Someone posted this on another thread: https://github.com/iodide-project/pyodide

What I gather is that it's compiling CPython to WASM and ships it along with some other utilities. It looks pretty cool.

WASM plans on adding GC, polymorphic inline cache, direct DOM access, etc. That would allow for Python without having to download the whole runtime. Whenever that happens, it might become more mainstream. Similar for other languages.

There have been js/asm.js CPython builds demoed since early days of emscripten, and emscripten is the main WebAssembly toolchain, so it's "just worked" since the beginning.