Originally published on danholloran.me

If you've ever added a scroll progress bar to a page or built a "reveal on scroll" effect, you've probably reached for Intersection Observer, a scroll event listener, or a library like GSAP. They work fine, but they run on the main thread, add weight to your bundle, and often require careful cleanup. The browser has a better idea.

CSS scroll-driven animations are now stable across Chrome, Edge, and Safari 18, with Firefox catching up fast. They let you tie CSS animations directly to scroll position — no JavaScript involved. The result is compositor-thread animations that stay buttery-smooth even when your main thread is busy.

The Two Timeline Types

The core of the API is the animation-timeline property, which replaces the default time-based animation timeline with a scroll-based one. There are two flavors: scroll() and view().

scroll() tracks how far a scroll container has been scrolled from top (0%) to bottom (100%). It's the right tool for things that should reflect the document's overall scroll progress — like a reading progress bar.

@keyframes grow-bar {
  from {
    transform: scaleX(0);
  }
  to {
    transform: scaleX(1);
  }
}

.progress-bar {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 4px;
  background: oklch(60% 0.2 250);
  transform-origin: left;

  animation: grow-bar linear;
  animation-timeline: scroll(root block);
  animation-fill-mode: both;
}

That's it. scroll(root block) tells the browser to use the root scrolling container's block (vertical) axis as the timeline. No scroll listeners. No requestAnimationFrame. The browser handles it entirely at the compositor level.

view() tracks an element's visibility within its scroll container as it enters and exits the viewport. It's purpose-built for reveal animations — animating each card or section as it scrolls into view.

@keyframes fade-up {
  from {
    opacity: 0;
    translate: 0 40px;
  }
  to {
    opacity: 1;
    translate: 0 0;
  }
}

.card {
  animation: fade-up linear both;
  animation-timeline: view();
  animation-range: entry 0% entry 40%;
}

The animation-range property is the secret weapon here. It lets you specify which phase of the element's journey through the viewport should drive the animation. entry 0% entry 40% means "animate during the first 40% of the element's entrance into the viewport" — a crisp reveal that finishes before the element is fully on screen.

Scroll-Triggered Animations (Chrome 145+)

There's a newer addition worth knowing about: scroll-triggered animations. Unlike scroll-driven animations (which continuously update based on position), scroll-triggered animations fire once when a scroll threshold is crossed — similar to IntersectionObserver, but declarative in CSS.

@keyframes highlight-nav {
  to {
    background: oklch(25% 0 0);
    box-shadow: 0 2px 12px oklch(0% 0 0 / 30%);
  }
}

header {
  animation: highlight-nav linear forwards;
  animation-timeline: scroll(root);
  animation-range: 80px 81px;
}

This changes the header background once the user has scrolled past 80px and holds that state. It's a one-liner replacement for the classic sticky-nav-scroll-listener pattern.

Progressive Enhancement

Browser support is solid but not universal — Firefox still has partial support behind a flag. Always wrap scroll-driven animations in a @supports check so the experience degrades gracefully:

@supports (animation-timeline: scroll()) {
  .progress-bar {
    animation: grow-bar linear;
    animation-timeline: scroll(root block);
    animation-fill-mode: both;
  }
}

Without the @supports guard, browsers that don't understand animation-timeline will simply ignore those declarations. For the progress bar case that's fine — no bar is better than a broken one. For critical reveal animations, ensure elements are visible by default and only animate them when the feature is supported.

When to Reach for This

Scroll-driven animations shine when the effect is purely visual and directly tied to scroll position: reading indicators, sticky header transitions, parallax backgrounds, element reveals, and image carousels. For anything that needs to trigger side effects — updating state, fetching data, analytics events — you still want Intersection Observer or a scroll event. CSS animations can't call your JavaScript.

MDN's scroll-driven animations guide is the most complete reference, and scroll-driven-animations.style has a fantastic playground of real-world examples to steal from. Give it a scroll.

This post was originally published on danholloran.me. Follow along there for more frontend and dev content.