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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>
<title>CSS Device Adaptation</title>
<link href="http://dev.w3.org/csswg/default.css" rel="stylesheet" type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel="stylesheet" type="text/css">
<link href="default.css" rel="stylesheet" type="text/css">
</head>
<body class="draft">
<div class="head">
<!--logo-->
<h1>CSS Device Adaptation</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
<dt>This version:</dt>
<dd><a href="http://dev.w3.org/csswg/css-device-adapt/">http://dev.w3.org/csswg/css-device-adapt/</a></dd>
<dt>Latest version:</dt>
<dd><a href="http://dev.w3.org/csswg/css-device-adapt/">http://dev.w3.org/csswg/css-device-adapt/</a></dd>
<dt>Previous version:</dt>
<dd>-</dd>
<dt>Editor:</dt>
<dd><a href="mailto:rune@opera.com">Rune Lillesveen</a> (<a href="http://www.opera.com/">Opera Software</a>)</dd>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc">Abstract</h2>
<p>This specification provides a way for an author to
specify, in CSS, the size, zoom factor, and orientation of the viewport
that is used as the base for the initial containing block.</p>
<h2 class="no-num no-toc">Status of this document</h2>
<!--status-->
<h2 class="no-num no-toc">Table of contents</h2>
<!--toc-->
<h2>Introduction</h2>
<p><em>This section is not normative.</em></p>
<p>CSS 2.1 [[!CSS21]]</a> specifies an
<a href="http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">
initial containing block</a> for continuous media that has the dimensions
of the <a href="http://www.w3.org/TR/CSS21/visuren.html#viewport">
viewport</a>. Mobile/handheld device browsers have a viewport that is
generally a lot narrower than a desktop browser window at a zoom level
that gives a CSS pixel size
<a href="http://www.w3.org/TR/CSS21/syndata.html#pixel-units">recommended
</a> by CSS 2.1.</p>
<p>The narrow viewport is a problem for documents designed to look good in
desktop browsers. The result is that mobile browser vendors use a fixed
initial containing block size that is different from the viewport size,
and close to that of a typical desktop browser window. In addition to
scrolling or panning, zooming is often used to change between an overview
of the document and zoom in on particular areas of the document to read
and interact with.</p>
<p>Certain DOCTYPEs (for instance XHTML Mobile Profile) are used to recognize
mobile documents which are assumed to be designed for handheld devices,
hence using the viewport size as the initial containing block size.</p>
<p>Additionally, an HTML <code class="html">META</code> tag has been introduced for allowing an
author to specify the size of the initial containing block, and the initial
zoom factor directly. It was first implemented by Apple for the Safari/iPhone
browser, but has since been implemented for the Opera, Android, and Fennec
browsers. These implementations are not fully interoperable and this
specification is an attempt at standardizing the functionality provided by
the viewport <code class="html">META</code> tag in CSS.</p>
<h2>Values</h2>
<p>This specification follows the <a
href="http://www.w3.org/TR/css3-syntax/#property-defs">CSS property
definition conventions</a> from [[!CSS3SYN]].</p>
<p>Value types are defined in [[!CSS3VAL]].</p>
<h2>The viewport</h2>
<p>This specification introduces a way of overriding the size of the viewport
provided by the user agent (UA). Because of this, we need to introduce the
difference between the <span>initial viewport</span> and the
<span>actual viewport</span>.</p>
<dl>
<dt><dfn>initial viewport</dfn></dt>
<dd>This refers to the viewport before any UA or author styles have
overridden the viewport given by the window or viewing area of the UA.
Note that the initial viewport size will change with the size of the
window or viewing area.</dd>
<dt><dfn>actual viewport</dfn></dt>
<dd>This is the viewport you get after the cascaded viewport properties, and the
following <a href="#constraining-procedure">constraining procedure</a>
have been applied.</dd>
</dl>
<p>When the <span>actual viewport</span> cannot fit inside the window or
viewing area, either because the <span>actual viewport</span> is larger
than the <span>initial viewport</span> or the zoom factor causes only parts
of the <span>actual viewport</span> to be visible, the UA should offer a
scrolling or panning mechanism.</p>
<p>It is recommended that initially the upper-left corners of the <span>
actual viewport<span> and the window or viewing area are aligned if the
base direction of the document is ltr. Similarly, that the upper-right
corners are aligned when the base direction is rtl. The base direction for
a document is defined as the computed value of the
<code class="property">direction</code> property for the first
<code class="html">BODY</code> element of an HTML or XHTML document. For
other document types, it is the computed
<code class="property">direction</code> for the root element.</p>
<h2>The <code class="css">@viewport</code> rule</h2>
<p>The <code class="css">@viewport</code> <a
href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">at-rule</a>
consists of the @-keyword followed by a block of property
declarations describing the viewport.</p>
<p>The property declarations inside an <code class="css">@viewport</code>
rule are per document properties and there is no inheritance involved. Hence
declarations using the ‘<code class="css">inherit</code>’
keyword will be dropped. They work similar to <code class="css">@page</code>
properties and follow the cascading order of CSS. Hence, properties in
<code class="css">@viewport</code> rules will override properties from
preceding rules. The declarations allow !important which will affect
cascading of properties accordingly.
</p>
<div class="example">
<p>This example sets the viewport to fit the width of the device.
Note that it is enough to set the width as the zoom and height
will be resolved from the width.</p>
<pre><!--
-->@viewport {
<!-- --> width: device-width;
<!-- -->}<!--
--></pre>
</div>
<h3>Syntax</h3>
<p>The syntax for the <code class="css">@viewport</code> rule is as follows (using
the notation from the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar appendix</a> of CSS
2.1 [[!CSS21]]):</p>
<pre><!--
-->viewport
<!----> : VIEWPORT_SYM S*
<!----> '{' S* declaration? [ ';' S* declaration? ]* '}' S*<!--
-->;</pre>
<p>with the new token:</p>
<pre>@{V}{I}{E}{W}{P}{O}{R}{T} {return VIEWPORT_SYM;}</pre>
<p>
where:
</p>
<pre>V v|\\0{0,4}(56|76)(\r\n|[ \t\r\n\f])?|\\v
<!------>W w|\\0{0,4}(57|77)(\r\n|[ \t\r\n\f])?|\\w</pre>
<p>The <code>viewport</code> non-terminal is added to the <code>stylesheet</code>
production along with the <code>ruleset</code>, <code>media</code>, and
<code>page</code> non-terminals:</p>
<pre><!--
-->stylesheet
<!-- --> : [ CHARSET_SYM STRING ';' ]?
<!-- --> [S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
<!-- --> [ [ ruleset | media | page | viewport ] [ CDO S* | CDC S* ]* ]*
<!-- --> ;<!--
--></pre>
<p>It is also added to media production to allow
<code class="css">@viewport</code> rules nested inside
<code class="css">@media</code> rules <span class="note">This is extending
the CSS 2.1 syntax. A draft of CSS3 Paged Media also allows page inside
@media.</span>:</p>
<pre><!--
-->media
<!-- --> : MEDIA_SYM S* media_list LBRACE S* [ ruleset | viewport ]* '}' S*
<!-- --> ;<!--
--></pre>
<h2>Viewport properties</h2>
<p>This section presents the properties that are allowed inside an
<code class="css">@viewport</code> rule. Other properties than those
listed here will be dropped.</p>
<p>Relative length values are resolved against initial values. For instance
'em's are resolved against the initial value of the font-size property.</p>
<h3>The
‘<code class="property">min-width</code>’ and
‘<code class="property">max-width</code>’ properties</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>min-width</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td><span><viewport-length></span></td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto!!length">auto<span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>Refer to the width of the <span>initial viewport</span></td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto!!length">auto</code>’,
‘<code class="css">device-width</code>’,
‘<code class="css">device-height</code>’, an
absolute length, or a percentage as specified</td>
</tr>
</tbody>
</table>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>max-width</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td><span><viewport-length></span></td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto!!length">auto</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>Refer to the width of the <span>initial viewport</a></td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto!!length">auto</code>’,
‘<code class="css">device-width</code>’,
‘<code class="css">device-height</code>’, an
absolute length, or a percentage as specified</td>
</tr>
</tbody>
</table>
<p>Specifies the minimum and maximum width of the
<a href="http://www.w3.org/TR/CSS21/visuren.html#viewport">viewport</a>
that is used to set the size of the
<a href="http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">
initial containing block</a> where</p>
<p style="margin: 1em"><var><dfn><viewport-length></dfn></var> = auto | device-width | device-height | <var><length></var> | <var><percentage></var></p>
<p>and the values have the following meanings:</p>
<dl>
<dt>‘<code class="css"><dfn title="auto!!length">auto</dfn></code>’</dt>
<dd>The used value is calculated from the other property values
according to the
<a href="#constraining-procedure">constraining procedure</a>.</dd>
<dt>‘<code class="css"><dfn>device-width</dfn></code>’</dt>
<dd>The width of the screen in CSS pixels at zoom factor 1.0.</dd>
<dt>‘<code class="css"><dfn>device-height</dfn></code>’</dt>
<dd>The height of the screen in CSS pixels at zoom factor 1.0.</dd>
<dt><var><length></var></dt>
<dd>
<p>A positive absolute or relative length.</p>
</dd>
<dt><var><percentage></var></dt>
<dd>
<p>A percentage value relative to the width or height of the
<span>initial viewport</span> at zoom factor 1.0,
for horizontal and vertical lengths respectively. Must be positive.
</p>
</dd>
</dl>
<p>The min-width and max-width properties are inputs to the
<a href="#constraining-procedure">constraining procedure</a>.
The width will initially be set as close as possible to the
<span>initial viewport</span> width within the min/max
constraints.</p>
<h3>The
‘<code class="property">width</code>’
shorthand property</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>width</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td><span><viewport-length></span>{1,2}</td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td>See individual properties</td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>See individual properties</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>See individual properties</td>
</tr>
</tbody>
</table>
<p>This is a shorthand property for setting both min-width and max-width.
One <span><viewport-length></span> value will set both min-width and max-width
to that value. Two <span><viewport-length></span> values will set min-width to
the first and max-width to the second.</p>
<h3>The ‘<code class="property">min-height</code>’ and
‘<code class="property">max-height</code>’ properties</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>min-height</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td><span><viewport-length></span></td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto!!length">auto</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>Refer to the height of the <span>initial viewport</span></td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto!!length">auto</code>’,
‘<code class="css">device-width</code>’,
‘<code class="css">device-height</code>’, an
absolute length, or a percentage as specified</td>
</tr>
</tbody>
</table>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>max-height</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td><span><viewport-length></span></td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto!!length">auto</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>Refer to the height of the <span>initial viewport</span></td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto!!length">auto</code>’,
‘<code class="css">device-width</code>’,
‘<code class="css">device-height</code>’, an
absolute length, or a percentage as specified</td>
</tr>
</tbody>
</table>
<p>Specifies the minimum and maximum height of the
<a href="http://www.w3.org/TR/CSS21/visuren.html#viewport">viewport</a>
that is used to set the size of the
<a href="http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">
initial containing block</a>.</p>
<p>The min-height and max-height properties are inputs to the
<a href="#constraining-procedure">constraining procedure</a>.
The height will initially be set as close as possible to the
<span>initial viewport</span> height within the min/max
constraints.</p>
<h3>The ‘<code class="property">height</code>’ shorthand property</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>height</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td><span><viewport-length></span>{1,2}</td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td>See individual properties</td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>See individual properties</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>See individual properties</td>
</tr>
</tbody>
</table>
<p>This is a shorthand property for setting both min-height and max-height.
One <span><viewport-length></span> value will set both min-height and max-height
to that value. Two <span><viewport-length></span> values will set min-height to
the first and max-height to the second.</p>
<h3>The ‘<code class="property" title="zoom!!property">zoom</code>’ property</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn title="zoom!!property">zoom</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td>auto | <number> | <percentage></td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto!!zoom">auto</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>The zoom factor itself</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto!!zoom">auto</code>’, or a positive
number or percentage as specified.</td>
</tr>
</tbody>
</table>
<p>Specifies the initial zoom factor for the window or viewing area.</p>
<p>Values have the following meanings:</p>
<dl>
<dt>‘<code class="css"><dfn title="auto!!zoom">auto</dfn></code>’</dt>
<dd>The used value is calculated from the other property values according
to the <a href="#constraining-procedure">constraining procedure</a>.</dd>
<dt><var><number></var></dt>
<dd>
<p>A positive number used as a zoom factor. A factor of 1.0 means
that no zooming is done. Values larger than 1.0 gives a zoomed-in
effect and values smaller than 1.0 a zoomed-out effect.</p>
</dd>
<dt><var><percentage></var></dt>
<dd>
<p>A positive percentage value used as a zoom factor. A factor of
100% means that no zooming is done. Values larger than 100% gives a
zoomed-in effect and values smaller than 100% a zoomed-out effect.</p>
</dd>
</dl>
<h3>The ‘<code class="property">min-zoom</code>’ property</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>min-zoom</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td>auto | <number> | <percentage></td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto!!min-zoom">auto</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>The zoom factor itself</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto!!min-zoom">auto</code>’, or a positive
number or percentage as specified.</td>
</tr>
</tbody>
</table>
<p>Specifies the smallest allowed zoom factor. It is used as input to the
<a href="#constraining-procedure">constraining procedure</a>, but also to
limit the allowed zoom factor that can be set through user interaction.</p>
<p>Values have the following meanings:</p>
<dl>
<dt>‘<code class="css"><dfn title="auto!!min-zoom">auto</dfn></code>’</dt>
<dd>The used value is calculated from the other property values
according to the <a href="#constraining-procedure">constraining procedure</a>.</dd>
<dt><var><number></var></dt>
<dd>
<p>A positive number limiting the minimum value of the zoom factor.</p>
</dd>
<dt><var><percentage></var></dt>
<dd>
<p>A positive percentage limiting the minimum value of the zoom factor.</p>
</dd>
</dl>
<h3>The ‘<code class="property">max-zoom</code>’ property</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>max-zoom</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td>auto | <number> | <percentage></td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto!!max-zoom">auto</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>The zoom factor itself</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto!!max-zoom">auto</code>’, or a positive
number or percentage as specified.</td>
</tr>
</tbody>
</table>
<p>Specifies the largest allowed zoom factor. It is used as input to the
<a href="#constraining-procedure">constraining procedure</a>, but also to
limit the allowed zoom factor that can be set through user interaction.</p>
<p>Values have the following meanings:</p>
<dl>
<dt>‘<code class="css"><dfn title="auto!!max-zoom">auto</dfn></code>’</dt>
<dd>The used value is calculated from the other property values according
to the <a href="#constraining-procedure">constraining procedure</a>.</dd>
<dt><var><number></var></dt>
<dd>
<p>A positive number limiting the maximum value of the zoom factor.</p>
</dd>
<dt><var><percentage></var></dt>
<dd>
<p>A positive percentage limiting the maximum value of the zoom factor.</p>
</dd>
</dl>
<h3>The ‘<code class="property">user-zoom</code>’ property</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>user-zoom</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td>zoom | fixed</td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="zoom!!value"
A8A6
;>zoom</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="zoom!!value">zoom</code>’ or
‘<code class="css">fixed</code>’ as specified.</td>
</tr>
</tbody>
</table>
<p>Specifies if the zoom factor can be changed by user interaction or not.</p>
<p>Values have the following meanings:</p>
<dl>
<dt>‘<code class="css"><dfn title="zoom!!value">zoom</dfn></code>’</dt>
<dd>The user can interactively change the zoom factor.</dd>
<dt>‘<code class="css"><dfn>fixed</dfn></code>’</dt>
<dd>The user cannot interactively change the zoom factor.</dd>
</dl>
<h3>The ‘<code class="property">orientation</code>’ property</h3>
<table class="propdef">
<tbody>
<tr>
<td><em>Name:</em></td>
<td><dfn>orientation</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td>auto | portrait | landscape</td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td><span title="auto-orientation">auto</span></td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual, continuous</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>‘<code class="css" title="auto-orientation">auto</code>’,
‘<code class="css">portrait</code>’, or
‘<code class="css">landscape</code>’ as specified.</td>
</tr>
</tbody>
</table>
<p>This property is used to request that a document is displayed in portrait
or landscape mode. For a UA/device where the orientation is changed upon
tilting the device, an author can use this property to inhibit the
orientation change.</p>
<p>Values have the following meanings:</p>
<dl>
<dt>‘<code class="css"><dfn title="auto-orientation">auto</dfn></code>’</dt>
<dd>The UA automatically chooses the orientation based on the device's
normal mode of operation. The UA may choose to change the orientation
of the presentation when the device is tilted.</dd>
<dt>‘<code class="css"><dfn>portrait</dfn></code>’</dt>
<dd>The document should be locked to portrait presentation.</dd>
<dt>‘<code class="css"><dfn>landscape<dfn></code>’</dt>
<dd>The document should be locked to landscape presentation.</dd>
</dl>
<h2>Constraining viewport property values</h2>
<h3>Definitions</h3>
<p>For the procedure below:</p>
<p>Computed values refer to the computed values from the property
definitions. Other values refer to the values resolved/constrained
to at that point in the procedure. The values are initially resolved
to their computed values.
</p>
<p><code class="index" title="width!!resolved">width</code> and
<code class="index" title="height!!resolved">height</code> refer to the
resolved viewport size and not the shorthand properties. They are both
initially ‘<code class="css" title="auto!!length">auto</code>’.
</p>
<p>
<code>MIN/MAX</code> computations where one of the arguments is
‘<code class="css">auto</code>’ resolve to the other argument.
For instance, <code>MIN(0.25, 'auto') = 0.25</code>, and
<code>MAX(5, 'auto') = 5</code>.
</p>
<p>
<code class="index" title="width!!initial">initial-width</code> is the
width of the <span>initial viewport</span> in pixels at zoom factor 1.0.
<span class="note">On a device this is typically (<code>device-width
- decorations</code>). On Safari/iPhone this is identical to the
device-width.</span>
</p>
<p>
<code class="index" title="height!!initial">initial-height</code> is the
height of the <span>initial viewport</span> in pixels at zoom factor 1.0.
<span class="note">Typically (<code>device-height - decorations</code>).</span>
</p>
<h3 id="constraining-procedure">The procedure</h3>
<p>The used values are resolved from the computed values going
through the steps below. <span class="note">The procedure is closely
modelled after the behavior of the Safari/iPhone browser. We have run
tests on the Android SDK browser and a Fennec nightly build for Windows to
see where the various implementations are the same and where they differ.
We found that the Safari implementation is by far the most
consistent and predictable, and decided to ignore the other
implementations for this specification. This procedure, and the
<a href="#parsing-algorithm">parsing algorithm</a>, are backed up by a
compliance test suite currently consisting of 120-130 tests. All but one
pass in the <a href="#tested-safari">tested Safari browser</a>.</span></p>
<p>User agents are expected, but not required, to re-run this procedure and
re-layout the document, if necessary, in response to changes in the user
environment, for example if the device is tilted from landscape to
portrait mode or the window that forms the
<span>initial viewport</span> is resized.</p>
<h4 class="no-num no-toc">Resolve non-‘<code class="css" title="auto!!length">auto</code>’ lengths to pixel lengths</h4>
<ol>
<li>Resolve relative and absolute lengths, percentages, and
keywords (‘<code class="css">device-width</code>’,
‘<code class="css">device-height</code>’) to pixel
values for the ‘<code class="property">min-width</code>’,
‘<code class="property">max-width</code>’,
‘<code class="property">min-height</code>’ and
‘<code class="property">max-height</code>’
properties</li>
</ol>
<h4 class="no-num no-toc">Resolve initial <code title="width!!resolved">width</code> and
<code title="height!!resolved">height</code> from min/max properties</h4>
<ol start="2">
<li>If <code title="min-width!!resolved">min-width</code> or
<code title="max-width!!resolved">max-width</code> is not
‘<code class="css" title="auto!!length">auto</code>’, set
<code>width = MAX(min-width, MIN(max-width, initial-width))</code></li>
<li>If <code title="min-height!!resolved">min-height</code> or
<code title="max-height!!resolved">max-height</code> is not
‘<code class="css" title="auto!!length">auto</code>’, set
<code>height = MAX(min-height, MIN(max-height, initial-height))</code></li>
</ol>
<h4 class="no-num no-toc">Resolve <code title="min-zoom!!resolved">min-zoom</code>
and <code title="max-zoom!!resolved">max-zoom</code> values</h4>
<ol start="4">
<li>If <code title="min-zoom!!resolved">min-zoom</code> is not
‘<code class="css" title="auto!!min-zoom">auto</code>’ and
<code title="max-zoom!!resolved">max-zoom</code> is not
‘<code class="css" title="auto!!max-zoom">auto</code>’,