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>
    <revisions rvcontinue="16" />
  </query-continue>
  <query>
    <pages>
      <page pageid="1" ns="0" title="Main Page">
        <revisions>
          <rev user="MediaWiki default" anon="" timestamp="2010-02-01T00:12:25Z" comment="" />
          <rev user="Tanner" timestamp="2010-02-01T00:34:15Z" comment="Add the beginning of the nice index page while retaining some stuff that may be helpful as I learn how to use a wiki." />
          <rev user="Tanner" timestamp="2010-02-01T00:53:10Z" comment="Remove a few useless links." />
          <rev user="Tanner" timestamp="2010-02-01T01:00:22Z" comment="" />
          <rev user="Tanner" timestamp="2010-02-01T01:00:49Z" comment="Fix the links" />
        </revisions>
      </page>
    </pages>
  </query>
</api>