Must “feel like” DocBook
Most existing documents should still be valid or it should be possible to transform them in simple, mechanical ways into valid documents
Must enforce as many constraints as possible in the schema. Some additional constraints are expressed with Schematron rules
Must simplify many content models
Must give users the flexibility to extend or subset the schema in an easy and straightforward way
Must allow the generation of a (non-normative) XML DTD and W3C XML Schema.