> Negative values must cause the outline to shrink into the border box. Both the height and the width of 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.
There's a lot of RFC 2119 SHOULD there, unfortunately.
> Negative values must cause the outline to shrink into the border box. Both the height and the width of 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.
There's a lot of RFC 2119 SHOULD there, unfortunately.