Fuckit, if anything more fancy than HTML + CGI is needed, there's always VRML.

....holy shit, it just dawned on me.. VRML + CGI? Today's PCs should have no issue rendering it, and server-side should be the same as any other CGI page. I gotta do some experimenting with this.

UPDATE: I actually did some research into this, and the possibilities are actually quite extensive. In theory it should be possible to make an extremely basic multiplayer FPS game.