The following is a guest post by Michał Sajnóg, a front end developer at Netguru. Michał has created one of those "when you scroll to here, trigger this animation" libraries. One of the things I like about it is that it leaves as much as it can to CSS for creating and controlling the animation themselves. Not to mention it's proved itself by working well on a number of production sites. I'll let Michał walk you through it.