Extensible Hypertext Markup Language (XHTML) is a reformulation of HTML 4.01 in XML.

Extensible Markup Language (XML) is a simple and flexible text format derived from Standard Generalized Markup Language (SGML). It offers a basic syntax that facilitates the exchange of a wide variety of data between different kinds of computers, different applications, and different organizations without needing to pass through many layers of conversion.

Cross-referencing is the act of creating connections between different related objects in memory that are used to construct a wide variety of data structures.