[css3-backgrounds] Should a non-zero border-radius create a new stacking context ?

The specification expects [1] that "effects that clip to the border or padding edge (such as overflow other than 'visible') also must clip to the curve".
In practice, most browsers have issues with this today so this requirement is somewhat at risk; today only Firefox 4 seems to clip relatively positioned 
children to the curve when overflow is hidden, for instance.

Without the creation of a new stacking context, one can construct odd scenarios where a relatively positioned child is clipped to the curve while elements 
outside its parent are layered in z-order underneath it. (see attached testcase in latest Firefox 4 Beta).

It's not  clear to me why this is useful or desirable. Should we treat curved borders as a composition effect similar to opacity or transforms ?

[1] http://dev.w3.org/csswg/css3-background/#corner-clipping