>> They implement the effect by using a rather large background image and a mask image and move the background image around via Javascript. Scrolling the backg

>> They implement the effect by using a rather large background image and a mask image and move the background image around via Javascript. Scrolling the background image position with javascript can be really slow on even recent browsers, pushing the CPU when software rendering. Mobile Safari also shows image "seams" between mask images when zoomed out.