JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">
</div>

CSS

.box{
  background: lavender;
  height: 200px;
  width: 200px;
  position: absolute;
  left: 50%;
  transform: translateX(-50%);
  z-index: 2;
}
.box:before{
  content: '';
  width: 100%;
  height: 100%;
  border: 1px solid #000;
  position: absolute;
  right: -20px;
  bottom: -20px;
  z-index: -1;
}