Inside the Document: DOCTYPE and Namespaces

Unstable Identifiers

DOCTYPE This is supposed to be where XML documents announce their structure. Pluses: Widely supported, well understood. Minuses: Internal subset open to abuse, non-validating parsers can skip external references.
Namespaces Namespaces don't actually identify document types, but they do identify element types. Used on the root element, that could be about the same. Pluses: Unique identifiers allow mixing of document types. Minues: Identification is the only function supported by namespaces.
Incompatible Toolsets A significant minus of both of these options is that they are incompatible in a well-known group of cases.