I’m having hard time even to start with this, what is horizontal and how to set the left offset?
Horizontal movement is left to right or right to left movement. In two-dimensional space, it would be movement along the x-axis. Vertical movement would be along the y-axis and would be up or down movement.
skewY: left; it would be the answer?
skeyY? The instructions tell you to add the left offset property. See the example where they used the top offset property. The values to be used for the left offset property are 0 pixels at 0%, 25 pixels at 50% and -25 pixels at 100%.