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>
    <messagegroups>
      <group id="page-Main Page" label="Main Page" description="Translation of the wiki page [[:Main Page|Main Page]]." class="WikiPageMessageGroup" exists="1" />
      <group id="page-Overview" label="Overview" description="Translation of the wiki page [[:Overview|Overview]]." class="WikiPageMessageGroup" exists="1" />
      <group id="!additions" label="Recent additions" description="This message group shows new and changed messages." class="RecentAdditionsMessageGroup" exists="1" />
      <group id="!recent" label="Recent translations" description="This message group shows all recent translations into this language.&#10;Most useful for the review tasks." class="RecentMessageGroup" exists="1" />
    </messagegroups>
  </query>
</api>