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="Main_other" />
  </query-continue>
  <query>
    <alltransclusions>
      <t ns="10" title="Template:Case preserving encode" />
      <t ns="10" title="Template:Documentation" />
      <t ns="10" title="Template:Documentation/docspace" />
      <t ns="10" title="Template:Documentation/end box2" />
      <t ns="10" title="Template:Documentation/start box" />
      <t ns="10" title="Template:Documentation/start box2" />
      <t ns="10" title="Template:Documentation/template page" />
      <t ns="10" title="Template:Error" />
      <t ns="10" title="Template:ISO 639 name" />
      <t ns="10" title="Template:ISO 639 name undefined" />
    </alltransclusions>
  </query>
</api>