I'm glad you liked my project!
Interesting, do you remember what did you do before it fell through the ground? also what was the fps of the game in average?
The falling on the ground issue only happens in web. when switching between tabs or windows, the browser limits the resources it's spending on unused programs. like my game. this results in a very low fps, and accordingly, a higher delta time, and accordingly bigger move steps and falling through the ground.
I fixed this by running the game loop only when the window has focus. but the issue persists in different situations, for example when the player scrolls down the page of the game, it happens.
So now that I think about it I think maybe running the game loop only when the window has mouse focus would fix it?