- To create a dependable, configurable and logical method for manipulating the position of DOM elements relative to scroll position without the use of JS.
- Anything in addition to position control is a bonus.
It's easy to achieve scroll effects with JS but it usually comes at a high repaint/redraw cost when elements are being altered post render. There are ways to curb these effects but it would be preferable to eliminate JS in as many of these scenarios as possible.
- CSS is largely unaware of the scroll position of a document. Currently the only explicit reference to scroll I know of is the
position: stickyattribute. It's still in working draft but has potential for further control of these elements.