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="1" pages="0" files="0" subcats="1" xml:space="preserve">Advanced Micro Devices products</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Advanced Micro Devices x86 microprocessors</c>
      <c size="1" pages="0" files="0" subcats="1" xml:space="preserve">Computer hardware navigational boxes</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Lists of microprocessors</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Stránky s odkazy na neexistující soubory</c>
    </allcategories>
  </query>
</api>