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="Doc" />
  </query-continue>
  <query>
    <pages>
      <page ns="10" title="Template:Collapsible option" missing="" />
      <page ns="10" title="Template:Country data" missing="" />
      <page ns="10" title="Template:Country showdata" missing="" />
      <page pageid="23" ns="10" title="Template:Birth date and age" />
      <page pageid="26" ns="10" title="Template:Br separated entries" />
      <page pageid="28" ns="10" title="Template:Category handler" />
      <page pageid="31" ns="10" title="Template:Citation/core" />
      <page pageid="218" ns="10" title="Template:Clear" />
      <page pageid="52" ns="10" title="Template:DMCA" />
      <page pageid="53" ns="10" title="Template:Dated maintenance category" />
    </pages>
  </query>
</api>