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/Brandschutz_Tiefbahnhof/Chronologie|2042" />
  </query-continue>
  <query>
    <pages>
      <page pageid="18" ns="4" title="WikiReal:Über" />
      <page pageid="420" ns="0" title="Main Page" />
      <page pageid="421" ns="8" title="MediaWiki:Main Page" />
      <page pageid="1618" ns="0" title="Stuttgart 21/Personenzugänge/Entfluchtung" />
      <page pageid="1810" ns="0" title="Deutsche Bahn/Fremdsteuerung" />
      <page pageid="1919" ns="0" title="Stuttgart 21/Anhörung PFA 1.3/DB 09.2015/Analyse" />
      <page pageid="1920" ns="0" title="Stuttgart 21/Anhörung PFA 1.3/Analyse 09.2015" />
      <page pageid="1939" ns="4" title="WikiReal:Über WikiReal" />
      <page pageid="1974" ns="0" title="Schlauer Fernsehen/Netztipps" />
      <page pageid="2037" ns="0" title="Stuttgart 21/Brandschutz" />
    </pages>
  </query>
</api>