e.g. <include src="http://www.xyz.com/standard-footer.html">
This would have the same structure as IMG but would allow something
from the server to be included BEFORE the page is processed.
This would allow lumps of "boiler-plate" HTML to be cached at the client
end rather than being inserted at the server and sent each time
(as for "server-side includes").
More interesting, would be to include small pieces of generated text.
This would allow the client to cache the overall (fixed) page and just
re-load the dynamic parts (which could be marked as short-lived).
e.g. <include src="http://www.xyz.com/object-abc?current-price">