what exactly do you mean by 'browser based'?
A wallet or miner?
both are possible but not practical...
you can even mine most (if not all) of the coins out there with some hard work to port the miner to javascript.
if the coin is so easy to mine that a browser can do it with ease, then the coin is worthless to mine with a browser...
kind of... a paradox situation here.
in short, it is pointless to create a 'web' based coin.
It would be more of an experiment with the Bitcoin protocol than an actually coin. The code could be cannibalized for something useful. We have already seen something like BitMessage come out of the Bitcoin idea. Furthermore it could be used as a microcurrency, something that maybe a small group of people could use as a simple exchange system.