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="DoubleTheRubble.jpg|318" />
  </query-continue>
  <query>
    <pages>
      <page pageid="1" ns="0" title="Main Page" />
      <page pageid="6" ns="0" title="The simplest possible guide to writing MediaWiki" />
      <page pageid="146" ns="0" title="The History of Tux the Linux Penguin" />
      <page pageid="185" ns="3" title="User talk:SteveBaker" />
      <page pageid="315" ns="0" title="Realistic Space Game" />
      <page pageid="317" ns="0" title="RSG: Networking" />
      <page pageid="337" ns="0" title="RSG: Reference imagery" />
    </pages>
  </query>
</api>