Preload lets you tell the browser about critical resources that you want to load as soon as possible, before they are discovered in HTML. This is especially useful for resources that are not easily discoverable, such as fonts included in stylesheets, background images, or resources loaded from a script.
Core Web Vitals focuses on three aspects of User Experience – Loading performance (LCP), Interactivity (FID) and visual stability (CLS).
The web-vitals library is a tiny (~1K), modular library for measuring all the Web Vitals metrics on real users, in a way that accurately matches how they’re measured by Chrome and reported to other Google tools (e.g. Chrome User Experience Report, Page Speed Insights, Search Console’s Speed Report).
These web-vitals JS help you generate Report and display in a histogram and timeline of each Web Vitals metric.