Using a good library is not enough. For a simple example, try setting “height: 100vh;” on the body, and then open the page on an iPhone. You may be surprised that the height of the body does not fit into the screen and a scroll appears (I was, when I learnt about it). Similarly, there are many small differences that exist between Desktop <> Mobile or Chrome <> Safari. Sometimes the differences are significant enough to make your page appear completely broken.