[css-borders] allow negative values for `border-image-outset`

The spec states that negative border-image-outset values are invalid but doesn't give an explanation as to why (https://drafts.csswg.org/css-backgrounds/#border-image-outset). This functionality seems useful. Granted one can build the "inset" into the image itself but it would be better to be able to do this in CSS and not have to hard code it into an image.

outline-offset (https://drafts.csswg.org/css-ui/#outline-offset) already does some very similar to this but does not have the restriction on negative values. The outline-offset spec listing does give an explanation about limitations. I wonder if the same sort of thing could be done for border-image-outset?

Negative values must cause the outline to shrink into the border box. Both the height and the width of the outside of the shape drawn by the outline should not become smaller than twice the computed value of the outline-width property to make sure that an outline can be rendered even with large negative values. User agents should apply this constraint independently in each dimension. If the outline is drawn as multiple disconnected shapes, this constraint applies to each shape separately.

Start by comparing the border-image-outset section with the referenced outline-offset specification and the CSS Backgrounds draft. Determine whether negative values can be specified consistently and what rendering constraints are needed; done means a resolved CSSWG issue with agreed specification wording.