How to make a triangle in pure CSS?

In this post I'll show you a trick to draw triangles in pure CSS. Why should you want to draw triangles in CSS instead of using images? Because CSS needs less bytes, it's easier to modify colors and size and it can be enlarged easily (which would lead to horrible pixelation when using images).
First we should understand the method behind this trick, so- everything is based on drawing rectangles with big border.

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