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="BookImage|4267" />
  </query-continue>
  <query>
    <pages>
      <page pageid="2973" ns="6" title="File:Winning Ways for Your Mathematical Plays.png" />
      <page pageid="2974" ns="0" title="Winning Ways for Your Mathematical Plays" />
      <page pageid="2981" ns="0" title="Wheels, Life, and Other Mathematical Amusements" />
      <page pageid="2982" ns="6" title="File:Wheels, Life, and Other Mathematical Amusements.png" />
      <page pageid="3039" ns="6" title="File:The Recursive Universe.png" />
      <page pageid="3040" ns="0" title="The Recursive Universe" />
      <page pageid="4266" ns="6" title="File:Lifeline vol 1.png" />
      <page pageid="5780" ns="0" title="Game of Life Cellular Automata" />
      <page pageid="12555" ns="0" title="0Ⅹ" />
      <page pageid="12598" ns="0" title="Genius at Play" />
    </pages>
  </query>
</api>