HTML is based on SGML. SGML has always supported this sort of thing.
HTML browser vendors have chosen not to implement it. You can do it,
but Netscape and Mosaic will not display it correctly. The SGML feature
you are looking for is known as an entity. You can learn all about it from
any SGML book, but it won't do you much good on the World Wide Web.
While I am on the topic: a question for the readers.
Should the HTML WG specify a mapping from SYSTEM identifiers in HTML
documents to URLs? Wouldn't that be a step towards external entity
support? The HTML standard seems to completely ignore the concept of
external entities. For instance I would _much_ rather use external
entities to deal with pictures than CDATA attributes. Of course the
CDATA attributes are not going to go away, but shouldn't the SGML entity
way be also allowed?
Paul Prescod