The web server (like Apache or Nginx) must be explicitly told to "parse" .shtml files for SSI commands. If this is disabled, the server treats it as a plain text or HTML file.
"Here’s the problem," Elena pointed to the screen. "The mod_include module is loaded, but the AddOutputFilter directive for .shtml is missing. The server doesn't know it needs to process those files." view shtml fix
Finding a fix for .shtml files that won't display properly usually boils down to one simple thing: isn't enabled on your web server. The web server (like Apache or Nginx) must
Nginx does not parse SSI by default. The ssi on; directive explicitly tells Nginx to scan SHTML files for include directives. view shtml fix