Read attribute of a self closing tag using pure javascript?

How can I read the attribute of self closing tag in an XML document using pure javascript? eg. get value of "url" in the xml element below?

<channel>     
  <item> 
    <title>News Title Goes Here<title>
    <media:thumbnail width="144" height="81" url="http://ift.tt/1IyiOf6"/> 
  </item>  
<channel>

The javascript call getElementsByTagName works on the title node, but not on the self closing media:thumbnail node