This repository is the `web-vitals` JavaScript library by Google Chrome, designed to measure Core Web Vitals (LCP, CLS, INP) and other performance metrics (FCP, TTFB) in real-user monitoring (RUM) scenarios. It provides a tiny, deferred-load-friendly API that aligns with Chrome User Experience Report (CrUX) methodology, supporting bfcache restores, prerendering activation, soft navigations, and attribution data for debugging. The library is written in TypeScript, compiled to multiple formats (ESM, UMD, IIFE), and includes both a standard build and an attribution build with additional debugging fields. Its architecture is modular: each metric function (e.g., `onLCP`, `onCLS`) uses shared internal utilities (`observe`, `bindReporter`, `initMetric`, `getVisibilityWatcher`) and metric-specific state managers (`LayoutShiftManager`, `InteractionManager`, `LCPEntryManager`). The codebase demonstrates high craftsmanship with clean abstractions, thorough cross-browser handling, and a comprehensive test suite covering unit tests and end-to-end WebDriverIO tests across Chrome, Firefox, and Safari. It is actively maintained, with recent commits and issue triage, though it tracks rapidly evolving browser APIs (soft navigations, Long Animation Frames) which introduces some technical drift risk. Overall, it is a well-engineered library suitable for production use and integration into analytics pipelines.