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="Baden-Baden_Casino_18.jpg|4" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Baden-Baden 01 Stadtansicht.jpg" />
      <f ns="6" title="File:Baden-Baden 03 Stadtansicht.jpg" />
      <f ns="6" title="File:Baden-Baden 04 Kurhaus.jpg" />
      <f ns="6" title="File:Baden-Baden 07 Casino.jpg" />
      <f ns="6" title="File:Baden-Baden 09 Lichtentaler Allee.jpg" />
      <f ns="6" title="File:Baden-Baden 10 Lichtentaler Allee.jpg" />
      <f ns="6" title="File:Baden-Baden 11 Caracalla.jpg" />
      <f ns="6" title="File:Baden-Baden 13 Museum Frieder Burda.jpg" />
      <f ns="6" title="File:Baden-Baden 14 Theater.jpg" />
      <f ns="6" title="File:Baden-Baden Caracalla 02.jpg" />
    </allfileusages>
  </query>
</api>