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="Benutzer-Export" />
  </query-continue>
  <query>
    <allcategories>
      <c size="9" pages="0" files="0" subcats="9" xml:space="preserve">!Hauptkategorie</c>
      <c size="88" pages="58" files="30" subcats="0" xml:space="preserve">Algebra</c>
      <c size="3" pages="0" files="3" subcats="0" xml:space="preserve">Analytische Geometrie</c>
      <c size="1" pages="0" files="1" subcats="0" xml:space="preserve">Arbeitsblatt</c>
      <c size="1" pages="0" files="1" subcats="0" xml:space="preserve">Architektur</c>
      <c size="11" pages="11" files="0" subcats="0" xml:space="preserve">Aufgaben (Wintersemester 2016/2017)</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Aufgaben für EntdeckerInnen</c>
      <c size="11" pages="11" files="0" subcats="0" xml:space="preserve">Auftrag der Woche (Wintersemester 2016/2017)</c>
      <c size="1" pages="0" files="1" subcats="0" xml:space="preserve">Auftrag der Woche Sommersemester 2021</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Begriffsklärung</c>
    </allcategories>
  </query>
</api>