eXtended server-side includes (XSSI) is part of Apache, a freely distributed Web-server application that's used by many sites, including HotWired. The XSSI module (mod_include) defines a set of commands that Apache will parse after a page is requested, but before it's sent to the browser. Unlike client-side scripting languages such as JavaScript, XSSI isn't dependent on the capabilities of the browser and is relatively inexpensive for the server. XSSI allows a variety of variables to pass according to set parameters (this is where the \"extended\" part comes in), while server-side includes are usually limited to including HTML fragments into a served document.