<div class="container">DIV</div>

.container {
  position: relative;
  width: 100px;
  height: 50px;
  line-height: 50px;
  text-align: center;
  border: 1px solid rgb(100, 100, 100, 0.3);
  font-size: 20px;
  margin-left: 50px;
  margin-top: 50px;
}

.container:before,
.container:after {
  content: "";
  position: absolute;
  display: block;
  width: 25px;
  height: 25px;
  border: 5px solid #000000;
}

.container:before {
  top: -5px;
  left: -5px;
  border-right: none;
  border-bottom: none;
}

.container:after {
  top: 25px;
  left: 75px;
  border-left: none;
  border-top: none;
}