The lack of a standard formalism for expressing the semantics of an XML vocabulary is a major obstacle to the development of high-function interoperable digital libraries. XML document type de.nitions (DTDs) provide a mechanism for specifying the syntax of an XML vocabulary, but there is no comparable mechanism for specifying the semantics of that vocabulary — where semantics simply means the basic facts and relationships represented by the occurrence of XML constructs. A substantial loss of functionality and interoperability in digital libraries results from not having a common machine-readable formalism for expressing these relationships for the XML vocabularies currently being used to encode content. Recently a number of projects and standards have begun taking up related topics. We describe the problem and our own project.