> I never understood why there wasn't an <include>
    > element in HTML, that would just paste the content of
    > the linked document. Why wasn't such an element added
    > in the late 90's or early 2000?

As far as I know, for SGML (and therefore old fashioned XML, too), 'include' gets realized via parsing of external &entities;. That's how the Mozilla XUL applications (Seamonkey, Firefox, Thunderbird) etc. realized localization, for example.

I don't know, when XInclude has first been ratified (the current, 2nd revision, of the spec is from 2006), but with it, a generic inclusion mechanism exists for XML, therefore also for XHTML.

With XHTML you could (in theory, as said, the spec is there) use XInclude.

    <?xml version='1.0'?>
    <html xmlns="http:www.w3.org/1999/xhtml" 
          xmlns:xi="http://www.w3.org/2001/XInclude">
      <head>        
      </head>
      <body>
        <p>120 MHz is adequate for an average home user.</p>
        <xi:include href="disclaimer.xml"/>
      </body>
    </html>

I am not quite sure, yet, how namespace mixins are handled in XHTML, I think, one must define a new schema (which, honestly is a crappy requirement and should be abandoned), effectively creating an XHTML+XInclude document type, but that may be only a real issue for strict validation.