title: Vertically centering a flex item works in Chrome but not Firefox
body: <p>I've written this simple code for centering my inner <code>div</code>:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div style="display: flex;align-items: center;width: 100%;height: 600px;background-color: gray;"&gt;
    &lt;div style="background-color: white;width: 100px;height: 300px;position: absolute;"&gt;&lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>And it works fine in Chromium. but not in Firefox.</p>
  
  <p>In Firefox the inner div is not centered.</p>
  
  <p>Firefox:</p>
  
  <p><a href="https://i.stack.imgur.com/JFoLG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JFoLG.png" alt="enter image description here"></a></p>
  
  <p>Chromium:</p>
  
  <p><a href="https://i.stack.imgur.com/qHEWz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qHEWz.png" alt="enter image description here"></a></p>
  
  <p>I need the inner div to remain absolutely positioned.</p>