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by mikegerwitz 5154 days ago
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.