JSFiddle - React, Tailwind, and code Playground

HTML

<a class="link_back" href="#">Retour au site</a>

CSS

.link_back:before {
  position: absolute;
  z-index: -1;
  left: 20px;

  display: inline-block;

  width: 15em;
  height: 15em;
  margin-top: -1.2em;

  content: '';
  transform: rotate(45deg);
  transform-origin: center right;

  background-color: #3786bd;
}