RFC 2119: Key words for use in RFCs to Indicate Requirement Levels March 1997
Extensible Markup Language (XML) 1.0 (Fourth Edition) 16 August 2006
XML Information Set (Second Edition) 4 February 2004
Namespaces in XML (Second Edition) 16 August 2006
Resource Description Framework (RDF): Concepts and Abstract Syntax 10 February 2004
RDF/XML Syntax Specification (revised) 10 February 2004
Mathematical Markup Language (MathML) Version 2.0 (Second Edition) 21 October 2003
XML Linking Language (XLink) Version 1.0 27 June 2001