The Extensible HyperText Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax.
A file is considered to be a XHTML document when the root element name is a html.
The schema used for these documents is located in ${frameworks}/xhtml/dtd/xhtml1-strict.dtd, where ${frameworks} is a subdirectory of the Oxygen XML Developer install directory.