How do I add client-side form validation to a Drupal form? How do I add client-side form validation to a Drupal form?  For example, in the comment module.  If the vistor don't enter the email address, I want to popup a window that says: "please enter the email address".

I hope someone can give me an example of how I do this.
I'm biased since i created http://drupal.org/project/clientside_validation, but it is using http://bassistance.de/jquery-plugins/jquery-plugin-validation/ to do all validations, the benefit of the module is that you don't have to write any code