How do CSS triangles work?

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There’re plenty of different CSS shapes over at CSS Tricks – Shapes of CSS and I’m particularly puzzled with a triangle:

CSS Triangle

#triangle-up {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
}
<div id="triangle-up"></div>

How and why does it work?

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CSS Triangles: A Tragedy in Five Acts

As alex said, borders of equal width butt up against each other at 45 degree angles:

borders meet at 45 degree angles, content in middle

When you have no top border, it looks like this:

no top border

Then you give it a width of 0…

no width

…and a height of 0…

no height either

…and finally, you make the two side borders transparent:

transparent side borders

That results in a triangle.

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