|server side includes (ssi)|
An SSI page is looked over by the web server on the way out. Specially formatted comments are treated as commands.
Normally the Apache web server (if configured to serve SSI) will treat a file named .shtml as an SSI page.
The most popular uses are to include the date of last update of the page directly within the page itself, and to include a header or footer directly withing the page.
Downside. An ssi which performs an "include" insert a space into the html which may effect formatting.