From http://www.w3schools.com (Copyright Refsnes Data)

XHTML Modularization

The XHTML modularization-model defines the modules of XHTML.

Why XHTML Modularization?

XHTML contains most of the functionality a web developer will need.

For some purposes XHTML is too large and complex, and for other purposes it's too simple.

By splitting XHTML into modules, the W3C (World Wide web Consortium) has created small and well-defined sets of XHTML elements that can be used separately for small devices, or combined with other XML standards in more complex applications.

With modular XHTML, designers can:

Choose the elements to be supported by a device
Simplify XHTML for small devices
Extend XHTML for complex applications by adding new XML functionality (like MathML, SVG, Voice and Multimedia)
Define XHTML profiles like XHTML Basic (a subset of XHTML for mobile devices)

XHTML Modules

W3C has split the definition of XHTML into 28 modules:

Module name	Description
Applet Module	Defines the deprecated* applet element
Base Module	Defines the base element
Basic Forms Module	Defines the basic forms elements
Basic Tables Module	Defines the basic table elements
Bi-directional Text Module	Defines the bdo element
Client Image Map Module	Defines browser side image map elements
Edit Module	Defines the editing elements del and ins
Forms Module	Defines all elements used in forms
Frames Module	Defines the frameset elements
Hypertext Module	Defines the a element
Iframe Module	Defines the iframe element
Image Module	Defines the img element
Intrinsic Events Module	Defines event attributes like onblur and onchange
Legacy Module	Defines deprecated* elements and attributes
Link Module	Defines the link element
List Module	Defines the list elements ol, li, ul, dd, dt, and dl
Metainformation Module	Defines the meta element
Name Identification Module	Defines the deprecated* name attribute
Object Module	Defines the object and param elements
Presentation Module	Defines presentation elements like b and i
Scripting Module	Defines the script and noscript elements
Server Image Map Module	Defines server side image map elements
Structure Module	Defines the elements html, head, title and body
Style Attribute Module	Defines the style attribute
Style Sheet Module	Defines the style element
Tables Module	Defines the elements used in tables
Target Module	Defines the target attribute
Text Module	Defines text container elements like p and h1

* Deprecated elements should not be used in XHTML.