DocBook (SGML/XML)
http://www.allegrotechindexing.com/news014.htm

Brown, Fred
Allegro Time!
2001

Abstract:

The DocBook document type definition (DTD) was developed during the 1990s to provide an application independent method for creating computer documentation. Versions of the DocBook DTD have been created for both SGML and XML. You can create an embedded index in DocBook using index elements.