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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=iso-8859-1" http-equiv=Content-Type>
<title>CSS3 Values and Units</title>
<style type="text/css">
.issue { color: red }
table { border-collapse: collapse; border: 1px solid black; margin: 1em 0 }
th, td { border: 1px solid black; padding: 0.2em; text-align: left }
tt.declaration { white-space: nowrap }
.del { text-decoration: line-through}
.say { font-style: italic }
</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 id=css3-template>CSS3 Values and Units</h1>
<h2 class="no-num no-toc" id=w3c-working>W3C Working Draft 17 January 2009</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2009/ED-css3-values-20090117">http://www.w3.org/TR/2009/ED-css3-values-20090117</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-values">http://www.w3.org/TR/css3-values</a>
<dt>Previous version:
<dd><a
href="http://www.w3.org/TR/2005/WD-css3-values-20050726">http://www.w3.org/TR/2005/WD-css3-values-20050726</a>
<dt>Editors:
<dd>Håkon Wium Lie, Opera Software <howcome@opera.com>
<dd>Chris Lilley, W3C <chris@w3.org>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2009 <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.org/"><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" (which is what the cascading process yields) 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">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>This is a draft of a <a href="http://www.w3.org/TR/css3-roadmap/">module
of CSS level 3</a>. It will probably be bundled with some other modules
before it becomes a <a href="http://www.w3.org/TR/#About">W3C
Recommendation</a>.
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#dependencies-on-other-modules"><span class=secno>1
</span>Dependencies on other modules</a>
<li><a href="#introduction"><span class=secno>2 </span>Introduction</a>
<li><a href="#values"><span class=secno>3 </span>Values</a>
<ul class=toc>
<li><a href="#keywords"><span class=secno>3.1 </span>Keywords</a>
<li><a href="#numbers"><span class=secno>3.2 </span>Numbers</a>
<li><a href="#numbers-with-unit-identifiers"><span class=secno>3.3
</span>Numbers with unit identifiers</a>
<ul class=toc>
<li><a href="#lengths"><span class=secno>3.3.1 </span>Lengths</a>
<li><a href="#absolute-length-units"><span class=secno>3.3.2
</span>Absolute length units</a>
<li><a href="#relative-length-units"><span class=secno>3.3.3
</span>Relative length units</a>
<ul class=toc>
<li><a href="#the-em-unit"><span class=secno>3.3.3.1. </span>The
‘<code class=property>em</code>’ unit</a>
<li><a href="#the-ex-unit"><span class=secno>3.3.3.2. </span>The
‘<code class=property>ex</code>’ unit</a>
<li><a href="#the-px-unit"><span class=secno>3.3.3.3. </span>The
‘<code class=property>px</code>’ unit</a>
<li><a href="#the-rem-unit"><span class=secno>3.3.3.4. </span>The
‘<code class=property>rem</code>’ unit</a>
<li><a href="#the-vw-unit"><span class=secno>3.3.3.5. </span>The
‘<code class=property>vw</code>’ unit</a>
<li><a href="#the-vh-unit"><span class=secno>3.3.3.6. </span>The
‘<code class=property>vh</code>’ unit</a>
<li><a href="#the-vm-unit"><span class=secno>3.3.3.7. </span>The
‘<code class=property>vm</code>’ unit</a>
</ul>
<li><a href="#fractions"><span class=secno>3.3.4 </span>Fractions</a>
<ul class=toc>
<li><a href="#the-fr-unit"><span class=secno>3.3.4.1. </span>The
‘<code class=property>fr</code>’ unit</a>
</ul>
<li><a href="#grids"><span class=secno>3.3.5 </span>Grids</a>
<ul class=toc>
<li><a href="#the-gr-unit"><span class=secno>3.3.5.1. </span>The
‘<code class=property>gr</code>’ unit</a>
</ul>
<li><a href="#percentages"><span class=secno>3.3.6
</span>Percentages</a>
<li><a href="#angles"><span class=secno>3.3.7 </span>Angles</a>
<li><a href="#times"><span class=secno>3.3.8 </span>Times</a>
<li><a href="#frequencies"><span class=secno>3.3.9
</span>Frequencies</a>
</ul>
<li><a href="#strings"><span class=secno>3.4 </span>Strings</a>
<li><a href="#functional-notation"><span class=secno>3.5
</span>Functional notation</a>
<ul class=toc>
<li><a href="#attribute"><span class=secno>3.5.1 </span>The
‘<code class=property>attr</code>’ function</a>
<li><a href="#uris"><span class=secno>3.5.2 </span>The ‘<code
class=property>url</code>’ function</a>
<li><a href="#the-counter-function"><span class=secno>3.5.3 </span>The
‘<code class=property>counter</code>’ function</a>
<li><a href="#calc"><span class=secno>3.5.4 </span>The ‘<code
class=property>calc</code>’ function</a>
</ul>
<li><a href="#special-cases"><span class=secno>3.6 </span>Special
cases</a>
<ul class=toc>
<li><a href="#font-families"><span class=secno>3.6.1 </span>Font
families</a>
<li><a href="#colors"><span class=secno>3.6.2 </span>Colors</a>
</ul>
</ul>
<li><a href="#specified-computed-and-actual-values"><span class=secno>4
</span>Specified, computed, and actual values</a>
<ul class=toc>
<li><a href="#finding-the-specified-value"><span class=secno>4.1
</span>Finding the specified value</a>
<li><a href="#finding-the-computed-value"><span class=secno>4.2
</span>Finding the computed value</a>
<li><a href="#finding-the-actual-value"><span class=secno>4.3
</span>Finding the actual value</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=dependencies-on-other-modules><span class=secno>1
</span>Dependencies on other modules</h2>
<p>This CSS3 module depends on the following other CSS3 modules:
<ul>
<li><cite>Cascading and Inheritance</cite> <a href="#CSS3CASCADE"
rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a>, which describes
how to find the <cite>resulting value</cite> from several competing
values. Also, it describes how to replace the "inherit" keyword with the
inherited value.
<li><cite>Syntax</cite> <a href="#CSS3SYN"
rel=biblioentry>[CSS3SYN]<!--{{!CSS3SYN}}--></a>, which describes the
syntax of values at the lexical level.
<li><cite>Color</cite> <a href="#CSS3COLOR"
rel=biblioentry>[CSS3COLOR]<!--{{!CSS3COLOR}}--></a>, which defines
additional values used for some properties that control color of text,
borders, etc
<li><cite>Fonts</cite> <a href="#CSS3FONT"
rel=biblioentry>[CSS3FONT]<!--{{!CSS3FONT}}--></a>, which describes
additional values used for font-related properties
</ul>
<h2 id=introduction><span class=secno>2 </span>Introduction</h2>
<p>By setting property values on elements in a document, style sheets
express the appearance of the document. In order to express rich designs,
a wide range of values and associated units are necessary. This
specifiction describes the various types of values and units that can be
used in CSS style sheets.
<h2 id=values><span class=secno>3 </span>Values</h2>
<p>Values in CSS can be split into eight groups:
<ol>
<li>keywords (e.g. "pitch-range: inherit")
<li>numbers (e.g. "orphans: 3")
<li>non-negative numbers (e.g. "orphans: 3")
<li>numbers with unit identifiers (e.g. "border-left: -1.2em")
<li>non-negative numbers with unit identifiers (e.g. "border-width:
0.2em")
<li>strings (e.g. "content: ‘<code class=css>Figure:
</code>’")
<li>functional notations (e.g. "background:
url(http://www.example.org/image)")
<li>special cases (e.g. "color: #F00" and "font-family: Helvetica,
sans-serif")
</ol>
<p>Most properties accept values from several of the above types. Some
properties accept space- or comma-separated lists of values.
Comma-separated lists have optional whitespace before and after the comma.
<p class=issue>The syntax module should perhaps describe comma- and
space-separated lists?
<p>Each CSS property has a formal definition of what types of values it
accepts which can be found in the description of the property. The syntax
used to describe acceptable values is <a
href="http://www.w3.org/TR/css3-syntax/#property-def-value">defined</a> in
<a href="#CSS3SYN" rel=biblioentry>[CSS3SYN]<!--{{!CSS3SYN}}--></a>.
<h3 id=keywords><span class=secno>3.1 </span>Keywords</h3>
<p>In the formal definition of property values, keywords appear literally.
<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>
<p>All CSS3 properties accept the keyword values <span
class=css>‘<code class=property>inherit</code>’</span> and
<span class=css>‘<code class=property>initial</code>’</span>.
Keywords must not be quoted.
<p class=issue>Would it be useful to have a ‘<code
class=property>default</code>’ value, defined to be equivalent to
<span class=css>‘<code class=property>inherit</code>’</span>
for properties that are inherited by default and equivalent to <span
class=css>‘<code class=property>initial</code>’</span> for
properties that are not inherited by default? This might be easier for
authors to use than <span class=css>‘<code
class=property>initial</code>’</span> and <span
class=css>‘<code class=property>inherit</code>’</span> since
it wouldn't require thinking about whether a property is inherited by
default or not (which isn't obvious for some properties, such as
text-decoration and visibility).
<h3 id=numbers><span class=secno>3.2 </span>Numbers</h3>
<p>Number values can either be integer or real numbers Integer values are
denoted by <dfn id=ltintegergt><integer></dfn> and real number
values are denoted by <dfn id=ltnumbergt><number></dfn>. Integers
and real numbers are specified in decimal notation only. An
<integer> consists of one or more digits "0" to "9". A
<number> can either be an <integer>, or it can be zero or more
digits followed by a dot (.) followed by one or more digits. Both integers
and real numbers may be preceded by a "-" or "+" to indicate the sign, and
the sign character is considered part of the <number> value.
<p>Many properties that allow an integer or real number as a value actually
restrict the value to some range. If the restriction is to a non-negative
value, then a non-negative number type is used.
<p>Non-negative number values can either be integer or real numbers.
Integer values are denoted by <dfn
id=ltnon-negative-integergt><non-negative integer></dfn> and real
number values are denoted by <dfn
id=ltnon-negative-numbergt><non-negative number></dfn>. The syntax
for non-negative numbers is the same as for other numbers execept that the
"-" sign is not allowed.
<h3 id=numbers-with-unit-identifiers><span class=secno>3.3 </span>Numbers
with unit identifiers</h3>
<p>Many CSS values consist of a number with a unit identifier. The most
common types is lengths, but percentages, angles, times, and frequences
also belong to this group.
<h4 id=lengths><span class=secno>3.3.1 </span>Lengths</h4>
<p>Lengths refer to horizontal or vertical measurements. The format of a
length value (denoted by <dfn id=ltlengthgt><length></dfn>) is a
<number> immediately followed by a unit identifier (e.g.,
‘<code class=property>px</code>’, ‘<code
class=property>pt</code>’). After the ‘<code
class=css>0</code>’ length, the unit identifier is optional.
<p>Some properties allow negative length values, but this may complicate
the formatting and there may be implementation-specific limits. If a
negative length value is allowed but cannot be supported, it should be
converted to the nearest value that can be supported.
<p>There are several types of length units. Absolute <dfn
id=relative-length-units0>Relative length units</dfn> specify a length
relative to other lengths. <!--Style sheets that use relative units will
more easily scale from one medium to another (e.g., from a computer
display to a laser printer).-->
<h4 id=absolute-length-units><span class=secno>3.3.2 </span>Absolute length
units</h4>
<p><dfn id=absolute-length>Absolute length</dfn> units are useful when the
physical properties of the output medium are known. The absolute units
are:
<table>
<tbody>
<tr>
<th>unit
<th>definition
<tr>
<td><span class=css>in</span>
<td>inches; 1 inch is equal to 2.54 centimeters.
<tr>
<td><span class=css>cm</span>
<td>centimeters
<tr>
<td><span class=css>mm</span>
<td>millimeters
<tr>
<td><span class=css>pt</span>
<td>points; 1pt is equal to 1/72 inch.
<tr>
<td><span class=css>pc</span>
<td>picas; 1 pica is equal to 12 points.
</table>
<div class=example>
<p style=display:none>Examples:</p>
<pre>
h1 { margin: 0.5in } /* inches */
h2 { line-height: 3cm } /* centimeters */
h3 { word-spacing: 4mm } /* millimeters */
h4 { font-size: 12pt } /* points */
h4 { font-size: 1pc } /* picas */</pre>
</div>
<p>In cases where the specified length cannot be supported, user agents
must approximate it in the actual value.
<h4 id=relative-length-units><span class=secno>3.3.3 </span>Relative length
units</h4>
<p id=absrel-units>
<p>Relative units are:
<table>
<tbody>
<tr>
<th>unit
<th>relative to
<tr>
<td><span class=css>em</span>
<td>the font size of the element (or, to the parent element's font size
if set on the <span class=property>‘<code
class=property>font-size</code>’</span> property)
<tr>
<td><span class=css>ex</span>
<td>the <em><a href="#ex">x-height</a></em> of the element's font
<tr>
<td><span class=css>px</span>
<td>viewing device <!--
<tr><td><span class="css">gd</span><td>the grid defined by <span class="property">‘<code class=property>layout-grid</code>’</span> described in the CSS3 Text module !CSS3TEXT
-->
<tr>
<td><span class=css>rem</span>
<td>the font size of the root element
<tr>
<td><span class=css>vw</span>
<td>the viewport's width
<tr>
<td><span class=css>vh</span>
<td>the viewport's height
<tr>
<td><span class=css>vm</span>
<td>the viewport's height or width, whichever is smaller of the two
<tr>
<td><span class=css>ch<span></span></span>
<td>The width of the "0" (ZERO, U+0030) glyph found in the font for the
font size used to render. If the "0" glyph is not found in the font,
the average character width may be used. <span class=issue>How is the
"average character width" found?</span>
</table>
<h5 id=the-em-unit><span class=secno>3.3.3.1. </span>The ‘<code
class=property>em</code>’ unit</h5>
<p>The <dfn id=em title="em (unit)|quad width">‘<code
class=property>em</code>’</dfn> unit is equal to the computed value
of the <span class=property>‘<code
class=property>font-size</code>’</span> property of the element on
which it is used. The exception is when ‘<code
class=property>em</code>’ occurs in the value of the <span
class=property>‘<code class=property>font-size</code>’</span>
property itself, in which case it refers to the font size of the parent
element. On the root element, It may be used for vertical or horizontal
measurement. (This unit is also sometimes called the quad-width in
typographic texts.)
<p>The rule:
<pre class=example>
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 the elements. On the other hand:
<pre class=example>
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.
<div class=example>
<pre>
h1 { margin: 0.5em } /* em */
h1 { margin: 1ex } /* ex */
p { font-size: 12px } /* px */
</pre>
</div>
<h5 id=the-ex-unit><span class=secno>3.3.3.2. </span>The ‘<code
class=property>ex</code>’ unit</h5>
<p>The <dfn id=ex title="x-height|ex (unit)">‘<code
class=property>ex</code>’</dfn> unit is defined by 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 ‘<code
class=property>ex</code>’ is defined even for fonts that don't
contain an "x". <span class=issue>Should we say that ex is 0.5em if no
better value exists?</span>
<p>When specified on the <span class=property>‘<code
class=property>font-size</code>’</span> property of the root
element, ‘<code class=property>em</code>’ and ‘<code
class=property>ex</code>’ units refer to the property's <em>initial
value</em>.
<h5 id=the-px-unit><span class=secno>3.3.3.3. </span>The ‘<code
class=property>px</code>’ unit</h5>
<p>The pixel unit, <dfn id=px title="pixel unit|px (pixel)::definition
of">‘<code class=property>px</code>’</dfn> is relative to the
resolution of the viewing device. For example, the viewing device can be a
computer display or a printer. Normally, the pixel unit refers to physical
pixels of the viewing device. However, if the pixel density of the output
device is very different from that of a typical computer display, the user
agent should rescale pixel values. It is recommended that the <dfn
id=reference-pixel>reference pixel</dfn> be the visual angle of one pixel
on a device with a pixel density of 96dpi and a distance from the reader
of an arm's length. For a nominal arm's length of 28 inches, the visual
angle is therefore about 0.0213 degrees.
<p>For reading at arm's length, 1px thus corresponds to about 0.26mm
(1/96 inch). When printed on a laser printer, meant for reading at a
little less than arm's length (55 cm, 21 inches), 1px is about
0.21mm. On a 300 dots-per-inch (dpi) printer, that may be rounded up to 3
dots (0.25 mm); on a 600 dpi printer, it can be rounded to 5
dots.
<p>The two images below illustrate the effect of viewing distance on the
size of a pixel and the effect of a device's resolution. In the first
image, a reading distance of 71cm (28 inch) results in a px of
0.26mm, while a reading distance of 3.5m (12 feet) requires a px of
1.3mm.
<div class=figure>
<p><img alt="Showing that pixels must become larger if the viewing
distance increases" src=pixel1></p>
</div>
<p>In the second image, an area of 1px by 1px is covered by a single dot in
a low-resolution device (a computer screen), while the same area is
covered by 16 dots in a higher resolution device (such as a 400 dpi
laser printer).
<div class=figure>
<p><img alt="Showing that more device pixels (dots) are needed to cover a
1px by 1px area on a high-resolution device than on a low-res one"
src=pixel2></p>
</div>
<!--
<p>Grid units, <dfn title="grid units|gd (grid)::definition
of">‘<code class=property>gd</code>’</dfn>, are frequently used in East Asian typography,
especially for margins. The existence of a grid in an element makes it
possible to express various measurements in terms of grid units.
<div class="example">
<p style="display:none">Example(s):
<pre>
p { layout-grid: strict both 20pt 15pt; margin: 1gd 3gd 1gd 2gd }
</pre>
<p>The <span class="property">‘<code class=property>layout-grid</code>’</span> property, described
in the CSS3 Text module <a href="#CSS3TEXT" rel="biblioentry">[CSS3TEXT]<!--{{!CSS3TEXT}}-->
<p>, establishes the grid. In the example above, <code class=html>p</code>
elements acquire a 15pt top margin, a 60pt right margin, a 15pt bottom
margin and a 40pt left margin.
<p>If no grid is specified on the element, the ‘<code
class=property>gd</code>’ unit is treated as the ‘<code
class=property>em</code>’ unit. --><!--
<p>Relative values are not inherited. Instead, <em><a
href="#computed1">computed values</a></em> are passed on to child
elements.
<div class="example">
<p style="display:none">Example(s):</p>
<p>In the following rules, the computed <span
class="property">‘<code class=property>text-indent</code>’</span> value of H1 elements will be 36pt, not
45pt, if H1 is a child of the BODY element.</p>
<pre>
body {
font-size: 12pt;
text-indent: 3em; /* i.e., 36pt */
}
h1 { font-size: 15pt }
</pre>
</div>
<p>There are some exceptions to this rule. For more information on
inheritance, see the "Cascading and Inheritance" module.
-->
<h5 id=the-rem-unit><span class=secno>3.3.3.4. </span>The ‘<code
class=property>rem</code>’ unit</h5>
<p>The <dfn id=rem title="root em|em (grid)::definition of">‘<code
class=property>rem</code>’</dfn> unit (‘<code class=css><a
href="#rem">root em</a></code>’) is relative to the computed value
of the <span class=property>‘<code
class=property>font-size</code>’</span> value of the root element.
If ‘<code class=property>rem</code>’ occurs in the value of
<span class=property>‘<code
class=property>font-size</code>’</span> on the root element itself,
it is relative to the ‘<code class=property>medium</code>’
font-size.
<h5 id=the-vw-unit><span class=secno>3.3.3.5. </span>The ‘<code
class=property>vw</code>’ unit</h5>
<p>The <dfn id=vw title="vw::definition of">‘<code
class=property>vw</code>’</dfn> unit is relative to the viewport's
width. The viewport's width is equal to 100 ‘<code
class=property>vw</code>’ units.
<div class=example>
<p style=display:none>Example:
<pre>
h1 { font-size: 8vw }
</pre>
</div>
<p>If the width of the viewport is 200mm, the font size of <code
class=html>h1</code> elements will be 16mm ((8×200)/100). When the
width of the viewport is changed (for example, when the browser window is
enlarged), lengths specifed in the ‘<code
class=property>vh</code>’ unit is scaled propertionally.
<h5 id=the-vh-unit><span class=secno>3.3.3.6. </span>The ‘<code
class=property>vh</code>’ unit</h5>
<p>The <dfn id=vh title="vh::definition of">‘<code
class=property>vh</code>’</dfn> unit is relative to the viewport's
height. The viewport's width is equal to 100 ‘<code
class=property>vh</code>’ units. When the height of the viewport is
changed (for example, when the browser window is enlarged), lengths
specifed in the ‘<code class=property>vh</code>’ unit are
scaled proportionally.
<h5 id=the-vm-unit><span class=secno>3.3.3.7. </span>The ‘<code
class=property>vm</code>’ unit</h5>
<p>The <dfn id=vm title="vm::definition of">‘<code
class=property>vm</code>’</dfn> unit is relative to the viewport's
height or width, whichever of the two is smaller. The minimum of the
viewport's width/height is equivalent to 100 ‘<code
class=property>vm</code>’ units. When the height or width of the
viewport is changed, lengths specified in the ‘<code
class=property>vm</code>’ unit are scaled proportionally.
<h4 id=fractions><span class=secno>3.3.4 </span>Fractions</h4>
<p>Some properties accept a series of length values that, in sum, should
add up to a certain length. To take up any remaining space, fractions can
be used.
<h5 id=the-fr-unit><span class=secno>3.3.4.1. </span>The ‘<code
class=property>fr</code>’ unit</h5>
<p>The <dfn id=fr title="fr::definition of">‘<code
class=property>fr</code>’</dfn> unit is used to distribute any
remaining space in a series of length values. If multiple fractions are
specified, they take up space proportionally to their numeric value.
<div class=example>
<pre>
border-parts: 10px 1fr 10px;
border-parts: 10px 1fr 10px 1fr 10px;
border-parts: 10px 2fr 10px 2fr 10px;
</pre>
</div>
<p>The ‘<code class=property>fr</code>’ unit can only be used
in combination with regular length units.
<h4 id=grids><span class=secno>3.3.5 </span>Grids</h4>
<p>A grid is a set of invisible vertical and horizontal lines that can be
used to align content. In CSS3, a grid lines can be established implicitly
or explicitly <a href="#CSS3COL"
rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a> <a href="#CSS3GRID"
rel=biblioentry>[CSS3GRID]<!--{{!CSS3GRID}}--></a>. In any case, the
distance between grid lines can be referred to by the ‘<code
class=property>gr</code>’ unit.
<h5 id=the-gr-unit><span class=secno>3.3.5.1. </span>The ‘<code
class=property>gr</code>’ unit</h5>
<p>The ‘<code class=property>gr</code>’ unit is used to
position elements in relation to grid lines.
<div class=example>
<pre>
img {
float: top left multicol;
float-offset: 2gr;
width: 1gr }
}
</pre>
</div>
<p>Grid lines can be laid out in uneven patterns. Therefore, the
‘<code class=property>gr</code>’ unit is not linear.
<div class=example>
<p>For example, "2gr" is not necessarily twice as long as "1gr".
</div>
<h4 id=percentages><span class=secno>3.3.6 </span>Percentages</h4>
<p>The format of a percentage value (denoted by <dfn id=ltpercentagegt
title="<percentage>::definition of"><percentage></dfn> in this
specification) is a <a class=index href="#ltnumbergt"
id=ltnumbergt0><number></a> immediately followed by ‘<code
class=css>%</code>’.
<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 <em>containing block</em>). When
a percentage value is set for a property of the <em>root</em> element and
the percentage is defined as referring to the inherited value of some
property, the resultant value is the percentage times the <em>initial
value</em> of that property.
<div class=example>
<p style=display:none>Example(s):</p>
<p>Since child elements (generally) inherit the <em>computed values</em>
of their parent, in the following example, the children of the P element
will inherit a value of 12pt for <span class=property>‘<code
class=property>line-height</code>’</span>, not the percentage value
(120%):</p>
<pre>
p { font-size: 10pt }
p { line-height: 120% } /* 120% of 'font-size' */
</pre>
</div>
<p class=issue>Do we need a "non-negative percentage", e.g. for
"font-size"?
<h4 id=angles><span class=secno>3.3.7 </span>Angles</h4>
<p>Angle values (denoted by <dfn id=ltanglegt
title="<angle>::definition of"><angle></dfn> in the text) are
used with aural cascading style sheets.
<p>Their format is a <a class=index href="#ltnumbergt"
id=ltnumbergt1><number></a> immediately followed by an angle unit
identifier.
<p>Angle unit identifiers are:
<ul>
<li><dfn id=deg title="deg (degrees)">deg:</dfn> degrees
<li><dfn id=grad title="grad (grads)">grad:</dfn> grads
<li><dfn id=rad title="rad (radians)">rad:</dfn> radians
<li><dfn id=turn title=turn>turn:</dfn> turns
</ul>
<p>Angle values should be normalized to the range 0-360deg by the user
agent. For example, -10deg and 350deg are equivalent.
<p>For example, a right angle is ‘<code class=css>90deg</code>’
or ‘<code class=css>100grad</code>’ or ‘<code
class=css>1.570796326794897rad</code>’.
<h4 id=times><span class=secno>3.3.8 </span>Times</h4>
<p>Time values (denoted by <dfn id=lttimegt title="<time>::definition
of"><time></dfn> in the text) are used with aural cascading style
sheets.
<p>Their format is a <a class=index href="#ltnumbergt"
id=ltnumbergt2><number></a> immediately followed by a time unit
identifier.
<p>Time unit identifiers are:
<ul>
<li><dfn id=ms title="ms (milliseconds)">ms:</dfn> milliseconds
<li><dfn id=s title="s (seconds)">s:</dfn> seconds
</ul>
<p>Time values are always positive.
<h4 id=frequencies><span class=secno>3.3.9 </span>Frequencies</h4>
<p>Frequency values (denoted by <dfn id=ltfrequencygt
title="<frequency>::definition of"><frequency></dfn> in the
text) are used with aural cascading style sheets.
<p>Their format is a <a class=index href="#ltnumbergt"
id=ltnumbergt3><number></a> immediately followed by a frequency unit
identifier.
<p>Frequency unit identifiers are:
<ul>
<li><dfn id=hz title="Hz (Hertz)">Hz:</dfn> Hertz
<li><dfn id=khz title="kHz (kilohertz)">kHz:</dfn> kilohertz
</ul>
<p>For example, 200Hz (or 200hz) is a bass sound, and 6kHz (or 6khz) is a
treble sound.
<p>Frequency values are always positive.
<h3 id=strings><span class=secno>3.4 </span>Strings</h3>
<p><dfn id=strings0 title="<string>, definition of">Strings</dfn> can
either be written with double quotes or with single quotes. Double quotes
cannot occur inside double quotes, unless escaped (as ‘<code
class=css>\"</code>’ or as ‘<code
class=css>\22</code>’). Analogously for single quotes ("\'" or
"\27").</p>
<!--"-->
<div class=example>
<p style=display:none>Example(s):</p>
<p></p>
<pre>
"this is a ‘<code class=property>string</code>’"
"this is a \"string\""
‘<code class=css>this is a "string"</code>’
‘<code class=css>this is a \</code>’string\''
</pre>
</div>
<p>A string cannot directly contain a <span class=index
id=newline>newline</span>. 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). See the <span
class=property>‘<code class=property>content</code>’</span>
property for an example.
<p>It is possible to break strings over several lines, for esthetic 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>
<h3 id=functional-notation><span class=secno>3.5 </span>Functional notation</h3>
<p>The functional notation is used to denote colors, attributes and URIs in
CSS3. The syntax of the functional notation is: the name of the functions
(e.g. ‘<code class=property>rgb</code>’) followed by a left
parenthesis followed by optional whitespace followed by the argument(s) to
the functions followed by optional whitespace followed by a right
parenthesis. If a function takes more than one argument, the arguments are
separated by a comma (‘<code class=css>,</code>’) with
optional whitespace before and after the comma.
<h4 id=attribute><span class=secno>3.5.1 </span>The ‘<code
class=property>attr</code>’ function</h4>
<p>In CSS2.1 <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>,
the ‘<code class=css>attr()</code>’ expression always returns
a string. In CSS3, the ‘<code class=css>attr()</code>’
expression can return many different types. The new syntax for the attr()
expression is:
<pre>
‘<code class=css>attr(</code>’ ident [ ‘<code class=css>,</code>’ <type> [ ‘<code class=css>,</code>’ <value> ]? ]? ‘<code class=css>)</code>’
</pre>
<p>The first argument represents the attribute name. The value of the
attribute with that name on the element whose computed values are being
computed is used as the value of the expression, according to the rules
given below.
<p>The first argument accepts an optional namespace prefix to identify the
namespace of the attribute. The namespace prefix and the attribute name is
separated by ‘<code class=css>|</code>’, with no whitespace
before or after the separator <a href="#CSS3NAMESPACE"
rel=biblioentry>[CSS3NAMESPACE]<!--{{CSS3NAMESPACE}}--></a>.
<p>The second argument (which is optional but must be present if the third
argument is present) is a <type> and tells the UA how to interpret the
attribute value. It may be one of the values from the list below.
<p>The third argument (which is optional) is a CSS value which must be
valid where the attr() expression is placed. If it is not valid, then the
whole attr() expression is invalid.
<p>If the attribute named by the first argument is missing, cannot be
parsed, or is invalid for the property, then the value returned by attr()
will be the third argument, or, if the third argument is absent, will be
the value given as the default for the relevant type in the list below.
<dl>
<dt>string
<dd>The attribute value will be interpreted as the contents of a CSS
string. The default is the empty string.