DocBook XML DTD

The purpose of this committee [OASIS DocBook Technical Committee] is to develop and maintain the DocBook family of specifications. In particular, this TC maintains the XML and SGML DocBook DTDs, a suite of extension modules, and a simplified authoring subset of DocBook. The TC may also support other schema languages in the future and may develop additional modules and derived document types. The DocBook TC will also develop specifications based on previously published standards for use in the XML version of DocBook and other related XML specifications; the first such specification will be based on previously published ISO 9573 entity sets (themselves an extension of the ISO 8879 entity sets). [Revised OASIS DocBook TC Charter, 2001-08-23]

http://xml.coverpages.org/docBook.html