[ Thanks to jmalasko for this link.
]
“Client-side validation is useful because it can reduce
or even eliminate the cases when the form is returned to the user
for correcting errors. Nevertheless, the JavaScript-based
validation is not sufficient because it can be bypassed if the data
is submitted to the server with a program or if JavaScript is
disabled in the user’s browser. This article shows how to implement
both client-side and server-side validation.”