Mainly:

I fixed a bug in level 4 that told the player to do something they've already done

I also made the first boss overall much better than before. It now is not as unpredictable, and less boring/tedious.

Also:

I retextured a couple things in the main (train) cart to hopefully improve readability and let the player better understand where to go.

I adjusted the difficulty in a secret level so that the player can replenish health more easily

I also became aware of a potential issue where the save data may not be preserving in the browser version all the time (and in general, the browser version is less reliable when it comes to save data) so to be safe, I'd recommend downloading the game.