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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="Hatnote_templates_for_user_pages" />
  </query-continue>
  <query>
    <allcategories>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Article message boxes</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Articles to be expanded</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Articles to be expanded with sources</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Articles which use infobox templates with no data rows</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Correct title templates</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Disambiguation and redirection templates</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Documentation subpages without corresponding pages</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Graphic templates</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Hatnote templates</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Hatnote templates documentation</c>
    </allcategories>
  </query>
</api>