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="DataAccess|1421" />
  </query-continue>
  <query>
    <pages>
      <page pageid="1369" ns="0" title="Administration Client" />
      <page pageid="1376" ns="0" title="Database Server" />
      <page pageid="1377" ns="0" title="Presence Server" />
      <page pageid="1409" ns="0" title="Heartbeat Server" />
      <page pageid="1414" ns="0" title="Remote Program Call Server" />
      <page pageid="1415" ns="0" title="Heartbeat Client" />
      <page pageid="1416" ns="0" title="HTTP Server" />
      <page pageid="1417" ns="0" title="Object Monitor Nodes" />
      <page pageid="2587" ns="0" title="Menu Options (Admin Client)" />
      <page pageid="2588" ns="0" title="Toolbar Items" />
    </pages>
  </query>
</api>