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="Executives" />
  </query-continue>
  <query>
    <allcategories>
      <c size="146" pages="146" files="0" subcats="0" xml:space="preserve">Alumni</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles pending deletion</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles that need Images</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Articles with things to do</c>
      <c size="10" pages="10" files="0" subcats="0" xml:space="preserve">Articles with unsourced statements</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Category:Templates</c>
      <c size="14" pages="14" files="0" subcats="0" xml:space="preserve">Chronicles</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Deceased Person</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Disambiguation pages</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Electronics</c>
    </allcategories>
  </query>
</api>