HTML 5 will support some kind of raw-ish TCP connection (accessible from JavaScript). I think most games able to run performantly in the browser will be just fine without UDP.
yes, most games would be, we've got a realtime fps running (in flash) over standard tcp at the moment. But udp would let us scale from 2 players to 16 per game room in a much smoother way.
The one issue we have with lag is an occassional dropped packed that holds everything up.
Not to mention flash 10 will allow direct peer to peer connections, bypassing several hops...
