[css3-background] 'border-radius' and 'overflow'

http://dev.w3.org/csswg/css3-background/#the-border-radius says:
  # Backgrounds, but not the border-image, are clipped to the
  # appropriate curve (as determined by ‘background-clip’). Other
  # effects that clip to the border or padding edge (such as
  # ‘overflow’) also must clip to the curve. 

What does it mean for 'overflow: scroll' or 'overflow: auto' to clip
to the curve?  I have no idea how that's supposed to work; there
aren't standard mechanisms for drawing curved scrollbars.

Then it goes on to say:
  # It is recommended that the UA style sheet apply overflow: hidden
  # to elements (such as the <img> element in HTML) that are
  # expected to be replaced elements so that their corners
  # automatically trim to the border radius. 

I'm pretty skeptical of this advice; 'overflow: hidden' is a pretty
heavyweight mechanism (it involves the ability to scroll
programatically).  I suspect we'd be unable to make this change
because it would cause a performance regression, although I haven't
tried.

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/