On the other hand, emulating a machine of this complexity was beyond imagination when gameboy came out. On the other hand, Virtualbox can emulate much more complicate machines. On the other hand, this one lives entirely and safely inside your browser.
Virtualization without extensions is possible, but tricky on x86. VMWare has historically used binary translation to deal with the tricky bits. (I.e. go through the binary before you run it, look for instructions that don't virtualize properly, replace them with sequences that can be virtualized and either do the same thing, or trap into the virtual machine monitor.)
