Nice game, I enjoyed it!
Is there a reason you allow access to Floor 3 - Normal in the demo but it is completely empty? Doesn't seem to make much sense. Also why have challenges for this floor in the Demo if you can't complete any of them?
Also, at one point in the game, it just went to a completely black screen (Windows 11, Firefox 150.0.2) with just the music playing. Controls didn't seem to do anything. Had to reload the site and then load from my savegame to continue.
Edit:
Was just playing Floor 2-3 and the monsters were on top of me and the game froze. Only the music continued playhing. The mouse pointer was not visible while over the game screen and I could not click on anything. Had to reload the page and restore by savegame.
Edit 2:
I converted essence to coins in the Currency Exchange. It showed that my total coins went from 50 to 75 at the top, however when I went to the Shop, it still showed that I only had 50 coins, although it let me buy things that cost more than 50 coins and after I bought something the total was updated to reflect the correct value.