[rsperberg]

Your point about external links is well taken.

Still I could imagine some texts intended for a school research project that might have a short-enough shelf-life that they would skirt this issue.

Or I could envision linking to a Wikipedia article or some other long-lived piece, like a newspaper article. But, as you say, the risk of demise is large enough that you'd have to be really selective about those.

And, sorry, I did miss the x reference in xhtml. I didn't realize (never looked closely at it, either) that the XHTML 1.1 recommendation allowed namespaces