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="Extensions" />
  </query-continue>
  <query>
    <pages>
      <page pageid="904" ns="10" title="Template:CSS" />
      <page pageid="598" ns="10" title="Template:Case Study" />
      <page pageid="873" ns="10" title="Template:Case Study/fr" />
      <page pageid="122" ns="10" title="Template:Category" />
      <page pageid="887" ns="10" title="Template:Contact Address" />
      <page pageid="5" ns="10" title="Template:Contact Us" />
      <page pageid="948" ns="10" title="Template:Development" />
      <page pageid="972" ns="10" title="Template:Development Comments" />
      <page pageid="459" ns="10" title="Template:Edit" />
      <page pageid="16" ns="10" title="Template:Extension DPL" />
    </pages>
  </query>
</api>