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="Hc" />
  </query-continue>
  <query>
    <pages>
      <page ns="10" title="Template:Documentation" missing="" />
      <page pageid="3258" ns="10" title="Template:Bot Navbox" />
      <page pageid="1568" ns="10" title="Template:Br" />
      <page pageid="2690" ns="10" title="Template:ChangeLogBox" />
      <page pageid="3252" ns="10" title="Template:Collapsible option" />
      <page pageid="3138" ns="10" title="Template:DevDesk" />
      <page pageid="3354" ns="10" title="Template:EPtip" />
      <page pageid="1218" ns="10" title="Template:Editors:Fin Kename" />
      <page pageid="1219" ns="10" title="Template:Editors:Mark McDoogle" />
      <page pageid="3260" ns="10" title="Template:Faction Navbox" />
    </pages>
  </query>
</api>