For centuries, whenever you scrolled down a page full of content, nothing exciting happened. The content you scrolled into view just appeared. The content that you scrolled out of view just disappeared. The most exciting thing really was being able to use your fingers to scroll on touch devices as opposed to relying only on those gray scroll bars that annoyingly clung to the edges of your screen. Blech.
With improvements in CSS and the DOM APIs, something even more exciting became possible. To see this for yourself, visit http://bit.ly/scroll_animation_example or check out the following video:
Notice what happens once you start scrolling through the content. Depending on how far and how fast you are scrolling and what content is currently visible, you are going to see all sorts of awesomeness in the form of animations where your background color changes, content slides in from all directions, things fade into view, and so on. There is a name for all of these animations that play as you are scrolling, and that awfully boring name is scroll-activated animations.
In the following sections, you’ll learn all about scroll-activated animations and the various techniques we can use to bring scrolled elements to life. The way we’ll do that is by first taking a bird’s-eye view of the problem and what our solution will look like. Then, ...