Form Validation
http://developer.apple.com/internet/webcontent/validation.html

Apple Inc.
2006

Abstract:

Any sort of interactive site is going to have form inputs — a place where your users input who they are, what they want to buy, where they live, and so forth. This data is passed to whatever handles your back end — a Perl CGI script, a PHP engine, a database like Oracle, or some other technology you’ve invested in. Whatever system is back there, you can bet that it doesn’t appreciate having its time wasted with bogus information, and chances are the user doesn’t appreciate it either. If the data the user submits to the CGI contains an error, there will be a noticeable lag — typically several seconds — before the information travels over the Internet to the server, is examined on the server, and then returns to the user along with an irritating error message. If you run a little preliminary validation of the user’s form input before the form is submitted, there will be no wait time. Client-side validation is instantaneous because it doesn’t have to transmit any data. JavaScript catches any erroneous data the user enters before it goes anywhere.