Tools used here:

0) Subversion.

1) The rfc2629tools suit and reference library.

2) Python and the Python lxml library.

3) Trang.

See rsync://subvert-ietf.hactrn.net/rfc2629tools/INSTALLING for
details on how to install the rfc2629tools package.

xml2rfc already uses Python and lxml, so you probably have them.

Trang is available as a port (textproc/trang) on FreeBSD, and as a
Homebrew package (trang) on MacOSX.  On Debian and Ubuntu Linux it's
the libtrang-java package; if you're on some other platform, you may
have to hunt a bit, but it's probably available.