I've been quite frustrated with the state of WebAssembly. It still feels like a platform that's in beta and not ready for production. Partially this is the slow pace of standardization, partially this is the lack of investment into tooling by various stakeholders.

This plays out precisely as the blog post details with the split between WASI and Web Platform. Say you want to compile a Rust + C codebase to Wasm and run in the browser. You have three targets: wasm32-unknown-emscripten, wasm32-unknown-unknown, and wasm32-wasi. Emscripten is relatively old and not maintained. It'll work but you get some old JS that doesn't play well with newer stuff. wasm32-unknown-unknown has an ABI incompatibility which means you cannot interoperate between C compiled to Wasm and Rust compiled to Wasm. wasm32-wasi works, but now you have to have a WASI implementation in the browser and that's still very immature. Tools like wasm-bindgen or wasm-pack don't work with wasm32-wasi either.

Basically you have the Web Platform backend (wasm32-unknown-unknown) that plays well with the browser part, but does not play well with the interoperability across languages part. And you have the WASI backend (wasm32-wasi) that plays well with the interoperability across languages part but does not play well with browser part.