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"?>
    <alltransclusions gatcontinue="Cc-by-3.0" />
      <page ns="10" title="Plantilla:Borrar" missing="" />
      <page ns="10" title="Plantilla:CC-BY-SA" missing="" />
      <page ns="10" title="Plantilla:COSL Nav" missing="" />
      <page ns="10" title="Plantilla:Caso de estudio" missing="" />
      <page pageid="4489" ns="10" title="Plantilla:BD/Mysql" />
      <page pageid="4848" ns="10" title="Plantilla:BD/Oracle" />
      <page pageid="3229" ns="10" title="Plantilla:Barra porcentaje" />
      <page pageid="4814" ns="10" title="Plantilla:Bases Datos/Modelo Logico" />
      <page pageid="4840" ns="10" title="Plantilla:Bases datos/Nivel Conceptual" />
      <page pageid="760" ns="10" title="Plantilla:CPDGA ContentInfobox" />