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="Bibliography|3019" />
  </query-continue>
  <query>
    <pages>
      <page pageid="1524" ns="0" title="Sandbox" />
      <page pageid="1539" ns="0" title="Circular Sail" />
      <page pageid="3009" ns="0" title="Deep Space Probes: To the Outer Solar System and Beyond" />
      <page pageid="3010" ns="0" title="Gossamer Spacecraft: Membrane and Inflatable Structures Technology for Space Applications" />
      <page pageid="3011" ns="0" title="Recent Advances in Gossamer Spacecraft" />
      <page pageid="3012" ns="0" title="Large Space Structures Formed by Centrifugal Forces" />
      <page pageid="3014" ns="0" title="The Mote in God&#039;s Eye" />
      <page pageid="3015" ns="0" title="Planet of the Apes" />
      <page pageid="3016" ns="0" title="Project Solar Sail" />
      <page pageid="3018" ns="0" title="Rocheworld" />
    </pages>
  </query>
</api>