Overall, I like the game and its simplicity. But you should add some explanatory text.
- add health bars for damaged enemies
- I felt like sometimes, the bullets didn’t harm the enemies, because it took WAY too long to kill a green one; I’m not sure if this is a bug, but I didn’t find any other bugs
- you should add some retro sound effects to improves the game’s feel
- at first, I didn’t know killing enemies would give me buffs, so maybe add a main menu page and a tutorial page where you explain this (and other things)
Good job!