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="Begin_Proof" />
  </query-continue>
  <query>
    <alltransclusions>
      <t ns="10" title="Template:Bad Page" />
      <t ns="10" title="Template:Begin Answer" />
      <t ns="10" title="Template:Begin Blue Notebox" />
      <t ns="10" title="Template:Begin Blue Notebox Content" />
      <t ns="10" title="Template:Begin Example" />
      <t ns="10" title="Template:Begin Example Body" />
      <t ns="10" title="Template:Begin Inline Proof" />
      <t ns="10" title="Template:Begin Inline Theorem" />
      <t ns="10" title="Template:Begin Notebox" />
      <t ns="10" title="Template:Begin Notebox Content" />
    </alltransclusions>
  </query>
</api>