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/.
There's more information about the current binary format here: https://github.com/WebAssembly/design/blob/master/BinaryEnco...
There's a prototype spec interpreter here: https://github.com/WebAssembly/spec/tree/master/ml-proto
There's a build waterfall testing multiple tools here: 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