Created attachment 218008 [details] Test-case Floats with width expressed in percents are sized according to the named flow width instead of the width of the region(s) they're in.
Note that this bug is concerned with the fragmentation aspects of floats relative sizing. There's also bug 124205 for the simpler scenario with floats that are *not* fragmented
The other bug is actually bug 123795 (not 124205 as stated in previous comment)
CSS Regions were removed in Bug 174978.