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="BabbageHouse_011.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="4874" ns="6" title="File:BabbageHouse 001.jpg" />
      <page pageid="4875" ns="6" title="File:BabbageHouse 002.jpg" />
      <page pageid="4876" ns="6" title="File:BabbageHouse 003.jpg" />
      <page pageid="4877" ns="6" title="File:BabbageHouse 004.jpg" />
      <page pageid="4878" ns="6" title="File:BabbageHouse 005.jpg" />
      <page pageid="4879" ns="6" title="File:BabbageHouse 006.jpg" />
      <page pageid="4880" ns="6" title="File:BabbageHouse 007.jpg" />
      <page pageid="4881" ns="6" title="File:BabbageHouse 008.jpg" />
      <page pageid="4882" ns="6" title="File:BabbageHouse 009.jpg" />
      <page pageid="4883" ns="6" title="File:BabbageHouse 010.jpg" />
    </pages>
  </query>
</api>