Althrough, I think you have some problems with your rockets/laser gun mechanic.
I didn't read the rules. Because players won't read them, it needs to be intuitive.
And it wasn't really, for some reasons.
Your rockets are fine, but the "remaining rocket" thing is not clear enough, maybe it needs to be bigger, I dunno..
Also, to make that more clear, you could do something like a shop at the end of lvls, to make the player able to buy rockets with his score, maybe? So he would know that he has a limited amount of rockets.
And with the laser gun I was feeling like "hey it's not doing anything when I hold the click". So I think it would be better if it was a clearly visible Beam, insteat of these little ray that are almost colorless, cause here they really are barely visible.
Also, I think you need more visual feedbacks for the UFO and actually for everything that is not instant killed by the laser gun. It feels like I'm not reaching it and it's frustrating! like, a little explosion if I touch them would be nice!
Anyway, your game is quite fun already (well, it's kinda classic, but it's well made. Maybe you could add some more specifics mechanics? Like the shop I mentionned earlier? It asks a bit of work but it makes the game more meaningful, more interresting to play I think, the player feels more involved..), it feels pretty good
Have a nice day