Run Unix processes inside your browser
It can run C++? Cool. Can you run a web browser instance in it? Then you'd have a web browser, running inside an operating system, running on top of a web browser, running on top of an operating system. Potentially, this operating system is a virtual machine, inside another operating system. But wait! There's more! This operating system is running inside of a browser. It's JavaScript all the way down! That's right we're running inside of a an advanced computer simulation written in JavaScript. Now about that XDS (cross-dimensional scripting) attack..