title: How to get the src for a video with multiple sources?
body: <p>I'd like to be able to get the src value that is actually used for a video element like the following:</p>
  
  <pre><code>&lt;video&gt;
    &lt;source src="foo.mp4"  type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'&gt;
    &lt;source src="foo.webm" type='video/webm; codecs="vp8, vorbis"'&gt;
    &lt;source src="foo.ogv"  type='video/ogg; codecs="theora, vorbis"'&gt;
  &lt;/video&gt;
  </code></pre>
  
  <p>In Firefox (at least), src is defined for the source elements but not for the video element.</p>
  
  <p>How can I find which source is used?</p>