Back Tolerance Area (Tolerance = 100px)

Initially no options are passed to the :in-viewport selector.

Hence all visible div(s) are light grey coloured as default tolerance is height of window.

onscroll, in-viewport is called with tolerance as 100

and debug as true for the div with class `js-hook` ie the 2nd div.

Therefore 2nd div turns light grey only when it is inside tolerance area.