JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block"></div>

CSS

body {
  padding: 50px;
}

.block {
  width: 200px;
  height: 100px;
  background-color: #ffffff;
  border: 1px solid #cccccc;
  position: relative;
}

.block::after {
  content: "";
  width: 20px;
  height: 20px;
  background-color: #ffffff;
  border-top: solid 1px #cccccc;
  border-left: solid 1px #cccccc;
  display: block;
  transform: rotate(45deg);
  position: absolute;
  top: -11px;
  right: 20px;
}