DocBook is a general purpose XML schema particularly well suited to books and
papers about computer hardware and software (though it is by no means limited
to these applications).

The Version 5.0 release is a complete rewrite of DocBook in RELAX NG.
The intent of this rewrite is to produce a schema that is true to the spirit
of DocBook while simultaneously removing inconsistencies that have arisen as
a natural consequence of DocBook's long, slow evolution. The Technical
Committee has taken this opportunity to simplify a number of content models
and tighten constraints where RELAX NG makes that possible.

The Technical Committee provides the DocBook 5.0 schema in other schema
languages, including W3C XML Schema and an XML DTD, but the RELAX NG Schema
is now the normative schema.

WWW: http://www.docbook.org/specs/docbook-5.0-spec-cd-04.html