Really impressive! You managed to nail the GB feel, I specially liked the intro title which appears in the same way the Nintendo logo did on the GB.
It's also very well designed!
I encountered a problem however, I clicked outside the game window once and the game glitched out and the character wouldn't stop going to the right afterwards. In fact, pressing D would stop the character.
Also I think the control scheme wasn't the best for this kind of game, because the Enter key is pretty far from the other keys, I personally prefer Z, X and the arrow keys for 2D games.
I still loved it, very good job!