I’ve just tried https://play.webrcade.com and it’s working really fine through Falkon.
If you own some roms, you can also play your favorite console games 
Below SuperBomberman with sound through jack :
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What’s the point though? Haiku already has many emulators and I don’t think browser emus have the best user experience. However it’s cool that it works.
Mmm do you experience a good emulator under Haiku for retrograming ? I didn’t find one working fine so far but I might miss it.