Yesterday, I checked in a JavaScript IRC library, along with a sample
client (in HTML) and 'bot (in JavaScript) to mozilla/extensions/irc.
The client's output window, including individual messages, is fully
CSS stylable based on Channel, User, and Message Type, (as well as a
few more obscure properties) in any combination.  Screenshots of the
client in action are at http://www.ndcico.com/jsirc/screenshots .

The code hasn't hit lxr (lxr.mozilla.org) yet, but a bonsai query (
http://cvs-mirror.mozilla.org/webtools/bonsai/cvsqueryform.cgi?cvsroo...
) for rginda%ndcico.com, showing checkins in the last week, should
turn up a complete log.

To build and run the client on Unix...

cd mozilla/extensions
cvs up -d
make makefiles DIR=irc   (ignore the errors)
cd irc
make
cd ../../dist/bin
../mozilla-apprunner.sh resource:///irc/tests/test2.html

Makefiles for Windows are in the works.  Mac Makefiles still need a
volunteer.

Some of the ideas discussed so far include...

HTMLized chat message support,
  Includes the ability to send messages marked up in HTML over some
  out-of-band communication channel, such as a parallel channel
  (#mozilla and #mozilla-html) or a parallel IRC server.

Per-Channel "Topic" style support,
  Includes the ability to push CSS styles to clients, in order to
  affect the appearance of the channel.  Possibly through a bot on
  the channel, or through the topic, This could include a default
  background image,  text styles (even per-user styles) and possibly
  non-CSS data such as a list of URLs.  User's could presumably
  alter the "topic" style similar to the way the existing channel
  topic is changed.

Please feel free to contribute ideas (hopefully in a new thread) and
code (send diffs!) to the project.  I will be updating the Mozilla
realtime-messaging ( http://www.mozilla.org/projects/chat/ ) webpage
shortly.