Responsive Tip Calculator App

The JS part of the challenge was a bit complicated for me, mainly because the calculator doesn't have any calculate/submit button (which is strange). As a result, I managed to handle the inputs with focusout event, so that the tip amount is calculated each time all 3 inputs are filled in and focused out. Not sure whether it was an optimal solution, so if you have any suggestions on how to do it better, please tell me 🙂 Thanks in advance)