<!--#command tag="value" >
Where #command can be any of various commands supported by the Web server. The simplest command is #include, which inserts the contents of another file. This 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 insert the results. They therefore represent 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 filename extension enables the Web server to differentiate those pages that need to be processed before they are sent to the browser.