Forms
The UX rule for submit button state when a form is invalid, and why it matters.
A very important note here is that you should NOT make submit buttons disabled when the form is not valid.
If a design shows that a submit button is disabled, this is likely out of date. DO NOT MAKE SUBMIT BUTTONS DISABLED.
When a submit button is disabled, that just tells the user "the form is invalid" (though this isn't completely true, it could indicate some other reason that they can't submit the form which may not be because of their input) but knowing that is not useful to them. They want to know how to fix the form. Instead, when the user clicks the submit button and the form is invalid it makes all of the validation messages visible. (A note: What this means is that users will only see the validation messages when they've indicated "I think what I've entered here is correct" by them focusing and then blurring the field or attempting to submit the form because we don't want to bombard them with messages when they're still filling it in and have done nothing wrong but if they have said "I think what I've entered here is correct," we should then show them why their input is not correct.)