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-continue>
    <allcategories accontinue="Articles_with_invalid_date_parameter_in_template" />
  </query-continue>
  <query>
    <allcategories>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">All articles to be expanded</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">All articles with unsourced statements</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">All pages needing factual verification</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">All plants in database</c>
      <c size="20" pages="20" files="0" subcats="0" xml:space="preserve">Aquatic mammals</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Article message boxes</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Article types</c>
      <c size="6" pages="5" files="0" subcats="1" xml:space="preserve">Articles</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Articles to be expanded</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles with 'species' microformats</c>
    </allcategories>
  </query>
</api>