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="0" pages="0" files="0" subcats="0" xml:space="preserve">1 - Aspects sociaux et économiques de la GIZC</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">5 - Evolution géomorphologique à long terme et impacts du changement climatique</c>
      <c size="21" pages="19" files="0" subcats="2" xml:space="preserve">Acteurs</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Activités par thème</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Activités portuaires et dragages</c>
      <c size="18" pages="18" files="0" subcats="0" xml:space="preserve">Adaptation de l'habitat &amp; des infrastructures aux aléas</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Adaptation transfrontalière</c>
      <c size="9" pages="9" files="0" subcats="0" xml:space="preserve">Agriculture</c>
      <c size="2" pages="1" files="0" subcats="1" xml:space="preserve">Air</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Allier</c>
    </allcategories>
  </query>
  <query-continue>
    <allcategories accontinue="Alsace" />
  </query-continue>
</api>