 Scroll-driven animations are a common UX pattern on the web. A scroll-driven animation is linked to the scroll position of a scroll container. This means that as you scroll up or down, the linked animation goes forward or backward in direct response. You can see examples on the screen like reading indicators which move as you scroll or an element that fades in as they come into view. The scroll-driven animation specification defines two new types of timelines that you can use, the scroll-progress timeline and the view-progress timeline. Here's a code sample that uses an anonymous scroll-progress timeline to create a reading progress indicator fixed to the top of the viewport.