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="Accounting" />
  </query-continue>
  <query>
    <allcategories>
      <c size="9" pages="9" files="0" subcats="0" xml:space="preserve">2nd ADempiere European Conference</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">3.8 Trunk Candidates</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">ADempiereWiki users</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">ADempiereWiki users by country of residence</c>
      <c size="1" pages="0" files="0" subcats="1" xml:space="preserve">ADempiereWiki users by job</c>
      <c size="21" pages="11" files="0" subcats="10" xml:space="preserve">ADempiereWiki users by spoken language</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">ADempiere European Conference</c>
      <c size="6" pages="6" files="0" subcats="0" xml:space="preserve">ADempiere Virtual Appliance</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">ADempiere Wiki</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">ADempiere World Conference 2011</c>
    </allcategories>
  </query>
</api>