JSFiddle - React, Tailwind, and code Playground

HTML

<div class="elem"></div>

CSS

.elem {
  position: relative;
  width: 200px;
  height: 200px;
  border: 1px solid #fff;
  margin: 20px;
}

.elem::after {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  margin: 0 auto;
  transform: translateY(-50%) rotate(45deg);
  width: 31px;
  height: 31px;
  border-bottom: 1px solid #fff;
  border-right: 1px solid #fff;
  background: green;
}

body {
  background: green;
}