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>
    <allcategories accontinue="Astronomical_object" />
  </query-continue>
  <query>
    <allcategories>
      <c size="2" pages="1" files="0" subcats="1" xml:space="preserve">All articles with unsourced statements</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Allegedly an Actual Pilot</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">Allegedly an actual scientist</c>
      <c size="5" pages="5" files="0" subcats="0" xml:space="preserve">Americans</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Armchair moderator</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Art and Culture</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Article with unsupported titties</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles that correspond with the Star Trek universe</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Articles that correspond with the real world</c>
      <c size="35" pages="35" files="0" subcats="0" xml:space="preserve">Articles with unsupported titles</c>
    </allcategories>
  </query>
</api>