JSFiddle - React, Tailwind, and code Playground

HTML

<div>

</div>

CSS

div{
  width:200px;
  height:200px;
  position:relative;
}
div:after, div:before{
  content:"";
  position:absolute;
  width:180px;
  height:180px;
}
div:after{
  right:0;
  bottom:0;
  border-right:3px solid #000;
  border-bottom:3px solid #000;
}
div:before{
  left:0;
  top:0;
  border-left:3px solid #000;
  border-top:3px solid #000;
}