.box{
  border: 4px dashed rgba(0,0,0,.2);
  padding: 10px;
  margin: 5px 0;
}

.box-1{
  background-color: yellow;
  position: relative;
}

.box-1:after{
  content: '';
  position: absolute;
  background-color: blue;
  left: -4px;
  right: -4px;
  bottom: -4px;
  height: 4px;
  opacity: .5;
}

.box-2 {
  background-color: blue;
}