HTML 4, XHTML 1.0 and XHTML 1.1 Documents that include images that can not be described in a short text alternative.
User Agent and Assistive Technology Support Notes
Some older assistive technologies do not support the longdesc attribute. Supported in leading Assistive technologies now.
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.
Resources are for information purposes only, no endorsement implied.
Check that a longdesc attribute exists.
Check that the link in the longdesc attribute is valid
Check that the long description describes the original non-text content associated with it.
#1 through #3 are all true