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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="Speedy_deletion_templates" />
  </query-continue>
  <query>
    <allcategories>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">All articles with unsourced statements</c>
      <c size="13" pages="13" files="0" subcats="0" xml:space="preserve">Articles</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles that need work</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles with invalid date parameter in template</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Candidates for speedy deletion</c>
      <c size="13" pages="13" files="0" subcats="0" xml:space="preserve">Pages that need an admin</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Pages with broken file links</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Requests for Deletion</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Spam pages for speedy deletion</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Speedy deletion</c>
    </allcategories>
  </query>
</api>