Game viewport in browser is a bit too big for a browser window, but luckily you can go fullscreen, so it's ok there
And also, spaming LMB might be a bit hurting after some time(I'll consider shooting when you hold instead of press, because there are more and more of them, so practically you're not stopping)
You could add maybe some kind of combos, when you shoot enemies one by one and the beat changes a bit depending on how many you killed quickly.
Or just bonus for hitting in certain times of the beat. But sure, to implement more time is needed than that. But anyway, as a 3 hours entry, is cool :)