Here is a link to someone's [link|http://www.dansteinman.com/dynduo/en/source.html|explanation] of old solutions.

Here something much better: [link|http://www.w3.org/TR/html401/struct/objects.html#h-13.1|W3C HTML 4.01 Spec]
To include one HTML document in another, authors may use either the new IFRAME element or the OBJECT element. In both cases, the embedded document remains independent of the main document. Visual user agents may present the embedded document in a distinct window within the main document. Please consult the notes on embedded documents for a comparison of OBJECT and IFRAME for document inclusion.
W3C HTML 4.01 Spec [link|http://www.w3.org/TR/html401/struct/objects.html#h-13.5|Notes on Embedded Documents.]
...text before...
<OBJECT data="embed_me.html">
Warning: embed_me.html could not be embedded.
</OBJECT>
...text after...

Note: I checked this out and it works with Mozzila.

HTH