服务器端/客户端或两者都用户输入验证

What is the best way to use validation on your site when I want to give people client side "helper" validation such as password not long enough, email is incorrect format but also do server side validation and return errors such as username already exists and have both client and server validation messages visually be displayed the same to the user with the minimal amount of duplication. What is the best way to use validation on your