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="Booster_Erweiterung2text.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="777" ns="6" title="File:Ballonnen.jpg" />
      <page pageid="1526" ns="6" title="File:Betrieb.png" />
      <page pageid="1527" ns="6" title="File:BlBl.png" />
      <page pageid="751" ns="6" title="File:Blitzer 1x2.png" />
      <page pageid="567" ns="6" title="File:Blitzer 2013 V2.png" />
      <page pageid="747" ns="6" title="File:Blitzer 2x3.png" />
      <page pageid="742" ns="6" title="File:Blitzer 3x2.png" />
      <page pageid="661" ns="6" title="File:Booster-Anschluss-Reihe.png" />
      <page pageid="957" ns="6" title="File:Booster-Prinzip2.jpg" />
      <page pageid="971" ns="6" title="File:Booster Erweiterung2.png" />
    </pages>
  </query>
</api>