una cosa del tipo:

codice:
<!DOCTYPE pictures [
   <!ELEMENT pictures (photo,photo)>
   <!ELEMENT photo (floor*)>
   <!ELEMENT floor (link*)>
   <!ELEMENT link (#PCDATA)>
   <!ATTLIST photo 
       name CDATA #IMPLIED 
       src CDATA #REQUIRED>
   <!ATTLIST floor
       name CDATA #IMPLIED>
   <!ATTLIST link url CDATA #REQUIRED>
]>