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>
    <allredirects garcontinue="Stuttgart_21/Faktencheck" />
  </query-continue>
  <query>
    <pages>
      <page pageid="399" ns="0" title="Deutsche Bahn/Strategie" />
      <page pageid="1771" ns="0" title="Empfohlene Dokumentationen" />
      <page pageid="1" ns="0" title="Hauptseite" />
      <page pageid="1920" ns="0" title="Stuttgart 21/Anhörung PFA 1.3/Analyse 09.2015" />
      <page pageid="1802" ns="0" title="Stuttgart 21/Anhörung PFA 1.3/DB 09.2015 Analyse" />
      <page pageid="2041" ns="0" title="Stuttgart 21/Brandschutz/Entfluchtung" />
      <page pageid="1624" ns="0" title="Stuttgart 21/Brandschutz Tiefbahnhof" />
      <page pageid="1623" ns="0" title="Stuttgart 21/Brandschutz Tiefbahnhof/Chronologie" />
      <page pageid="328" ns="0" title="Stuttgart 21/Brandschutz Tiefbahnhof/Entfluchtung" />
      <page pageid="1937" ns="0" title="Stuttgart 21/Brandschutz Tunnel" />
    </pages>
  </query>
</api>