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="City-building_games" />
  </query-continue>
  <query>
    <allcategories>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">1995 video games</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">4X video games</c>
      <c size="11" pages="11" files="0" subcats="0" xml:space="preserve">Action-adventure games</c>
      <c size="16" pages="16" files="0" subcats="0" xml:space="preserve">Action games</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">Action role-playing games</c>
      <c size="48" pages="48" files="0" subcats="0" xml:space="preserve">Adventure games</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Arcade games</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Artillery games</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Board game games</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Boxing games</c>
    </allcategories>
  </query>
</api>