First (Contentful) Paint

first-contentful-paint.js

const perfObserver = new PerformanceObserver((performanceEntryList) => {
  for (const performanceEntry of performanceEntryList.getEntries()) {
    console.log(performanceEntry.name); // 'first-paint' or 'first-contentful-paint'
    console.log(performanceEntry.startTime); // DOMHighResTimeStamp
  }
});
perfObserver.observe({ entryTypes: ["paint"] });