The Essence of XML

The World-Wide Web Consortium (W3C) promotes XML and related standards, including XML Schema, XQuery, and XPath. This paper describes a formalization of XML Schema. A formal semantics based on these ideas is part of the official XQuery and XPath specification, one of the first uses of formal methods by a standards body. XML Schema features both named and structural types, with structure based on tree grammars. While structural types and matching have been studied in other work (notably XDuce, Relax NG, and a previous formalization of XML Schema), this is the first work to study the relation between named types and structural types, and the relation between matching and validation.