XHTML Modularization is a decomposition of XHTML 1.0, and by reference HTML 4, into a collection of abstract modules that provide specific types of functionality. These abstract modules are
implemented in this specification using the XML Schema and XML Document Type Definition languages. The rules for defining the abstract modules, and for implementing them using XML Schemas and XML
DTDs, are also defined in this document.

These modules may be combined with each other and with other modules to create XHTML subset and extension document types that qualify as members of the XHTML-family of document types.

The modularization of XHTML refers to the task of specifying well-defined sets of XHTML elements that can be combined and extended by document authors, document type architects, other XML
standards specifications, and application and product designers to make it economically feasible for content developers to deliver content on a greater number and diversity of platforms.

Over the last couple of years, many specialized markets have begun looking to HTML as a content language. There is a great movement toward using HTML across increasingly diverse computing
platforms. Currently there is activity to move HTML onto mobile devices (hand held computers, portable phones, etc.), television devices (digital televisions, TV-based Web browsers, etc.), and
appliances (fixed function devices). Each of these devices has different requirements and constraints.

Modularizing XHTML provides a means for product designers to specify which elements are supported by a device using standard building blocks and standard methods for specifying which building
blocks are used. These modules serve as "points of conformance" for the content community. The content community can now target the installed base that supports a certain collection of modules,
rather than worry about the installed base that supports this or that permutation of XHTML elements. The use of standards is critical for modularized XHTML to be successful on a large scale. It is
not economically feasible for content developers to tailor content to each and every permutation of XHTML elements. By specifying a standard, either software processes can autonomously tailor content
to a device, or the device can automatically load the software required to process a module.

Modularization also allows for the extension of XHTML's layout and presentation capabilities, using the extensibility of XML, without breaking the XHTML standard. This development path provides a
stable, useful, and implementable framework for content developers and publishers to manage the rapid pace of technological change on the Web.