TODO where is it specified? http://www.w3.org/CGI/ says it's not maintained anymore.
Protocol of how a server communicates with a CGI script.
A CGI script is simply a script/executable that outputs the part of HTTP response.
This part includes some last header lines which the server delegates to it, notably content type, followed by \n\n, followed by the entire body.
The server passes information to the script through environment variables only.