first of all let me tell you that i'm a big lover of arena bossfight games so kudos on chosing that genre.
now about this particular game:
encountered this bug after the second boss: https://gyazo.com/cc082499fe8ca5ff9df2a22a3b922839
character felt floaty and attacks didn't had well defined feedback so they didn't felt good. start screen looked stretched in my browser idk if that's browsers fault or not (i use Opera GX). art is very good indeed. it's great that you didn't forget to include music for your first game. good job, had moderate fun with it.