It's a rareity when Dave posts on his blog. He spends more time breaking and fixing it, and tinkering with the design, adding new plugins, but when he does post he does it well. He's written this time (month/year/decade) about the longdesc attribute used in the Image tag. Now I for one just thought it was an extension of the alt attribute and should be used in conjunction with alt but to explain in more detail about the image. As it happens I couldn't be further from the truth! So if you're interested in good XHTML, Semantics and Accessibility I'd highly recommend you at least read the post on it, if not bookmark the site (and return in maybe a month for another post!).

Direct Link: longdesc, it's not just alternate text.