Really cool concept and fun to play. I agree with some other people that tweaking the scaling might be a good idea in the future since taking damage is inevitable at the higher levels.
In my browser it seemed to be a problem of the browser deselecting the game itself so it wouldn't take input. I got past this by fullscreening the game immediately after clicking "Run Game". After that the game is started by pressing "Z" (which is Button 1 according to the instructions)
