Pretty fun game. Minimalist art and sound mix nicely, music is pleasing and the levels can get challenging. Boss has nice patterns that can get harder with his movement. Personally, I think that the player damage is a bit too low (or enemies have higher health than expected) and in the first levels the enemies are stationary with easy patterns, so you don't move much and just spam them. But fourth and fifth levels are great!
I had the same problem as JokubasG, movement inputs stopped working when I clicked accidentally outside of the game. Focusing on it again didn't fix it and I had to reload the page.