It's pretty terrible. It's not hard to require that the user select a ROM from his or her local machine. Web browsers go to the trouble of supporting blob URLs for a reason.
http://taisel.github.io/IodineGBA/ is one where you load your own in. You need the BIOS ROM in addition to the game ROM, as it's all LLE emulation, no BIOS SWI HLE'ing.