<div class="demo"></div>

.demo {
  width: 300px;
  height: 200px;
  background: linear-gradient(#000, #000),
              linear-gradient(#000, #000),
              linear-gradient(#000, #000),
              linear-gradient(#000, #000);
  background-position: top left 10px, 
                       top 10px left,
                       top right 10px,
                       bottom 10px left;
  background-size: 2px 100%, 100% 2px, 2px 100%, 100% 2px;
  background-repeat: no-repeat;
}