Seems like they're usually based on hardware emulators like qemu, bochs or similar which are compiled to javascript using tools like emscripten. IO hardware ne

Seems like they're usually based on hardware emulators like qemu, bochs or similar which are compiled to javascript using tools like emscripten. IO hardware needs to be emulated in JS, a framebuffer is drawn to a canvas, mouse input events are translated to the native hardware events, I'm sure there's a million details.