Is the extension you have in mind processed by the server or the client?
For the server, such a feature already exists in NCSA's HTTP server. When
returning an HTML document, it will process include directives for HTML
text.
For handling on the client end, things get more complicated. The core
issue is: what does it mean to "include" a document of type X into an HTML
document? If the included document is HTML, you get stuff like this:
<head><title>Outer</title></head>
<body>
<head><title>Inner included document</title></head>
<body>Here it is!</body>
</body>
Is this meaningful? What does mean to have more than one [pick your favorite
construct] in an HTML document?
If the included document is some other type, what does it mean to include
it? What if I include a Microsoft Excel spreadsheet document--what does
that look like?
-andy