Responsive design mobile first inclusive. flexbox and grid.

difficult is to manage the content of the main div not to overflow the div when resizing the window. I couldn't figure out how this could be done. I tried a min height or min width but when I applied those css rules the main container was way too big when I resized the window to a smaller size. So I still struggle with this. Was a nice challenge I really enjoyed it because I can now fully understand CSS Grid. Thanks !