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="Begin_Proof" />
  </query-continue>
  <query>
    <pages>
      <page pageid="101" ns="10" title="Template:Bad Page" />
      <page pageid="323" ns="10" title="Template:Begin Answer" />
      <page pageid="488" ns="10" title="Template:Begin Blue Notebox" />
      <page pageid="489" ns="10" title="Template:Begin Blue Notebox Content" />
      <page pageid="143" ns="10" title="Template:Begin Example" />
      <page pageid="144" ns="10" title="Template:Begin Example Body" />
      <page pageid="419" ns="10" title="Template:Begin Inline Proof" />
      <page pageid="418" ns="10" title="Template:Begin Inline Theorem" />
      <page pageid="482" ns="10" title="Template:Begin Notebox" />
      <page pageid="483" ns="10" title="Template:Begin Notebox Content" />
    </pages>
  </query>
</api>