Field Validation and Your Users

Now, if you have a field in a form that only accepts certain values don’t you think that it would be a good idea to tell the users of the form what those values are before they’ve filled them in? I am sick of the “invalid email” error after a form has been submitted. Why is so hard to indicate what valid values are beside the field? Or why can’t this be validated in JavaScript? It’s simply a lack of thought by the page designer/programmer.