Aww, I like this one a lot. Gameboy aesthetics tug on my heartstrings, and I love a good puzzle platformer. I ran into a little finickyness interacting with the gameboy the first time. Got stuck and had to reload to get it to work. Might be a browser thing, I don't know. Took me a minute to realize only left shift would work to change carts, as well.
But yeah, really solid all around. Nice take on the theme, cool core mechanic, good art, good music, good "hup" sound.