No &-References to Unparsed Entities

In other words, you can't do the following:

<!ENTITY Madonna SYSTEM 'Madonna.jpg' NDATA Gif>
...
<p>The picture: &Madonna;</p>

The reason is that nobody really understands what it means to embed unparsed (possibly binary, as in the example) data in the middle of some text.

Back-link to spec

Copyright © 1998, Tim Bray. All rights reserved.