This example uses editable fields that are normally hidden from sight (not the more common hidden field that is not editable). I heard about this method from Murray, Expression Web MVP. The code used for this page is here.
If the hidden fields are completed by the user, then either the user is a spambot, or the browser does not support CSS.
In this demonstration if the hidden fields are changed by the user an error is generated.
Note: This form does not submit any data to anywhere. It is for demonstration purposes only.