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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.
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<?xml version="1.0"?>
<api>
  <query-continue>
    <allcategories accontinue="Animal_models" />
  </query-continue>
  <query>
    <allcategories>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">1950 births</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">1964 births</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">2012 controversies</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">2012 in American politics</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">2012 in Missouri</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Academia</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">American political catch phrases</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">American political slogans</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">American science fiction magazines</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">American writers</c>
    </allcategories>
  </query>
</api>