title: Why doesn't percentage padding / margin work on flex items in Firefox and Edge?
body: <p>I want to have a square div inside a flexbox. So I use:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.outer {
    display: flex;
    width: 100%;
    background: blue;
  }
  .inner {
    width: 50%;
    background: yellow;
    padding-bottom: 50%;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="outer"&gt;
    &lt;div class="inner"&gt;
      &lt;a&gt;hehe&lt;/a&gt;
    &lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>This works fine in Chrome. But in Firefox, the parent squeezes to just one line. </p>
  
  <p>How do I solve this in Firefox? I use version 44.</p>
  
  <p>You can also view the code at <a href="https://jsbin.com/lakoxi/edit?html,css" rel="noreferrer">https://jsbin.com/lakoxi/edit?html,css</a></p>