I still don't get it. What do you mean that "borders are styled in such a way that they angle toward the edges if the border is large enough"? What causes the angle?
Ultimately, how the border is joined at the corners is undefined by the specification:
http://www.w3.org/TR/css3-background/#border-style
> Note: This specification does not define how borders of different styles should be joined in the corner. [...]Many rendering engines will join borders at an angle, producing those diagonal lines.