Hmm, interesting... How was it made? Is it just a mimicry of a game launched on GAMEBOY console, of built-in emulator with ROM attached?
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It's made in an engine called GB Studio. It can export in html and as a ROM that works on GameBoy as well. The GameBoy running the game in the screenshots is a standard GameBoy without any modifications. I've also had it run on an original GameBoy Advance. Not sure 100% on what GB Studio does on its end, but the builds can run on classic GameBoy hardware.