Created attachment 59937 [details]
http://trac.webkit.org/changeset/62035 introduced a new method of drawing border-radius using paths. Right now, this new code is only enabled for some platforms…if you want to know if your favorite platform has the new code path enabled, see if it has been added to #define HAVE_PATH_BASED_BORDER_RADIUS_DRAWING in RenderObject.h.
I think there are some edge cases where we do not match up the radius edges as the W3C suggests. Attached is a test case demonstrating what I think is an incorrect rendering, and also a diagram demonstrating the correct rendering method.
Created attachment 59938 [details]
Image explaining strategy for radius drawing
Note that the radius strategy here does not conform to the CSS3 specification at http://www.w3.org/TR/css3-background/#corner-transitions
Specifically, the spec says "The center of color and style transitions between adjoining borders is at the point on the curve that is at an angle that is proportional to the ratio of the border widths."
I took a shot at interpreting just what the spec means at http://jerssoftwarehut.com/generalized-round-corner-diagram.xhtml
I just read <http://jerssoftwarehut.com/generalized-round-corner-diagram.xhtml> (nice!).
I'd encourage you to send feedback to the www-style mailing list highlighting the issues you saw in the spec.