Online Emulation
Recently I’ve become fascinated with online emulation of various old bits of kit.
It appears that modern PC’s are so much more powerful than their ancestors (and by a good factor) that emulator writers no longer need to resort to assembly language or C++ to emulate 8 bit computer hardware. You can emulate a Spectrum in Java, or indeed in Javascript!
If you want to investigate further, I would invite you to check out The World of Spectrum page on emulators:
http://www.worldofspectrum.org/emulators.html
My weapon of choice is Qaop by Jan Bobrowski; he has written Qaop in Java but also includes a Javascript Spectrum emulator on his site. I’ve written a very rough-and-ready emulator page which can be found by clicking here.