As sunfish mentions below, this spec document is being conservative in describing things that are still being discussed. There are experimental implementations

As sunfish mentions below, this spec document is being conservative in describing things that are still being discussed. There are experimental implementations in Firefox, Chrome and Edge. See https://webassembly.github.io/demo/ https://webassembly.github.io/demo/. There's more information about the current binary format here: https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md https://github.com/WebAssembly/design/blob/master/BinaryEnco... There's a prototype spec interpreter here: https://github.com/WebAssembly/spec/tree/master/ml-proto https://github.com/WebAssembly/spec/tree/master/ml-proto There's a build waterfall testing multiple tools here: https://wasm-stat.us/console https://wasm-stat.us/console, including the experimental LLVM toolchain, the emscripten-wasm toolchain[0], binaryen[1], and sexpr-wasm[2]. WebAssembly is still under development, but I don't think it's fair to call it an early prototype. [disclaimer: I'm a WebAssembly contributor] [0]: https://github.com/kripken/emscripten/wiki/WebAssembly https://github.com/kripken/emscripten/wiki/WebAssembly [1]: https://github.com/WebAssembly/binaryen https://github.com/WebAssembly/binaryen [2]: https://github.com/WebAssembly/sexpr-wasm-prototype/ https://github.com/WebAssembly/sexpr-wasm-prototype/