When building forms, it’s important to be able to monitor or control the content users enter into the form fields. This is called validation. For quality assurance, you should conduct validation on both the client and server side of your application.To help with validation, the HTML input tag has a placeholder option. You can use this to effectively direct users to the exact information you need from them. However, placeholders aren’t enough on their own to alert users when they’ve entered invalid information. That’s why developers often write scripts to prompt the user to avoid entering invalid characters.