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="ModTweakerNavigation|37" />
  </query-continue>
  <query>
    <pages>
      <page pageid="2" ns="0" title="Aura Cascade" />
      <page pageid="8" ns="0" title="ContentTweaker:BlockItem Support" />
      <page pageid="9" ns="0" title="ContentTweaker:Fluid Support" />
      <page pageid="10" ns="0" title="ContentTweaker:TConstruct Support" />
      <page pageid="25" ns="0" title="ModTweaker" />
      <page pageid="29" ns="0" title="ModTweaker:Applied Energistics 2 Support" />
      <page pageid="34" ns="0" title="ModTweaker:Botania Support" />
      <page pageid="35" ns="0" title="ModTweaker:Botania Support:Lexica Botania" />
      <page pageid="212" ns="10" title="Template:Mod" />
    </pages>
  </query>
</api>