According to an HTML Working Group Decision, the longdesc attribute will not be included in the HTML5 specification. Not everybody is happy with that decision, so whether longdesc is still out when HTML5 is finished remains to be seen.
In case you aren’t familiar with the longdesc attribute, here is how it is described in the HTML 4.01 specification:
This attribute specifies a link to a long description of the image. This description should supplement the short description provided using the alt attribute.
This provides a way of describing images to non-sighted users in detail, with more text than what is suitable for the alt attribute (which is really more of a text alternative to an image than a description of it). The longdesc attribute explicitly associates this description with the image.
Posted in Accessibility, HTML 5.