Principles of XML Schemas
  - define shared markup vocabularies and allow machines to carry out rules
    made by people
- provide a means for defining the structure, content and semantics of
    XML documents
- provide better integration with XML Namespaces -> inheritance (reuse
    with overriding)
- identify parts of a document where a schema may apply or which parts of
    a document may be validated
- XML Schemas are XML documents themselves, they may be managed by XML
    authoring tools
- W3C Candidate Recommendation 24 October 2000