cssgeometrypolygoncss-shapes

How do CSS triangles work?

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?

Solution

  • 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.