...

... Changing standard_error_message does not seem change this error page.

Maybe, you made a mistake?

Yes, I made a mistake! In standard_error_message: <dtml-if error_message> <dtml-var error_message> <dtml-else> <h2>My TEST Site Error</h2> ... 404 errors hit the dtml-if and other errors, like attribute errors, go to the dtml-else. I was just modifying the else. Thanks!