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</span>References</a>
<ul class="toc">
<li class="tocline"><a class="tocxref" href="#normative-references"><span
class="secno">B.1
</span>Normative references</a></li>
<li class="tocline"><a class="tocxref" href="#informative-references"><span
class="secno">B.2
</span>Informative references</a></li>
</ul>
</li>
<li class="tocline"><a class="tocxref" href="#changes"><span class="secno">C.
- </span>Change History</a></li>
</ul>
</section>
<section id="intro" class="introductory">
<h1>Introduction</h1>
<p>HTML 4 provided a <code>longdesc</code> attribute for the <code>img</code>
element. This enabled a machine-discoverable description of an image to
be linked to the image. This extension specification defines the same
functionality for HTML5-based content.</p>
<pre class="example"><code><br><br><!-- The description is an element on the same page as the image, with the id "graph1Explained" -->
<img src="http://example.com/graph1" alt="(Drinks are getting sweeter)" title="Figure One" <strong>longdesc="#graph1Explained"</strong>>
<!-- The description is a page on its own -->
<img src="figure1" alt="figure 1" <strong>longdesc="http://example.com/desc1"</strong>>
<!-- The description is one of several within an external page -->
<img src="ExampleImage" alt="example" <strong>longdesc="http://example.com/descs#item3"</strong>>
<!-- The description is included in a data: URI -->
<img src="logo" alt="W3C" <strong>longdesc="data:text/html;charset=utf-8;,%3C!DOCTYPE%20html%3E%3Chtml%3E%3Chead%3E
%3Ctitle%3EDescription%20of%20the%20W3C%20Logo%3C/title%3E%3C/head%3E%3Cbody%3E%3Cp%3EA%20blue%20capital%20letter<br>%20%22W%22%20with%20kerning%20so%20it%20touches%20a%20blue%203%2C%20followed%20by%20a%20black%20shadow%20<br>of%20a%20white%20capital%20letter%20C%20all%20on%20a%20white%20background%3C/body%3E%3C/html%3E"</strong>></code></pre>
</section>
<section id="conformance">
<h2><span class="secno">1. </span>Conformance</h2>
<p> As well as sections marked as non-normative, all authoring guidelines,
diagrams, examples, and notes in this specification are non-normative.
Everything else in this specification is normative. </p>
<p> The key words <em title="must" class="rfc2119">must</em>, <em title="should"
class="rfc2119">should</em>,
<em title="should not" class="rfc2119"></em><em title="recommended" class="rfc2119"></em>and
+ </span>Change History</a></li>
</ul>
</section>
<section id="intro" class="introductory">
<h1>Introduction</h1>
<p>HTML 4 provided a <code>longdesc</code> attribute for the <code>img</code>
element. This enabled a machine-discoverable description of an image to
be linked to the image. This extension specification defines the same
functionality for HTML5-based content.</p>
<pre class="example"><code><br><br><!-- The description is an element on the same page as the image, with the id "graph1Explained" -->
<img src="http://example.com/graph1" alt="(Drinks are getting sweeter)" title="Figure One" <strong>longdesc="#graph1Explained"</strong>>
<!-- The description is a page on its own -->
<img src="figure1" alt="figure 1" <strong>longdesc="http://example.com/desc1"</strong>>
<!-- The description is one of several within an external page -->
<img src="ExampleImage" alt="example" <strong>longdesc="http://example.com/descs#item3"</strong>>
<!-- The description is included in a data: URI -->
<img src="logo" alt="W3C" <strong>longdesc="data:text/html;charset=utf-8;,%3C!DOCTYPE%20html%3E%3Chtml%3E%3Chead%3E
%3Ctitle%3EDescription%20of%20the%20W3C%20Logo%3C/title%3E%3C/head%3E%3Cbody%3E%3Cp%3EA%20blue%20capital%20letter<br>%20%22W%22%20with%20kerning%20so%20it%20touches%20a%20blue%203%2C%20followed%20by%20a%20black%20shadow%20<br>of%20a%20white%20capital%20letter%20C%20all%20on%20a%20white%20background%3C/body%3E%3C/html%3E"</strong>></code></pre>
</section>
<section id="conformance">
<h2><span class="secno">1. </span>Conformance</h2>
<p> As well as sections marked as non-normative, all diagrams, examples,
and notes in this specification are non-normative. Everything else in
this specification is normative. </p>
<p> The key words <em title="must" class="rfc2119">must</em>, <em title="should"
class="rfc2119">should</em>,
<em title="should not" class="rfc2119"></em><em title="recommended" class="rfc2119"></em>and
<em title="may" class="rfc2119">may</em><em title="optional" class="rfc2119"></em>
in this specification are to be interpreted as described in [<cite><a class="bibref"
href="#bib-RFC2119">RFC2119</a></cite>].</p>
<section id="dependencies">
<h3><span class="secno">1.1 </span>Dependencies and definitions</h3>
<p>The IDL fragments in this specification <em title="must" class="rfc2119">must</em>
be interpreted as required for conforming IDL fragments, as described
in the Web IDL specification. [<cite><a class="bibref" href="#bib-WEBIDL">WEBIDL</a></cite>]</p>
<p>The terms "<a href="http://www.w3.org/TR/HTML5/#valid-url-potentially-surrounded-by-spaces">valid
non-empty URL potentially surrounded by spaces</a>", "<a href="http://www.w3.org/TR/HTML5/#hyperlink">hyperlink"</a>,
and "<a href="http://www.w3.org/TR/HTML5/#reflect">reflect</a>", used
in this specification are defined in [<cite><a class="bibref" href="#bib-HTML5">HTML5</a></cite>].</p>
<p>This document does not define the term "accessible" nor
accessibility, but uses them with the sense they have in [<a href="#bib-WCAG">WCAG</a>]</p>
</section>
</section>
<section id="UCnR">
<h2><span class="secno">2. </span>Use Cases and Requirements</h2>
<p>There are many ways users can successfully interact with content even
if they cannot see, or see well, images included in that content. The <code>alt</code>
attribute is designed to ensure that for everyday work, a user has
enough information to replace an image, and often this is more helpful
than a detailed description of every image. The <code>longdesc</code>
attribute is designed to complement this functionality, to meet the
following use cases. </p>
<h3>Use Cases</h3>
<dl>
<dt>Identifying a well-known image</dt>
</dl>
<dl>
<dd>There are many well-known images which are widely reproduced and
well known. In some cases, such as "Mona Lisa", "Gerníka", Cubbin's
"Lost", Leunig's "ramming the shears", the most effective way to refer
identify an image to another person is by the creator, title, and
similar information that would be found in a catalogue. But different
people know the images by different titles, and some images have no
widely known titles. Identifying an image is often an important part
of framing a discussion. Where the image itself cannot be seen, a
description can often be used to offer easier recognition than the
cataloguing information, for example "Washington crossing some river,
standing heroically in the boat, while soldiers do the hard work", or
"the Da Vinci picture of the guy in a square and a circle".</dd>
<dd><em>Requires</em>: Discoverability</dd>
<dd><em>Helped by</em>: Inline, Reuse</dd>
<dt>Describing a complex diagram</dt>
<dd>In many environments diagrams are used to explain a concept or
transmit information efficiently. Where a user has a reduced ability
to see the image (poor contrast vision and other conditions are as
relevant here as a complete lack of vision), a description can enable
that user to understand the information being presented</dd>
<dd><em>Requires</em>: Structured Markup, Inline, Reuse, Simple Return</dd>
<dt>Teaching Accessible Development</dt>
<dd>In many environments producing content that is accessible to users
regardless of disability is a legal and/or market requirement. </dd>
<dd><em>Requires</em>: Maintenance, Backwards compatibility</dd>
<dt>A self-describing artistic work</dt>
<dd>For many pages the visual design of the page is an important part of
the message it conveys to a fully-sighted user, but the author would
like to convey as much of that design as possible to a user with low
or no vision available. </dd>
<dd><em>Requires</em>: No visual encumbrance, Structured Markup</dd>
<dd><em>Helped by</em>: Reuse, Simple Return</dd>
<dt>Referring to an existing description</dt>
<dd>Many well-known images are already described by other sources. The
copyright on those sources may not be compatible with re-publishing
the description, but there is little value in making a new one. </dd>
<dd><em>Requires</em>: Reuse, Structured Markup, Simple Return, Optional
Consumption </dd>
<dd><em>Helped by</em>: No Visual Encumbrance</dd>
<dt>Linking to a description included within a page</dt>
<dd>In some cases an image already has a description included within a
page. Making the linkage explicit can provide further clarity for a
user navigating the page who is not able to interpret the default
layout, e.g. because they have forced a re-layout of the page
elements, or because they do not see the default visual relationship
between the element and its description.</dd>
<dd>This practice also enables description to be provided for all users.</dd>
<dd>By keeping the association clear the content maintainer can more
easily check that the description and link are actually correct.</dd>
<dd><em>Requires</em>: Inline, Simple Return, Structured Markup,
Optional Consumption </dd>
<dt>Localising descriptions</dt>
<dd>When content is localised to multiple languages, it is important
that metadata intended for human consumption, such as image
descriptions, can be readily localised.</dd>
<dd><em>Helped by</em>: Structured Markup, Reuse, Maintenance.</dd>
</dl>
<h3>Requirements for an Image Description functionality </h3>
<dl>
<dt>No visual encumbrance</dt>
<dd>It <em class="rfc2119">must</em> be possible to provide a
description for an image without any visual encumbrance on the page.</dd>
<dt>Structured Markup</dt>
<dd>It <em class="rfc2119">must</em> be possible to include rich markup
(e.g. HTML5) in the description of the image.</dd>
<dt>Inline</dt>
<dd>It <em class="rfc2119">must</em> be possible to associate a
description in the body of a page with an image in that page. </dd>
<dt>Reuse</dt>
<dd>It <em class="rfc2119">must</em> be possible to re-use a single
description for multiple occurrences of an image.</dd>
<dt>Maintenance</dt>
<dd>It <em class="rfc2119">must</em> be simple to maintain a library of
images and descriptions for dynamic assembly, or dis-assembly and
re-assembly, of content. </dd>
<dt>Discoverability </dt>
<dd class="rfc2119">It <em class="rfc2119">must</em> be simple for a
user agent to automatically discover a description provided for a
given image.</dd>
<dd>A user <em class="rfc2119">should</em> be able to determine that
there is a description available for a given image. </dd>
<dt>Optional consumption</dt>
<dd>A user <em class="rfc2119">must</em> be able to choose not to read
the long description of a given image.</dd>
<dt>Simple return</dt>
<dd>A user <em class="rfc2119">must</em> be able to return from the
description to the image. </dd>
<dt>Backwards Compatibility </dt>
<dd>It <em class="rfc2119">should</em> be possible to use existing
tools and techniques to associate an image with its description. </dd>
</dl>
</section>
<section id="longdesc">
<h2><span class="secno"></span>3. The <code>longdesc</code> attribute</h2>
<h3>3.0 Implementation</h3>
<h4>3.0.1 Authors and Conformance checkers</h4>
<section id="parsing">
<p class="rfc2119">Zero or one <code>longdesc</code> attributes <em class="rfc2119">may</em>
be added to any <code>img</code> element.</p>
<p>The <code>longdesc</code> attribute <em class="rfc2119">must</em>
contain a <a href="http://dev.w3.org/html5/spec/urls.html#valid-url-potentially-surrounded-by-spaces">valid
non-empty URL potentially surrounded by spaces</a>. The URL is a <a
href="http://dev.w3.org/html5/spec/links.html#hyperlink">hyperlink</a>
to a description of the image that its parent <code>img</code>
element represents. </p>
<p>The linked description <em class="rfc2119">should</em> be a broadly
accessible format. (For example, a link to another version of the
image is almost certainly not a useful description, and plain text is
an extremely limiting format).</p>
<div class="note">
<p><em>This section is informative</em></p>
<p>Best practices for checking descriptions of images are beyond the
scope of this document, but there are many resources available.
For further guidance tool developers can consult e.g. [<a href="#bib-ATAGB3">ATAGB3</a>].