A few tips for working with masks (clipping layers) and rounded corners.
You used to need a special force2d property for this to work, but in the latest WebKit this seems to be solved. So you can drop that entirely.
If you use rounded corners with a mask, you can run into situations where the anti aliasing looks like it mixes up colors. You can avoid this by using an extra layer for your background color, instead of setting it directly on the mask.