You literally can't write a portable "hello world" command line app in WASM. It's the worst "write once, run everywhere" of anything. Which is expected because

You literally can't write a portable "hello world" command line app in WASM. It's the worst "write once, run everywhere" of anything. Which is expected because it distinctly does not provide any standard APIs or syscalls. There's no standard library. At all. Someone may attempt to add a batteries-include system that uses WASM with a bunch of platform-abstraction libraries, but WASM itself does not provide that. And isn't going to provide it. You can make a portable library with WASM, assuming you have zero dependencies on anything, but that's about it.