You probably know already how to shape a div into a triangle. If you're not familiar with this technique, read this.
I was discussing with a friend some time ago that this is a perfect solution to add an arrow on top of a popup for instance, but he kept saying that it's still impossible to add a border to the triangular div.
Well, with this technique it's really easy to achieve just that. It's a pure CSS solution.
See how it looks in this here.