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="!sandbox" label="Sandbox messages" description="Suggests messages to translate for sandboxed users" class="SandboxMessageGroup" exists="" />
      <group id="!additions" label="最近添加" description="此信息组显示新出现和已变更的信息。" class="RecentAdditionsMessageGroup" exists="" />
      <group id="!recent" label="最近翻译" description="本信息组显示该语言的所有最近翻译。对复核任务很实用。" class="RecentMessageGroup" exists="" />
    </messagegroups>
  </query>
</api>