Yelp

Yelp is the help viewer in Gnome. It natively views Mallard, DocBook, man, info, and HTML documents. It can locate documents according to the freedesktop.org help system specification.

Yelp development has led to the development of various tools, and the Mallard and DocBook transformations live in standalone XSLT module. All of these are under the umbrella name Yelp.