title: Margin left/right behaving differently on Chrome & Firefox
body: <p>I have a div (Facebook comments plugin, for what it's worth) styled with only the following CSS.</p>
  
  <pre><code>div.fb-comments
  {
  margin-left:30%;
  margin-right:30%;
  }
  </code></pre>
  
  <p>On Chrome, this puts the <code>&lt;div&gt;</code> in the center of the screen. However, on Firefox, it remains at the left. Why might this be?</p>