<!doctype html>
<title>CSS test reference</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<style>
.test {
  display: flex;
}
.test::before {
  content: "";
  display: block;
  width: 100px;
  height: 100px;
  background-color: green;
}
</style>
<div class="test"></div>