By default, the web server checks for the presence of SSI instructions only files with the extension *.shtml. For example, put the SSI commands in the index.shtml file and it will work right away. If there is a need to check for the presence of SSI other files, the corresponding extensions for checking can be added by placing the following instruction in the .htaccess file:

AddHandler server-parsed .ext1

.ext1 here is the file extension that should be checked for the presence of SSI in them. It is not recommended to add extensions .html and .htm, since in this case, the server will check all html files for SSI inserts. This can lead to a completely senseless load on the server.