In this case, the RFC is mismatched from the actual behaviour of browsers (as described and codified by WhatWG).  It was surprising to me that urljoin() didn't do what I percieved as "the right thing" (and I expect other users would too).

I would personally expect urljoin to do "the thing that everybody else does".  Is there a sensible way to reduce this mismatch?

For reference, Java's stdlib does what I would expect here:

    URI base = URI.create("https://example.com/?a=b");
    URI rel = base.resolve("?");
    System.out.println(rel);

https://example.com/?