hello, what did you use to run the Genesis ROM on itch.io in the browser?
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This one use an HTML5 port of retroarch by ToadKing, but it's quite ancient now: https://toadking.com/retroarch/genesis_plus_gx.html.
There is now an updated version called "Emulator.JS" that allow you to embed emulators for many more consoles (loads of cores supported), and with more configurable options, that I recommend (I used this one in my latest releases):
Hope that helps!