XHTML modules provide means for subsetting and extending XHTML. XHTML modules are used for creating new flavors of XHTML, such as XHTML Basic for mobile devices.

Core modules are

 * Structure (html, head, body, title...)
 * Text (h1, h2, h3... p, pre...)
 * Hypertext (a)
 * List (ul, li...)
Other modules include applet, image, forms and basic forms.

External Links

Modularization of XHTML - W3 Consortium. In addition to interesting view on modules, this document contains all of the actual tag definitions.