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="!Predefinições_de_edição" />
  </query-continue>
  <query>
    <allcategories>
      <c size="6" pages="6" files="0" subcats="0" xml:space="preserve">!Ajuda</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">!Anti-vandal tool</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">!Artigos em edição</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">!Avisos para artigos com problemas</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">!Fichas de acções</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">!Mensagens pré-definidas para utilizadores</c>
      <c size="3" pages="0" files="0" subcats="3" xml:space="preserve">!Predefinições</c>
      <c size="2" pages="0" files="0" subcats="2" xml:space="preserve">!Predefinições Twiki</c>
      <c size="6" pages="6" files="0" subcats="0" xml:space="preserve">!Predefinições da página principal</c>
      <c size="11" pages="1" files="0" subcats="10" xml:space="preserve">!Predefinições da wiki</c>
    </allcategories>
  </query>
</api>