JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box"></div>

CSS

html, body {
  margin: 0;
}
div {
  width: 50vw;
  height: 50vw;
  background-color: red;
    tranform: rotate(45deg);

}
.box:after {
  content: ' ';
  border-top: 50vw solid transparent;
  border-left: 50vw solid transparent;
  width: 0;
  position: absolute;
}