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Bumps on corners when border-width is larger than border-radius #11
When the border-width for an element is larger than the border-radius for one or more of its corners, then that corner will be rendered with some extra "bumps" sticking out from it. This is due to the fact that the calculated path ends up crossing itself in the corners, and the quadratic curve gets drawn concave rather than convex.
This is easily seen in some of the IE9 border-radius testcases, e.g. border-radius-sum-of-radii-001.htm