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>
    <allpages apcontinue="OBS_v2" />
  </query-continue>
  <query>
    <allpages>
      <p pageid="21" ns="0" title="Data Sources" />
      <p pageid="13" ns="0" title="Dbf, prj, shp, shx." />
      <p pageid="15" ns="0" title="Documentos Grupo Trabajo Observatorio" />
      <p pageid="14" ns="0" title="Glosario" />
      <p pageid="34" ns="0" title="Guia GeoNode en Linea" />
      <p pageid="5" ns="0" title="Guias" />
      <p pageid="458" ns="0" title="Indicadores Sustentables de Barrios" />
      <p pageid="444" ns="0" title="Informacion - Proyectos CEDEUS" />
      <p pageid="50" ns="0" title="Informativo de Observatorio - Agenda de Ediciones" />
      <p pageid="2" ns="0" title="Metadatos requeridos" />
    </allpages>
  </query>
</api>