csswg-drafts/css-ui/Overview.bs at 81300f58292e0323acfe978c6d22881594c0ae1f · w3c/csswg-drafts · GitHub

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<h1>CSS Basic User Interface Module Level 3 (CSS3 UI)</h1>

<pre class='metadata'>
ED: http://dev.w3.org/csswg/css-ui/
TR: http://www.w3.org/TR/css3-ui/
Previous Version: http://www.w3.org/TR/2012/WD-css3-ui-20120117/
Shortname: css-ui
Level: 3
Group: csswg
!Issue Tracking: http://wiki.csswg.org/spec/css3-ui
Status: ED
Editor: Tantek &Ccedil;elik, Mozilla http://www.mozilla.org, tantek@cs.stanford.edu, http://tantek.com

Abstract: This specification describes user interface related selectors,
          properties and values that are proposed for CSS level&nbsp;3
          to style HTML and XML (including XHTML).
          It includes and extends user interface related features from the selectors,
          properties and values of CSS level&nbsp;2 revision 1 and Selectors specifications.
          It uses various selectors,
          properties and values to style basic user interface elements in a document.
Deadline: 2012-02-14
Link Defaults: css-color-3 (property) color
Link Defaults: selectors-4 (selector) :indeterminate
Link Defaults: css21 (property) border-width
Link Defaults: css21 (property) border-style
Ignored Terms: outline-top
Ignored Terms: outline-left
At Risk: ::value ::choices ::repeat-item ::repeat-index pseudo-elements
At Risk: 'box-sizing' property value: padding-box
At Risk: 'content' property value: icon
At Risk: 'icon' property
At Risk: 'ime-mode' property
At Risk: 'nav-index' property
At Risk: 'text-overflow' property value: &lt;string&gt;
At Risk: 'text-overflow' property 2-value syntax and definition
</pre>

<!--
At risk due to only one implementation, or obsolete dependency:
XForms is defunct on the web.
<li>XForms needs:	:default :valid :invalid :in-range :out-of-range :required :optional :read-only :read-write ::value ::choices ::repeat-item ::repeat-index

<li>box-sizing: padding-box - only FF supports in prefixed version

<li>ime-mode: new - not sure about IE5+ vs FF3+ interop

<li>nav-index property
	implemented for sure only by Opera, http://www.opera.com/docs/specs/presto22/#css
	possibly obsolete: Tasman v1 internal implementation
	possibly current: MSTV Tasman
	required (depended on) by non-web DVB-HTML, ATSC standards
	 - unknown if any DVB-HTML or ATSC simulators
</li>

Not at risk:
<li>CSS2.1:  		:hover :active :focus
<li>Selectors:		:enabled :disabled :checked
<li>:indeterminate implemented by FF3.6, IE9, Opera 10.6, Saf3

<li>box-sizing implemented by IE5/Mac, Opera
	-moz-box-sizing implemented by Mozilla

<li>outline, outline-width, outline-color, outline-style all in CSS 2.1
<li>outline-offset property
	implemented by Safari 1.2

<li>cursor property:
	CSS2.1: auto | default | help | pointer | wait | crosshair | text |
e-resize | n-resize | ne-resize | nw-resize | s-resize | se-resize | sw-resize | w-resize
	IE5/Mac implements: none | progress | cell
	IE6/Windows implements: <uri> | progress | not-allowed | no-drop | vertical-text | all-scroll | col-resize | row-resize | move |
	Mozilla implements: progress | cell(as -moz-cell) | alias(as -moz-alias) | copy(as -moz-copy) | context-menu(as -moz-context-menu) |
	freedesktop.org copied all the cursors from the CSS3-UI LC Working Draft
		by FredrikHoeglund - 14 Oct 2003
		http://freedesktop.org/Standards/cursor-spec

<li>'cursor' property values: ew-resize | ns-resize | nesw-resize | nwse-resize
implemented by Firefox and Safari

<li>'resize' property
implemented by Firefox 4 and Safari
<li>nav-up, nav-down, nav-right, nav-left properties
implemented by Opera/Presto desktop with "shift+arrow", and Blink, as well as Samsung in webkit

-->

<hr title="Separator for header">

<h2 class="no-num no-toc" id="summary">Overview</h2>

This section is <em>informative</em>.

This document is one of the "modules" for the upcoming CSS3
specification. It not only describes the user interface related
properties and values that already exist in
<a href="http://www.w3.org/TR/REC-CSS1">CSS1</a> [[CSS1]]
and <a href="http://www.w3.org/TR/CSS21">CSS2.1</a> [[!CSS21]], but introduces
new properties and values for CSS3 as well.
The Working Group doesn't expect that all implementations of CSS3 will implement
all properties or values.
Instead, there will probably be a small number of variants of CSS3, so-called "profiles".

This document is the result of the merging of relevant parts of
the following Recommendations and Working Drafts, and the addition of some new features.
<ul>
 <li>Cascading Style Sheets, level 2, revision 1 [[!CSS21]]
 <li><a href="http://www.w3.org/TR/2000/WD-css3-userint-20000216">User Interface for CSS3 (16 February 2000)</a> [[CSSUI]]
</ul>

This specification contains:
<ul>
<li>Pseudo-classes and pseudo-elements to style
user interface states and element fragments respectively.

<li>Additions to the user interface features in
<a href="http://www.w3.org/TR/CSS21">CSS2.1</a>.
<li>Directional focus navigation properties.
<li>A mechanism to allow the styling of elements as icons for accessibility.
</ul>

<hr>

<h2 id="intro">Introduction</h2>

CSS3 is a set of modules, divided up and profiled in order to
simplify the specification,
and to allow implementors the flexibility of supporting
the particular modules appropriate for their implementations.

This module describes selectors and CSS properties which enable authors
to style user interface related states, element fragments, properties
and values.

<a href="http://www.w3.org/TR/REC-CSS1#anchor-pseudo-classes">Section 2.1 of CSS1</a> [[CSS1]]
and <a href="http://www.w3.org/TR/CSS2/ui.html">Chapter 18 of CSS2</a> [[CSS2]]
introduced several user interface related pseudo-classes, properties and values.
<a href="http://www.w3.org/TR/css3-selectors/#UIstates">Section 6.6.4 of Selectors</a> [[!SELECT]] also describes several additional user interface related pseudo-classes (and one pseudo-element).

This Working Draft extends them to provide the ability, through CSS,
to style elements based upon additional user interface states,
to style fragments of user interface elements, and to alter the
dynamic presentation of elements in ways previously only available through specific HTML4/XHTML1 elements and attributes.

<h3 id="purpose">Purpose</h3>

The purpose of this specification is to achieve the following objectives:
<ul>
<li>Extend the user interface features in CSS2.1.
<li>Provide additional CSS mechanisms to augment or replace other
dynamic presentation related features in HTML4/XHTML1.
<li>Introduce directional navigation properties to assist in the construction of
user interfaces which make use of a directional navigation model.
<li>Introduce properties and values to specify icon presentations for
elements to enhance accessibility.
</ul>

<h2 id="dependencies">Dependencies on other modules</h2>

This CSS3 module depends on the following other specifications.

<ul>
<li>[[!SELECT]]
<li>[[!CSS3COLOR]]
<li>[[!CSS21]]
<li>[[!CSS3VAL]]
<li>[[!CSS3-WRITING-MODES]]<!--for end, start, line-left, line-end-->
</ul>

<!--
It has non-normative (informative) references to the following
other specifications:
<ul>
</ul>
-->

The following work is related to the CSS Basic User Interface Module Level 3 (CSS3 Basic UI).

<ul>
<li>[[HTML401]]
<li>[[HTML5]]
<li>[[UAAG10]]
<li>[[XML10]]
<li>[[XHTML10]]
<li>[[XHTML11]]
<li>[[XFORMS11]]
</ul>

This specification does not define what is a form element.

<h2 id="selectors">User Interface Selectors</h2>

<h3 id="pseudo-classes">User interface states: pseudo-classes</h3>

The Selectors specification <a href="http://www.w3.org/TR/css3-selectors/#UIstates">defines several user interface selectors</a> ([[!SELECT]], sections 6.6.1 and 6.6.4)
which represent user interface states:
<ul><li id="pseudo-hover"><a href="http://www.w3.org/TR/css3-selectors/#useraction-pseudos">:hover</a>
	<li id="pseudo-active"><a href="http://www.w3.org/TR/css3-selectors/#useraction-pseudos">:active</a>
	<li id="pseudo-focus"><a href="http://www.w3.org/TR/css3-selectors/#useraction-pseudos">:focus</a>
	<li id="pseudo-enabled"><a href="http://www.w3.org/TR/css3-selectors/#enableddisabled">:enabled</a>
	<li id="pseudo-disabled"><a href="http://www.w3.org/TR/css3-selectors/#enableddisabled">:disabled</a>
	<li id="pseudo-checked"><a href="http://www.w3.org/TR/css3-selectors/#checked">:checked</a>
	<li id="pseudo-indeterminate"><a href="http://www.w3.org/TR/css3-selectors/#indeterminate">:indeterminate</a>
</ul>

These pseudo-classes as defined by [[!SELECT]] are
included in this specification by reference.

CSS 2.1 [[!CSS21]] specifies additional details
for some of the selectors mentioned, above and beyond Selectors.


<h4 id="active">:active details</h4>

In addition, on systems with more than one mouse button,
:active is clarified to apply only to the primary
or primary activation button (typically the "left" mouse button),
and any aliases thereof.

<h4 id="indeterminate">
The indeterminate-value pseudo-class '':indeterminate''</h4>

The '':indeterminate'' pseudo-class applies
to UI elements whose value is in an indeterminate state.
For example, radio and checkbox elements
can be toggled between checked and unchecked states,
but are sometimes in an indeterminate state,
neither checked nor unchecked.
Similarly a progress meter can be in an indeterminate state
when the percent completion is unknown.

Like the :checked pseudo-class,
'':indeterminate'' applies to all media.
Components of a radio-group initialized with no pre-selected choice,
for example, would be '':indeterminate''
even in a static display.


<h4 id="new-pseudo-classes" class="no-num no-toc">New user interface state pseudo-classes</h4>

In addition to the above-mentioned pseudo-classes,
this specification introduces several new pseudo-classes
to define additional user interface states.
<ul>
  <li>:default
  <li>:valid
  <li>:invalid
  <li>:in-range
  <li>:out-of-range
  <li>:required
  <li>:optional
  <li>:read-only
  <li>:read-write
</ul>

Specifically, these new states (except for :default) are provided

<h4 id="pseudo-default">:default</h4>

The :default selector applies to the one or more UI elements
that are the default among a set of similar elements.
This selector typically applies to context menu items,
buttons, and select lists/menus.

One example is the default submit button among a set of buttons.
Another example is the default option from a popup menu.
Multiple elements in a select-many group could have multiple :default elements,
like a selection of pizza toppings for example.

<h4 id="pseudo-validity"><span id="pseudo-valid">:valid</span>
and <span id="pseudo-invalid">:invalid</span></h4>

An element is :valid or :invalid when it is,
respectively, valid or invalid with respect to data validity semantics
defined by a different specification (e.g. [[XFORMS11]]).
An element which lacks data validity semantics is neither :valid nor :invalid.
This is different from an element which otherwise has no constraints.
Such an element would always be :valid.

<h4 id="pseudo-range"><span id="pseudo-in-range">:in-range</span> and <span id="pseudo-out-of-range">:out-of-range</span></h4>

The :in-range and :out-of-range pseudo-classes apply only to elements that have range limitations.
An element is :in-range or :out-of-range
when the value that the element is bound to is in range or out of range
of the presentation (e.g. visual or spoken representation) of the element respectively.
An element that lacks data range limits or is not a form control is neither :in-range nor :out-of-range.
E.g. a slider element with a value of 11
presented as a slider control that only represents the values from 1-10 is :out-of-range.
Another example is a menu element with a value of "E"
that happens to be presented as a popup menu that only has choices "A", "B" and "C".

<h4 id="pseudo-required-value"><span id="pseudo-required">:required</span>
and <span id="pseudo-optional">:optional</span></h4>

A form element is :required or :optional if a value for it
is, respectively, required or optional before the form it belongs to is submitted.
Elements that are not form elements are neither required nor optional.

<h4 id="pseudo-ro-rw"><span id="pseudo-read-only">:read-only</span>
and <span id="pseudo-read-write">:read-write</span></h4>

An element whose contents are not user-alterable is :read-only.
However, elements whose contents are user-alterable (such as text input fields)
are considered to be in a :read-write state.
In typical documents, most elements are :read-only.
However it may be possible (e.g. in the context of an editor) for any element to become :read-write.

<h3 id="pseudo-elements">User interface element fragments: pseudo-elements</h3>

In addition to the above-mentioned pseudo-element,
this specification introduces four new pseudo-elements
to provide access to additional user interface element fragments.
<ul>
  <li>::value
  <li>::choices
  <li>::repeat-item
  <li>::repeat-index
</ul>

Specifically, these new pseudo-elements are provided
as a way to style user interface fragments
as defined by XForms [[XFORMS11]].

Note: The ::value, ::choices, ::repeat-item, and ::repeat-index
pseudo-elements are all at risk.

<h4 id="pseudo-value">::value</h4>

A form element may contain both a label for its data value,
and the data value itself.
For such elements, the ::value pseudo-element
selects the representation of just the data value itself,
in order to style its appearance.

<div class="example"><p style="display:none">Example(s):
<h4 id="value-example" class="no-num no-toc">fictional markup and illustration</h4>

Here is an example which illustrates the ::value of a text input field
with fictional markup which is then styled with CSS.

sample XForms fragment with fictional markup:
<pre><code class="lang-markup">
&lt;input&gt;
  &lt;label&gt;Zip code&lt;label&gt;
  <em>&lt;input::value/&gt;</em>
&lt;/input&gt;
</code></pre>

sample CSS:
<pre><code class="lang-css">
input { border:dashed }
label { border:dotted }
input::value { border:solid }
</code></pre>

an HTML+CSS approximation of this example

<p>
<span style="border:dashed;display:inline-block;padding:10px">
 <label
  style="border:dotted;display:inline-block;padding:2px;margin:0;font-size:1em"
 >Zip code</label>
 <input type="text"
  style="border:solid;display:inline-block;padding:2px;margin:0;font-size:1em"
  value="94117">
</span>

Spacing (in the form of padding) has been added to the above approximation
to separate the borders and make the individual (pseudo-)elements easier to distinguish.
</div>

The ''::value'' pseudo-element is similar to an inline-level element,
but with certain restrictions.
The following properties apply to ''::value''
pseudo-element: font properties, color property, background properties,
'word-spacing', 'letter-spacing', 'text-decoration', 'vertical-align',
'text-transform', 'line-height'.  UAs may apply other properties as well.


<h4 id="pseudo-choices">::choices</h4>

Similarly, a form element which represents a list of options
may contain both a label for the list,
and the list of choices itself.
For such elements,
the ''::choices'' pseudo-element selects
the representation of just the list of choices themselves,
in order to style their appearance.

A list of radio buttons can also be selected with the ''::choices'' pseudo-element,
and the currently chosen radio button can be selected with the ''::value'' pseudo-element.

<h4 id="pseudo-repeat-item">::repeat-item</h4>

The ''::repeat-item'' pseudo-element represents a single item from a repeating sequence.
It is generated as a parent to all the elements in a single repeating item.
Each ''::repeat-item'' is associated with a particular instance data node,
and is affected by the model item properties (e.g. '<code>relevant</code>') found there,
as the related style properties will cascade to the child elements.

<h4 id="pseudo-repeat-index">::repeat-index</h4>

The ::repeat-index pseudo-element represents the current item of a repeating sequence.
It takes the place of the ::repeat-item as a parent of all the elements in the index repeating item.

Note: Any style declarations that an author wants to apply to all repeat items,
including the index,
must be done so by using both ::repeat-item and ::repeat-index selectors.
Styles that are only applied to ::repeat-item will not automatically be applied to the respective ::repeat-index.

<div class="example"><p style="display:none">Example(s):
<h4 id="repeat-item-index-example" class="no-num no-toc">::repeat-item and ::repeat-index fictional markup</h4>

Here is an example that illustrates both ::repeat-item and ::repeat-index,
since they are often both available and used at the same time.
Assume appropriate namespace declarations were made in a header somewhere
preceding the code in the example.

The following markup snippet uses XHTML and XForms:
<pre><code class="lang-markup">
&lt;html:table xforms:repeat-nodeset="..."&gt;
   &lt;html:tr&gt;
      &lt;html:td&gt;&lt;xforms:input ref="..."/&gt;&lt;xforms:input ref="..."/&gt;&lt;/html:td&gt;
   &lt;/html:tr&gt;
&lt;/html:table&gt;
</code></pre>

The following style rules are used to style all the repeated items and the current repeated item.
<pre><code class="lang-css">
html|tr::repeat-item { outline: medium solid; color:gray }
html|tr::repeat-index { outline: medium dashed; color:black }
</code></pre>

The following fictional markup shows
the state of the above markup
when through user interaction the XForm contains three of the repeated items,
where the third item is current.
<pre><code class="lang-markup">
&lt;html:table xforms:repeat-nodeset="..."&gt;
 <em>&lt;html:tr::repeat-item&gt;</em>
  &lt;html:tr&gt;
   &lt;html:td&gt;&lt;xforms:input ref="..."/&gt;&lt;xforms:input ref="..."/&gt;&lt;/html:td&gt;
  &lt;/html:tr&gt;
 <em>&lt;/html:tr::repeat-item&gt;</em>
 <em>&lt;html:tr::repeat-item&gt;</em>
  &lt;html:tr&gt;
   &lt;html:td&gt;&lt;xforms:input ref="..."/&gt;&lt;xforms:input ref="..."/&gt;&lt;/html:td&gt;
  &lt;/html:tr&gt;
 <em>&lt;/html:tr::repeat-item&gt;</em>
 <em>&lt;html:tr::repeat-index&gt;</em>
  &lt;html:tr&gt;
   &lt;html:td&gt;&lt;xforms:input ref="..."/&gt;&lt;xforms:input ref="..."/&gt;&lt;/html:td&gt;
  &lt;/html:tr&gt;
 <em>&lt;/html:tr::repeat-index&gt;</em>
&lt;/html:table&gt;
</code></pre>
</div>

Note: The ::repeat-index pseudo-element takes the place of the ::repeat-item
rather than being nested inside as a separate element.
Thus just like :link or :visited are mutually exclusive for selecting hyperlinks,
only one will exist and apply to a particular repeated item at any point.

<h2 id="icons">Element icons</h2>

<h3 id="content-addition">'content' property addition</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>content</dfn></td></tr>
<tr><th>New&nbsp;Value(s):</th><td title="">icon</td></tr>

<tr><th>Initial:</th><td>same as CSS 2.1</td></tr>
<tr><th>Applies to:</th><td>same as CSS 2.1</td></tr>
<tr><th>Inherited:</th><td>same as CSS 2.1</td></tr>
<tr><th>Percentages:</th><td>same as CSS 2.1</td></tr>
<tr><th>Media:</th><td>same as CSS 2.1</td></tr>
<tr><th>Computed&nbsp;value:</th><td>the keyword ''icon'' if specified as such, otherwise same as CSS 2.1</td></tr>
<tr><th>Animatable:</th><td>no</td></tr>
</tbody></table>

<dl>
<dt>icon</dt>
<dd>The (pseudo-)element is replaced in its entirety by the resource referenced by its 'icon' property, and treated as a replaced element.
</dd>
</dl>

Note: It is expected that the next draft of the CSS3 Generated Content module [[CSS3GENCON]]
will include and superset this functionality.

Note: The ''icon'' value is at risk.

<h3 id="icon">'icon' property</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>icon</dfn></td></tr>
<tr><th>Value:</th><td>auto | &lt;uri&gt; [, &lt;uri&gt;]* | inherit</td></tr>
<tr><th>Initial:</th><td>auto</td></tr>
<tr><th>Applies to:</th><td>all elements</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>all</td></tr>
<tr><th>Computed&nbsp;value:</th><td>as specified, except with any relative URLs converted to absolute</td></tr>
<tr><th>Animatable:</th><td>no</td></tr>
</tbody></table>

<dl>
<dt>auto</dt>
<dd>Use a default generic icon provided by the user agent.</dd>
<dt>&lt;uri&gt;</dt>
<dd>URIs (see [[!URI]], [[!RFC1738]] and [[!RFC1808]]) provide a way of identifying resources.
The &lt;uri&gt; value(s) in this property refer to one or more icons in a comma delimited list.
The user agent loads the referenced icons one by one until it finds one that it is able to render.
This permits the usage of multiple different icon formats for various platforms,
and various media for that matter.</dd>
</dl>

The 'icon' property provides the author
the ability to style any arbitrary element with an iconic equivalent.
An element's icon is not used/rendered
unless the 'content' property is set
to the value ''icon''(see above).
Documents whose elements have icons assigned to them
can be more easily viewed by users who find too much text distracting.

<div class="example"><p style="display:none">Example(s):
<h4 id="icon-example" class="no-num no-toc">Representing elements with icons</h4>

This example uses the above icon features to display icons in place of images and objects.
<pre><code class="lang-css">
img,object { content:icon }
  /* note that the CSS3 Generated Content module [[CSS3GENCON]]
     expands the 'content'property to apply to all elements. */

img { icon:url(imgicon.png); }
  /* provide a custom icon for images */

object { icon:url(objicon.png); }
  /* provide a different custom icon for objects */
</code></pre>
</div>

Note: The 'icon' property is at risk.

<h2 id="box-model">Box Model addition</h2>

<h3 id="box-sizing">'box-sizing' property</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>box-sizing</dfn></td></tr>
<tr><th>Value:</th><td>
content-box |
padding-box |
border-box |
inherit</td></tr>
<tr><th>Initial:</th><td>content-box</td></tr>
<tr><th>Applies to:</th><td>all elements that accept width or height</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th><td>specified value</td></tr>
<tr><th>Animatable:</th><td>no</td></tr>
</tbody></table>

<dl>
<dt>content-box</dt>
<dd>This is the behavior of width and height as specified by CSS2.1.
The specified width and height (and respective min/max properties)
apply to the width and height respectively of the content box of the element.
The padding and border of the element are laid out
and drawn outside the specified width and height.</dd>
<dt>padding-box</dt>
<dd>
The specified width and height (and respective min/max properties)
on this element determine the padding box of the element.
That is, any padding specified on the element is laid out
and drawn inside this specified width and height.
The content width and height are calculated by
subtracting the padding widths of the respective sides
from the specified 'width' and
'height' properties.
As the content width and height
<a href="http://www.w3.org/TR/CSS21/visudet.html#the-width-property">
cannot be negative</a> ([[!CSS21]], section 10.2),
this computation is floored at 0.
</dd>
<dt>border-box</dt>
<dd>The specified width and height (and respective min/max properties) on this element
determine the border box of the element.
That is, any padding or border specified on the element
is laid out and drawn inside this specified width and height.
The content width and height are calculated
by subtracting the border and padding widths of the respective sides
from the specified 'width'
and 'height' properties.
As the content width and height
<a href="http://www.w3.org/TR/CSS21/visudet.html#the-width-property">cannot be negative</a> ([[!CSS21]], section 10.2),
this computation is floored at 0.

Note: This is the behavior of width and height as commonly implemented
by legacy HTML user agents for replaced elements and input elements.
</dd>
</dl>


<div class="example"><p style="display:none">Example(s):
<h4 id="box-sizing-example" class="no-num no-toc">Using box-sizing to evenly share space</h4>

This example uses box-sizing to evenly horizontally split
two divs with fixed size borders inside a div container,
which would otherwise require additional markup.

sample CSS:
<pre><code class="lang-css">
div.container {
	width:38em;
	border:1em solid black;
}

div.split {
	box-sizing:border-box;
	width:50%;
	border:1em silver ridge;
	float:left;
}
</code></pre>

sample HTML fragment:
<pre><code class="lang-markup">
&lt;div class="container"&gt;
&lt;div class="split"&gt;This div occupies the left half.&lt;/div&gt;
&lt;div class="split"&gt;This div occupies the right half.&lt;/div&gt;
&lt;/div&gt;
</code></pre>

demonstration of sample CSS and HTML:
<div style="width:38em; border:1em solid black"><div style="box-sizing:border-box; width:50%; border:1em silver ridge; float:left">This div should occupy the left half.</div><div style="box-sizing:border-box; width:50%; border:1em silver ridge; float:left">This div should occupy the right half.</div>The two divs above should appear side by side, each (including borders) 50% of the content width of their container. If instead they are stacked one on top of the other then your browser does not support 'box-sizing'.
</div>
</div>

Note: The ''padding-box'' value is at risk.

<h2 id="outline-props">Outline properties</h2>

At times, style sheet authors may want to create outlines around
visual objects such as buttons, active form fields, image maps, etc.,
to make them stand out. Outlines differ from borders in the following
ways:

<ol>
<li>Outlines do not take up space.
<li>Outlines may be non-rectangular.
</ol>

The outline properties control the style of these dynamic outlines.

<h3 id="outline">'outline' property</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>outline</dfn></td></tr>
<tr><th>Value:</th><td>
[ &lt;'outline-color'&gt; || &lt;'outline-style'&gt; ||
&lt;'outline-width'&gt; ] |
inherit
</td></tr>
<tr><th>Initial:</th><td>see individual properties</td></tr>
<tr><th>Applies to:</th><td>all elements</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th><td>see individual properties</td></tr>
<tr><th>Animatable:</th><td>see individual properties</td></tr>
</tbody></table>

<h3 id="outline-width">'outline-width' property</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>outline-width</dfn></td></tr>
<tr><th>Value:</th><td><<'border-width'>> | inherit</td></tr>
<tr><th>Initial:</th><td>medium</td></tr>
<tr><th>Applies to:</th><td>all elements</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th><td>absolute length; ''0'' if the outline style is ''none''.</td></tr>
<tr><th>Animatable:</th><td>as <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-length">length</a></td></tr>
</tbody></table>

<h3 id="outline-style">'outline-style' property</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>outline-style</dfn></td></tr>
<tr><th>Value:</th><td>auto | <<'border-style'>> | inherit</td></tr>
<tr><th>Initial:</th><td>none</td></tr>
<tr><th>Applies to:</th><td>all elements</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th><td>specified value</td></tr>
<tr><th>Animatable:</th><td>no</td></tr>
</tbody></table>

<h3 id="outline-color">'outline-color' property</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>outline-color</dfn></td></tr>
<tr><th>Value:</th><td><<color>> | invert | inherit</td></tr>
<tr><th>Initial:</th><td>invert</td></tr>
<tr><th>Applies to:</th><td>all elements</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th><td>The computed value for ''invert'' is ''invert''. For &lt;color&gt; values, the computed value is as defined for the [[!CSS3COLOR]] 'color' property.</td></tr>
<tr><th>Animatable:</th><td>as <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-color">color</a></td></tr>
</tbody></table>

The outline created with the outline properties is drawn "over" a box,
i.e., the outline is always on top,
and doesn't influence the position or size of the box,
or of any other boxes.
Therefore, displaying or suppressing outlines does not cause reflow.

Outlines may be non-rectangular.
For example, if the element is broken across several lines,
the outline should be an outline or minimum set of outlines
that encloses all the element's boxes.
Each part of the outline should be fully connected
rather than open on some sides
(as borders on inline elements are when lines are broken).
The parts of the outline are not required to be rectangular.
The position of the outline may be affected by descendant boxes.
User agents should use an algorithm for determining
the outline that encloses a region appropriate
for conveying the concept of focus to the user.

The 'outline-width' property accepts
the same values as
          'border-width'
([[!CSS21]], section 8.5.1).

The 'outline-style' property accepts
the same values as
          'border-style'
([[!CSS21]], section 8.5.3), except that
''hidden'' is not a legal outline style.
In addition, in CSS3,
'outline-style'
accepts the value ''auto''.
The ''auto'' value permits the user agent
to render a custom outline style,
typically a style which is either a user interface default for the platform,
or perhaps a style that is richer
than can be described in detail in CSS,
e.g. a rounded edge outline with semi-translucent outer pixels
that appears to glow.
As such, this specification does not define how the
'outline-color'
is incorporated or used (if at all) when rendering
''auto'' style outlines.
User agents may treat ''auto'' as
''solid''.

The 'outline-color' property
accepts all colors, as well as the keyword <dfn>invert</dfn>.
''Invert'' is expected to perform a color inversion on the pixels on the screen.
This is a common trick to ensure the focus border is visible,
regardless of color background.

Conformant UAs may ignore the ''invert'' value
on platforms that do not support color inversion of the pixels on the screen.
If the UA does not support the ''invert'' value
then the initial value of the 'outline-color' property
is the ''currentColor'' [[!CSS3COLOR]] keyword.

The 'outline' property is a shorthand property,
and sets all three of 'outline-style',
'outline-width',
and 'outline-color'.

Note: The outline is the same on all sides.
In contrast to borders,
there are no 'outline-top' or 'outline-left' etc. properties.

This specification does not define how multiple overlapping outlines are drawn,
or how outlines are drawn for boxes that are partially obscured behind other elements.

<div class="example"><p style="display:none">Example(s):

Here's an example of drawing a thick outline around a BUTTON element:
<pre><code class="lang-css">
button { outline: thick solid }
</code></pre>
</div>

Graphical user interfaces may use outlines around elements
to tell the user which element on the page has the focus.
These outlines are in addition to any borders,
and switching outlines on and off should not cause the document to reflow.
The focus is the subject of user interaction in a document
(e.g., for entering text, selecting a button, etc.).

<div class="example"><p style="display:none">Example(s):

For example, to draw a thick black line around an element when it has the focus,
and a thick red line when it is active,
the following rules can be used:
<pre><code class="lang-css">
:focus  { outline: thick solid black }
:active { outline: thick solid red }
</code></pre>
</div>

Note: Since the outline does not affect formatting
(i.e., no space is left for it in the box model),
it may well overlap other elements on the page.

<h3 id="outline-offset">'outline-offset' property</h3>

By default, the outline is drawn starting just outside the border edge.
However, it is possible to offset the outline and draw it beyond the border edge.

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>outline-offset</dfn></td></tr>
<tr><th>Value:</th><td>&lt;length&gt; | inherit</td></tr>
<tr><th>Initial:</th><td>0</td></tr>
<tr><th>Applies to:</th><td>all elements</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th><td>&lt;length&gt; value in absolute units (px or physical).</td></tr>
<tr><th>Animatable:</th><td>as <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-length">length</a></td></tr>
</tbody></table>

If the computed value of 'outline-offset'
is anything other than 0,
then the outline is outset from the border edge by that amount.

<div class="example"><p style="display:none">Example(s):

For example,
to leave 2 pixels of space between a focus outline
and the element that has the focus or is active,
the following rule can be used:
<pre><code class="lang-css">
:focus,:active  { outline-offset: 2px }
</code></pre>
</div>


<h2 id="resizing-and-overflow">Resizing &amp; Overflow</h2>

CSS2.1 provides a mechanism for controlling the appearance of a scrolling mechanism
(e.g. scrollbars)
on block container elements.
This specification adds to that a mechanism for controlling
user resizability of elements as well as the ability to specify text overflow behavior.

<h3 id="resize">'resize' property</h3>

The 'resize' property allows the author
to specify whether or not an element is resizable by the user,
and if so, along which axis/axes.

<table class="propdef"><tbody>
<tr><th>Name:</th><td><dfn>resize</dfn></td></tr>
<tr><th>Value: </th><td>none | both | horizontal | vertical | inherit</td></tr>
<tr><th>Initial:</th><td>none</td></tr>
<tr><th>Applies to:</th><td>elements with 'overflow' other than visible</td></tr>
<tr><th>Inherited:</th><td>no</td></tr>
<tr><th>Percentages:</th><td>N/A</td></tr>
<tr><th>Media:</th><td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th><td>specified value.</td></tr>
<tr><th>Animatable:</th><td>no</td></tr>
</tbody></table>

<dl>
<dt>none</dt>
<dd>The UA does not present a resizing mechanism on the element,
and the user is given no direct manipulation mechanism to resize the element.</dd>
<dt>both</dt>
<dd>The UA presents a bidirectional resizing mechanism
to allow the user to adjust both the height and the width of the element.</dd>
<dt>horizontal</dt>
<dd>The UA presents a unidirectional horizontal resizing mechanism
to allow the user to adjust only the width of the element.</dd>
<dt>vertical</dt>
<dd>The UA presents a unidirectional vertical resizing mechanism
to allow the user to adjust only the height of the element.</dd>
</dl>

Currently it is possible to control the appearance of the scrolling mechanism (if any)
on an element using the 'overflow' property
(e.g. '<code class="lang-css">overflow: scroll</code>' vs. '<code class="lang-css">overflow: hidden</code>' etc.).
The purpose of the 'resize' property
is to also allow control over the appearance and function of the resizing mechanism
(e.g. a resize box or widget) on the element.

Note: The resizing mechanism is NOT the same as the scrolling mechanism.
The scrolling mechanism allows the user
to determine which portion of the contents of an element is shown.
The resizing mechanism allows the user
to determine the size of the element.

The 'resize' property applies to elements
whose computed 'overflow' value
is something other than ''visible''.

When an element is resized by the user,
the user agent keeps track of a resize factor
(which is initially 1.0) for the width and height,
which it then applies to the computed width and height
as part of determining the used width and height.
The element's contents (and surroundings) are reformatted as necessary.

The resize factor introduces a step in width/height calculations and formatting
as described in <a href="http://www.w3.org/TR/CSS21/visudet.html">chapter 10 of CSS2.1</a>.
Specifically the following step is inserted between steps 1 and 2
of the algorithm in <a href="http://www.w3.org/TR/CSS21/visudet.html#min-max-widths">section 10.4</a>
and <a href="http://www.w3.org/TR/CSS21/visudet.html#min-max-heights">10.7</a> in CSS2.1 [[!CSS21]],
where [dimension] is 'width' for 10.4 and 'height' for 10.7.
<blockquote><p style="text-indent:-2em">
1b. If the resize [dimension] factor is not 1.0,
 then the tentative used [dimension] is multiplied by that factor,
 and the rules above are applied again,
 but this time using the result of that multiplication as the computed value for '[dimension]'.
</blockquote>

With regard to interactivity and the Document Object Model (DOM),
the resize factor on an element lasts the lifetime of the element,
however, if the 'resize' property itself is altered
(e.g. via pseudo-class change or via DOM manipulation),
then the resize factor is reset to 1.0.

The precise direction of resizing
(i.e. altering the top left of the element or altering the bottom right)
may depend on a number of factors
including whether the element is absolutely positioned,
whether it is positioned using the 'right'
and 'bottom' properties,
whether the language of the element is right-to-left etc.
The precise direction of resizing is left to the UA to properly determine for the platform.

The user agent may restrict the resizing range to something suitable,
such as between the original formatted size of the element,
and large enough to encompass all the element's contents.

<div class="example"><p style="display:none">Example(s):

For example,
to make iframes scrollable <em>and</em> resizable,
the following rule can be used:
<pre><code class="lang-css">
iframe,object[type^="text/"],
object[type$="+xml"],object[type="application/xml"]
{
  overflow:auto;
  resize:both;
}
</code></pre>
</div>

<!--
too bad we don't have @viewport yet. otherwise this would be cool:
<pre><code class="lang-css">
@viewport {
       width: 100px;
       height: 100px;
       overflow: hidden;
       resize: none
     } /* display content in a non-resizable 100px by 100px window */
</code></pre>
-->


<h3 id="text-overflow">
    Overflow Ellipsis: the 'text-overflow' property</h3>

<table class="propdef"><tbody>
<tr><th>Name:</th>
    <td><dfn>text-overflow</dfn></td></tr>
<tr><th>Value:</th>
    <td> (
clip |
ellipsis |
<<string>>
){1,2} |
inherit
</td></tr>
<tr><th>Initial:</th>
    <td>clip</td></tr>
<tr><th>Applies to:</th>
    <td>block containers</td></tr>
<tr><th>Inherited:</th>
    <td>no</td></tr>
<tr><th>Percentages:</th>
    <td>N/A</td></tr>
<tr><th>Media:</th>
    <td>visual</td></tr>
<tr><th>Computed&nbsp;value:</th>
    <td>as specified</td></tr>
<tr><th>Animatable:</th>