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HTML is good for debugging, but probably is not suitable for your application.
See complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query>
    <allcategories>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Admin</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Pages with broken file links</c>
    </allcategories>
  </query>
</api>