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>
    <allimages gaicontinue="Unterrichten.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="879" ns="6" title="Datei:Unter der Lupe Direkte Instruktion.pdf" imagerepository="local">
        <imageinfo>
          <ii timestamp="2015-08-17T13:52:59Z" user="Fabian.steiner" />
        </imageinfo>
      </page>
      <page pageid="880" ns="6" title="Datei:Unter der Lupe Klarheit der Lehrperson.pdf" imagerepository="local">
        <imageinfo>
          <ii timestamp="2015-08-17T13:57:13Z" user="Fabian.steiner" />
        </imageinfo>
      </page>
      <page pageid="881" ns="6" title="Datei:Unter der Lupe Lehrer Schüler Beziehung.pdf" imagerepository="local">
        <imageinfo>
          <ii timestamp="2015-08-17T14:00:37Z" user="Fabian.steiner" />
        </imageinfo>
      </page>
      <page pageid="911" ns="6" title="Datei:Unterricht Lehrstrategien.png.jpg" imagerepository="local">
        <imageinfo>
          <ii timestamp="2018-07-19T15:14:41Z" user="Beywol" />
        </imageinfo>
      </page>
    </pages>
  </query>
</api>