To explain and demonstrate:
A CSS arrow is created by coloring 1 border side, and then moving the other 3 sides in towards the middle of the shape as transparent so they don't show but cut the remaining colored side into a triangle. The shorthand for this is TOP RIGHT BOTTOM LEFT. So to make a triangle pointing upwards you use the third property or bottom.
Using pseudo elements (incase you want the arrow added to another element) you need content:'' to "create" the pseudo element. I've set them as display: block so that they are in the flow and interact with eachother (rather than being laid on top of one another).
By giving the rectangle position: relative you can then use left: 30px (half of the triangle width) to position it in the middle of the triangle.
.arrowWrapper:before {
content: '';
display: block;
width: 0;
height: 0;
border-style: solid;
border-width: 0 60px 60px 60px;
border-color: transparent transparent black transparent;
/* border-color: TOP RIGHT BOTTOM LEFT; */
}
.arrowWrapper:after {
content: '';
position: relative;
display: block;
width: 60px;
height: 60px;
background: black;
left: 30px;
}
<div class="arrowWrapper"></div>