Resize Observer
Essential for responsive components that need to adapt to their container size (not just viewport). Powers container-query polyfills, responsive charts, and layout-aware components.
What It Defines
Asynchronous API to observe changes to an element's content rect or border box dimensions. Reports width, height, and box sizing via ResizeObserverEntry. Replaces polling or window resize listeners for element-level size changes.
Canonical (Normative)
Convenient (Practical)
Publishes web platform specs including CSS, accessibility, security policies, Service Workers, Web App Manifest, and many browser APIs. Also maintains some versioned HTML/DOM specs.
Related Specs
Every frontend framework (React, Vue, Svelte) ultimately produces DOM operations. Understanding the event model and tree structure prevents subtle bugs.
The standard way to implement lazy loading, infinite scroll, ad viewability tracking, and scroll-triggered animations. Far more performant than scroll event listeners — runs off the main thread.
CSS drives all visual layout. Key modules: Grid, Flexbox, Custom Properties, Cascade layers, Container Queries. The snapshot identifies what's stable vs experimental.