I agree with the linked quote - it captures an important reason why it is valuable to _enforce_ sanitisation at component boundaries, rather than merely documenting "please don't rely on isolated surrogates being preserved across component boundaries" (which would be a problem if we didn't enforce it, since an external component you don't control may be forced to internally sanitise the string if it relies on (e.g.) an API, language runtime, or storage mechanism that admits only well-formed strings).

EDIT: since a whole other paragraph was edited in as I replied, I will respond by saying that within a component, your string can have whatever invalid representation you want. Most written code will naturally be a single component (which could even be made up of both JS and Wasm composed together through the JS API). The code may interface with other components, and this discussion is purely about what enforcement is appropriate at that boundary.

EDIT2: please consider a further reply to my post, rather than repeatedly editing your parent post in response. It is disorientating for observers. In any case, my paragraph above did not claim that there will be one component per language, but that the code _one writes oneself_ within a single language (or a collection of languages/libraries which can be tightly coupled through an API/build system) will naturally form one component.