[no subject]

 "Relative URI references are distinguished from absolute URI in that
 they do not begin with a scheme name.  Instead, the scheme is
 inherited from the base URI,"

So your thing should be uri="./art500/art500.dtd".

Another marginally related question for the experts: If a relative URI is
specified in the catalog, what should it be relative to? To the location of
the catalog, or to the current directory of the current process? When I run
xmllint --valid on something, it uses the directory where it started from as
a base URI for every relative URI it encounters in the catalog. Is that
okay?

Ciao,
Igor