You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <allfileusages afcontinue="Building_Metal_Den.jpg" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Building Alliance Depot.jpg" />
      <f ns="6" title="File:Building Crystal Den.jpg" />
      <f ns="6" title="File:Building Crystal Mine.jpg" />
      <f ns="6" title="File:Building Crystal Storage.jpg" />
      <f ns="6" title="File:Building Deuterium Den.jpg" />
      <f ns="6" title="File:Building Deuterium Synthesizer.jpg" />
      <f ns="6" title="File:Building Deuterium Tank.jpg" />
      <f ns="6" title="File:Building Fusion Reactor.jpg" />
      <f ns="6" title="File:Building Jump Gate.jpg" />
      <f ns="6" title="File:Building Lunar Base.jpg" />
    </allfileusages>
  </query>
</api>