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.