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>
    <allpages>
      <p pageid="9" ns="0" title="Instalar SQLDeveloper" />
      <p pageid="2" ns="0" title="Mini-guía" />
      <p pageid="7" ns="0" title="Practica1 - Apartado 5" />
      <p pageid="15" ns="0" title="Pregunta examen" />
      <p pageid="53" ns="0" title="Problemas Máquina virtual" />
      <p pageid="51" ns="0" title="Prácticas ABD 2015" />
      <p pageid="1" ns="0" title="Página principal" />
      <p pageid="49" ns="0" title="Resumen comandos básicos mongoDB" />
      <p pageid="48" ns="0" title="Resumen comandos usados" />
      <p pageid="3" ns="0" title="Términos de servicio" />
    </allpages>
  </query>
</api>