AJAX form validating

Drupal has a wonderful form processing subsystem, especially in 5.x.

Now, hook_form_validate only gets involked after user pressed the submit button. But the form items which are rejected by hook_form_validated are not as striking as what we need. There are lots of average users who just asking, why the form remains the same after I press Submit.

If validating could be done via AJAX, the fields which have errors (set by form_set_error) should be blinking several times, or something else that will get huge impression to the user end.

ATM, ajax form validating should be compatible with 5.x, which means no more API changes.