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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="Bernd_Fischer&#039;s_PhD_students" />
  </query-continue>
  <query>
    <allcategories>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">1540 births</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">1627 deaths</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">2</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">2008 video games</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">3</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">4</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Acclaim Entertainment games</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Action games</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Aim</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">Andrew Allison's PhD students</c>
    </allcategories>
  </query>
</api>