I wonder if your question relates to WASM or things in the WASM-sphere like WASI et al?

I'm not sure if it's right but I think of WASM as just an ISA and I guess the documentation seems to cover instruction encodings and semantics.

Right. I was able to write custom wasm generation for browsers just going off the binary encoding spec. Started with using wasm as target IL for a Befunge JIT