title: AMP image is not appearing
body: <p>I can't get this <code>amp-img</code> to show up.  It works as an image, and when I click the src-link it loads on a separate page correctly.  Changing it to <code>amp-img</code> makes it disappear.  I set <code>background-color: red</code> on the <code>amp-img</code> and the box appears correctly, but it's an empty red box.</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;li&gt;
      &lt;a href="/"&gt;
      &lt;amp-img src="https://d12v9rtnomnebu.cloudfront.net/oursite/logo_white.png" alt="site logo" width="264" height="96"/&gt;
      &lt;/a&gt;
  &lt;/li&gt;
  </code></pre>
  
  <p>CSS (probably a lot of extraneous stuff that needs to be trimmed but don't know what's important until it works):</p>
  
  <pre><code>.amp .site-menu li {
      height: 32px;
      line-height: 32px;
  }
  
  .amp .site-menu li:first-child {
      float: left;
      min-width: 50px;
      max-width: 180px;
      width: 100%;
      display: block;
  }
  
  .amp .site-menu a {
      min-width: 50px;
      max-width: 200px;
      width: 100%;
      display: block;
  }
  
  .amp .site-menu amp-img {
      height: 32px;
      min-width: 50px;
      max-width: 200px;
      width: 100%;
      float: left;
      display: block;
      background-color: red;
  }
  </code></pre>