Dropdown isn't valid, does it work?

Just a simple question, the parameter ‘This input should no be empty’, according the reference, should mark an input as invalid if it is empty. However, seems it does not, I set a condition if it is invalid and still does nothing.
I checked it widely through the forum and it was already pointed in 2 posts, here my marktuff and here by eric1 just saying the same, surprisingly both were closed with no answer.