I think this ranks as one of the best games in the jam! Hopefully you do flesh this out into a full game, because this by itself is worth playing.
You really took advantage of the small screen space, instead of being limited by it. The fact that the enemy graphics say everything you need to know about how to kill them, and that the background is richly detailed, really shows a lot of finesse. It's a shame about the subpixels, but honestly the smooth animations do the game more of a service.
The fonts impressed me; my own game used a lot of text and I ended up with a 3 x 5 font that works well enough, but I think yours is really economical and yet readable.
I feel I should mention that on Windows 10 in Firefox, I didn't hear audio and had some control issues running the browser version of the game. The download works perfectly!