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="4" ns="0" title="Página principal" />
      <page pageid="32" ns="0" title="Dulce Chacón Gutiérrez" />
      <page pageid="47" ns="0" title="Hamlet Príncipe de Dinamarca" />
      <page pageid="49" ns="0" title="El Dador" />
      <page pageid="204" ns="0" title="El extraño caso del Dr. Jekyll y Mr. Hyde" />
      <page pageid="215" ns="12" title="Ayuda:Página no encontrada (Error 404)" />
      <page pageid="220" ns="0" title="Campos de fresas" />
    </pages>
  </query>
</api>