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Strictly speaking, one can use DTDs (to declare entities) in
conjunction with XSD to define everything else. After all, DTD
operates on XML syntax and XSD is defined on the Infoset. I suspect
that there are gotchas that make this difficult ... does anyone know
what they might be?
The main one is that some parsers use the presence of a DOCTYPE to
switch between validating and non validating mode.
So if you put all your real document constraints in a schema, and just
use a DTD for entity definitions you end up with (dtd) validation errors
on every element as they are all undefined. Most likely the parser has
some configuration option to force non-validation mode for the dtd while
still allowing schema validation but it's not always clear how to access
that from every context that might be using the parser.
The alternative is to make a "full" dtd but that ends up duplicating the
effort in building the schema, unless you can automatically derive one
from the other. But in the case that the dtd catches all constraints
expressed by the schema, there is no point in doing the schema
validation as the dtd validation occurs first.
David
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