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 afcontinue="Chap11_antialias_on.png|176" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Catherine Tut1 pic1.png" />
      <f ns="6" title="File:Catherine Tut1 pic2.png" />
      <f ns="6" title="File:Catherine Tut1 pic3.jpg" />
      <f ns="6" title="File:Catherine Tut1 pic4.jpg" />
      <f ns="6" title="File:Catherine Tut1 pic5.jpg" />
      <f ns="6" title="File:Catherine Tut1 pic6.jpg" />
      <f ns="6" title="File:Catherine Tut1 pic7.jpg" />
      <f ns="6" title="File:Catherine Tut1 pic8.jpg" />
      <f ns="6" title="File:Chaotica Launcher.jpg" />
      <f ns="6" title="File:Chap11 antialias off.png" />
    </allfileusages>
  </query>
</api>