Client Side Validation

ASP.NET automatically adds JavaScript code for client side validation.In this case, when the user clicks a CausesValidation button, the same error message will appear without the page needing to be submitted and returned from the server.This increase the representation of your web page.
However, even if the page validates successfully on the client side,ASP.NET still revalidates it when it's received at the server.This is because it's easy for an experienced user to circumvent client side validation. For example, a malicious user might delete the block of JavaScript validation. For example, a malicious user might delete the block of JavaScript validation code and continue working with the page.By performing the validation at both ends,ASP.NET makes sure your application can be responsive as possible while also remaining secure.