JSFiddle - React, Tailwind, and code Playground

HTML

<div class="square"></div>

CSS

.square,
.square:before,
.square:after {
  border: solid #000 3px;
}
.square {
  position: relative;
  width: 100px;
  height: 100px;
  border-left: 0;
  margin: 0 auto;
}
.square:before,
.square:after {
  content: '';
  display: block;
  position: absolute;
  border-right: 0;
}
.square:before {
  top: 0;
  right: 100%;
  width: 150%;
  height: 40px;
  margin: -3px -3px 0 0;
}
.square:after {
  left: 0;
  top: 40px;
  bottom: 0;
}