Openness

Ultimate openness The any element can be used to throw the gates wide open to any well-formed XML, whether or not its content has been declared in the schema.
Exclusionary openness The namespace attribute can be used to permit only elements without namespaces, only elements outside of the current namespace, or only elements of a particular namespace.
A more comprehensive solution for extensibility While previous models (like DTDs) have required all elements and attributes to be declared, perhaps in the internal subset, this mechanism lets developers create spaces in their documents for a much wider range of content.
Risky The ANY content model has been regarded with suspicion in XML 1.0 for a long time now; this even more open version deserves careful consideration before use.