XHTML is a simple, but large language. XHTML contains most of the functionality a web developer will need. 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 simple devices as well as combined with other XML standards into larger and more complex applications.
With modular XHTML, product and application designers can:

• Choose the elements to be supported by a device using standard XHTML building blocks.
• Add extensions to XHTML, using XML, without breaking the XHTML standard.
• Simplify XHTML for devices like hand held computers, mobile phones, TV, and home appliances.
• 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).
  

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.

• XHTML Syntax Rules • XHTML Modularization
• XHTML Standard Attributes