<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Test Reference</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:ecobos@igalia.com">
<style>
  .container {
    display: flex;
    width: 300px;
    height: 300px;
    align-items: center;
    border: 2px solid purple;
  }
  .container > div {
    width: 100px;
    height: 100px;
    background: blue;
    align-self: auto;
  }
</style>
<p>Test passes if there's a blue square vertically centered inside the box.</p>
<div class="container">
  <div></div>
</div>