This is the reason why it is a good practice to either put a note on the top of the form that indicates that all fields marked with an asterisk are required or to use required field markers.In case that all fields are required there is no need to place asterisks or markers in the form.This design pattern is also often called forgiving format UI design pattern.
Thus, validation has to ensure that the user provided all the necessary details in the web form and it has to fail if at least one of the fields is not provided.
[Links checked February/10/2017] User’s input can be validated on the server and on the client (web browser).
Thus we have server-side and client-side validation. In the server-side validation, information is being sent to the server and validated using one of server-side languages.
By combining server-side and client-side methods we can get the best of the two: fast response, more secure validation and better user experience.
There are several different types of validation you can perform: required fields, correct format and confirmation fields.