This website hosts Nintendo Game Boy games made by previous students of mine. This website is also running a GBA emulator. The controls are as follows (in GBA -> Keyboard pairs): A -> Z, B -> X, L -> A, R -> S, SELECT -> Backspace, START -> Enter, D-Pad: Arrow Keys. You can modify the mappings using the settings menu (click on the key in the list, then tap the key you want to map it to). Note: The GBA emulator does not work on mobile currently.
The source code for the website can be found here, which originally came from here.