Validate on Submit

Your visitor may not access all the fields in the form so validating each field individually is not enough.

As well as validating individual fields in a form you should also revalidate all of the fields again when the form is submitted just in case your visitor skipped over some fields and didn't trigger the individual field validation. This gives your page one last chance to tell them what they did wrong without having to wait for a response from the server. Of course you still need to validate again on the server just in case your visitor has JavaScript disabled but at least you can make the validation quicker for everyone else.

Validate on Submit