"Almost", because it doesn't use Base.fragment when R.frament is undefined.
a) Should we try describe the algorithm based on RFC 3986 ("do relative resolution as defined by ..., then, if the result doesn't have a fragment, add the one from the Base URI")?
b) Is this potentially an erratum for RFC 3986?
a) sounds good.
b) I'd call it an "update" rather than an erratum.