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>
    <allredirects garcontinue="Guide_to_GEOS-Chem_error_messages|10" />
  </query-continue>
  <query>
    <pages>
      <page pageid="813" ns="0" title="GCST" />
      <page pageid="1576" ns="0" title="List of diagnostics archived to netCDF format" />
      <page pageid="1582" ns="0" title="GEOS-Chem v11-03" />
      <page pageid="1583" ns="0" title="GEOS-Chem v11-03 benchmark history" />
      <page pageid="1708" ns="0" title="GEOS-Chem coding style guide" />
      <page pageid="1795" ns="0" title="Benchmark/" />
      <page pageid="1821" ns="0" title="GEOS-Chem Output Files" />
      <page pageid="1955" ns="0" title="Building GEOS-Chem with CMake" />
      <page pageid="2077" ns="0" title="Benchmark/GEOS-Chem 13.0.0" />
      <page pageid="2181" ns="0" title="Emissions and Deposition Working Group" />
    </pages>
  </query>
</api>