You are looking at the HTML representation of the XML format.
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>
    <allcategories>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles which use infobox templates with no data rows</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Navigational boxes without horizontal lists</c>
      <c size="38" pages="38" files="0" subcats="0" xml:space="preserve">Pages with broken file links</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Template documentation</c>
      <c size="6" pages="6" files="0" subcats="0" xml:space="preserve">Wikipedia pages with incorrect protection templates</c>
      <c size="5" pages="5" files="0" subcats="0" xml:space="preserve">Wikipedia protected templates</c>
    </allcategories>
  </query>
</api>