LLVM to JavaScript compilers

emscripten compiler

http://kripken.github.io/emscripten-site/

C/C++ to JavaScript (asm.js)

em-DOSbox

https://github.com/dreamlayers/em-dosbox

DOSBox compiled with emscripten

Windows 95 in your browser

https://win95.ajf.me/

Windows95 installed on em-DOSBox

Windows 2000 & Linux
https://bellard.org/jslinux/