you can write a script to send HTTP probe to a server and look at the response received.
Use the command 'socket waitfor' to look for the "Hello World" string.
To send the *reversed* response, you need to use a primary sorry server with a 2nd service.
This service could be a different server, or a different service on the same physical server or a redirect service to a different web page where you could have a file with the response that you want.
The CSS can't generate a response itself.
It can only get it from somewhere else.
Regards,
Gilles.