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<!doctype html>
<html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<title>CSS Values and Units Module Level 3</title>
<link href="../default.css" rel="stylesheet" type="text/css">
<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
<style>
body {
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
background-attachment: fixed;
color: black;
font-family: sans-serif;
margin: 0 auto;
max-width: 50em;
padding: 2em 1em 2em 70px;
}
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code, small { white-space: nowrap }
pre.value { font: inherit; white-space: pre-wrap; margin: 0; padding: 0; }
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</head>
<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="http://www.w3.org/">
<img alt="W3C" height="48" src="https://www.w3.org/Icons/w3c_home" width="72">
</a>
</p>
<h1 class="p-name no-ref" id="title">CSS Values and Units Module Level 3</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft,
<time class="dt-updated" datetime="2015-04-22">22 April 2015</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="http://dev.w3.org/csswg/css-values/">http://dev.w3.org/csswg/css-values/</a>
<dt>Latest version:
<dd><a href="http://www.w3.org/TR/css-values/">http://www.w3.org/TR/css-values/</a>
<dt>Previous Versions:
<dd><a href="http://www.w3.org/TR/2013/CR-css3-values-20130730/" rel="previous">http://www.w3.org/TR/2013/CR-css3-values-20130730/</a>
<dd><a href="http://www.w3.org/TR/2013/CR-css3-values-20130404/" rel="previous">http://www.w3.org/TR/2013/CR-css3-values-20130404/</a>
<dd><a href="http://www.w3.org/TR/2012/CR-css3-values-20120828/" rel="previous">http://www.w3.org/TR/2012/CR-css3-values-20120828/</a>
<dd><a href="http://www.w3.org/TR/2012/WD-css3-values-20120308/" rel="previous">http://www.w3.org/TR/2012/WD-css3-values-20120308/</a>
<dd><a href="http://www.w3.org/TR/2011/WD-css3-values-20110906/" rel="previous">http://www.w3.org/TR/2011/WD-css3-values-20110906/</a>
<dd><a href="http://www.w3.org/TR/2006/WD-css3-values-20060919" rel="previous">http://www.w3.org/TR/2006/WD-css3-values-20060919</a>
<dd><a href="http://www.w3.org/TR/2005/WD-css3-values-20050726" rel="previous">http://www.w3.org/TR/2005/WD-css3-values-20050726</a>
<dd><a href="http://www.w3.org/TR/2001/WD-css3-values-20010713/" rel="previous">http://www.w3.org/TR/2001/WD-css3-values-20010713/</a>
<dt>Feedback:
<dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss-values%5D%20feedback">www-style@w3.org</a> with subject line “<kbd>[css-values] <var>… message topic …</var></kbd>” (<a href="http://lists.w3.org/Archives/Public/www-style/" rel="discussion">archives</a>)</span>
<dt>Issue Tracking:
<dd><a href="http://www.w3.org/Style/CSS/Tracker/products/33">Tracker</a>
<dt class="editor">Editors:
<dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://xanthir.com/contact/">Tab Atkins</a> (<span class="p-org org">Google</span>)
<dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://fantasai.inkedblade.net/contact">fantasai</a>
<dt>Former Editors:
<dd>
<dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:howcome@opera.com">Håkon Wium Lie</a> (<span class="p-org org">Opera Software</span>)
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2015 <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>, <a href="http://ev.buaa.edu.cn/">Beihang</a>). 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="h
6DC6
ttp://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.
</p>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<div class="p-summary" data-fill-with="abstract">
<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.</p>
<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.</div>
<h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
<div data-fill-with="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>
<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=%5Bcss-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 “css-values” in the subject,
preferably like this:
“[css-values] <em>…summary of comment…</em>”
</p>
<p>
This document was produced by the <a href="http://www.w3.org/Style/CSS/members">CSS Working Group</a>
(part of the <a href="http://www.w3.org/Style/">Style Activity</a>).
</p>
<p>
This document was produced by a group operating under
the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/32061/status" rel="disclosure">public list of any patent disclosures</a>
made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a>
must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent Policy</a>.
</p>
<p>
This document is governed by the <a href="http://www.w3.org/2014/Process-20140801/" id="w3c_process_revision">1 August 2014 W3C Process Document</a>.
</p>
<p>
</p></div>
<div data-fill-with="at-risk">
<p>The following features are at-risk, and may be dropped during the CR period:
</p>
<ul>
<li><a class="css" data-link-type="maybe" href="#funcdef-toggle">toggle()</a>, <a class="css" data-link-type="maybe" href="#funcdef-attr">attr()</a>
</ul></div>
<h2 class="no-num no-toc no-ref heading settled" id="contents"><span class="content">Table of Contents</span></h2>
<div data-fill-with="table-of-contents" role="navigation">
<ul class="toc" role="directory">
<li><a href="#intro"><span class="secno">1</span> <span class="content">
Introduction</span></a>
<ul class="toc">
<li><a href="#placement"><span class="secno">1.1</span> <span class="content">
Module Interactions</span></a>
</ul>
<li><a href="#value-defs"><span class="secno">2</span> <span class="content">
Value Definition Syntax</span></a>
<ul class="toc">
<li><a href="#component-types"><span class="secno">2.1</span> <span class="content">
Component value types</span></a>
<li><a href="#component-combinators"><span class="secno">2.2</span> <span class="content">
Component value combinators</span></a>
<li><a href="#component-multipliers"><span class="secno">2.3</span> <span class="content">
Component value multipliers</span></a>
<li><a href="#combinator-multiplier-patterns"><span class="secno">2.4</span> <span class="content">
Combinator and Multiplier Patterns</span></a>
<li><a href="#component-whitespace"><span class="secno">2.5</span> <span class="content">
Component values and white space</span></a>
<li><a href="#value-examples"><span class="secno">2.6</span> <span class="content">
Property value examples</span></a>
</ul>
<li><a href="#textual-values"><span class="secno">3</span> <span class="content">
Textual Data Types</span></a>
<ul class="toc">
<li><a href="#keywords"><span class="secno">3.1</span> <span class="content">
Pre-defined Keywords</span></a>
<ul class="toc">
<li><a href="#common-keywords"><span class="secno">3.1.1</span> <span class="content">
CSS-wide keywords: <span class="css">initial</span>, <span class="css">inherit</span> and <span class="css">unset</span></span></a>
</ul>
<li><a href="#custom-idents"><span class="secno">3.2</span> <span class="content">
Author-defined Identifiers: the <span class="production"><custom-ident></span> type</span></a>
<li><a href="#strings"><span class="secno">3.3</span> <span class="content">
Quoted Strings: the <span class="production"><string></span> type</span></a>
<li><a href="#urls"><span class="secno">3.4</span> <span class="content">
Resource Locators: the <span class="production"><url></span> type</span></a>
<ul class="toc">
<li><a href="#relative-urls"><span class="secno">3.4.1</span> <span class="content">
Relative URLs</span></a>
<li><a href="#url-modifiers"><span class="secno">3.4.2</span> <span class="content">
URL Modifiers</span></a>
</ul>
</ul>
<li><a href="#numeric-types"><span class="secno">4</span> <span class="content">
Numeric Data Types</span></a>
<ul class="toc">
<li><a href="#integers"><span class="secno">4.1</span> <span class="content">
Integers: the <span class="production"><integer></span> type</span></a>
<li><a href="#numbers"><span class="secno">4.2</span> <span class="content">
Real Numbers: the <span class="production"><number></span> type</span></a>
<li><a href="#percentages"><span class="secno">4.3</span> <span class="content">
Percentages: the <span class="production" title="Expands to: "><percentage></span> type</span></a>
<li><a href="#dimensions"><span class="secno">4.4</span> <span class="content">
Numbers with Units: <i>dimensions</i></span></a>
</ul>
<li><a href="#lengths"><span class="secno">5</span> <span class="content">
Distance Units: the <span class="production" title="Expands to: "><length></span> type</span></a>
<ul class="toc">
<li><a href="#relative-lengths"><span class="secno">5.1</span> <span class="content">
Relative lengths</span></a>
<ul class="toc">
<li><a href="#font-relative-lengths"><span class="secno">5.1.1</span> <span class="content">
Font-relative lengths: the <span class="css">em</span>, <span class="css">ex</span>, <span class="css">ch</span>, <span class="css">rem</span> units</span></a>
<li><a href="#viewport-relative-lengths"><span class="secno">5.1.2</span> <span class="content">
Viewport-percentage lengths: the <span class="css">vw</span>, <span class="css">vh</span>, <span class="css">vmin</span>, <span class="css">vmax</span> units</span></a>
</ul>
<li><a href="#absolute-lengths"><span class="secno">5.2</span> <span class="content">
Absolute lengths: the <span class="css">cm</span>, <span class="css">mm</span>, <span class="css">q</span>, <span class="css">in</span>, <span class="css">pt</span>, <span class="css">pc</span>, <span class="css">px</span> units</span></a>
</ul>
<li><a href="#other-units"><span class="secno">6</span> <span class="content">
Other Quantities</span></a>
<ul class="toc">
<li><a href="#angles"><span class="secno">6.1</span> <span class="content">
Angle Units: the <span class="production"><angle></span> type and <span class="css">deg</span>, <span class="css">grad</span>, <span class="css">rad</span>, <span class="css">turn</span> units</span></a>
<li><a href="#time"><span class="secno">6.2</span> <span class="content">
Duration Units: the <span class="production"><time></span> type and <span class="css">s</span>, <span class="css">ms</span> units</span></a>
<li><a href="#frequency"><span class="secno">6.3</span> <span class="content">
Frequency Units: the <span class="production"><frequency></span> type and <span class="css">Hz</span>, <span class="css">kHz</span> units</span></a>
<li><a href="#resolution"><span class="secno">6.4</span> <span class="content">
Resolution Units: the <span class="production"><resolution></span> type and <span class="css">dpi</span>, <span class="css">dpcm</span>, <span class="css">dppx</span> units</span></a>
</ul>
<li><a href="#defined-elsewhere"><span class="secno">7</span> <span class="content">
Data Types Defined Elsewhere</span></a>
<ul class="toc">
<li><a href="#colors"><span class="secno">7.1</span> <span class="content">
Colors: the <span class="production" title="Expands to: steelblue
| skyblue
| salmon
| green
| lightblue
| powderblue
| darkgoldenrod
| grey
| aquamarine
| mintcream
| slategrey
| midnightblue
| paleturquoise
| mediumturquoise
| orchid
| peru
| pink
| slateblue
| dodgerblue
| cadetblue
| lawngreen
| transparent
| chartreuse
| brown
| yellow
| limegreen
| gray
| black
| blue
| tomato
| silver
| oldlace
| navajowhite
| saddlebrown
| mediumvioletred
| chocolate
| springgreen
| teal
| currentcolor
| ghostwhite
| mediumorchid
| mediumspringgreen
| turquoise
| darkseagreen
| antiquewhite
| blanchedalmond
| darkmagenta
| peachpuff
| firebrick
| lightseagreen
| dimgrey
| cornflowerblue
| navy
| snow
| cornsilk
| lightgrey
| rebeccapurple
| darksalmon
| darkviolet
| lavender
| darkgrey
| olive
| lime
| whitesmoke
| papayawhip
| lightgoldenrodyellow
| darkslategrey
| sandybrown
| darkslateblue
| lightgray
| lightsalmon
| palevioletred
| sienna
| ivory
| darkblue
| lightsteelblue
| crimson
| lightslategray
| gold
| aqua
| darkred
| burlywood
| yellowgreen
| tan
| honeydew
| darkgreen
| mediumpurple
| darkcyan
| lightgreen
| cyan
| forestgreen
| plum
| darkorange
| mediumslateblue
| azure
| coral
| rosybrown
| white
| aliceblue
| lightskyblue
| mediumblue
| lightcyan
| moccasin
| fuchsia
| mediumaquamarine
| indianred
| lightcoral
| lemonchiffon
| beige
| seashell
| slategray
| orange
| darkgray
| goldenrod
| deepskyblue
| olivedrab
| thistle
| darkslategray
| darkorchid
| lightyellow
| royalblue
| lightpink
| mediumseagreen
| mistyrose
| palegreen
| blueviolet
| lavenderblush
| gainsboro
| deeppink
| maroon
| khaki
| lightslategrey
| linen
| seagreen
| darkkhaki
| magenta
| indigo
| darkturquoise
| floralwhite
| greenyellow
| dimgray
| red
| palegoldenrod
| bisque
| purple
| orangered
| violet
| darkolivegreen
| wheat
| hotpink
"><color></span> type</span></a>
<li><a href="#images"><span class="secno">7.2</span> <span class="content">
Images: the <span class="production"><image></span> type</span></a>
<li><a href="#position"><span class="secno">7.3</span> <span class="content">
2D Positioning: the <span class="production"><position></span> type</span></a>
</ul>
<li><a href="#functional-notations"><span class="secno">8</span> <span class="content">
Functional Notations</span></a>
<ul class="toc">
<li><a href="#calc-notation"><span class="secno">8.1</span> <span class="content">
Mathematical Expressions: <span class="css">calc()</span></span></a>
<ul class="toc">
<li><a href="#calc-syntax"><span class="secno">8.1.1</span> <span class="content">
Syntax</span></a>
<li><a href="#calc-type-checking"><span class="secno">8.1.2</span> <span class="content">
Type Checking</span></a>
<li><a href="#calc-computed-value"><span class="secno">8.1.3</span> <span class="content">
Computed Value</span></a>
<li><a href="#calc-range"><span class="secno">8.1.4</span> <span class="content">
Range Checking</span></a>
</ul>
<li><a href="#toggle-notation"><span class="secno">8.2</span> <span class="content">
Toggling Between Values: <span class="css">toggle()</span></span></a>
<li><a href="#attr-notation"><span class="secno">8.3</span> <span class="content">
Attribute References: <span class="css">attr()</span></span></a>
</ul>
<li><a href="#iana"><span class="secno">9</span> <span class="content">
Appendix A: IANA Considerations</span></a>
<ul class="toc">
<li><a href="#about-invalid"><span class="secno">9.1</span> <span class="content">
Registration for the <code>about:invalid</code> URL scheme</span></a>
</ul>
<li><a href="#acknowledgments"><span class="secno"></span> <span class="content">
Acknowledgments</span></a>
<li><a href="#changes"><span class="secno"></span> <span class="content">
Changes</span></a>
<li><a href="#conformance"><span class="secno"></span> <span class="content">
Conformance</span></a>
<ul class="toc">
<li><a href="#conventions"><span class="secno"></span> <span class="content">
Document conventions</span></a>
<li><a href="#conformance-classes"><span class="secno"></span> <span class="content">
Conformance classes</span></a>
<li><a href="#partial"><span class="secno"></span> <span class="content">
Partial implementations</span></a>
<li><a href="#experimental"><span class="secno"></span> <span class="content">
Experimental implementations</span></a>
<li><a href="#testing"><span class="secno"></span> <span class="content">
Non-experimental implementations</span></a>
</ul>
<li><a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
<ul class="toc">
<li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
<li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
</ul>
<li><a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ul class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
</ul>
</ul></div>
<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">
Introduction</span><a class="self-link" href="#intro"></a></h2>
<p>The value definition field of each CSS property can contain keywords,
data types (which appear between <span class="css"><</span> and <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/selectors-4/#selectordef-child">></a>), and information on how
they can be combined.
Generic data types (<a class="production css" data-link-type="type" href="#length-value"><length></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., <a class="production css" data-link-type="type"><spacing-limit></a>)
are described in the corresponding modules.</p>
<h3 class="heading settled" data-level="1.1" id="placement"><span class="secno">1.1. </span><span class="content">
Module Interactions</span><a class="self-link" href="#placement"></a></h3>
<p>This module replaces and extends the data type definitions in <a data-link-type="biblio" href="#biblio-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>.</p>
<h2 class="heading settled" data-level="2" id="value-defs"><span class="secno">2. </span><span class="content">
Value Definition Syntax</span><a class="self-link" href="#value-defs"></a></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.</p>
<h3 class="heading settled" data-level="2.1" id="component-types"><span class="secno">2.1. </span><span class="content">
Component value types</span><a class="self-link" href="#component-types"></a></h3>
<p>Component value types are designated in several ways:</p>
<ol>
<li>
<a href="#keywords">keyword</a> values (such as <span class="css">auto</span>, <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-counter-styles-3/#disc">disc</a>, etc.),
which appear literally, without quotes (e.g. <code>auto</code>)
<li>
basic data types, which appear between <span class="css"><</span> and <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/selectors-4/#selectordef-child">></a>
(e.g., <a class="production css" data-link-type="type" href="#length-value" title="Expands to: "><length></a>, <a class="production css" data-link-type="type" href="#percentage-value"><percentage></a>, etc.).
<li>
types that have the same range of values as a property bearing the same name
(e.g., <a class="production" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#border-width"><‘border-width’></a>, <a class="production" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#background-attachment"><‘background-attachment’></a>, 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 class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-inherit">inherit</a>.
<li>
non-terminals that do not share the same name as a property.
In this case, the non-terminal name appears between <span class="css"><</span> and <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/selectors-4/#selectordef-child">></a>,
as in <a class="production css" data-link-type="type"><spacing-limit></a>.
Notice the distinction between <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css2/box.html#value-def-border-width"><border-width></a> and <a class="production" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#border-width"><‘border-width’></a>:
the latter is defined as the value of the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#border-width">border-width</a> property,
the former requires an explicit expansion elsewhere.
The definition of a non-terminal is typically located near its first appearance in the specification.
</ol>
<p>Some property value definitions also include the slash (/),
the comma (,),
and/or parentheses as literals.
These represent their corresponding tokens.
Other non-keyword literal characters that may appear in a component value,
such as “+”,
must be written enclosed in single quotes.</p>
<p><dfn data-dfn-type="grammar" data-export="" data-lt="," id="comb-comma">Commas<a class="self-link"
1D64
href="#comb-comma"></a></dfn> specified in the grammar are <strong>implicitly omittable</strong> in some circumstances,
when used to separate optional terms in the grammar.
Within a top-level list in a property or other CSS value,
or a function’s argument list,
a comma specified in the grammar must be omitted if:</p>
<ul>
<li>
all items preceding the comma have been omitted
<li>
all items following the comma have been omitted
<li>
multiple commas would be adjacent (ignoring whitespace/comments),
due to the items between the commas being omitted.
</ul>
<div class="example">
For example, if a function can accept three arguments in order,
but all of them are optional,
the grammar can be written like:
<pre class="prod">example( first<a data-link-type="grammar" href="#mult-opt">?</a> <a data-link-type="grammar" href="#comb-comma">,</a> second<a data-link-type="grammar" href="#mult-opt">?</a> <a data-link-type="grammar" href="#comb-comma">,</a> third<a data-link-type="grammar" href="#mult-opt">?</a> )
</pre>
<p>Given this grammar,
writing <span class="css">example(first, second, third)</span> is valid,
as is <span class="css">example(first, second)</span> or <span class="css">example(first, third)</span> or <span class="css">example(second)</span>.
However, <span class="css">example(first, , third)</span> is invalid, as one of those commas are no longer separating two options;
similarly, <span class="css">example(,second)</span> and <span class="css">example(first,)</span> are invalid.
<span class="css">example(first second)</span> is also invalid,
as commas are still required to actually separate the options.</p>
<p>If commas were not implicitly omittable,
the grammar would have to be much more complicated
to properly express the ways that the arguments can be omitted,
greatly obscuring the simplicity of the feature.</p>
</div>
<p>All CSS properties also accept the <a href="#common-keywords">CSS-wide keyword values</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 class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#border-color">border-color</a>
is <code><color>{1,4} | inherit | initial | unset</code>
(even though it is listed as <code><color>{1,4}</code>).</p>
<p class="note" role="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,
<a class="css" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#background">background: url(corner.png) no-repeat, inherit;</a> is invalid.</p>
<h3 class="heading settled" data-level="2.2" id="component-combinators"><span class="secno">2.2. </span><span class="content">
Component value combinators</span><a class="self-link" href="#component-combinators"></a></h3>
<p>Component values can be arranged into property values as follows:</p>
<ul>
<li>Juxtaposing components means that
all of them must occur, in the given order.
<li>A double ampersand (<dfn data-dfn-type="grammar" data-export="" id="comb-all">&&<a class="self-link" href="#comb-all"></a></dfn>) separates two or more components,
all of which must occur, in any order.
<li>A double bar (<dfn data-dfn-type="grammar" data-export="" id="comb-any">||<a class="self-link" href="#comb-any"></a></dfn>) separates two or more options:
one or more of them must occur, in any order.
<li>A bar (<dfn data-dfn-type="grammar" data-export="" id="comb-one">|<a class="self-link" href="#comb-one"></a></dfn>) 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:</p>
<pre> a b | c || d && e f
[ a b ] | [ c || [ d && [ e f ]]]
</pre>
<p>For re-orderable combinators (||, &&),
ordering of the grammar does not matter:
components in the same grouping may be interleaved in any order.
Thus, the following lines are equivalent:</p>
<pre>a || b || c
b || a || c
</pre>
<h3 class="heading settled" data-level="2.3" id="component-multipliers"><span class="secno">2.3. </span><span class="content">
Component value multipliers</span><a class="self-link" href="#component-multipliers"></a></h3>
<p>Every type, keyword, or bracketed group may be followed by one of
the following modifiers:</p>
<ul>
<li>An asterisk (<dfn data-dfn-type="grammar" data-export="" id="mult-zero-plus">*<a class="self-link" href="#mult-zero-plus"></a></dfn>) indicates that the preceding type, word, or
group occurs zero or more times.
<li>A plus (<dfn data-dfn-type="grammar" data-export="" id="mult-one-plus">+<a class="self-link" href="#mult-one-plus"></a></dfn>) indicates that the preceding type, word, or group
occurs one or more times.
<li>A question mark (<dfn data-dfn-type="grammar" data-export="" id="mult-opt">?<a class="self-link" href="#mult-opt"></a></dfn>) indicates that the preceding type, word, or
group is optional (occurs zero or one times).
<li>A single number in curly braces (<dfn data-dfn-type="grammar" data-export="" id="mult-num">{<var>A</var>}<a class="self-link" href="#mult-num"></a></dfn>)
indicates that the preceding type, word, or group occurs <var>A</var> times.
<li>A comma-separated pair of numbers in curly braces (<dfn data-dfn-type="grammar" data-export="" id="mult-num-range">{<var>A</var>,<var>B</var>}<a class="self-link" href="#mult-num-range"></a></dfn>)
indicates that the preceding type, word, or group occurs at least
<var>A</var> and at most <var>B</var> times.
The <var>B</var> may be omitted ({<var>A</var>,})
to indicate that there must be at least <var>A</var> repetitions,
with no upper bound on the number of repetitions.
<li>A hash mark (<dfn data-dfn-type="grammar" data-export="" id="mult-comma">#<a class="self-link" href="#mult-comma"></a></dfn>) indicates that the preceding type, word, or
group occurs one or more times, separated by comma tokens
(which may optionally be surrounded by white space and/or comments).
It may optionally be followed by the curly brace forms, above,
to indicate precisely how many times the repetition occurs,
like <span class="css"><length>#{1,4}</span>.
<li>An exclamation point (<dfn data-dfn-type="grammar" data-export="" id="mult-req">!<a class="self-link" href="#mult-req"></a></dfn>) after a group indicates that the group is required
and must produce at least one value;
even if the grammar of the items within the group would otherwise allow the entire contents to be omitted,
at least one component value must not be omitted.
</ul>
<p>For repeated component values (indicated by <span class="css">*</span>, <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/selectors-4/#selectordef-adjacent">+</a>, or <span class="css">#</span>),
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.</p>
<h3 class="heading settled" data-level="2.4" id="combinator-multiplier-patterns"><span class="secno">2.4. </span><span class="content">
Combinator and Multiplier Patterns</span><a class="self-link" href="#combinator-multiplier-patterns"></a></h3>
<p>There are a small set of common ways to combine multiple independent <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-syntax-3/#component-value">component values</a> in particular numbers and orders.
In particular, it’s common to want to express that,
from a set of component value,
the author must select zero or more, one or more, or all of them,
and in either the order specified in the grammar or in any order.</p>
<p>All of these can be easily expressed using simple patterns of <a href="#component-combinators">combinators</a> and <a href="#component-multipliers">multipliers</a>:</p>
<table class="data">
<thead>
<tr>
<th>
<th>in order
<th>any order
<tbody>
<tr>
<th>zero or more
<td>
<pre>A? B? C?</pre>
<td>
<pre> A? || B? || C?</pre>
<tr>
<th>one or more
<td>
<pre> [ A? B? C? ]!</pre>
<td>
<pre>A || B || C</pre>
<tr>
<th>all
<td>
<pre>A B C </pre>
<td>
<pre>A && B && C</pre>
</table>
<p>Note that all of the "any order" possibilities are expressed using combinators,
while the "in order" possibilities are all variants on juxtaposition.</p>
<h3 class="heading settled" data-level="2.5" id="component-whitespace"><span class="secno">2.5. </span><span class="content">
Component values and white space</span><a class="self-link" href="#component-whitespace"></a></h3>
<p>Unless otherwise specified,
white space and/or comments may appear before, after, and/or between
components combined using the above
<a href="#component-combinators">combinators</a> and
<a href="#component-multipliers">multipliers</a>.</p>
<p class="note" role="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 <span class="css">1em2em</span> would be parsed as a single <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-syntax-3/#typedef-dimension-token"><dimension-token></a>
with the number <span class="css">1</span> and the identifier <span class="css">em2em</span>,
which is an invalid unit.
In this case, a space would be required before the <span class="css">2</span>
to get this parsed as the two lengths <span class="css">1em</span>and <span class="css">2em</span>.</p>
<h3 class="heading settled" data-level="2.6" id="value-examples"><span class="secno">2.6. </span><span class="content">
Property value examples</span><a class="self-link" href="#value-examples"></a></h3>
<p>Below are some examples of properties with their corresponding value
definition fields</p>
<div class="example">
<table class="data" id="propvalues">
<thead>
<tr>
<th>Property
<th>Value definition field
<th>Example value
<tbody>
<tr>
<td><a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-break-3/#orphans">orphans</a>
<td><integer>
<td><span class="css">3</span>
<tr>
<td><a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-text-3/#propdef-text-align">text-align</a>
<td>left | right | center | justify
<td><a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-text-3/#valdef-text-align-center">center</a>
<tr>
<td><a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/box.html#propdef-padding-top">padding-top</a>
<td><length> | <percentage>
<td><span class="css">5%</span>
<tr>
<td><a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-ui-3/#propdef-outline-color">outline-color</a>
<td><color> | invert
<td><span class="css">#fefefe</span>
<tr>
<td><a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-text-decor-3/#propdef-text-decoration">text-decoration</a>
<td>none | underline || overline || line-through || blink
<td><span class="css">overline underline</span>
<tr>
<td><a class="css" data-link-type="property" href="http://dev.w3.org/csswg/css-fonts-3/#propdef-font-family">font-family</a>
<td>[ <family-name> | <generic-family> ]#
<td><span class="css">"Gill Sans", Futura, sans-serif</span>
<tr>
<td><a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#border-width">border-width</a>
<td>[ <length> | thick | medium | thin ]{1,4}
<td><span class="css">2px medium 4px</span>
<tr>
<td><a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-text-decor-3/#propdef-text-shadow">text-shadow</a>
<td>[ inset? && [ <length>{2,4} && <color>? ] ]# | none
<td><span class="css">3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0 0 4px inset</span>
</table>
</div>
<h2 class="heading settled" data-level="3" id="textual-values"><span class="secno">3. </span><span class="content">
Textual Data Types</span><a class="self-link" href="#textual-values"></a></h2>
<p>An <dfn data-dfn-type="dfn" data-noexport="" id="identifier">identifier<a class="self-link" href="#identifier"></a></dfn> is a sequence of characters conforming to the <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-syntax-3/#typedef-ident-token"><ident-token></a> grammar. <a data-link-type="biblio" href="#biblio-css3syn">[CSS3SYN]</a>
Identifiers cannot be quoted;
otherwise they would be interpreted as a string.</p>
<h3 class="heading settled" data-level="3.1" id="keywords"><span class="secno">3.1. </span><span class="content">
Pre-defined Keywords</span><a class="self-link" href="#keywords"></a></h3>
<p>In the value definition fields,
keywords with a pre-defined meaning appear literally.
Keywords are CSS <i>identifiers</i>
and are interpreted <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-syntax-3/#ascii-case_insensitive">ASCII case-insensitively</a>
(i.e., \[a-z] and [A-Z] are equivalent).</p>
<div class="example">
For example, here is the value definition for the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/tables.html#propdef-border-collapse">border-collapse</a>
property:
<pre>Value: collapse | separate</pre>
<p>And here is an example of its use:</p>
<pre>table { border-collapse: separate }</pre>
</div>
<h4 class="heading settled" data-level="3.1.1" id="common-keywords"><span class="secno">3.1.1. </span><span class="content">
CSS-wide keywords: <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-initial">initial</a>, <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-inherit">inherit</a> and <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-unset">unset</a></span><a class="self-link" href="#common-keywords"></a></h4>
<p>As defined <a href="#component-types">above</a>,
all properties accept the <dfn data-dfn-type="dfn" data-export="" id="css_wide-keywords">CSS-wide keywords<a class="self-link" href="#css_wide-keywords"></a></dfn>,
which represent value computations common to all CSS properties.</p>
<p>The <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-initial">initial</a> keyword represents the value specified as the property’s initial value.
The <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-inherit">inherit</a> keyword represents the computed value of the property on the element’s parent.
The <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-unset">unset</a> keyword acts as either <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-inherit">inherit</a> or <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-initial">initial</a>,
depending on whether the property is inherited or not.
All of these keywords are normatively defined in the Cascade module. <a data-link-type="biblio" href="#biblio-css3cascade">[CSS3CASCADE]</a></p>
<p>Other CSS specifications can define additional CSS-wide keywords.</p>
<h3 class="heading settled" data-level="3.2" id="custom-idents"><span class="secno">3.2. </span><span class="content">
Author-defined Identifiers: the <a class="production css" data-link-type="type" href="#identifier-value"><custom-ident></a> type</span><a class="self-link" href="#custom-idents"></a></h3>
<p>Some properties accept arbitrary author-defined identifiers as a component value.
This generic data type is denoted by <dfn class="css" data-dfn-type="type" data-export="" id="identifier-value"><custom-ident><a class="self-link" href="#identifier-value"></a></dfn>,
and represents any valid CSS <i>identifier</i>
that would not be misinterpreted as a pre-defined keyword in that property’s value definition.