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>
    <pages>
      <page pageid="1" ns="0" title="Main Page">
        <linkshere>
          <lh pageid="80" ns="0" title="GEOS-Chem newsletters" />
          <lh pageid="188" ns="0" title="GEOS-Chem Newsletter February 2010" />
          <lh pageid="214" ns="0" title="GEOS-Chem Newsletter March 2010" />
          <lh pageid="1515" ns="0" title="GCHP v11-02" />
          <lh pageid="1641" ns="0" title="GEOS-Chem 12" />
          <lh pageid="1827" ns="0" title="GEOS-Chem overview" />
          <lh pageid="1836" ns="0" title="Guide to compilers for GEOS-Chem" />
          <lh pageid="1852" ns="0" title="Guide to using Git with GEOS-Chem" />
          <lh pageid="1949" ns="0" title="Guide to GEOS-Chem simulations" />
          <lh pageid="2263" ns="0" title="GEOS-Chem in CESM" />
        </linkshere>
      </page>
    </pages>
  </query>
</api>