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 gafcontinue="Building_Metal_Den.jpg" />
  </query-continue>
  <query>
    <pages>
      <page ns="6" title="File:Building Crystal Den.jpg" missing="" />
      <page ns="6" title="File:Building Deuterium Den.jpg" missing="" />
      <page pageid="43" ns="6" title="File:Building Alliance Depot.jpg" />
      <page pageid="44" ns="6" title="File:Building Crystal Mine.jpg" />
      <page pageid="46" ns="6" title="File:Building Crystal Storage.jpg" />
      <page pageid="49" ns="6" title="File:Building Deuterium Synthesizer.jpg" />
      <page pageid="51" ns="6" title="File:Building Deuterium Tank.jpg" />
      <page pageid="52" ns="6" title="File:Building Fusion Reactor.jpg" />
      <page pageid="59" ns="6" title="File:Building Jump Gate.jpg" />
      <page pageid="75" ns="6" title="File:Building Lunar Base.jpg" />
    </pages>
  </query>
</api>