growt's link does an excellent job of illustrating it. Ultimately, how the border is joined at the corners is undefined by the specification: http://www.w3.

growt's link does an excellent job of illustrating it. 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.