Notations and the Web

<!DOCTYPE image [
  <!NOTATION jpeg SYSTEM "http://www.example.org/jpeg">
  <!NOTATION png SYSTEM "http://www.example.org/png">
  <!ENTITY mypic SYSTEM "foo.jpg" NDATA jpeg>
  <!ELEMENT image (#PCDATA)*>
  <!ATTLIST image
    src ENTITY (jpeg|png) #REQUIRED
  >
]>
<image src="mypic">my picture</image>