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>
    <allfileusages gafcontinue="CLASSIFICATION_02.PNG|1420" />
  </query-continue>
  <query>
    <pages>
      <page pageid="341" ns="10" title="Template:Ombox" />
      <page pageid="409" ns="10" title="Template:Navbox" />
      <page pageid="570" ns="10" title="Template:Ombox/doc" />
      <page pageid="760" ns="10" title="Template:Navbox/doc" />
      <page pageid="1270" ns="500" title="HowTo:Customize Logo and Background on Mobile Client" />
      <page pageid="1420" ns="500" title="HowTo:Assign a Classification to a MetaClass" />
      <page pageid="3390" ns="0" title="XAML Basics" />
      <page pageid="3391" ns="0" title="XAML Best practices" />
      <page pageid="3587" ns="0" title="Configure EventNotification Endpoint and Enterprise Service for CUI" />
    </pages>
  </query>
</api>