Only for certain kinds of URLs

[mnot]

The introduction should explicitly point out that this mechanism is only for URLs that use application/x-www-form-urlencoded query strings; other formats aren't supported.

[domenic]

It contains

Section 6 gives the key algorithm for comparing if two URLs are equivalent under the influence of the header; notably, it leans on the decomposition of the query component into keys and values given by the application/x-www-form-urlencoded format specified in [WHATWG-URL].

I guess you're suggesting an additional sentence after that, something like

(As such, this header is not useful for URLs whose query component does not follow that format.)

?

[mnot]

That would help.