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-Fréttinga" label="Fréttinga" description="Translation of the wiki page [[:Fréttinga|Fréttinga]]." class="WikiPageMessageGroup" exists="1" />
      <group id="page-Guifi - Media-pool Common Repository:About" label="Guifi - Media-pool Common Repository:About" description="Translation of the wiki page [[:Guifi - Media-pool Common Repository:About|Guifi - Media-pool Common Repository:About]]." 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" />
      <group id="!sandbox" label="Sandbox messages" description="Suggests messages to translate for sandboxed users" class="SandboxMessageGroup" exists="1" />
    </messagegroups>
  </query>
</api>