JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.box {
  position: relative;
  height: 100px;
  width: 100px;
  background-color: #000;
}

.box::before {
  content: '';
  position: absolute;
  left: 0;
  top: 0;
  height: 40px;
  width: 40px;
  background-color: #fff;
}