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="Articles" />
  </query-continue>
  <query>
    <allcategories>
      <c size="36" pages="27" files="9" subcats="0" xml:space="preserve">1st millennium</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">2nd millenium</c>
      <c size="19" pages="17" files="2" subcats="0" xml:space="preserve">2nd millennium</c>
      <c size="5" pages="5" files="0" subcats="0" xml:space="preserve">3rd millennium</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">4th millennium</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">5th millennium</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">6th millennium</c>
      <c size="3" pages="0" files="0" subcats="3" xml:space="preserve">Akana</c>
      <c size="8" pages="2" files="0" subcats="6" xml:space="preserve">Akana by period</c>
      <c size="4" pages="3" files="1" subcats="0" xml:space="preserve">Anatolionesian languages</c>
    </allcategories>
  </query>
</api>