This section is normative.
XHTML is more than just a recasting of HTML into XML. It is also an extensible architecture that permits the ready definition of new document types. The W3C envisions that client manufacturers, document authors, and content providers may all use this architecture to define document types that are specific to their needs. The XHTML Modularization specification defines a collection of modules and a framework that make the definition of these new document types relatively easy.
That architecture by itself may not be sufficient for the needs of all document type creators. In particular, people who are defining new functionality or combining new functionality with existing elements need a way to define that functionality. The XHTML method for doing this is through the definition of an XHTML module.
XHTML modules define elements and their attributes, add attributes to elements defined in other modules, add values to the set of values available to an attribute defined in other modules, define content models, or some combination of these things. The expression of a module is done through the creation of a prose functional description of the module, an abstract definition of the module's contents, and then one or more implementations of the module. The remainder of this document defines the way in which these steps should be conducted.
An XHTML document type is defined as a set of modules. Each XHTML module has an abstract definition that generally indicates the facilities made available through the module and way those facilities are minimally integrated with each other and with an (eventual) document type.
An XML DTD module consists of a set of element types, a set of attribute list declarations, and a set of content model declarations, where any of these three sets may be empty. An attribute list declaration in an XML DTD module may modify an element type outside the element types in the module, and a content model declaration may modify an element type outside the element type set.