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="Bahrain_International_Circuit_-_Paddock_Layout.png" />
  </query-continue>
  <query>
    <pages>
      <page pageid="4578" ns="6" title="File:B11.png" />
      <page pageid="4579" ns="6" title="File:B21.png" />
      <page pageid="4580" ns="6" title="File:B31.png" />
      <page pageid="2232" ns="6" title="File:BA Racing 2000.png" />
      <page pageid="4395" ns="6" title="File:BMW K=Xtreme.jpg" />
      <page pageid="4392" ns="6" title="File:BMW SimRace SA Livery2.jpg" />
      <page pageid="4229" ns="6" title="File:BackMarker Brigade logo 2015.jpg" />
      <page pageid="4231" ns="6" title="File:BackMarker Brigadeside2.png" />
      <page pageid="4230" ns="6" title="File:Backmarker1 2015.jpg" />
      <page pageid="1180" ns="6" title="File:Bahrain International Circuit.png" />
    </pages>
  </query>
</api>