As a backend developer who build a register or login page should consider these for coding the form.

1. Don't allow the user details to be very long. In terms of them writing their first name,last name and other name, it's unnecessary. Instead of a user should write their full name in a input.
2. Don't put validation in the username. In terms of "your username is not greater 8 character" avoid unnecessary validation. Let the user choose his/her choice of username.
3. Always allow the user to know their mistake before submitting the form. In terms of user inputting their password and confirm password and make a message box to show their error before submitting into the form.
4. If the user input details is long is in form of stack step try as much as possible with multiple step. Thanks you for following us give us a feedback.