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

[1]: https://github.com/WebAssembly/binaryen

[2]: https://github.com/WebAssembly/sexpr-wasm-prototype/

Indeed; WebAssembly as a whole is still being designed. For example, it doesn't have an official text format yet, and while there is a binary format implemented in multiple browsers now, it's still evolving. The TODOs mark places where there are ongoing design discussions.

And the project is in search of a logo. You can find a bunch of proposals here: https://github.com/WebAssembly/design/issues/112