It seems that having a well-constructed, durable note here would be friendly, since this is indeed an odd design, so some form of "sic!" marker would be nice.

[Note 1: If you find this surprising, you were paying attention. Good job! --end note]

I don't understand the rationale offered here. "floatfield" could never be "hex" in C++2003. Maybe in some implementations, the numeric value of hex conflicted with scientific or fixed?

Well, to make

str.setf(ios_­base​::hex, ios_­base​::​floatfield);

do anything, ios_base::hex must have bits in common with ios_base::floatfield. But ios_base::basefield is defined as dec|oct|hex, so the result would be that floatfield and basefield would overlap, which seems bad.