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 and 5.1 schemas in other schema languages, including W3C XML Schema and an XML DTD, but the RELAX NG Schema is the normative schema.
DocBook V5.2 is under development. There have been no official releases.
DocBook V5.1 is OASIS Committee Specification and it has been submitted to become OASIS standard.
DocBook 5.0 is an OASIS standard.
The XML Schema version of DocBook V5.x is now maintained separately; it is no longer mechanically generated from the RELAX NG version.
This is an attempt to craft an XML Schema versions of DocBook V5.0 that is reasonably useful. It's impossible to accurately capture all of the grammatical constraints of DocBook in XML Schema, but these versions (should) accept all valid documents. They may also accept some invalid ones; always use the normative RELAX NG grammars to be sure.
These initial releases are experimental, they have no normative standing.