shape-outside (CSS Shapes 1) and shape-inside (CSS Shapes 2) are defined with this prose:

<basic-shape>: The shape is computed based on the values of one of inset(), circle(), ellipse() or polygon().

It is not clear to me if xywh(), rect(), path(), should be invalid.

In SVG 2, <basic-shape> is even further restricted in shape-inside and shape-substract, but I assume these are superseded by CSS Shapes.

^{This was previously reported in the multi-faceted issue #7390.}