Fades the element in from opacity: 0 to opacity: 1 between 10% and 30% of the viewport height.
Fades the element out from opacity: 1 to opacity: 0.5 between 20% and 40% of the viewport height.
Changes the opacity from 0.3 to 0.7 as the element moves between 15% and 25% of the viewport height
Fades the element from opacity: 0 to opacity: 1 between 10% and 30% of the viewport on screens larger than 768px. On screens smaller than 768px, the opacity change will happen at half the speed.
