This question about web page ‘inclusion’ triggered my curiosity. The XML allows you to specify a reference to an ‘external’ portion. Can you do the same in XHTML, then?
XML example:
<!ENTITY foo SYSTEM "http://www.mydomain.com/logo.gif" NDATA GIF87A>
...
<SOMETHING>&foo;</SOMETHING>
Yes but…
text/htmlmime type (goodbye IE) or in any browser which doesn’t do DTD processing (probably goodbye to most other browsers).