See Understanding Techniques for WCAG Success Criteria for important information about the usage of these informative techniques and how they relate to the normative WCAG 2.0 success criteria. The Applicability section explains the scope of the technique, and the presence of techniques for a specific technology does not imply that the technology can be used in all situations to create content that meets WCAG 2.0.
HTML and XHTML documents that include images that cannot be described in a short text alternative.
This technique relates to:
The objective of this technique is to provide information in a file designated by the
longdesc attribute when a short text alternative does not adequately convey the function
or information provided in the image. The longdesc attribute is a URI, the target of
which contains a long description of the non-text content.
Authors can provide a description for an image by including text in a separate resource or within the text of the page containing the image. An advantage of using a separate resource for the description is that it is easily reusable for multiple instances of the same image, it does not add on-page visual clutter to the original document, and the description's end-point is apparent to the user. An advantage of providing the description within the same page as the image is that all users can access the description. A limitation of the on-page method, as well as in providing multiple descriptions on a single separate page, is that current implementations supporting longdesc do not identify the long description's end-point. Authors can solve this by providing a well-formed description, which identifies the where the description ends.
Example Code:
<p><img src="chart.gif" alt="a complex chart" longdesc="chartdesc.html"/></p>Example Code:
<img longdesc="thispage.html#desc" alt="Line graph of the number of subscribers" src="http://www.company/images/graph.png">
<div id="desc">
<h3>Long Description: Line graph of the number of subscribers</h3>
<!-- Full Description of Graph -->
<p>Long description ends.</p>
<div>Resources are for information purposes only, no endorsement implied.
Check that the img element has a longdesc attribute.
Check that the value of the longdesc attribute is a valid URI of an existing resource.
Check that the content at the target of that URI contains a long description describing the original non-text content associated with it.
#1 through #3 are all true
If this is a sufficient technique for a success criterion, failing this test procedure does not necessarily mean that the success criterion has not been satisfied in some other way, only that this technique has not been successfully implemented and can not be used to claim conformance.