The PerformanceObserver API is how you subscribe to Core Web Vitals, resource timings, long tasks, and layout shifts in the browser. Here is every entry type, buffered replay, and a production-ready RUM pipeline.
Fix Cumulative Layout Shift fast: capture per-element attribution with web-vitals, replay shifts in Chrome DevTools, and ship one-line fixes for the five sources that cause 90% of real CLS.
HTTP 103 Early Hints lets servers preload critical resources during backend think-time, cutting LCP by 100-400ms. A practical 2026 guide for Cloudflare, Fastly, and Node.js origins.