[css-images-4][css-shapes-1] Allow a single `*-corner` keyword to define an ellipse?

[cdoublev]

The <radial-size> extension in CSS Images 4 does not define the result from an ellipse defined with a *-corner keyword and a *-side keyword, or with two *-corner keywords.

This comment says:

X-corner Y-corner creates an ellipse with the same aspect-ratio as X-side Y-side that passes through both corners

How can it be achieved without moving the center of the ellipse?

Visual examples would be great.