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<h1>Media Queries Level 4</h1>
<pre class='metadata'>
Group: csswg
Shortname: mediaqueries
Level: 4
Status: ED
Work Status: Exploring
ED: http://dev.w3.org/csswg/mediaqueries4/
TR: http://www.w3.org/TR/mediaqueries-4/
Previous Version: http://www.w3.org/TR/2014/WD-mediaqueries-4-20140605/
Editor: Florian Rivoal, Invited Expert, florian@rivoal.net, http://florian.rivoal.net
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
Former Editor: Håkon Wium Lie, Opera, howcome@opera.com
Former Editor: Tantek Çelik, Mozilla, tantek@cs.standard.edu
Former Editor: Daniel Glazman, Disruptive Innovations, daniel@glazman.org
Former Editor: Anne van Kesteren, Mozilla, annevk@annevk.nl
Abstract: <a>Media Queries</a> allow authors to test and query values or features of the user agent or display device, independent of the document being rendered. They are used in the CSS @media rule to conditionally apply styles to a document, and in various other contexts and languages, such as HTML and Javascript.
Abstract:
Abstract: Media Queries Level 4 describes the mechanism and syntax of media queries, media types, and media features. It extends and supersedes the features defined in Media Queries Level 3.
Ignored Terms: min-resolution, max-resolution, none, view-mode, mediaText, DOMString
Link Defaults: css-break-3 (property) break-inside
</pre>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
HTML4 [[HTML401]] defined a mechanism to support media-dependent style sheets,
tailored for different <a>media types</a>.
For example, a document may use different style sheets for screen and for print.
In HTML, this can be written as:
<div class="example">
<pre>
<link rel="stylesheet" type="text/css" media="screen" href="style.css">
<link rel="stylesheet" type="text/css" media="print" href="print.css">
</pre>
</div>
CSS adapted and extended this functionality with its ''@media'' and ''@import'' rules,
adding the ability to query the value of individual features:
<div class="example">
Inside a CSS style sheet,
one can declare that sections apply to certain <a>media types</a>:
<pre>
@media screen {
* { font-family: sans-serif }
}
</pre>
Similarly, stylesheets can be conditionally imported based on media queries:
<pre>@import "print-styles.css" print;</pre>
</div>
<a>Media queries</a> can be used with HTML, XHTML, XML [[XMLSTYLE]] and the @import and @media rules of CSS.
<div class="example">
Here is the same example written in HTML, XHTML, XML, @import and @media:
<pre>
<link media="screen and (color), projection and (color)"
rel="stylesheet" href="example.css">
<link media="screen and (color), projection and (color)"
rel="stylesheet" href="example.css" />
<?xml-stylesheet media="screen and (color), projection and (color)"
rel="stylesheet" href="example.css" ?>
@import url(example.css) screen and (color), projection and (color);
@media screen and (color), projection and (color) { … }
</pre>
Note: The [[XMLSTYLE]] specification has not yet been updated to
use media queries in the <code>media</code> pseudo-attribute.
</div>
<h3 id="placement">
Module interactions</h3>
This module replaces and extends the Media Queries, Media Type and Media Features
defined in [[!CSS21]] sections 7 and in [[!MEDIAQ]].
<h3 id="values">
Values</h3>
Value types not defined in this specification, such as <<integer>>,
<<number>> or <<resolution>>, are defined in [[!CSS3VAL]]. Other CSS
modules may expand the definitions of these value types.
This specification also introduces some new value types.
The <dfn type><ratio></dfn> value type is a positive (not zero or negative)
<<integer>> followed by optional whitespace, followed by a solidus ('/'),
followed by optional whitespace, followed by a positive <<integer>>.
<<ratio>>s can be ordered or compared by transforming them into the number
obtained by dividing their first <<integer>> by their second <<integer>>.
The <dfn type><mq-boolean></dfn> value type is an <<integer>>
with the value ''0'' or ''1''.
Any other integer value is invalid.
<span class='note'>Note that ''-0'' is always equivalent to ''0'' in CSS,
and so is also accepted as a valid <<mq-boolean>> value.</span>
<h3 id="units">
Units</h3>
The units used in media queries are the same as in other parts of CSS, as
defined in [[!CSS3VAL]]. For example, the pixel unit represents CSS pixels and
not physical pixels.
Relative units in media queries are based on the initial value, which means
that units are never based on results of declarations. For example, in HTML,
the ''em'' unit is relative to the initial value of 'font-size',
defined by the user agent or the user's preferences,
not any styling on the page.
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<h2 id="media">
Media Queries</h2>
A <dfn export>media query</dfn> is a method of testing certain aspects of the user agent
or device that the document is being displayed in.
<a>Media queries</a> are (almost) always independent of the contents of the document,
its styling,
or any other internal aspect;
they're only dependent on “external” information
unless another feature explicitly specifies that it affects the resolution of Media Queries,
such as the ''@viewport'' rule.
The syntax of a <a>media query</a> consists of
an optional <a>media query modifier</a>,
an optional <a>media type</a>,
and zero or more <a>media features</a>:
<pre class='railroad'>
Or:
N: media condition
And:
Or: 1
T: only
S:
T: not
N: media type
Opt:
And:
T: and
N: media condition
</pre>
A <a>media query</a> is a logical expression that is either true or false.
A media query is true if:
* the <a>media type</a>,
if specified,
matches the media type of the device where the user agent is running, and
* the <a>media condition</a> is true.
Statements regarding media queries in this section assume the <a href="#mq-syntax">syntax section</a> is followed.
Media queries that do not conform to the syntax are discussed in [[#error-handling]].
I.e. the syntax takes precedence over requirements in this section.
<div class="example">
Here is a simple example written in HTML:
<pre><link rel="stylesheet" media="screen and (color)" href="example.css" /></pre>
This example expresses that a certain style sheet
(<code>example.css</code>) applies to devices of a certain media type
(''screen'') with certain feature (it must be a color screen).
93D4
Here is the same media query written in an @import-rule in CSS:
<pre>@import url(example.css) screen and (color);</pre>
</div>
User agents must re-evaluate <a>media queries</a> in response to changes in the user environment that they're aware of,
for example if the device is tiled from landscape to portrait orientation,
and change the behavior of any constructs dependent on those <a>media queries</a> accordingly.
Unless another feature explicitly specifies that it affects the resolution of Media Queries, it is never necessary to apply a style sheet in order to evaluate expressions.
Note: CSS Device Adaptation [[CSS-DEVICE-ADAPT]]]
defines how ''@viewport'' rules interact with Media Queries.
<h3 id='mq-list'>
Combining Media Queries</h3>
Several <a>media queries</a> can be combined into a comma-separated <dfn export>media query list</dfn>.
<pre class='railroad'>
Star:
N: media query
T: ,
</pre>
A <a>media query list</a> is true if <em>any</em> of its component <a>media queries</a> are true,
and false only if <em>all</em> of its component <a>media queries</a> are false.
<div class="example">
For example, the following <a>media query list</a> is true if either
the <a>media type</a> is ''screen'' and it's a color device,
<strong>or</strong> the <a>media type</a> is ''projection'' and it's a color device:
<pre>
@media screen and (color), projection and (color) { … }
</pre>
</div>
An empty <a>media query list</a> evaluates to true.
<div class="example">
For example, these are equivalent:
<pre>
@media all { … }
@media { … }
</pre>
</div>
<h3 id='mq-prefix'>
Media Query Modifiers</h3>
A <a>media query</a> may optionally be prefixed by a single <dfn export>media query modifier</dfn>,
which is a single keyword which alters the meaning of the following <a>media query</a>.
<h4 id='mq-not'>
Negating a Media Query: the ''not'' keyword</h4>
An individual <a>media query</a> can have its result negated
by prefixing it with the keyword <dfn value for="@media">not</dfn>.
If the <a>media query</a> would normally evaluate to true,
prefixing it with ''not'' makes it evaluate to false,
and vice versa.
<div class="example">
For example, the following will apply to everything except color-capable screens.
Note that the entire media query is negated,
not just the <a>media type</a>.
<pre><link rel="stylesheet" media="not screen and (color)" href="example.css" /></pre>
</div>
<h4 id='mq-only'>
Hiding a Media Query From Legacy User Agents: the ''only'' keyword</h4>
The concept of <a>media queries</a> originates from HTML4 [[HTML401]].
That specification only defined <a>media types</a>,
but had a forward-compatible syntax that accommodated the addition of future concepts like <a>media features</a>:
it would consume the characters of a <a>media query</a> up to the first non-alphanumeric character,
and interpret that as a <a>media type</a>,
ignoring the rest.
For example, the <a>media query</a> ''screen and (color)''
would be truncated to just ''screen''.
Unfortunately, this means that legacy user agents using this error-handling behavior
will ignore any <a>media features</a> in a <a>media query</a>,
even if they're far more important than the <a>media type</a> in the query.
This can result in styles accidentally being applied in inappropriate situations.
To hide these <a>media queries</a> from legacy user agents,
the <a>media query</a> can be prefixed with the keyword <dfn value for="@media">only</dfn>.
The ''only'' keyword <strong>has no effect</strong> on the <a>media query</a>’s result,
but will cause the <a>media query</a> to be parsed by legacy user agents
as specifying the unknown <a>media type</a> “only”,
and thus be ignored.
<div class="example">
In this example, the stylesheet specified by the <code><link></code> element
will not be used by legacy user agents,
even if they would normally match the ''screen'' <a>media type</a>.
<pre><link rel="stylesheet" media="only screen and (color)" href="example.css" /></pre>
</div>
Note: Note that the ''only'' keyword can only be used before a <a>media type</a>.
A <a>media query</a> consisting only of <a>media features</a>,
or one with another <a>media query modifier</a> like ''not'',
will be treated as false by legacy user agents automatically.
Note: At the time of publishing this specification,
such legacy user agents are extremely rare,
and so using the ''only'' modifier is rarely, if ever, necessary.
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<h3 id='media-types'>
Media Types</h3>
A <dfn export>media type</dfn> is a broad category of user-agent devices
on which a document may be displayed.
The original set of <a>media types</a> were defined in HTML4,
for the <code>media</code> attribute on <code><link></code> elements.
Unfortunately, <a>media types</a> have proven insufficient as a way of discriminating between devices with different styling needs.
Some categories which were originally quite distinct,
such as ''screen'' and ''handheld'',
have blended significantly in the years since their invention.
Others, such as ''tty'' or ''tv'',
expose useful differences from the norm of a full-featured computer monitor,
and so are potentially useful to target with different styling,
but the definition of <a>media types</a> as mutually exclusive
makes it difficult to use them in a reasonable manner;
instead, their exclusive aspects are better expressed as <a>media features</a>
such as 'grid' or 'scan'.
As such, the following <a>media types</a> are defined for use in <a>media queries</a>:
<dl dfn-type=value dfn-for="@media">
<dt><dfn>all</dfn>
<dd>Matches all devices.
<dt><dfn>print</dfn>
<dd>Matches printers, and devices intended to reproduce a printed display,
such as a web browser showing a document in “Print Preview”.
<dt><dfn>screen</dfn>
<dd>Matches all devices that aren't matched by ''print'' or ''speech''.
<dt><dfn>speech</dfn>
<dd>Matches screenreaders and similar devices that “read out” a page.
</dl>
In addition, the following <strong>deprecated</strong> <a>media types</a> are defined.
Authors must not use these <a>media types</a>;
instead, it is recommended that they select appropriate <a>media features</a>
that better represent the aspect of the device that they are attempting to style against.
User agents must recognize the following <a>media types</a> as valid,
but must make them match nothing.
<ul dfn-type=value dfn-for="@media">
<li><dfn>tty</dfn>
<li><dfn>tv</dfn>
<li><dfn>projection</dfn>
<li><dfn>handheld</dfn>
<li><dfn>braille</dfn>
<li><dfn>embossed</dfn>
<li><dfn>aural</dfn>
</ul>
Note: It is expected that all of the media types will also be deprecated in time,
as appropriate <a>media features</a> are defined which capture their important differences.
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<h3 id='mq-features'>
Media Features</h3>
A <dfn export>media feature</dfn> is a more fine-grained test than <a>media types</a>,
testing a single, specific feature of the user agent or display device.
Syntactically, <a>media features</a> resemble CSS properties:
they consist of a feature name, a colon, and a value to test for.
They may also be written in boolean form as just a feature name,
or in range form with a comparison operator.
<pre class='railroad'>
T: (
Choice:
And:
N: feature name
T: :
N: feature value
N: feature name
And:
N: range form
C: (see below)
T: )
</pre>
There are, however, several important differences between properties and media features:
<ul>
<li>
Properties are used to give information about how to present a document.
Media features are used to describe requirements of the output device.
<li>
Media features are always wrapped in parentheses
and combined with the ''and'' keyword,
like ''(color) and (min-width: 600px)'',
rather than being separated with semicolons.
<li>
A media feature may be given with <em>only</em> its name
(omitting the colon and value)
to evaluate the feature in a <a>boolean context</a>.
This is a convenient shorthand for features that have a reasonable value representing 0 or “none”.
For example, ''(color)'' is true is the 'color' <a>media feature</a> is non-zero.
<li>
<a>Media features</a> with “range” type can be written in a <a>range context</a>,
which uses standard mathematical comparison operators rather than a colon,
or have their feature names <a href=#mq-min-max>prefixed with “min-” or “max-”</a>.
<li>
Properties sometimes accept complex values,
e.g., calculations that involve several other values.
<a>Media features</a> only accept single values: one keyword, one number, etc.
</ul>
If a <a>media feature</a> references a concept which does not exist on the device where the UA is running
(for example, speech UAs do not have a concept of "width"),
the <a>media feature</a> must always evaluate to false.
<div class="example">
The media feature ''device-aspect-ratio'' only applies to
visual devices. On an ''speech'' device, expressions involving
''device-aspect-ratio'' will therefore always be false:
<pre>
<link media="speech and (device-aspect-ratio: 16/9)"
rel="stylesheet" href="example.css">
</pre>
</div>
<h4 id='mq-ranges'>
Media Feature Types: “range” and “discrete”</h4>
Every media feature defines its “type” as either “range” or “discrete” in its definition table.
“Discrete” media features,
like 'light-level' or 'scripting',
take their values from a set.
The values may be keywords
or boolean numbers (0 and 1),
but the common factor is that there's no intrinsic “order” to them--
none of the values are “less than” or “greater than” each other.
“Range” media features like 'width', on the other hand,
take their values from a range.
Any two values can be compared to see which is lesser and which is greater.
The only significant difference between the two types is that “range” <a>media features</a>
can be evaluated in a <a>range context</a>
and accept “min-” and “max-” prefixes on their name.
Doing either of these changes the meaning of the feature--
rather than the <a>media feature</a> being true when the feature exactly matches the given value,
it matches when the feature is greater than/less than/equal to the given value.
<div class='example'>
A <span class=css data-link-type=maybe>(width >= 600px)</span> <a>media feature</a> is true
when the viewport's width is ''600px'' <em>or more</em>.
On the other hand, ''(width: 600px)'' by itself is only true
when the viewport's width is <em>exactly</em> ''600px''.
If it's less or greater than ''600px'', it'll be false.
</div>
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<h4 id='mq-boolean-context'>
Evaluating Media Features in a Boolean Context</h4>
While <a>media features</a> normally have a syntax similar to CSS properties,
they can also be written more simply as just the feature name,
like ''(color)''.
When written like this, the <a>media feature</a> is evaluated in a <dfn export>boolean context</dfn>.
If the feature would be true for any value
<em>other than</em> the number ''0'',
a <a spec=css-values>dimension</a> with the value ''0'',
or the keyword ''none'',
the <a>media feature</a> evaluates to true.
Otherwise, it evaluates to false.
<div class='example'>
Some <a>media features</a> are designed to be written like this.
For example, 'scripting' is typically written as ''(scripting)'' to test if scripting is enabled,
or ''not (scripting)'' to see if it's disabled.
It can still be given an explicit value as well,
with ''(scripting: enabled)'' equal to ''(scripting)'',
and ''(scripting: none)'' equal to ''not (scripting)''.
</div>
<div class='example'>
Some numeric <a>media features</a>, like 'width',
are rarely if ever useful to evaluate in a <a>boolean context</a>,
as their values are almost always greater than zero.
Others, like 'color', have meaningful zero values:
''(color)'' is identical to ''(color > 0)'',
indicating that the device is capable of displaying color at all.
</div>
<div class='example'>
Only some of the <a>media features</a> that accept keywords are meaningful in a <a>boolean context</a>.
For example, ''(pointer)'' is useful,
as 'pointer' has a ''pointer/none'' value to indicate there's no pointing device at all on the device.
On the other hand, ''(scan)'' is just always true or always false
(depending on whether it applies at all to the device),
as there's no value that means “false”.
</div>
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<h4 id="mq-range-context">
Evaluating Media Features in a Range Context</h4>
<a>Media features</a> with a “range” type can be alternately written in a <dfn export>range context</dfn>
that takes advantage of the fact that their values are ordered,
using ordinary mathematical comparison operators:
<pre class='railroad'>
T: (
Choice:
Seq:
N: feature name/value
Choice: 4
T: =
T: <
T: <=
T: >
T: >=
N: feature value/name
Seq:
N: value
Choice:
T: <
T: <=
N: feature name
Choice:
T: <
T: <=
N: value
Seq:
N: value
Choice:
T: >
T: >=
N: feature name
Choice:
T: >
T: >=
N: value
T: )
</pre>
The basic form,
consisting of a feature name,
a comparison operator,
and a value,
returns true if the relationship is true.
<div class='example'>
For example, ''(height > 600px)''
(or ''(600px < height)'')
returns true if the viewport height is greater than ''600px''.
</div>
The remaining forms,
with the feature name nested between two value comparisons,
returns true if both comparisons are true.
<div class='example'>
For example, ''(400px < width < 1000px)'' returns true if the viewport width is between ''400px'' and ''1000px''
(but not equal to either).
</div>
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<h4 id='mq-min-max'>
Using “min-” and “max-” Prefixes On Range Features</h4>
Rather than evaluating a “range” type <a>media feature</a> in a range context,
as described above,
the feature may be written as a normal <a>media feature</a>,
but with a “min-” or “max-” prefix on the feature name.
This is equivalent to evaluating the feature in a <a>range context</a>,
as follows:
<ul>
<li>
Using a “min-” prefix on a feature name is equivalent to using the “>=” operator.
For example, ''(min-height: 600px)'' is equivalent to <span class=css data-link-type=maybe>(height >= 600px)</span>.
<li>
Using a “max-” prefix on a feature name is equivalent to using the “<=” operator.
For example, ''(max-width: 40em)'' is equivalent to <span class=css data-link-type=maybe>(width <= 40em)</span>.
</ul>
“Discrete” type properties do not accept “min-” or “max-” prefixes.
Adding such a prefix to a “discrete” type <a>media feature</a> simply results in an unknown feature name.
<div class='example'>
For example,
''(min-grid: 1)'' is invalid,
because 'grid' is a “discrete” <a>media feature</a>,
and so doesn't accept the prefixes.
(Even though the 'grid' <a>media feature</a> appears to be numeric,
as it accepts the values ''0'' and ''1''.)
</div>
Attempting to evaluate a min/max prefixed <a>media feature</a> in a <a>boolean context</a>
is invalid and a syntax error.
Combining Media Features {#media-conditions}
--------------------------------------------
Multiple <a>media features</a> can be combined together into a <dfn export>media condition</dfn>
using full boolean algebra
(not, and, or).
Issue: TODO: Fill this in.
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<h2 id='mq-syntax'>
Syntax</h2>
Informal descriptions of the media query syntax appear in the prose and railroad diagrams in previous sections.
The formal media query syntax is described in this section,
with the rule/property grammar syntax defined in [[!CSS3SYN]] and [[!CSS3VAL]].
To parse a <dfn><media-query-list></dfn> production,
<a>parse a comma-separated list of component values</a>,
then parse each entry in the returned list as a <<media-query>>.
Its value is the list of <<media-query>>s so produced.
Note: This explicit definition of <<media-query-list>> parsing
is necessary to make the error-recovery behavior of <a>media query lists</a> well-defined.
Note: This definition of <<media-query-list>> parsing intentionally accepts an empty list.
<pre>
<dfn><media-query></dfn> = <<media-condition>>
| [ not | only ]? <<media-type>> [ and <<media-condition-without-or>> ]?
<dfn><media-type></dfn> = <<ident>>
<dfn><media-condition></dfn> = <<media-not>> | <<media-and>> | <<media-or>> | <<media-in-parens>>
<dfn><media-condition-without-or></dfn> = <<media-not>> | <<media-and>> | <<media-in-parens>>
<dfn><media-not></dfn> = not <<media-in-parens>>
<dfn><media-and></dfn> = <<media-in-parens>> [ and <<media-in-parens>> ]+
<dfn><media-or></dfn> = <<media-in-parens>> [ or <<media-in-parens>> ]+
<dfn><media-in-parens></dfn> = ( <<media-condition>> ) | <<media-feature>> | <<general-enclosed>>
<dfn><media-feature></dfn> = ( [ <<mf-plain>> | <<mf-boolean>> | <<mf-range>> ] )
<dfn><mf-plain></dfn> = <<mf-name>> : <<mf-value>>
<dfn><mf-boolean></dfn> = <<mf-name>>
<dfn><mf-range></dfn> = <<mf-name>> [ '<' | '>' ]? '='? <<mf-value>>
| <<mf-value>> [ '<' | '>' ]? '='? <<mf-name>>
| <<mf-value>> '<' '='? <<mf-name>> '<' '='? <<mf-value>>
| <<mf-value>> '>' '='? <<mf-name>> '>' '='? <<mf-value>>
<dfn><mf-name></dfn> = <<ident>>
<dfn><mf-value></dfn> = <<number>> | <<dimension>> | <<ident>> | <<ratio>>
<dfn><general-enclosed></dfn> = [ <<function-token>> <<any-value>> ) ] | ( <<ident>> <<any-value>> )
</pre>
The <<media-type>> production does not include the keywords ''only'', ''not'', ''and'', and ''or''.
A <dfn><dimension></dfn> is a <a spec=css-values>dimension</a>.
An <dfn><ident></dfn> is an <a>identifier</a>.
No whitespace is allowed between the "<" or ">" <<delim-token>>s and the following "=" <<delim-token>>,
if it's present.
Whitespace <em>must</em> be present between a ')' character and a ''not'', ''and'', or ''or'' keyword,
and between a ''not'', ''and'', or ''or'' keyword and a '(' character.
When parsing the <<media-in-parens>> production,
the <<general-enclosed>> branch must only be chosen if the input does not match either of the preceding branches.
<span class='note'><<general-enclosed>> exists to allow for future expansion of the grammar in a reasonably compatible way.</span>
In addition to conforming to the syntax, each media query needs to use
media types and media features according to their respective specification
in order to be considered conforming.
<div class="example">
Only the first media query is conforming in the example below because the
"example" media type does not exist.
<pre>
@media all { body { background:lime } }
@media example { body { background:red } }
</pre>
</div>
Each of the major terms of <<media-condition>> or <<media-condition-without-or>> is associated with a boolean result, as follows:
<dl>
<dt><<media-condition>>
<dt><<media-condition-without-or>>
<dt><<media-in-parens>>
<dd>
The result is the result of the child term.
<dt><<media-not>>
<dd>
The result is the negation of the <<media-in-parens>> term.
The negation of unknown is unknown.
<dt><<media-and>>
<dd>
The result is true if all of the <<media-in-parens>> child terms are true,
false if at least one of the <<media-in-parens>> child terms are false,
and unknown otherwise.
<dt><<media-or>>
<dd>
The result is false if all of the <<media-in-parens>> child terms are false,
true if at least one of the <<media-in-parens>> child terms are true,
and unknown otherwise.
<dt><<general-enclosed>>
<dd>
The result is unknown.
Authors must not use <<general-enclosed>> in their stylesheets.
<span class='note'>It exists only for future-compatibility,
so that new syntax additions do not invalidate too much of a <<media-condition>> in older user agents.</span>
<dt><<media-feature>>
<dd>
The result is the result of evaluating the specified media feature.
</dl>
If the result of any of the above productions
is used in any context that expects a two-valued boolean,
"unknown" must be converted to "false".
Note: This means that,
for example,
when a <a>media query</a> is used in a ''@media'' rule,
if it resolves to "unknown" it's treated as "false"
and fails to match.
<div class="note">
Media Queries use a three-value logic where terms can be "true", "false", or "unknown".
Specifically, it uses the <a href="http://en.wikipedia.org/wiki/Three-valued_logic#Kleene_and_Priest_logics">Kleene 3-valued logic</a>.
In this logic, "unknown" means "either true or false, but we're not sure which yet".
In general, an unknown value showing up in a formula will cause the formula to be unknown as well,
as substituting "true" for the unknown will give the formula a different result than substituting "false".
The only way to eliminate an unknown value is to use it in a formula that will give the same result
whether the unknown is replaced with a true or false value.
This occurs when you have "false AND unknown" (evaluates to false regardless)
and "true OR unknown" (evaluates to true regardless).
This logic was adopted because <<general-enclosed>> needs to be assigned a truth value.
In standard boolean logic, the only reasonable value is "false",
but this means that ''not unknown(function)'' is true,
which can be confusing and unwanted.
Kleen's 3-valued logic ensures that unknown things will prevent a <a>media query</a> from matching,
unless their value is irrelevant to the final result.
</div>
<h3 id="error-handling">
Error Handling</h3>
A media query that does not match the grammar in the previous section must be replaced by ''not all'' during parsing.
Note: Note that a grammar mismatch does <strong>not</strong> wipe out an entire <a>media query list</a>,
just the problematic <a>media query</a>.
The parsing behavior defined above automatically recovers at the next top-level comma.
<div class='example'>
<pre>
@media (example, all,), speech { /* only applicable to speech devices */ }
@media &test, speech { /* only applicable to speech devices */ }
</pre>
Both of the above <a>media query lists</a> are turned into ''not all, speech'' during parsing,
which has the same truth value as just ''speech''.
Note that error-recovery only happens at the top-level of a <a>media query</a>;
anything inside of an invalid parenthesized block
will just get turned into ''not all'' as a group.
For example:
<pre>
@media (example, speech { /* rules for speech devices */ }
</pre>
Because the parenthesized block is unclosed,
it will contain the entire rest of the stylesheet from that point
(unless it happens to encounter an unmatched ")" character somewhere in the stylesheet),
and turn the entire thing into a ''not all'' <a>media query</a>.
</div>
An unknown <<media-type>> must be treated as not matching.
<div class='example'>
For example, the media query ''unknown'' is false,
as ''unknown'' is an unknown <a>media type</a>.
But ''not unknown'' is true, as the ''not'' negates the false media type.
</div>
<div class='example'>
Remember that some keywords aren't allowed as <<media-type>>s
and cause parsing to fail entirely:
the media query ''or and (color)'' is turned into ''not all'' during parsing,
rather than just treating the ''or'' as an unknown <a>media type</a>.
</div>
An unknown <<mf-name>> or <<mf-value>>, or disallowed <<mf-value>>,
results in the value "unknown".
A <<media-query>> whose value is "unknown" must be replaced with ''not all''.
<div class="example">
<pre><link media="screen and (max-weight: 3kg) and (color), (color)"
rel="stylesheet" href="example.css" /></pre>
As ''max-weight'' is an unknown <a>media feature</a>,
this <a>media query list</a> is turned into ''not all, (color)'',
which is equivalent to just ''(color)''.
</div>
<div class="example">
<pre>@media (min-orientation:portrait) { … }</pre>
The 'orientation' feature does not accept prefixes,
so this is considered an unknown <a>media feature</a>,
and turned into ''not all''.
</div>
<div class="example">
The media query ''(color:20example)'' specifies an unknown value for the 'color' media feature
and is therefore turned into ''not all''.
</div>
<div class="example">
This media query is turned into ''not all'' because negative lengths are not allowed for the 'width' media feature:
<pre>@media (min-width: -100px) { … }</pre>
</div>
<div class='note'>
Note that <a>media queries</a> are also subject to the parsing rules of the host language.
For example, take the following CSS snippet:
<pre> @media test;,all { body { background:lime } }</pre>
The media query ''test;,all'' is, parsed by itself,
equivalent to ''not all, all'', which is always true.
However, CSS's parsing rules cause the ''@media'' rule,
and thus the <a>media query</a>,
to end at the semicolon.
The remainder of the text is treated as a style rule
with an invalid selector and contents.
</div>
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<h2 id="mf-dimensions">
Screen/Device Dimensions Media Features</h2>
<h3 id="width">
width</h3>
<pre class='descdef mq'>
Name: width
Value: <<length>>
For: @media
Type: range
</pre>
The '@media/width' media feature describes the width of the targeted display area of the output device.
For continuous media, this is the width of the viewport
(as described by CSS2, section 9.1.1 [[!CSS21]])
including the size of a rendered scroll bar (if any).
For paged media, this is the width of the page box
(as described by CSS2, section 13.2 [[!CSS21]]).
A specified <<length>> cannot be negative.
<div class="example">
For example, this media query expresses that the style sheet is usable on printed output wider than 25cm:
<pre><link rel="stylesheet" media="print and (min-width: 25cm)" href="http://…" /></pre>
</div>
<div class="example">
This media query expresses that the style sheet is usable on devices with viewport
(the part of the screen/paper where the document is rendered)
widths between 400 and 700 pixels:
<pre>@media (400px <= min-width <= 700px) { … }</pre>
</div>
<div class="example">
This media query expresses that style sheet is usable if the width of the viewport is greater than 20em.
<pre>@media (min-width: 20em) { … }</pre>
The ''em'' value is relative to the initial value of 'font-size'.
</div>
<h3 id="height">
height</h3>
<pre class='descdef mq'>
Name: height
Value: <<length>>
For: @media
Type: range
</pre>
The 'height' media feature describes the height of the targeted display area of the output device.
For continuous media, this is the height of the viewport including the size of a rendered scroll bar (if any).
For paged media, this is the height of the page box.
A specified <<length>> cannot be negative.
<h3 id='aspect-ratio'>
aspect-ratio</h3>
<pre class='descdef mq'>
Name: aspect-ratio
Value: <<ratio>>
For: @media
Type: range
</pre>
The 'aspect-ratio' media feature is defined as the ratio of the value of the 'width' media feature
to the value of the 'height' media feature.
<h3 id='orientation'>
orientation</h3>