The problem with XSD and DTD is they only offer primitive ways to validate data, and it takes significant effort to validate some data (eg,[ https://stackoverflow.com/questions/3382944 ]). As a result, there have been a bunch of other XML schema validators created to counter these problems, but we should really ask why we need to keep inventing new languages when the existing ones turn out to be insufficient.
If we start out instead with something that's turing complete and simple to begin with (perhaps S-expressions?), we can (often trivially) write our own validators/type-checkers, or any other processing tool to verify the document structure, with few or no constraints, and without requiring the effort and expertise to parse complex syntax.
If we start out instead with something that's turing complete and simple to begin with (perhaps S-expressions?), we can (often trivially) write our own validators/type-checkers, or any other processing tool to verify the document structure, with few or no constraints, and without requiring the effort and expertise to parse complex syntax.