CSS ScrollSpy built with Scroll-Driven Animations (SDA) is a modern, performance-optimized method for tracking a user’s scroll position and dynamically updating navigation indicators entirely through CSS, requiring no JavaScript.
Historically, creating a “ScrollSpy” effect (such as highlighting the active dot on a carousel or the current header in a sidebar) required expensive JavaScript scroll event listeners or an IntersectionObserver. With the advent of the CSS Scroll-Driven Animations API, browsers natively map @keyframes progression directly to a container’s physical scroll offset or a subject’s visibility in the viewport, completely eliminating main-thread “jank”. The Core Concept of ScrollSpy via CSS
Instead of calculating scroll pixels in JavaScript, you bind standard CSS keyframe animations to a scroll progress timeline. As the user scrolls from 0% to 100% of a container, the browser handles the animation state natively on the compositor thread.
@keyframes highlight-dot { 0%, 100% { background-color: var(–inactive-color); } 50% { background-color: var(–active-color); scale: 1.2; } } Use code with caution. Key Technical Mechanics A guide to Scroll-driven Animations with just CSS | WebKit
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