of a
non-terminal is located near its first appearance in the specification.
</ol>
<p>Some property value definitions also include the slash (/) and/or the
comma (,) as literals. These represent their corresponding tokens.
<p>All CSS properties also accept the keyword values ‘<code
class=css>inherit</code>’ and ‘<code
class=css>initial</code>’ as their property value, but for
readability these are not listed explicitly in the property value syntax
definitions. These keywords cannot be combined with other component values
in same declaration; such a declaration is invalid. For example,
‘<code class=css>background: url(corner.png) no-repeat,
inherit;</code>’ is invalid.
<h3 id=component-combinators><span class=secno>2.2. </span> Component value
combinators</h3>
<p>Component values can be arranged into property values as follows:
<ul>
<li>Several juxtaposed words mean that all of them must occur, in the
given order.
<li>A double ampersand (&&) separates two or more components, all of which
must occur, in any order.
<li>A double bar (||) separates two or more options: one or more of them
must occur, in any order.
<li>A bar (|) separates two or more alternatives: exactly one of them must
occur.
<li>Brackets ([ ]) are for grouping.
</ul>
<p>Juxtaposition is stronger than the double ampersand, the double
ampersand is stronger than the double bar, and the double bar is stronger
than the bar. Thus, the following lines are equivalent:
<pre>
<!----> a b | c || d && e f
<!---->[ a b ] | [ c || [ d && [ e f ]]]</pre>
<h3 id=component-multipliers><span class=secno>2.3. </span> Component value
multipliers</h3>
<p>Every type, keyword, or bracketed group may be followed by one of the
following modifiers:
<ul>
<li>An asterisk (*) indicates that the preceding type, word, or group
occurs zero or more times.
<li>A plus (+) indicates that the preceding type, word, or group occurs
one or more times.
<li>A question mark (?) indicates that the preceding type, word, or group
is optional.
<li>A pair of numbers in curly braces ({<var>A</var>,<var>B</var>})
indicates that the preceding type, word, or group occurs at least
<var>A</var> and at most <var>B</var> times.
<li>A hash mark (#) indicates that the preceding type, word, or group
occurs one or more times, separated by comma tokens.
</ul>
<p>For repeated component values (indicated by ‘<code
class=css>*</code>’, ‘<code class=css>+</code>’, or
‘<code class=css>#</code>’), UAs must support at least 30
repetitions of the component. If a property value contains more than the
supported number of repetitions, the declaration must be ignored as if it
were invalid.
<h3 id=component-whitespace><span class=secno>2.4. </span> Component values
and white space</h3>
<p>Component values are specified in terms of tokens, as described in <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">Chapter 4</a>
of <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. As the
grammar allows spaces between tokens in the components of the
<code>value</code> production, spaces may appear between tokens in
property values.
<p class=note>Note: In many cases, spaces will in fact be <em>required</em>
between tokens in order to distinguish them from each other. For example,
the value ‘<code class=css>1em2em</code>’ would be parsed as a
single <code>DIMEN</code> token with the number ‘<code
class=css>1</code>’ and the identifier ‘<code
class=css>em2em</code>’, which is an invalid unit. In this case, a
space would be required before the ‘<code class=css>2</code>’
to get this parsed as the two lengths ‘<code
class=css>1em</code>’ and ‘<code class=css>2em</code>’.
<h3 id=value-examples><span class=secno>2.5. </span> Property value
examples</h3>
<p>Below are some examples of properties with their corresponding value
definition fields
<div class=example>
<table class=data id=propvalues>
<thead>
<tr>
<th>Property
<th>Value definition field
<th>Example value
<tbody>
<tr>
<td>‘<code class=property>orphans</code>’
<td><integer>
<td>‘<code class=css>3</code>’
<tr>
<td>‘<code class=property>text-align</code>’
<td>left | right | center | justify
<td>‘<code class=css>center</code>’
<tr>
<td>‘<code class=property>padding-top</code>’
<td><length> | <percentage>
<td>‘<code class=css>5%</code>’
<tr>
<td>‘<code class=property>outline-color</code>’
<td><color> | invert
<td>‘<code class=css>#fefefe</code>’
<tr>
<td>‘<code class=property>text-decoration</code>’
<td>none | underline || overline || line-through || blink
<td>‘<code class=css>overline underline</code>’
<tr>
<td>‘<code class=property>font-family</code>’
<td><family-name>#
<td>‘<code class=css>"Gill Sans", Futura,
sans-serif</code>’
<tr>
<td>‘<code class=property>border-width</code>’
<td>[ <length> | thick | medium | thin ]{1,4}
<td>‘<code class=css>2px medium 4px</code>’
<tr>
<td>‘<code class=property>text-shadow</code>’
<td>[ inset? && [ <length>{2,4} && <color>? ] ]# | none
<td>‘<code class=css>3px 3px rgba(50%, 50%, 50%, 50%),
lemonchiffon 0 0 4px inset</code>’
<tr>
<td>‘<code class=property>voice-pitch</code>’
<td>
<pre class=value><frequency> && absolute |
<!-- -->[[x-low | low | medium | high | x-high] ||
<!-- --> [<frequency> | <semitones> | <percentage>]]</pre>
<td>‘<code class=css>-2st x-low</code>’
</table>
</div>
<h2 id=textual-values><span class=secno>3. </span> Textual Data Types</h2>
<p>An <dfn id=identifier>identifier</dfn> is a sequence of characters
conforming to the <code>IDENT</code> token in the <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
<a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> Identifiers
cannot be quoted; otherwise they would be interpreted as a string.
<h3 id=keywords><span class=secno>3.1. </span> Pre-defined Keywords</h3>
<p>In the value definition fields, keywords with a pre-defined meaning
appear literally. Keywords are CSS <i>identifiers</i> and are interpreted
case-insensitively within the ASCII range (i.e., [a-z] and [A-Z] are
equivalent).
<div class=example>
<p>For example, here is the value definition for the ‘<code
class=property>border-collapse</code>’ property:</p>
<pre>Value: collapse | separate</pre>
<p>And here is an example of its use:</p>
<pre>table { border-collapse: separate }</pre>
</div>
<h4 id=common-keywords><span class=secno>3.1.1. </span> CSS-wide keywords:
‘<code class=css>initial</code>’ and ‘<code
class=css>inherit</code>’</h4>
<p>As defined <a href="#component-types">above</a>, all properties accept
the ‘<code class=css>initial</code>’ and ‘<code
class=css>inherit</code>’ keywords, which represent value
computations common to all CSS properties.
<p>The <dfn id=inherit>‘<code class=css>inherit</code>’</dfn>
keyword is <a
href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">defined</a>
in <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<p>The <dfn id=initial>‘<code class=css>initial</code>’</dfn>
keyword represents the specified value that is designated as the
property's initial value. <a href="#CSS3CASCADE"
rel=biblioentry>[CSS3CASCADE]<!--{{CSS3CASCADE}}--></a>
<h3 id=identifiers><span class=secno>3.2. </span> User-defined Identifiers:
the ‘<a href="#identifier-value"><code
class=css><identifier></code></a>’ type</h3>
<p>Some properties accept arbitrary user-defined identifiers as a component
value. This generic data type is denoted by <dfn
id=identifier-value><code><identifier></code></dfn>, and represents
any valid CSS <a href="#identifier"><i>identifier</i></a> that does not
otherwise appear as a pre-defined keyword in that property's value
definition. Such identifiers are fully case-sensitive, even in the ASCII
range (e.g. ‘<code class=css>example</code>’ and ‘<code
class=css>EXAMPLE</code>’ are two different, unrelated user-defined
identifiers).
<p class=issue>What should be the minimum required size for identifiers?
(This depends on the encoding, which means a character may take up to 6
bytes in utf-8.)
<h3 id=strings><span class=secno>3.3. </span> Quoted Strings: the ‘<a
href="#string-value"><code class=css><string></code></a>’ type</h3>
<p>Strings are denoted by <dfn
id=string-value><code><string></code></dfn> and consist of a
sequence of characters delimited by double quotes or single quotes. They
correspond to the <code>STRING</code> token in the <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
<a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
<div class=example>
<p>Double quotes cannot occur inside double quotes, unless escaped (as
<code>"\""</code> or as <code>"\22"</code>). Analogously for single
quotes (<code>'\''</code> or
<code>'\27'</code>).
<pre>
<!-- -->content: "this is a 'string'.";
<!-- -->content: "this is a \"string\".";
<!-- -->content: 'this is a "string".';
<!-- -->content: 'this is a \'string\'.';</pre>
</div>
<p>It is possible to break strings over several lines, for aesthetic or
other reasons, but in such a case the newline itself has to be escaped
with a backslash (\). The newline is subsequently removed from the string.
For instance, the following two selectors are exactly the same:
<div class=example>
<p style="display:none">Example(s):</p>
<pre>
<!-- -->a[title="a not s\
<!-- -->o very long title"] {/*...*/}
<!-- -->a[title="a not so very long title"] {/*...*/}</pre>
</div>
<p>Since a string cannot directly represent a newline, to include a newline
in a string, use the escape "\A". (Hexadecimal A is the line feed
character in Unicode (U+000A), but represents the generic notion of
"newline" in CSS.)
<p class=issue>What should be the minimum required size for strings? (This
depends on the encoding, which means a character may take up to 6 bytes in
utf-8.)
<h3 id=urls><span class=secno>3.4. </span> Resource Locators: the ‘<a
href="#url-value"><code class=css><url></code></a>’ type</h3>
<p>A <dfn id=url>URL</dfn> is a pointer to a resource and is a <a
href="http://www.w3.org/TR/CSS21/syndata.html#uri">specially-parsed</a> <a
href="#functional-notation">functional notation</a> denoted by <dfn
id=url-value><code><url></code></dfn>. It corresponds to the
<code>URI</code> token in the <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
<a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
<div class=example>
<p>Below is an example of a URL being used as a background image:
<pre>body { background: url("http://www.example.com/pinkish.gif") }</pre>
<p>The same example can be written without quotes:
<pre>body { background: url(http://www.example.com/pinkish.gif) }</pre>
</div>
<p class=note>Note that in some CSS syntactic contexts (as defined by that
context), a URL can be represented as a <a
href="#string-value"><code><string></code></a> rather than by <a
href="#url-value"><code><URL></code></a>. An example of this is the
<a href="http://www.w3.org/TR/CSS21/cascade.html#at-import">‘<code
class=css>@import</code>’ rule</a>.
<p>Parentheses, whitespace characters, single quotes (‘<code
class=css>) and double quotes (") appearing in a URL must be escaped with
a backslash so that the resulting value is a valid <a
href="#url"><code>URL</code></a> token, e.g.
</code>’url(open\(parens)‘<code class=css>,
</code>’url(close\)parens)‘<code class=css>. Depending on the
type of URL, it might also be possible to write these characters as
URI-escapes (where <code>(</code> = <code>%28</code>, <code>)</code> =
<code>%29</code>, etc.) as described in <a href="#URI"
rel=biblioentry>[URI]<!--{{URI}}--></a>. Alternatively a URL containing
such characters may be represented as a quoted <a
href="#strings">string</a> within the </code>’‘<a
href="#url"><code class=css>url()</code></a>’‘<code class=css>
notation. </code>
<p>In order to create modular style sheets that are not dependent on the
absolute location of a resource, authors should use relative URIs.
Relative URIs (as defined in <a href="#URI"
rel=biblioentry>[URI]<!--{{URI}}--></a>) are resolved to full URIs using a
base URI. RFC 3986, section 3, defines the normative algorithm
for this process. For CSS style sheets, the base URI is that of the style
sheet, not that of the source document.
<div class=example>
<p>For example, suppose the following rule:
<pre>body { background: url("tile.png") }</pre>
<p>is located in a style sheet designated by the URL:</p>
<pre>http://www.example.org/style/basic.css</pre>
<p>The background of the source document’s <code><body></code>
will be tiled with whatever image is described by the resource designated
by the URL:
<pre>http://www.example.org/style/tile.png</pre>
<p>The same image will be used regardless of the URL of the source
document containing the <code><body></code>.
</div>
<p class=issue>What should be the minimum required size for urls? (This
depends on the encoding, which means a character may take up to 6 bytes in
utf-8.)
<h2 id=numeric-types><span class=secno>4. </span> Numeric Data Types</h2>
<h3 id=integers><span class=secno>4.1. </span> Integers: the ‘<a
href="#integer-value"><code class=css><integer></code></a>’
type</h3>
<p>Integer values are denoted by <dfn
id=integer-value><code><integer></code></dfn>. An <dfn
id=integer>integer</dfn> is one or more decimal digits ‘<code
class=css>0</code>’ through ‘<code class=css>9</code>’
and corresponds to a subset of the <a
href="#number"><code>NUMBER</code></a> token in the <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
Integers may be immediately preceded by ‘<code
class=css>-</code>’ or ‘<code class=css>+</code>’ to
indicate the sign.
<p>Properties may restrict the integer value to some range. If the value is
outside the allowed range, the declaration is invalid and must be <a
href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>. For
unrestricted values, UAs must support at least up to ±2<sup>30</sup>;
unsupported values must be clamped to the closest supported value.
<h3 id=numbers><span class=secno>4.2. </span> Numbers: the ‘<a
href="#number-value"><code class=css><number></code></a>’ type</h3>
<p>Number values are denoted by <dfn
id=number-value><code><number></code></dfn>. A <dfn
id=number>number</dfn> is either an <a href="#integer"><i>integer</i></a>,
or zero or more decimal digits followed by a dot (.) followed by one or
more decimal digits. It corresponds to the <a
href="#number"><code>NUMBER</code></a> token in the <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
Like integers, numbers may also be immediately preceded by ‘<code
class=css>-</code>’ or ‘<code class=css>+</code>’ to
indicate the sign.
<p>Properties may restrict the number value to some range. If the value is
outside the allowed range, the declaration is invalid and must be <a
href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>. For
unrestricted values, UAs must support at least up to ±2<sup>30;
unsupported values must be clamped to the closest supported value.</sup>
<h3 id=percentages><span class=secno>4.3. </span> Percentages: the
‘<a href="#percentage-value"><code
class=css><percentage></code></a>’ type</h3>
<p>A <dfn id=percentage>percentage</dfn> value is denoted by <dfn
id=percentage-value><code><percentage></code></dfn>, consists of a
<a href="#number-value"><i><number></i></a> immediately followed by
a percent sign ‘<code class=css>%</code>’. It corresponds to
the <a href="#percentage"><code>PERCENTAGE</code></a> token in the <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
<p>Percentage values are always relative to another value, for example a
length. Each property that allows percentages also defines the value to
which the percentage refers. The value may be that of another property for
the same element, a property for an ancestor element, or a value of the
formatting context (e.g., the width of a <i>containing block</i>). When a
percentage value is set for a property of the <i>root</i> element and the
percentage is defined as referring to the inherited value of some
property, the resultant value is the percentage times the <i>initial
value</i> of that property.
<p>Properties may restrict the percentage value to some range. If the value
is outside the allowed range, the declaration is invalid and must be <a
href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>. For
unrestricted values, UAs must support at least up to ±2<sup>30%;
unsupported values must be clamped to the closest supported value.</sup>
<h2 id=lengths><span class=secno>5. </span> Distance Units: the ‘<a
href="#length-value"><code class=css><length></code></a>’ type</h2>
<p>Lengths refer to distance measurements and are denoted by <dfn
id=length-value><code><length></code></dfn> in the property
definitions. A length is a <a href="#dimension"><i>dimension</i></a>. A
zero length may be represented instead as the <a
href="#number-value"><code><number></code></a> ‘<code
class=css>0</code>’. (In other words, for zero lengths the unit
identifier is optional.)
<p>A <dfn id=dimension>dimension</dfn> is a <a
href="#number"><i>number</i></a> immediately followed by a unit
identifier. It corresponds to the <a
href="#dimension"><code>DIMENSION</code></a> token in the <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
<a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> Like
keywords, unit identifiers are case-insensitive within the ASCII range.
<p class=issue>We need a minimum required range/precision for dimensions as
well. What should it be?
<p>Properties may restrict the length value to some range. If the value is
outside the allowed range, the declaration is invalid and must be <a
href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
<p>While some properties allow negative length values, this may complicate
the formatting and there may be implementation-specific limits. If a
negative length value is allowed but cannot be supported, it must be
converted to the nearest value that can be supported.
<p>In cases where the <a href="#used-value">used</a> length cannot be
supported, user agents must approximate it in the <a
href="#actual-value">actual</a> value.
<p>There are two types of length units: relative and absolute.
<h3 id=relative-lengths><span class=secno>5.1. </span> Relative lengths</h3>
<p><dfn id=relative-length-units title="relative length">Relative length
units</dfn> specify a length relative to another length. Style sheets that
use relative units can more easily scale from one output environment to
another.
<p>The relative units are:
<table class=data>
<caption>Informative Summary of Relative Units</caption>
<thead>
<tr>
<th>unit
<th>relative to
<tbody>
<tr>
<td>‘<a href="#em-unit"><code class=css>em</code></a>’
<td>font size of the element
<tr>
<td>‘<a href="#ex-unit"><code class=css>ex</code></a>’
<td><i>x-height</i> of the element's font
<tr>
<td>‘<a href="#ch-unit"><code class=css>ch</code></a>’
<td>width of the "0" glyph in the element's font
<tr>
<td>‘<a href="#rem-unit"><code class=css>rem</code></a>’
<td>font size of the root element
<tr>
<td>‘<a href="#vw-unit"><code class=css>vw</code></a>’
<td>viewport's width
<tr>
<td>‘<a href="#vh-unit"><code class=css>vh</code></a>’
<td>viewport's height
<tr>
<td>‘<a href="#vmin-unit"><code class=css>vmin</code></a>’
<td>minimum of the viewport's height and width
</table>
<p>Child elements do not inherit the relative values as specified for their
parent; they inherit the <a href="#computed-value">computed values</a>.
<h4 id=font-relative-lengths><span class=secno>5.1.1. </span> Font-relative
lengths: the ‘<a href="#em-unit"><code
class=css>em</code></a>’, ‘<a href="#ex-unit"><code
class=css>ex</code></a>’, ‘<a href="#ch-unit"><code
class=css>ch</code></a>’, ‘<a href="#rem-unit"><code
class=css>rem</code></a>’ units</h4>
<p>Aside from ‘<a href="#rem-unit"><code
class=css>rem</code></a>’ (which refers to the font-size of the root
element), the font-relative lengths refer to the computed font metrics of
the element on which they are used. The exception is when they occur in
the value of the ‘<code class=property>font-size</code>’
property itself, in which case they refer to the font metrics of the
parent element (or the font metrics corresponding to the initial values of
the ‘<code class=property>font</code>’ property, if the
element has no parent).
<dl>
<dt><dfn id=em-unit title=em>em unit</dfn>
<dd>
<p>Equal to the computed value of the ‘<code
class=property>font-size</code>’ property of the element on which
it is used.
<div class=example>
<p>The rule:</p>
<pre>h1 { line-height: 1.2em }</pre>
<p>means that the line height of <code>h1</code> elements will be 20%
greater than the font size of <code>h1</code> element. On the other
hand:
<pre>h1 { font-size: 1.2em }</pre>
<p>means that the font size of <code>h1</code> elements will be 20%
greater than the font size inherited by <code>h1</code> elements.</p>
</div>
<dt><dfn id=ex-unit title=ex>ex unit</dfn>
<dd>
<p>Equal to the font's x-height. The x-height is so called because it is
often equal to the height of the lowercase "x". However, an ‘<a
href="#ex-unit"><code class=css>ex</code></a>’ is defined even for
fonts that do not contain an "x".
<p>The x-height of a font can be found in different ways. Some fonts
contain reliable metrics for the x-height. If reliable font metrics are
not available, UAs may determine the x-height from the height of a
lowercase glyph. One possible heuristic is to look at how far the glyph
for the lowercase "o" extends below the baseline, and subtract that
value from the top of its bounding box. In the cases where it is
impossible or impractical to determine the x-height, a value of 0.5em
must be assumed.
<dt><dfn id=ch-unit title=ch>ch unit</dfn>
<dd>
<p>Equal to the advance measure of the "0" (ZERO, U+0030) glyph found in
the font used to render it.
<dt><dfn id=rem-unit title=rem>rem unit</dfn>
<dd>
<p>Equal to the computed value of ‘<code
class=property>font-size</code>’ on the root element.
<p>When specified on the ‘<code
class=property>font-size</code>’ property of the root element, the
‘<a href="#rem-unit"><code class=css>rem</code></a>’ units
refer to the property's <em>initial value</em>.</p>
</dl>
<h4 id=viewport-relative-lengths><span class=secno>5.1.2. </span>
Viewport-percentage lengths: the ‘<a href="#vw-unit"><code
class=css>vw</code></a>’, ‘<a href="#vh-unit"><code
class=css>vh</code></a>’, ‘<a href="#vmin-unit"><code
class=css>vmin</code></a>’ units</h4>
<p>The viewport-percentage lengths are relative to the size of the initial
containing block. When the height or width of the viewport is changed,
they are scaled accordingly.
<dl>
<dt><dfn id=vw-unit title=vw>vw unit</dfn>
<dd>Equal to 1% of the width of the initial containing block.
<div class=example>
<p>In the example below, if the width of the viewport is 200mm, the font
size of <code>h1</code> elements will be 16mm (i.e.
(8×200mm)/100).
<pre>h1 { font-size: 8vw }</pre>
</div>
<dt><dfn id=vh-unit title=vh>vh unit</dfn>
<dd>Equal to 1% of the height of the initial containing block.
<dt><dfn id=vmin-unit title=vmin>vmin unit</dfn>
<dd>Equal to the smaller of ‘<a href="#vw-unit"><code
class=css>vw</code></a>’ or ‘<a href="#vh-unit"><code
class=css>vh</code></a>’.
</dl>