[css-shapes-1] Proposal: Option for the radii of `round` on `<basic-shape-rect>` to resolve percentages against the rectangle itself instead of the reference box

[bernhardf-ro]

The border radii specified after the round identifier of <basic-shape-rect> functions resolve percentage values against the reference box.

However, in many cases authors prefer radius percentages to be resolved against the rectangle itself, i.e. the dimensions computed from the <basic-shape-rect> function.

We propose an additional or alternative identifier that changes how those percentages are resolved.

We are not yet sure about good names, so we will go with self and round-self for the following examples.

For example, the following would create border radii that are always about a third of the related side of the rectangle, leaving the remaining middle third of each side straight, regardless of the font-size, and with it the resolved em values:

xywh(10mm 5mm 4em 3em round self 33%) /* additional identifier */
xywh(10mm 5mm 4em 3em round-self 33%) /* alternative identifier */

Of course em values are not a common case. But even with absolute lengths, such percentages are easier to read and understand for authors and maintainers.