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From the old checklink TODO list: Currently, the URIs are extracted from a defined set of elements and attributes, whereas the right thing to do would be to parse the DTD and get the list elements and attributes to extract from from the DTD.
This isn't really doable. The link checker checks links, not URIs. DTDs do not contain enough information to decide whether something is a link or not. Nor do XML Schemas; they do have a anyURI datatype, but that's just that, an URI. Being an URI does not necessarily imply a "link".