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>
    <pages>
      <page pageid="653" ns="8" title="MediaWiki:Uploadtext" />
      <page pageid="709" ns="4" title="Tudwiki:Alle MediaWiki-Meldungen" />
      <page pageid="2060" ns="0" title="Betriebssysteme:Aufgaben V" />
      <page pageid="2074" ns="10" title="Vorlage:Info" />
      <page pageid="2080" ns="10" title="Vorlage:Löschantrag" />
      <page pageid="2081" ns="0" title="Gthi Klausur 21.02.2007" />
      <page pageid="2092" ns="0" title="Mathe:3.Übungsblatt 2007 Ganter" />
      <page pageid="2138" ns="0" title="Studium:Nebenfach:BMGT:DGT" />
    </pages>
  </query>
</api>