Standardizing WASI: A system interface to run WebAssembly outside the web

Just as WebAssembly is an assembly language for a conceptual machine, WebAssembly needs a system interface for a conceptual operating system, not any single operating system. So you can’t use any single OS’s system interface inside the WebAssembly implementation of the standard library. In the same way, WebAssembly needs a system interface for a conceptual operating system, not a real operating system.

Source: hacks.mozilla.org