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="B25.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="466" ns="6" title="File:B10.jpg" />
      <page pageid="1673" ns="6" title="File:B10 3d.gif" />
      <page pageid="467" ns="6" title="File:B17.jpg" />
      <page pageid="424" ns="6" title="File:B17 3d.gif" />
      <page pageid="468" ns="6" title="File:B18.jpg" />
      <page pageid="1674" ns="6" title="File:B18 3d.gif" />
      <page pageid="580" ns="6" title="File:B1bis.jpg" />
      <page pageid="1859" ns="6" title="File:B1bis 3d.gif" />
      <page pageid="582" ns="6" title="File:B1ter.jpg" />
      <page pageid="1860" ns="6" title="File:B1ter 3d.gif" />
    </pages>
  </query>
</api>