This is a very frequently asked question.
I hate to beg, but...
would one of the knowledgeable and capable HTML book/intro/tutorial
writers research this issue and submit an answer to the WWW FAQ
maintainer?
Currently, that would be  Thomas Boutell <[email protected]>
Basically, the options are:
existing technologies:
	server-side includes (see NCSA server doc)
	cpp/m4/text preprocessor of your choice
possible developments:
	<a href="included.html" REL=INCLUDE>...</a>
		(we've got a student implementing this, by the way!
			Hi Alex!)
	Real SGML Entities (tm)
Daniel W. Connolly        "We believe in the interconnectedness of all things"
Research Associate, MIT/W3C   PGP: EDF8 A8E4 F3BB 0F3C  FD1B 7BE0 716C FF21 
<[email protected]>             http://www.w3.org/hypertext/WWW/People/Connolly