Takeaway: Reducing processing overhead can improve the performance of Web sites with heavy traffic patterns. Using Client Side Includes to perform functions on the client side is one technique developers can use to reduce overhead. Here's how it works.
SSI files themselves can have any extension, but by default a page that uses one or more of them has the extension .SHTML to signify to the Web server that this page needs to be handled differently to a standard HTML file.
<!--#include virtual="/includes/new.txt" -->
A Client Side Include (CSI) is usually a short JavaScript function or routine contained in an External JavaScript file and is called in the same place as the SSI would be. CSIs can be contained either in a stand-alone file or included within a global JavaScript file which contains other useful client side JavaScript functions that the site may require.
The JavaScript version is much, much faster, and it places no more load on the server than an ordinary Web page.
clipped from: articles.techrepublic.com.com