title: <hr> inside container with display flex become corrupted
body: <p>This behavior is a little odd and weird. If there is a container with its <code>display</code> property set to <code>flex</code> and <code>flex-direction</code> to <code>column</code>, the orientation of the <code>&lt;hr&gt;</code> elements inside it will change and become vertical and its height will decrease to fit the height of the line.</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>html, body {
    height: 100%;
  }
  body {
    font-family: sans-serif;
  }
  .container {
    padding: 20px;
    height: 100%;
    display: flex;
    flex-direction: column;
    flex-grow: 1;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="container"&gt;
    &lt;h3&gt;Title&lt;/h3&gt;
    &lt;hr/&gt;
    &lt;div class="content"&gt;
      &lt;p&gt;This is some content&lt;/p&gt;
    &lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>Check out <a href="http://codepen.io/ahmadalfy/pen/bEpjyY">this pen</a>.</p>
  
  <p>This behavior is confirmed on Firefox and Chrome. I didn't find any explanation for it. How can I fix it?</p>