There's a lot of effort in here and I appreciate it. You have a very good programming result here.
I like the "insert coin" idea, but not with the 'i' key... maybe a button where to click on, just like the 'green flag'.
The increasing-difficulty-over-time was very good. Also the background and the mouse-moving-response were good. Oh, and I didn't see the full-screen option much, nice one there.
Yet: there's no scoreboard, only a timer and a 300-bullet counter.
Also, there's this red unidentified button that basically makes you lose if you click on it in mid-game, and there's no pause button. You should try and improve those things.
About the shooting: I couldn't do it fluidly. Like, after a few bullets (between 10 and 15) it would take some time before starting shooting again; after I would run out of the 300 bullets, it would take the same amount of time to "reload" and reset the counter. Maybe it's only in my computer, maybe not. Fix that, it allowed me to simply hold the click and wait for the bullets to come out, making the 300 counter completely useless, and annoyed me.
And... some music - sounds, maybe?
Last thing: this is not something new at all. Not yet, at least. Still, congratulations.