How fast are computers nowadays? Fast enough to run 12 year old games through your browser with no plugins whatsoever. Sure, an old Pentium II might run the game faster natively, but where’s the fun in that?
We are as yet unable to provide a public demo link. The Quake II code is GPL licensed, but the demo resources (textures, models, sounds, et al) are not, so we cannot simply upload them to a server. We are pursuing legitimate avenues to do so, though — stay tuned.