inline form validation when user tabs out of an empty field

Question

My question is similar to Should inline validation check empty fields if user only tab the field but do nothing and skip to next field?, but a bit different.

Let's say I have a form containing a few text fields. Each of these fields are mandatory and information about that "mandatoriness" is very obvious in copy as well as in the code. if a field is filled with invalid data, an error message shall appear along that field.

The error message can be triggered either if

1. The user blur/tabs out of an empty field, or
2. The user types something incorrect and then blur/tabs out of the field.

When a keyboard-user tabs trough this form, (s)he will get an error message on each field just by navigating in the form. It seems odd to show an error message when a user has not yet done anything.

Question: When shall we do that inline validation? On 1 or 2?

Answer 1

I'd vouch for showing the error message even if a user is only navigating through a form.

Users are less likely to unintentionally skip required fields

Leaving optional fields empty even if interacted with, results in a blank field with no error messages. Doing the same for fields that do require input, leads to a potential misunderstanding that the field is optional and can be ignored. This is not the case, so it's better to be clear about that and prevent users from submitting the form and getting a ton of errors back.

Users can locate 'mistakes' easier

Telling users about any errors right away ensures users won't be confused when the form is unable to be sent later on. Many forms disable the submit button if a required field is ignored. Sure, users can scroll back up scan for blank fields, but they will be once again be subjected to the mental load of processing forms. We want to make users take less time filling in our forms, not more.

These same principles apply to inline validation in general.