David Woolley <david@djwhome.demon.co.uk> wrote:

[client side inclusion]

>It does define the object element that will, in conforming browsers,
>provide much of this capability, although I'm not sure that the semantics
>of links in objects are well defined.

HTML embedded with the object element as defined in HTML 4 should render
in a separate viewport. Only full HTML documents should be embedded this
way. What the OP is asking for is including code *fragments* in way that
currently Server Side Inclusion provides for. These code fragments would
become an integral part of the original document. (please ignore me if
the <object> element has been redefined in the XHTML2 proposals to allow
code fragment inclusion)

Needless to say that such a feature would break every existing UA, and
it should only be considered for a language that is not backward
compatible. Whether or not that includes XHTML 2 is unclear to me given
the presence of legacy elements such as <img> apparently for reasons of
backward compatibility.

>My own solution to this problem would be to take HTML back to the its
>roots and not to try and use it as a page description language, but,
>rather, have browsers make appropriate use of link elements to display
>associated navigation, etc.

The OP specifically mentioned headers and footers.

-- 
Spartanicus