XHTML - Extensible Hypertext Markup Language
XHTML is a markup language written in XML; therefore, it is an XML application.
XHTML uses three XML namespaces (used to qualify element and attributes names by associating them with namespaces identified by URI references. Namespaces prevent identically custom-named tags that may be used in different XML documents from being read the same way), which correspond to three HTML 4.0 DTDs: Strict, Transitional, and Frameset.
XHTML markup must conform to the markup standards defined in a HTML DTD.
When applied to Net devices, XHTML must go through a modularization process. This enables XHTML pages to be read by many different platforms.
A device designer, using standard building blocks, will specify which elements are supported. Content creators will then target these building blocks--or modules.
Because these modules conform to certain standards, XHTML's extensibility ensures that layout and presentation stay true-to-form over any platform.