JSFiddle - React, Tailwind, and code Playground

HTML

<div id="corner"></div>

CSS

#corner {
      height: 200px;
      width: 200px;
      position: absolute;
      z-index: 999;
      left: -7.5em;
      top: -7.5em;
      background-color: red;
      transform: rotateZ(-45deg);
      border-bottom: 5px solid #0c0c0c;
    }