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>
    <allimages gaicontinue="ATAT_009.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="641" ns="6" title="File:100pxLogo.png" />
      <page pageid="3188" ns="6" title="File:28mmHMSSurprisePlans.jpg" />
      <page pageid="5105" ns="6" title="File:ATAT 001.jpg" />
      <page pageid="5106" ns="6" title="File:ATAT 002.jpg" />
      <page pageid="5107" ns="6" title="File:ATAT 003.jpg" />
      <page pageid="5108" ns="6" title="File:ATAT 004.jpg" />
      <page pageid="5109" ns="6" title="File:ATAT 005.jpg" />
      <page pageid="5110" ns="6" title="File:ATAT 006.jpg" />
      <page pageid="5111" ns="6" title="File:ATAT 007.jpg" />
      <page pageid="5112" ns="6" title="File:ATAT 008.jpg" />
    </pages>
  </query>
</api>