The Reference Build source code is written in DocBook XML. This is then transformed into HTML by way of the DocBook XSL stylesheets. Additionally, a separate XSLT stylesheet is used to extract a set of build commands for each package. These "scriptlets" are immensely useful when directly integrated into build scripts. See the gsbuild scripts for an example.
The Docbook XML sources are kept in CVS. You can check out the read-only CVS sources anonymously:
cvs -z9 -d:pserver:email@example.com:/cvs/diy co refbuild
All development discussion takes place on the development mailing list.