Keith Clark explains the core concept behind parallax scrolling (e.g. different elements scroll at different rates) with just CSS. It’s a devilishly clever (yet simple) combination of 3D transforms to push the element “away” from the screen, affecting it’s scroll rate, while adjusting the scale so the element appears at it’s original size.
Scott Kellum and Brenna O’Brien worked up a Sass @mixin for it a while back which makes it even easier.
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Pure CSS parallax scrolling websites is a post from CSS-Tricks
