Prinzhorn/skrollr ? reverse-engineered prompt

Build me a lightweight parallax scrolling library for mobile and desktop that works with plain JavaScript, not jQuery.

I want elements to animate as the page scrolls, using simple HTML data attributes on each element so the animation settings live right next to the markup. It should support changing any CSS property over scroll positions, including things like color, position, size, and transforms, and it should smoothly interpolate between keyframes. It would be great if it also supports easing for specific properties and can use viewport based points like when an element reaches the top of the screen.

Please include a simple way to initialize it on page load, a few demo pages showing common effects, and basic documentation that explains how to use it with plain HTML and CSS. Make it work in a way that feels designer friendly and easy to drop into a website. If you need to check current browser behavior or docs while implementing, look things up online first.