What is the current recommendation for providing long descriptions for complex graphics?

In HTML (Hypertext Markup Language), the longdesc attribute of the image element was specifically designed to allow for long descriptions of complex images such as charts and graphs. This contrasts with the alt attribute, which describes relatively simple images using a minimal amount of text. The longdesc attribute allows authors to link to a separate page (for example, longdesc="img1description.php") in which the image is described in detail.

<img src="sales_march.gif" alt="march sales graph" longdesc="march_sales.php" />

The longdesc attribute was for a long time unsupported by assistive technologies, so work-around techniques (such as linking to the separate long description page using just the letter D as link text) were used in its place. However, recent versions of most screen reader applications now support the longdesc attribute. Screen readers typically announce the presence of a long description when available, and provide users with the option of reading it by executing a specified keystroke.

Since longdesc is now supported by screen readers, it is the recommended technique for providing long descriptions of images. However, in using "longdesc" be sensitive to the fact that there might still be some users with technologies that do not support it, and consider summarizing the most critical concepts from the image within the text content of the document. This practice helps all readers since it reinforces the author's ideas through multiple modes of presentation.

A useful description of the appropriate use of the long description attribute can be found at Jim Thatcher's Web site in the Text Equivalents page.