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Among the checks that DTDs can't perform, veryfying the syntax of URIs for attributes with value type URI would be welcome. This is a subset of Bug 69.
A brick-by-brick explanation of how this could be done is at: http://lists.w3.org/Archives/Public/www-validator/2007Aug/0057.html 1) find of write a perl parser/checker for a URI string 2) list all the elements/attribute pairs for URI type 3) include in the validator
I propose to update the severity of this bug to "critical", see <http://idn.icann.org/Talk:IDNwiki#Invalid_XHTML_1.0>.