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>
    <blocks>
      <block id="5" user="0983867547" by="Nguyenthephuc" timestamp="2011-06-28T13:35:06Z" expiry="infinity" reason="Unacceptable username" nocreate="" autoblock="" allowusertalk="" />
      <block id="4" user="Pranavwright" by="Cao Xuân Hiếu" timestamp="2011-05-23T06:43:37Z" expiry="infinity" reason="spammer" nocreate="" autoblock="" noemail="" allowusertalk="" />
      <block id="3" user="113.168.15.204" by="Nguyenthephuc" timestamp="2010-08-10T03:09:35Z" expiry="infinity" reason="Spamming links to external sites" anononly="" nocreate="" allowusertalk="" />
      <block id="2" user="AlanaDowning" by="WikiSysop" timestamp="2010-03-19T04:54:57Z" expiry="infinity" reason="" nocreate="" autoblock="" noemail="" allowusertalk="" />
      <block id="1" user="Giaoandddddddddddddddddddd" by="Nguyenthephuc" timestamp="2008-09-05T15:02:24Z" expiry="infinity" reason="Cấm vô thời hạn, lí do: tên tài khoản thiếu nghiêm túc" nocreate="" autoblock="" allowusertalk="" />
    </blocks>
  </query>
</api>