Just to clarify, you can use SSI's to include ANY valid data - some useful examples -
- Plain text file (no headers required, a single word, even)
- output from a script or a compiled program (see SSI man. for exec directive). In this case, you must print content-type: text/html headers from your external program (or other valid content-types.)
- RSS/XML (but your document has to be a valid XML/XHTML doctype and you have to provide XML headers)
- image or other binary data
Basically anything you can output in a page you can include in the page using SSI. It doesn't have to be a valid HTML/XHTML document - in fact, doing so might screw up a perfectly valid document:
<!-- below is my include, do the math! -->
<p>Two plus Two.</p>
<!-- End of my include -->
The data you include becomes part of the page, so you just validate the entire .shtml with all the data imported.
In older server O.S's the use of SSI's presented serious security flaws and it was a memory hog (because it parses files before reading instead of straight serving) but it's no longer the case.