Backgrounds and shadows are well-defined, outlines aren't:

This specification does not define the exact position or shape of the outline
This specification does not define how multiple overlapping outlines are drawn or how outlines are drawn for boxes that are partially obscured behind other elements.

In fact outline-style: auto is entirely left up to the UA.

I'm afraid that with multiple outlines an author may expect a certain visual composition, but in other browsers it will be utterly broken.

Then I amend my proposal to first define the exact position and shape of the outline, as well as how multiple overlapping outlines are drawn, and then allow multiple outlines per element. 😁