Validation of user input has always been an important aspect of user interface programming. You have to make sure that the values a user provides to your program are of the right type, in the right range, etc. and you should provide meaningful feedback to help users correct any errors.
I have often blogged my opinion that validation rules are business logic, and that they should not reside in the presentation layer. The presentation layer may apply validation rules, but developers should not have to modify presentation layer code (including JSPs) when validation rules change.