From the abstract:
  The XHTML Basic document type includes the minimal set of modules
  required to be an XHTML host language document type, and in addition
  it includes images, forms, basic tables, and object support. It
  is designed for Web clients that do not support the full set of
  XHTML features; for example, Web clients such as mobile phones,
  PDAs, pagers, and settop boxes. The document type is rich enough for
  content authoring.

  XHTML Basic is designed as a common base that may be extended. For
  example, an event module that is more generic than the traditional
  HTML 4 event system could be added or it could be extended by
  additional modules from XHTML Modularization such as the Scripting
  Module. The goal of XHTML Basic is to serve as a common language
  supported by various kinds of user agents.

The document type definition is implemented using XHTML
modules as defined in "Modularization of XHTML", found in
ports/textproc/xhtml-modularization.