Background is not clipped according to border-radius, when border-image is also set. This happens even with a solid color background. For testcase, look at the attached html file (will be uploaded shortly after submission).
Created attachment 26540 [details] Testcase for bug #23198
<rdar://problem/6482825>
From the latest draft of the spec: "Backgrounds, but not the border-image, are clipped to the inner, resp., outer curve of the border if ‘background-clip’ is ‘padding-box’ resp., ‘border-box’. Other effects that clip to the border or padding edge (such as ‘overflow’) also must clip to the curve." So what's making this look like a bug to you is that the border-image got incorrectly clipped to the curve. Therefore its 4px corners aren't visible, and it looks like the background is running up too far to the edge.
Created attachment 28929 [details] Patch to stop clipping border-image The test case in this bug actually uses a border-image that is purely an alpha component, so even with this change to match the spec, the test case will still look exactly the same (since the underlying background clips to the outer border radius edge).
Comment on attachment 28929 [details] Patch to stop clipping border-image So lame. Can't we lobby www-style? The code looks fine, but I prefer the old behavior.
Fixed in r41976. I'll continue to complain on www-style.