Thanks for playing it! I'm glad you liked it.
I have an artist I commission for all the art—I've only done part of the UI, like buttons and a few small things here and there, but most of it was done by a hired artist.
The negative HP bar is actually intentional so you can see the overkill damage after you die, though I guess it can give the impression that something is bugged (I'll consider clamping it to 0 later).
The input delay between your attacks is also intentional to prevent spacebar spamming. You actually did exactly what I wanted players to do: timing your attacks and avoiding attacking at the same time as the enemy. The main objective of the game is to focus on parrying. Parries deal 100% of your ATK, whereas free attacks using the Space Bar only deal 25%.