HTML forms, which collect data from users, usually have more going on under the hood than meets the eye. They need to check the inputs to make sure they’re formatted correctly for the database. The server needs to validate the input, but validating input at the client makes for a better user experience. Let’s look at the what, why, and how of validating HTML forms on the client-side.

DavidVanVickle.com Javascript Validation Demo and Tutorial

webcheatsheet.com Validation Functions