css-overflow-3 allows the possibility that 'overflow' is 'clip' in one axis and 'visible' in the other. css-backgrounds-3 says that when 'overflow' is not 'visible', the clipping is done to the curve.

When the computed value of overflow is 'clip' in one axis and 'visible' in the other axis, is clipping done to the curve, nd if so, how?

I'd suggest that perhaps the easy answer is no: if you specify 'clip' in one axis and 'visible' in the other, then 'border-radius' is ignored when clipping.

(Noticed while trying to write edits for #6212.)