Great project.
I noticed that You are using absolute paths in certain places and relative in others. I think it should be relative everywhere.
This is a list of absolute paths in files:
index.html line 18
js/loaders.js line 20
js/loaders/level.js line 37
sprites/goomba.json line 2
sprites/koopa.json line 2
sprites/mario.json line 2
sprites/overworld.json line 2
sprites/underworld.json line 2

Good luck in the project.