Server Side Includes (SSI) is a simple interpreted server-side scripting language used almost exclusively for the World Wide Web.
[2] In order for a web server to recognize an SSI-enabled HTML file and therefore carry out these instructions, either the filename should end with a special extension, by default .shtml, .stm, .shtm, or, if the server is configured to allow this, set the execution bit of the file.
Apache tutorial on SSI stipulates the format requires a space character before the "-->" that closes the element.
The inclusion is not limited to files, and may also be the text output from a program, or the value of a system variable such as the current time.
They include the ubiquitous if-elif-else-endif flow control and variable writing as well as more exotic features like loops only found in some implementations.