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 atcontinue="Marco_Pagan_Books" />
  </query-continue>
  <query>
    <alltransclusions>
      <t ns="10" title="Template:Contrib" />
      <t ns="10" title="Template:Digby Smith Books" />
      <t ns="10" title="Template:French Army References" />
      <t ns="10" title="Template:French Drummer with Royal Livery" />
      <t ns="10" title="Template:French Gendarmerie Service" />
      <t ns="10" title="Template:Germania Figuren" />
      <t ns="10" title="Template:Hanoverian Army References" />
      <t ns="10" title="Template:Hesse-Kassel Army References" />
      <t ns="10" title="Template:Hungarian Line Infantry Colours" />
      <t ns="10" title="Template:Map of Schweidnitz" />
    </alltransclusions>
  </query>
</api>