Webkit renders rounded borders really strange if the border widths are different. The result in firefox is much more pleasant. I have attached a small html test file and screenshots in firefox 3 and webkit to illustrate it.
Created attachment 19251 [details]
Screenshot: left Firefox 3, right Webkit r30082
Created attachment 19252 [details]
HTML test file
See bug 9197 and the patch attached to it.
*** Bug 19655 has been marked as a duplicate of this bug. ***
*** Bug 30038 has been marked as a duplicate of this bug. ***