XML
* Extensible Markup Language
* Subset of Structured Generalized Markup Language (ISO8879, 1986)
* A document is well-formed if it satisfies XML grammar, as do all entities 
  included by reference.
* XML is only a syntax: the mark-up is not pre-specified.
* http://www.w3.org/TR/REC-xml.html
* http://www.w3.org/XML/1999/XML-in-10-points

Document Type Definition
* Constraints on document structure encoded in a DTD.
* DTD declared in the prolog to an XML doc.
* A document is valid if it satisfies its DTD.

DOM
* Document Object Model
* Provides OO API for XML documents.
* Formally defines the hierarchical structure.
* Unrelated to COM.

XML Schema Language 
* http://www.w3.org/TR/xmlschema-0/
* More expressive than XML DTDs;
* Expressed in XML;
* Self-describing;
* Usable by a wide variety of applications;
* straightforwardly usable on the Internet;
* optimized for interoperability;
* simple to implement;

Datatype Requirements
* Provide for primitive data typing, including byte, date, integer, sequence, 
  SQL & Java primitive data types;
* Define a type system that is adequate for import/export from database
  systems; 
* Allow creation of user-defined datatypes.

Structural Requirements
* Constrain document
  - structure (namespaces, elements, attributes) and
  - content (datatypes, entities, notations);
* Enable inheritance for element, attribute, and datatype definitions;
* URI reference to standard semantic understanding of a construct;