<div>12345</div>

div {
  height: 20px;
  border: 1 px solid black;
  position: relative;
}
div:after {
  content: '';
  position:absolute;
  width: 100%;
  height: 100%;
  top: 5px;
  left: 5px;
  background: red;
  z-index: -1;
}