JSFiddle - React, Tailwind, and code Playground

by RenaissanceDesign

HTML

<div class="diag">
  &nbsp;
</div>

CSS

body {
  background-color: #000;
}

.diag {
  position: relative;

  width: 500px;
  height: 500px;
  border: 1px solid #fff;
}

.diag::before {
  display: block;
  position: absolute;
  z-index: 2;
width: 0;
height: 0;
border-style: solid;
border-width: 0 200px 200px 0;
border-color: transparent #007bff transparent transparent;
}

.diag::after {
  width: 0;
height: 0;
border-style: solid;
border-width: 200px 0 0 200px;
border-color: transparent transparent transparent #007bff;

}