Great graphics, very classic retro looking pixel art. And good music too.
This is one game that would be really cool if it becomes a longer game, but if not, it's actually completed enough as it to feel like a full game with three bosses and all!
One thing I was thinking during the platforming section is that maybe it would need a way to recover health. Without any way to get health back, I mostly dash through the enemies without fighting because it's safer that way. Anyway, the sections are short enough to that be a real problem.
About the boss battles, I can actually "trick" the first and third bosses making them attack, then dashing and attacking them from behind. Still not completely trivial, specially with the third boss, you have to have a very good timing, but they won't try any other movement of their movement set doing that.