There's a project here: switch-gba by BFriedrichs for running a GBA emulator within the switch browser. I haven't tried it yet, the requirements are Docker + a PC with ROMs on the same network.
Because of that last requirement it's not going to be easy to have a simple link for playing a GBA game without having a locally networked PC serving the copyrighted content as well.
Also worth mentioning: the GB Operator is one legal option for getting a ROM file from a physical cartridge.