This section is normative.
XHTML is the reformulation of HTML 4.0 as an application of XML. XHTML 1.0 [XHTML1] specifies three XML document types that correspond to the three HTML 4.0 DTDs: Strict, Transitional, and Frameset. XHTML 1.0 is the basis for a family of document types that subset and extend HTML.
XHTML Modularization is a decomposition of XHTML 1.0, and by reference HTML 4.0, into a collection of abstract modules that provide specific types of functionality. These abstract modules are implemented in this specification using the XML Document Type Definition language, but an implementation using XML Schemas is expected. The mechanism for defining the abstract modules defined in this document, and for implementing them using XML DTDs, is defined in the document "Building XHTML Modules" [BUILDING].
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 permutation of XHTML elements 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.
An XHTML document type is defined as a set of abstract modules. A abstract module defines one kind of data that is semantically different from all others. Abstract modules can be combined into document types without a deep understanding of the underlying schema that define the modules.
A 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 a 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.
An XML DTD is a means of describing the structure of a class of XML documents, collectively known as an XML document type. XML document types are currently represented as DTDs, as described in the XML 1.0 Recommendation [XML]. Where possible, this document also allows for the potential use of other schema languages that are currently under consideration by the W3C XML Schema Working Group. (e.g. DCD, SOX, DDML, XSchema)
A hybrid document type is an XML DTD composed from a collection of XML DTDs or DTD Modules. The primary purpose of the modularization framework described in this document is to allow a DTD author to combine elements from multiple abstract modules into a hybrid document type, develop documents against that hybrid document type, and to validate that document against the associated hybrid document type definition.
One of the most valuable benefits of XML over SGML is that XML reduces the barrier to entry for standardization of element sets that allow communities to exchange data in an interoperable format. However, the relatively static nature of HTML as the content language for the Web has meant that any one of these communities have previously held out little hope that their XML document types would be able to see widespread adoption as part of Web standards. The modularization framework allows for the dynamic incorporation of these diverse document types within the XHTML family of document types, further reducing the barriers to the incorporation of these domain-specific vocabularies in XHTML documents.
The use of well-formed, but not valid, documents is an important benefit of XML. In the process of developing a document type, however, the additional leverage provided by a validating parser for error checking is important. The same statement applies to XHTML document types with elements from multiple abstract modules.
The general problem of fragment validation - validation of XML documents with different schemas from multiple XML Namespaces [XMLNAMES] in different portions of the document - is beyond the scope of this framework. An essential feature of this framework, however, is a collection of conventions for creating, from a set of abstract modules, hybrid DTDs.