CSS Triangles: How do they work?

A plain ol' div with four borders applied. Note the bevelled edges:

Let's use these bevelled borders to make a triangle with CSS. Give the div a width and height of "0" and the borders a width of 200px:

Make triangles by making the borders we don't want to see transparent:

Upward Pointing Arrow

Downward Pointing Arrow

Right Pointing Arrow

Left Pointing Arrow

You can use the ::before and ::after pseudo elements + positioning to make cool shapes!

Choosing border width:

You can use this same method to make a speech bubble!

Links to resources about CSS shapes:

CSS Tricks - CSS Triangle

The Internet of Shapes by Brenna O'Brien

Pure CSS Triangles Explained