A type of HTML comment that directs the Web server to dynamically generate data for the Web page whenever it is requested. The simplest command is #include, which inserts the contents of another file. It is especially useful for ensuring that boilerplate components, such as headers and footers, are the same on all pages throughout a Web site. To change a boilerplate element, you need only modify the include file, instead of updating every individual Web page.
SSIs can also be used to execute programs and to insert the results, and this makes them a powerful tool for Web developers. There is no official standard for SSIs, so every Web server is free to support different SSIs in different manners.
However, many SSI commands, such as #include and #exec, have become de facto standards. Web pages that contain SSIs often end with a .shtml extension, though this is not a requirement. The file name extension enables the Web server to differentiate those pages that need to be processed before they are sent to the browser.