You got it. And yes, it does seem that it isn't possible without intervening javascript before going to the cgi code. For doing simple forms, that's not so bad. But when you don't know whether data in a form isn't right until you've gone to the server side (which may require doing db searches, etc.), you're basically forced to redraw the page... and that may also be an equally expensive operation.

This is exactly why people use javascript to do client side data validation. It's an extremely common practice to have a bit of javascript triggered before you actually send the datato make sure it's all in the right format. Done correctly theres absolutely no server side work and no communication or time.

But of course you can't trust the client to validate correctly. Never trust the client. You still have to validate server side.