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)