JSFiddle - React, Tailwind, and code Playground

HTML

<div class="redSquare">
  <div class="blueLine" />
  <div class="greenLine" />
</div>

CSS

.redSquare {
  margin: 100px;
  width: 100px;
  height: 100px;
  background: red;
}

.blueLine {
  transform-origin: 0px 0px;
  transform: rotate(-45deg);
  width: 2px;
  height: 141px;
  background: blue;
}

.greenLine {
  transform-origin: 0px 0px;
  transform: rotate(45deg);
  width: 2px;
  height: 141px;
  background: green;
}