Okay, just tested in Chrome and I got the same issue. The worst part of it is whenever I press the space bar to shoot, the screen scrolls down and obscures the picture completely.
Having just done more testing, it seems that if you click the option to play the game in a new window it completely fixes the problem, so providing that's mentioned somewhere in description text you shouldn't have anything to worry about (phew!). Perhaps you've already mentioned it somewhere and I simply missed it?
Anyway, best of luck with everything - I'll test the game more later.