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>
    <alltransclusions gatcontinue="Box|610" />
  </query-continue>
  <query>
    <pages>
      <page pageid="581" ns="10" title="Vorlage:Quelle" />
      <page pageid="599" ns="0" title="Heinrich Abegg" />
      <page pageid="600" ns="0" title="Hans Abt" />
      <page pageid="601" ns="0" title="Otto Hans Abt" />
      <page pageid="602" ns="0" title="Angèle Adam" />
      <page pageid="604" ns="0" title="Jakob Adam" />
      <page pageid="605" ns="0" title="Jakob Joseph Adam" />
      <page pageid="606" ns="0" title="Maria Elisabeth Aebersold-Hufschmid" />
      <page pageid="607" ns="0" title="Johann Jakob Aenishänslin" />
      <page pageid="609" ns="0" title="Daniel August Alioth" />
    </pages>
  </query>
</api>