Positive feedback when valid

It is important to inform users when they made a mistake or when something went wrong, websites tell them al the time, but it is just as important to let them know when they did something right.

Therefor I would like to suggest to give positive feedback when a form field is valid. The span behind a form field is now visible when there is an error on a form field.

<span htmlfor="edit-subject" generated="true" class="error" style="display: none;">Subject required</span>

When a form field is valid there should be a something like a "tick" image displayed to give positive feedback to a user.

So I think the span behind a field should be more something like this example
<span htmlfor="edit-subject" generated="true" class="feedback error" style="display: none;">Subject required</span>
<span htmlfor="edit-subject" generated="true" class="feedback valid" style="display: inline;">Correct, well done!</span>
Or one span that changes to valid or error state depending on the situation, don't know what would make more sense in terms of the existing code.

Of course this can already be achieved by some CSS or other customization, but I think this should be a default feature inside this module and I don't expect it, correct me if i'm wrong, to be that hard to submit.

It might not be usefull for al fields so maybe it should be used on certain field types or be optional. I think it would be useful to have other people put it then there thoughts about this.