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="Destacados" />
  </query-continue>
  <query>
    <allcategories>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Arduino</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Bitcoin</c>
      <c size="49" pages="49" files="0" subcats="0" xml:space="preserve">Curso 2014-2015</c>
      <c size="55" pages="55" files="0" subcats="0" xml:space="preserve">Curso 2015-2016</c>
      <c size="14" pages="14" files="0" subcats="0" xml:space="preserve">Curso 2016-2017</c>
      <c size="30" pages="30" files="0" subcats="0" xml:space="preserve">Curso 2017-2018</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Curso 2017/2018</c>
      <c size="10" pages="9" files="0" subcats="1" xml:space="preserve">Curso 2018-2019</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Debian</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Derecho informático</c>
    </allcategories>
  </query>
</api>