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="16" pages="16" files="0" subcats="0" xml:space="preserve">Architektura</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Cyfrowe</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Czasopisma:2012</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Czasopisma 1992</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">Czasopisma 1993</c>
      <c size="7" pages="7" files="0" subcats="0" xml:space="preserve">Czasopisma 1994</c>
      <c size="10" pages="10" files="0" subcats="0" xml:space="preserve">Czasopisma 1995</c>
      <c size="12" pages="12" files="0" subcats="0" xml:space="preserve">Czasopisma 1996</c>
      <c size="22" pages="22" files="0" subcats="0" xml:space="preserve">Czasopisma 1997</c>
      <c size="27" pages="27" files="0" subcats="0" xml:space="preserve">Czasopisma 1998</c>
    </allcategories>
  </query>
  <query-continue>
    <allcategories accontinue="Czasopisma_1999" />
  </query-continue>
</api>