If the entered data follows all those rules, it is considered valid; if not, it is considered invalid.
When an element is valid: If you want to take control over the look and feel of native error messages, or if you want to deal with browsers that do not support HTML's built-in form validation, you must use Java Script.
If you have already created a Web site in Visual Studio (for example, by working with the topic ), you can use that Web site and skip to "Adding Controls" later in this walkthrough. In effect, you write client script that duplicates the logic that your server-side validation performs.
Writing client-side custom validation code is not always practical (for example, if the custom code validates a user's entry by looking in a server-side database).
You will also create a custom validator with code that you write, which illustrates how you can add your own logic to the validation framework in the page.
Finally, you will learn how you can validate user input conditionally, depending on choices that the user makes in a page.
In the walkthrough, you will create a page for a Web site that allows visitors to request a reservation.