Tim Bray: Two messages from James Clark that ought to be read. In a nutshell, James Clark argues that there shouldn't be a link from a document to a schema. He gives two reasons: one, that there might be multiple schema languages, and two, that there might be multiple schemas that a given instance may conform to.
I find myself agreeing with the argument but uncomfortable with the conclusion. By a similar argument, there shouldn't be a link from a weblog to its RSS feed. As long as one treats such information as an informative hint and not as a normative reference, I think it can be quite useful.
In general, the more links that there are in the world, the better off we all are.