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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 profile="http://dublincore.org/documents/2008/08/04/dc-html/ ">
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Values and Units Module Level 3</title>
<link href="http://purl.org/dc/terms/" rel=schema.dcterms>
<link href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=dcterms.rights>
<meta content="CSS Values and Units Module Level 3" name=dcterms.title>
<meta content=text name=dcterms.type>
<meta content=2013-01-23 name=dcterms.issued>
<meta content="http://dev.w3.org/csswg/css3-values/" name=dcterms.creator>
<meta content=W3C name=dcterms.publisher>
<meta content="http://www.w3.org/TR/2013/ED-css3-values-20130123/"
name=dcterms.identifier>
<style type="text/css">
code, small { white-space: nowrap }
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pre.value { font: inherit; white-space: pre-wrap; margin: 0; padding: 0; }
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</style>
<link href="../default.css" rel=stylesheet type="text/css">
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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 23 January
2013</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2013/ED-css3-values-20130123/">http://www.w3.org/TR/2013/ED-css3-values-20130123/</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-values/">http://www.w3.org/TR/css3-values/</a>
8096
<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/2012/WD-css3-values-20120308/">http://www.w3.org/TR/2012/WD-css3-values-20120308/</a>
<dt>Issue Tracking:
<dd><a
href="http://www.w3.org/Style/CSS/Tracker/products/33">http://www.w3.org/Style/CSS/Tracker/products/33</a>
<dt>Feedback:
<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)
<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> © 2013 <a href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><abbr
title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
href="http://www.ercim.eu/"><abbr
title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
<a href="http://www.keio.ac.jp/">Keio</a>), 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 common values and units that CSS
properties accept and the syntax used for describing them in CSS property
definitions.
<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. If you notice a conflict between this draft
and CSS 2.1 <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, please inform the editors!
<p>The CR period will last at least until 28 February 2013. At the time of
publication, no test suite and implementation report have yet been made.
They will be made available from the <a href="/Style/CSS/Test/">CSS test
suites page.</a> See the section <a href="#cr-exit-criteria">“CR exit
criteria”</a> for details.
<p>See the section <a href="#changes">“Changes”</a> for changes made to
this specification since the last Working Draft.
<p>The following features are at-risk and may be dropped during the CR
period: ‘<a href="#calc"><code class=css>calc()</code></a>’, ‘<a
href="#toggle"><code class=css>toggle()</code></a>’, ‘<a
href="#attr"><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="#strings"><span class=secno>3.2. </span> Quoted Strings:
the ‘<code class=css><string></code>’ type</a>
<li><a href="#urls"><span class=secno>3.3. </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>’, ‘<code class=css>vmax</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 <var><resolution></var> type and ‘<code
class=css>dpi</code>’, ‘<code class=css>dpcm</code>’, ‘<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-notation"><span class=secno>8.1. </span> Mathematical
Expressions: ‘<code class=css>calc()</code>’</a>
<ul class=toc>
<li><a href="#calc-syntax"><span class=secno>8.1.1. </span> Syntax</a>
<li><a href="#calc-type-checking"><span class=secno>8.1.2. </span>
Type Checking</a>
<li><a href="#calc-computed-value"><span class=secno>8.1.3. </span>
Computed Value</a>
<li><a href="#calc-range"><span class=secno>8.1.4. </span> Range
Checking</a>
</ul>
<li><a href="#toggle-notation"><span class=secno>8.2. </span> Toggling
Between Values: ‘<code class=css>toggle()</code>’</a>
<li><a href="#attr-notation"><span class=secno>8.3. </span> Attribute
References: ‘<code class=css>attr()</code>’</a>
</ul>
<li><a href="#iana"><span class=secno>9. </span> Appendix A: IANA
Considerations</a>
<ul class=toc>
<li><a href="#about-invalid"><span class=secno>9.1. </span> Registration
for the <code>about:invalid</code> URL scheme</a>
</ul>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#conformance"> Conformance</a>
<ul class=toc>
<li class=no-num><a href="#conventions"> Document conventions</a>
<li class=no-num><a href="#conformance-classes"> Conformance classes</a>
<li class=no-num><a href="#partial"> Partial implementations</a>
<li class=no-num><a href="#experimental"> Experimental
implementations</a>
<li class=no-num><a href="#testing"> Non-experimental
implementations</a>
<li class=no-num><a href="#cr-exit-criteria"> CR exit criteria</a>
</ul>
<li class=no-num><a href="#changes">Changes</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"><var><length></var></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.,
<var><spacing-limit></var>) 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"><var><length></var></a>, <a
href="#percentage-value"><var><percentage></var></a>, etc.).
<li>types that have the same range of values as a property bearing the
same name (e.g., <var><‘<code
class=property>border-width</code>’></var> <var><‘<code
class=property>background-attachment</code>’></var>, 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 <a
href="#common-keywords">CSS-wide keywords</a> such as ‘<a
href="#inherit"><code class=css>inherit</code></a>’.
<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
<var><spacing-limit></var>. Notice the distinction between
<var><border-width></var> and <var><‘<code
class=property>border-width</code>’></var>: 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 <a href="#common-keywords">CSS-wide
keyword values</a> ‘<a href="#inherit"><code
class=css>inherit</code></a>’ and ‘<a href="#initial"><code
class=css>initial</code></a>’ as the sole component of their property
value. For readability these are not listed explicitly in the property
value syntax definitions. For example, the full value definition of <a
href="http://www.w3.org/TR/css3-background/#border-color">‘<code
class=property>border-color</code>’</a> is ‘<code class=css>[
<color>{1,4} ] | inherit | initial&
133E
lt;/code>’ (even though it is listed
as ‘<code class=css><color>{1,4}</code>’).
<p class=note> Note: This implies that, in general, combining these
keywords with other component values in the same declaration results in an
invalid declaration. 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>
<!--
<p>
White space is allowed, but optional, between components of a combination.
-->
<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>
White space is allowed, but optional, between repetitions
and around the comma separator of ''#''-multiplied lists.
-->
<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 20 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>Unless otherwise specified, white space and/or comments may appear
before, after, and/or between components in a property value or
subcomponents in a <a href="#functional-notation">functional notation</a>
that is defined using these component <a
href="#component-combinators">combinators</a> and <a
href="#component-multipliers">multipliers</a>.
<p class=note>Note: In many cases, spaces will in fact be <em>required</em>
between components 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>‘<
9E70
;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> | <generic-family> ]#
<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 <a
href="#identifier"><i>identifiers</i></a> 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:
<pre>Value: collapse | separate</pre>
<p>And here is an example of its use:
<pre>table { border-collapse: separate }</pre>
</div>
<h4 id=common-keywords><span class=secno>3.1.1. </span> CSS-wide keywords:
‘<a href="#initial"><code class=css>initial</code></a>’ and ‘<a
href="#inherit"><code class=css>inherit</code></a>’</h4>
<p>As defined <a href="#component-types">above</a>, all properties accept
the ‘<a href="#initial"><code class=css>initial</code></a>’ and ‘<a
href="#inherit"><code class=css>inherit</code></a>’ 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>
<p class=note> Note, the <dfn id=default>‘<code
class=css>default</code>’</dfn> keyword is reserved for future use.
<h3 id=strings><span class=secno>3.2. </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><var><string></var></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):
<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.)
<h3 id=urls><span class=secno>3.3. </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><var><url></var></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"><var><string></var></a> rather than by <a
href="#url-value"><var><URL></var></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 (') 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 class=css>url(open\(parens)</code>’, ‘<code
class=css>url(close\)parens)</code>’. Depending on the type of URL, it
might also be possible to write these characters as URI-escapes (e.g.
‘<code class=css>url(open%28parens)</code>’ or ‘<code
class=css>url(close%29parens)</code>’) 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 class=css>url()</code>’
notation.
<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.
<p>When a <a href="#url-value"><i><url></i></a> appears in the computed
value of a property, it is resolved to an absolute URL, as described in
the preceding paragraph. The computed value of a URI that the UA cannot
resolve to an absolute URI is the specified value.
<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:
<pre>http://www.example.org/style/basic.css</pre>
<p>The background of the source document's <var><body></var> 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 <var><body></var>.
</div>
<h2 id=numeric-types><span class=secno>4. </span> Numeric Data Types</h2>
<p> Properties may restrict numeric values 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> CSS theoretically supports infinite precision and infinite ranges for
all value types; however in reality implementations have finite capacity.
UAs should support reasonably useful ranges and precisions. <!--
The recommended minimum ranges and precision,
and the required rounding and clamping rules,
are given in <a href="#required-ranges">Appendix A</a>.
-->
<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><var><integer></var></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>.
The first digit of an integer may be immediately preceded by ‘<code
class=css>-</code>’ or ‘<code class=css>+</code>’ to indicate the
integer's sign.
<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><var><number></var></dfn>. A <dfn
id=number>number</dfn> is either an <integer> 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>.
As with integers, the first character of a number may be immediately
preceded by ‘<code class=css>-</code>’ or ‘<code
class=css>+</code>’ to indicate the number's sign.
<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><var><percentage></var></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.
<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><var><length></var></dfn> in the property definitions.
A length is a <a href="#dimension"><i>dimension</i></a>. However, for zero
lengths the unit identifier is optional (i.e. can be syntactically
represented as the <a href="#number-value"><var><number></var></a>
‘<code class=css>0</code>’).
<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>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 <!--FIXME: href--><a>used</a> length cannot be
supported, user agents must approximate it in the
<!--FIXME: href--><a>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" (ZERO, U+0030) 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>1% of viewport's width
<tr>
<td>‘<a href="#vh-unit"><code class=css>vh</code></a>’
<td>1% viewport's height
<tr>
<td>‘<a href="#vmin-unit"><code class=css>vmin</code></a>’
<td>1% of viewport's smaller dimension
<tr>
<td>‘<a href="#vmax-unit"><code class=css>vmax</code></a>’
<td>1% of viewport's larger dimension
</table>
<p>Child elements do not inherit the relative values as specified for their
parent; they inherit the <!--FIXME: href--><a>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 computed font metrics of the parent element (or the computed
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
6D51
of the ‘<code
class=property>font-size</code>’ property of the element on which it
is used.
<div class=example>
<p>The rule:
<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 computed font size inherited by <code>h1</code>
elements.
</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>.
</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>’, ‘<a href="#vmax-unit"><code