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.