Modern specification-prose will use https://infra.spec.whatwg.org/ data structures. E.g., an algorithm would return a list of JavaScript strings or a byte sequence.

However, IDL does not work that way. It only knows about ECMAScript and IDL values which are 1:1. Whereas specification constructs can fit in various IDL values. E.g., a byte sequence could become a JavaScript string via ByteString or a Uint8Array.

Specification-prose is generally also not specific about numeric types and will assume appropriate casting happens at the boundary.

I'm not really sure how to solve this, but this came up in #664 and it seems out-of-scope to address there.