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.

XHTML 1.0
W3C Proposed Recommendation, August 1999. This specification defines XHTML 1.0, a reformulation of HTML 4.0 as an XML 1.0 application, and three DTDs corresponding to the ones defined by HTML 4.0.

XHTML Extended Forms Requirements
W3C Working Draft, September 6, 1999 describing a comprehensive set of requirements for W3C's work on XHTML forms.

internet.com's Web Development Channel
Contains links to internet.com sites that provide in-depth information, tutorials, and resources for Web development.

Modularization of XHTML
W3C Working Draft, April 10, 2001 specifying a modularization of XHTML.

XHTML Web Design for Beginners
This article teaches the user how to produce Web pages in XHTML. Prior knowledge of HTML is helpful.