When you work with tabs or sliders, you can make a field mandatory, but you can always proceed to the next tab or slide, even if a mandatory field is not filled in. That's very inconvenient. For example, it can happen that the customer forgets to tick a field (which has an amount attached to it) and still starts to pay.

So it would be nice if you can only go to the next tab or slide if mandatory fields are filled in or checked.