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="Cite_press_release" />
  </query-continue>
  <query>
    <pages>
      <page pageid="134" ns="10" title="Template:BLP sources" />
      <page pageid="136" ns="10" title="Template:BLP unsourced" />
      <page pageid="20" ns="10" title="Template:Category handler" />
      <page pageid="147" ns="10" title="Template:Check talk" />
      <page pageid="148" ns="10" title="Template:Check talk wp" />
      <page pageid="555" ns="10" title="Template:Citation" />
      <page pageid="487" ns="10" title="Template:Cite book" />
      <page pageid="460" ns="10" title="Template:Cite encyclopedia" />
      <page pageid="446" ns="10" title="Template:Cite journal" />
      <page pageid="21" ns="10" title="Template:Cite news" />
    </pages>
  </query>
</api>