Comment on CSS arrow pleaseparentComments−mikegerwitz14ygrowt'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.
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growt's link does an excellent job of illustrating it.
Ultimately, how the border is joined at the corners is undefined by the specification:
> 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.