Web standards should specify the semantics of HTML, CSS, etc, not dictate how exactly the browser operates. Limiting extensibility of all browsers just because there is some webextensions standard is ridiculous. Similarly, browsers should be able to ignore user-hostile parts of specs like the HSTS spec trying to dictate that users cannot override the certificate validitation.

In fact, even how websites are rendered should ultimately be up to how the user has configured his browser even if some spec says otherwise. This includes being able to install effective ad blockers that remove elements and deny requests that should be there according to the standard.

Think of web standards more as the definition of a common language between websites and browsers and not as a rulebook for either.