If you’re gonna put a form on your web page that checks to see what people are entering in the fields, and won’t let them move on to the next field unless they complete the first one properly, then…
…give the user feedback as to what they’re doing wrong so they can fix it!
Or… you know…. tell them how to do it right in the first place.
Otherwise, they may spend valuable time cursing you, trying multiple web browsers, and calling you, all to figure out why the hell they can’t use your form.