[ruslan_mart]

Я бы сделал так:

div {
  background: #FFF;
  height: 100px;
  position: relative;
  width: 100px;
}
div::before {
  border: 5px solid red;
  border-bottom: none;
  border-right: none;
  box-sizing: border-box;
  content: '';
  height: 20px;
  left: -7px;
  position: absolute;
  top: -7px;
  width: 20px;
}