css triangles

.arrow-down {
  width: 0;
  height: 0;
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;

  border-top: 20px solid #f00;
}

.arrow-top {
  width: 0;
  height: 0;
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;

  border-bottom: 20px solid #f00;
}

.arrow-right {
  width: 0;
  height: 0;
  border-top: 20px solid transparent;
  border-bottom: 20px solid transparent;

  border-left: 20px solid black;
}

.arrow-left {
  width: 0;
  height: 0;
  border-top: 20px solid transparent;
  border-bottom: 20px solid transparent;

  border-right: 20px solid black;
}