Currently, the grammar for clip-path is:

clip-path   = none | <url> | [ <basic-shape> || <geometry-box> ]

whereas the grammar for motion-path is:

motion-path = none | [ [ <ray()> | <url> |  <basic-shape> ] || <coord-box> ]

Ignoring the addition of ray(), the two differ in two ways:

1. clip-path uses <geometry-box> vs. motion-path which uses <coord-box>. (this means clip-path allows an extra keyword, margin-box)
2. clip-path does not allow a reference box to be specified with a <url> vs. motion-path does allow a reference box to be specified with a <url>.

I propose we change clip-path to allow specifying both a <url> and a <geometry-box>, updating its grammar to:

clip-path   = none | [ [ <url> | <basic-shape> ] || <geometry-box> ]

It would also be expanded to allow referencing either a clipPath element or, like motion-path, "an SVG shape element".

Like with motion-path, the reference box for a <url>, "defines the viewport and user coordinate system for the shape [or clipPath] element, with the origin (the 0,0 point) at the top left corner, and units being 1px in size."

Additionally, I propose a change to motion-path to use <geometry-box> instead of <coord-box> for its reference box, updating its grammar to:

motion-path   = none | [ [ <ray()> | <url> | <basic-shape> ] || <geometry-box> ]

With margin-box now a supported reference box.