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="Ball2.png|861" />
  </query-continue>
  <query>
    <pages>
      <page pageid="208" ns="0" title="Starter Pokémon" />
      <page pageid="209" ns="0" title="Route 29" />
      <page pageid="256" ns="0" title="Route 1" />
      <page pageid="260" ns="0" title="Pewter City" />
      <page pageid="497" ns="0" title="Water Laboratory" />
      <page pageid="736" ns="0" title="Viridian City" />
      <page pageid="739" ns="0" title="Route 22" />
      <page pageid="859" ns="0" title="Route 10" />
      <page pageid="2192" ns="0" title="Bagon" />
      <page pageid="2193" ns="0" title="Shelgon" />
    </pages>
  </query>
</api>