.one-pixel {
  position: relative;
  width: 20px;
  height: 20px;
  overflow: hidden;
  border: 1px solid black;
}

.one-pixel:before {
  position: absolute;
  content: '';
  display: block;
  top: -4px;
  left: 0;
  width: 4px;
  height: 4px;
  box-shadow: 0px 4px;
}