<!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>