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>
    <allpages apcontinue="Glossary" />
  </query-continue>
  <query>
    <allpages>
      <p pageid="139" ns="0" title="Blocked applications" />
      <p pageid="4" ns="0" title="CSV upload" />
      <p pageid="141" ns="0" title="Cell-id related web services" />
      <p pageid="43" ns="0" title="Collaboration with other projects" />
      <p pageid="137" ns="0" title="Commercial users" />
      <p pageid="11" ns="0" title="Community servers" />
      <p pageid="7" ns="0" title="Data sources" />
      <p pageid="15" ns="0" title="Editor support" />
      <p pageid="18" ns="0" title="FAQ" />
      <p pageid="147" ns="0" title="Future OpenCellID database content" />
    </allpages>
  </query>
</api>