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="Mann" />
  </query-continue>
  <query>
    <allcategories>
      <c size="3" pages="1" files="0" subcats="2" xml:space="preserve">BMW Z1</c>
      <c size="17" pages="12" files="3" subcats="2" xml:space="preserve">BMW Z3</c>
      <c size="73" pages="66" files="3" subcats="4" xml:space="preserve">BMW Z4</c>
      <c size="37" pages="37" files="0" subcats="0" xml:space="preserve">BMW Z4 FAQ</c>
      <c size="6" pages="0" files="6" subcats="0" xml:space="preserve">BMW Z4 Fotos</c>
      <c size="5" pages="2" files="1" subcats="2" xml:space="preserve">BMW Z8</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Deutscher</c>
      <c size="10" pages="9" files="0" subcats="1" xml:space="preserve">Firmenverzeichnis</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Geboren 1961</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Geologe</c>
    </allcategories>
  </query>
</api>