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="701" ns="0" title="Basic Troubleshooting" />
      <page pageid="702" ns="0" title="Curating Your Corpus for Better Analysis" />
      <page pageid="704" ns="0" title="Getting Lots of Data" />
      <page pageid="3" ns="0" title="Getting Started" />
      <page pageid="4" ns="0" title="Working with JSTOR Data for Research" />
    </pages>
  </query>
</api>