The longdesc attribute

The longdesc attribute, which can be added to an <img> tag, does nothing more than provide access to the separate page where a long description is available. The longdesc value must be the URL of the long description page. It should NOT contain the long description text itself. A longdesc works in the same way as the previous example, except that the link is typically not visible to sighted readers. The following example is the same graph with a longdesc attribute added:

While modern screen readers provide good support for longdesc, a problem arrises in using longdesc exclusively - browsers do not currently provide any visual indication that the image references a long description page. Many browsers do provide this information in the image details or context menu, but this is not readily apparent. The longdesc approach is a technique recommended in both the Web Content Accessibility Guidelines and the Section 508 guidelines. It would have much better utility if modern browsers and other assistive technologies provided a more apparent method of identifying and accessing long description pages. Because the long description page is useful to all users, if longdesc is used, it should be used in conjunction with a standard link to the long description page (method #2 above).

Here is the markup for the longdesc attribute: