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="10" pages="10" files="0" subcats="0" xml:space="preserve">DotP 2014</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">DotP 2015</c>
      <c size="33" pages="33" files="0" subcats="0" xml:space="preserve">DotP Editing</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">HQ Card Generator</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Language templates</c>
      <c size="2" pages="1" files="0" subcats="1" xml:space="preserve">MTG Forge</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">Magic Duels</c>
      <c size="2" pages="0" files="0" subcats="2" xml:space="preserve">Magic The Gathering</c>
      <c size="53" pages="53" files="0" subcats="0" xml:space="preserve">ManaLink</c>
      <c size="15" pages="15" files="0" subcats="0" xml:space="preserve">Online PlayTable</c>
    </allcategories>
  </query>
  <query-continue>
    <allcategories accontinue="Online_PlayTable/de" />
  </query-continue>
</api>