Implementation wise, it uses 75 image frames that are displayed according to percentage of the page scrolled. Before I looked, I thought this might be a HTML5 <video> controlled using the scroll events - wonder if that would be feasible?
Also submitted 19 hours ago, but with no discussion:<p><a href="https://news.ycombinator.com/item?id=6627541" rel="nofollow">https://news.ycombinator.com/item?id=6627541</a>