SSI, Server Side Include is a simple interpreted server-side script that used shtml to include some of the option from external file.
With SSI enabled, user may use include feature to include file or virtual directory.
To enable SSI, you may use .htaccess file command as below.
AddType text/html .shtml
AddHandler server-parsed .shtml
Options Indexes FollowSymLinks Includes
With the command as above it is to tell the server that the .shtml extension (Parsed HTML).
Either way, you may use .html to act as the server-parsed file.
AddHandler server-parsed .html