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="Cent_nav|4832" />
  </query-continue>
  <query>
    <pages>
      <page pageid="4747" ns="0" title="1st Century B.C.E." />
      <page pageid="4751" ns="0" title="2nd Century B.C.E." />
      <page pageid="4825" ns="0" title="1st Century" />
      <page pageid="4827" ns="0" title="2nd Century" />
      <page pageid="4828" ns="0" title="3rd Century" />
      <page pageid="4829" ns="0" title="4th Century" />
      <page pageid="4830" ns="0" title="5th Century" />
      <page pageid="4831" ns="0" title="6th Century" />
      <page pageid="4844" ns="4" title="Sciforums encyclopedia:Template messages" />
      <page pageid="4875" ns="14" title="Category:Template messages" />
    </pages>
  </query>
</api>