JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.link_back {
  font-size: 20px;
  font-weight: bold;
  line-height: 40px;

  position: relative;

  display: inline-block;
  overflow: hidden;

  height: 41px;
  padding: 0 .5em 0 2em;

  text-decoration: none;

  color: white;

  box-shadow: 1.4em 0.4em 0.2em 0 rgba(0, 0, 0, 0.2);
}

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

  display: inline-block;

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

  content: '';
  transition: background-color 1s linear;
  transform: rotate(45deg);
  transform-origin: center right;

  background-color: #3786bd;
}

.link_back:hover:before {
  background-color: red;
}