Jarallax is an open-source javascript library which makes adjusting css based on interaction easy. With Jarallax it's easy to create a parallax scrolling website.
jParallax turns a selected element into a 'window', or viewport, and all its children into absolutely positioned layers that can be seen through the viewport. These layers move in response to the mouse, and, depending on their dimensions (and options for layer initialisation), they move by different amounts, in a parallaxy kind of way.