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!