position: sticky lands in WebKit
position: sticky is a new way to position elements and is conceptually similar to position: fixed. The difference is that an element with position: sticky behaves like position: relative within its parent, until a given offset threshold is met in the viewport.
I’d love to see this implemented in every browser and without vendor prefixes as soon as possible with a real spec. At the moment it just works in Chrome 23.0.1247.0+ and WebKit nightly.
I had to build the same behavior with JS for the QUOTE.fm Labs and CSS would make it much easier and faster.