core-web-vitals

仓库: addyosmani/web-quality-skills
安装量: 3.2K
排名: #677

安装

npx skills add https://github.com/addyosmani/web-quality-skills --skill core-web-vitals

Core Web Vitals optimization

Targeted optimization for the three Core Web Vitals metrics that affect Google Search ranking and user experience.

The three metrics Metric Measures Good Needs work Poor LCP Loading ≤ 2.5s 2.5s – 4s > 4s INP Interactivity ≤ 200ms 200ms – 500ms > 500ms CLS Visual Stability ≤ 0.1 0.1 – 0.25 > 0.25

Google measures at the 75th percentile — 75% of page visits must meet "Good" thresholds.

LCP: Largest Contentful Paint

LCP measures when the largest visible content element renders. Usually this is:

Hero image or video Large text block Background image element Common LCP issues

  1. Slow server response (TTFB > 800ms)

Fix: CDN, caching, optimized backend, edge rendering

  1. Render-blocking resources

  1. Slow resource load times

Hero

Hero

  1. Client-side rendering delays

// ❌ Content loads after JavaScript useEffect(() => { fetch('/api/hero-text').then(r => r.json()).then(setHeroText); }, []);

// ✅ Server-side or static rendering // Use SSR, SSG, or streaming to send HTML with content export async function getServerSideProps() { const heroText = await fetchHeroText(); return { props: { heroText } }; }

LCP optimization checklist - [ ] TTFB < 800ms (use CDN, edge caching) - [ ] LCP image preloaded with fetchpriority="high" - [ ] LCP image optimized (WebP/AVIF, correct size) - [ ] Critical CSS inlined (< 14KB) - [ ] No render-blocking JavaScript in - [ ] Fonts don't block text rendering (font-display: swap) - [ ] LCP element in initial HTML (not JS-rendered)

LCP element identification // Find your LCP element new PerformanceObserver((list) => { const entries = list.getEntries(); const lastEntry = entries[entries.length - 1]; console.log('LCP element:', lastEntry.element); console.log('LCP time:', lastEntry.startTime); }).observe({ type: 'largest-contentful-paint', buffered: true });

INP: Interaction to Next Paint

INP measures responsiveness across ALL interactions (clicks, taps, key presses) during a page visit. It reports the worst interaction (at 98th percentile for high-traffic pages).

INP breakdown

Total INP = Input Delay + Processing Time + Presentation Delay

Phase Target Optimization Input Delay < 50ms Reduce main thread blocking Processing < 100ms Optimize event handlers Presentation < 50ms Minimize rendering work Common INP issues

  1. Long tasks blocking main thread

// ❌ Long synchronous task function processLargeArray(items) { items.forEach(item => expensiveOperation(item)); }

// ✅ Break into chunks with yielding async function processLargeArray(items) { const CHUNK_SIZE = 100; for (let i = 0; i < items.length; i += CHUNK_SIZE) { const chunk = items.slice(i, i + CHUNK_SIZE); chunk.forEach(item => expensiveOperation(item));

// Yield to main thread
await new Promise(r => setTimeout(r, 0));
// Or use scheduler.yield() when available

} }

  1. Heavy event handlers

// ❌ All work in handler button.addEventListener('click', () => { // Heavy computation const result = calculateComplexThing(); // DOM updates updateUI(result); // Analytics trackEvent('click'); });

// ✅ Prioritize visual feedback button.addEventListener('click', () => { // Immediate visual feedback button.classList.add('loading');

// Defer non-critical work requestAnimationFrame(() => { const result = calculateComplexThing(); updateUI(result); });

// Use requestIdleCallback for analytics requestIdleCallback(() => trackEvent('click')); });

  1. Third-party scripts

// ❌ Eagerly loaded, blocks interactions

// ✅ Lazy loaded on interaction or visibility const loadWidget = () => { import('https://heavy-widget.com/widget.js') .then(widget => widget.init()); }; button.addEventListener('click', loadWidget, { once: true });

  1. Excessive re-renders (React/Vue)

// ❌ Re-renders entire tree function App() { const [count, setCount] = useState(0); return (

{/ Re-renders on every count change /}
); }

// ✅ Memoized expensive components const MemoizedExpensive = React.memo(ExpensiveComponent);

function App() { const [count, setCount] = useState(0); return (

); }

INP optimization checklist - [ ] No tasks > 50ms on main thread - [ ] Event handlers complete quickly (< 100ms) - [ ] Visual feedback provided immediately - [ ] Heavy work deferred with requestIdleCallback - [ ] Third-party scripts don't block interactions - [ ] Debounced input handlers where appropriate - [ ] Web Workers for CPU-intensive operations

INP debugging // Identify slow interactions new PerformanceObserver((list) => { for (const entry of list.getEntries()) { if (entry.duration > 200) { console.warn('Slow interaction:', { type: entry.name, duration: entry.duration, processingStart: entry.processingStart, processingEnd: entry.processingEnd, target: entry.target }); } } }).observe({ type: 'event', buffered: true, durationThreshold: 16 });

CLS: Cumulative Layout Shift

CLS measures unexpected layout shifts. A shift occurs when a visible element changes position between frames without user interaction.

CLS Formula: impact fraction × distance fraction

Common CLS causes

  1. Images without dimensions

Photo

Photo

Photo

  1. Ads, embeds, and iframes
  1. Dynamically injected content

// ❌ Inserts content above viewport notifications.prepend(newNotification);

// ✅ Insert below viewport or use transform const insertBelow = viewport.bottom < newNotification.top; if (insertBelow) { notifications.prepend(newNotification); } else { // Animate in without shifting newNotification.style.transform = 'translateY(-100%)'; notifications.prepend(newNotification); requestAnimationFrame(() => { newNotification.style.transform = ''; }); }

  1. Web fonts causing FOUT

/ ❌ Font swap shifts text / @font-face { font-family: 'Custom'; src: url('custom.woff2') format('woff2'); }

/ ✅ Optional font (no shift if slow) / @font-face { font-family: 'Custom'; src: url('custom.woff2') format('woff2'); font-display: optional; }

/ ✅ Or match fallback metrics / @font-face { font-family: 'Custom'; src: url('custom.woff2') format('woff2'); font-display: swap; size-adjust: 105%; / Match fallback size / ascent-override: 95%; descent-override: 20%; }

  1. Animations triggering layout

/ ❌ Animates layout properties / .animate { transition: height 0.3s, width 0.3s; }

/ ✅ Use transform instead / .animate { transition: transform 0.3s; } .animate.expanded { transform: scale(1.2); }

CLS optimization checklist - [ ] All images have width/height or aspect-ratio - [ ] All videos/embeds have reserved space - [ ] Ads have min-height containers - [ ] Fonts use font-display: optional or matched metrics - [ ] Dynamic content inserted below viewport - [ ] Animations use transform/opacity only - [ ] No content injected above existing content

CLS debugging // Track layout shifts new PerformanceObserver((list) => { for (const entry of list.getEntries()) { if (!entry.hadRecentInput) { console.log('Layout shift:', entry.value); entry.sources?.forEach(source => { console.log(' Shifted element:', source.node); console.log(' Previous rect:', source.previousRect); console.log(' Current rect:', source.currentRect); }); } } }).observe({ type: 'layout-shift', buffered: true });

Measurement tools Lab testing Chrome DevTools → Performance panel, Lighthouse WebPageTest → Detailed waterfall, filmstrip Lighthouse CLI → npx lighthouse Field data (real users) Chrome User Experience Report (CrUX) → BigQuery or API Search Console → Core Web Vitals report web-vitals library → Send to your analytics import {onLCP, onINP, onCLS} from 'web-vitals';

function sendToAnalytics({name, value, rating}) { gtag('event', name, { event_category: 'Web Vitals', value: Math.round(name === 'CLS' ? value * 1000 : value), event_label: rating }); }

onLCP(sendToAnalytics); onINP(sendToAnalytics); onCLS(sendToAnalytics);

Framework quick fixes Next.js // LCP: Use next/image with priority import Image from 'next/image'; Hero

// INP: Use dynamic imports const HeavyComponent = dynamic(() => import('./Heavy'), { ssr: false });

// CLS: Image component handles dimensions automatically

React // LCP: Preload in head

// INP: Memoize and useTransition const [isPending, startTransition] = useTransition(); startTransition(() => setExpensiveState(newValue));

// CLS: Always specify dimensions in img tags

Vue/Nuxt

References web.dev LCP web.dev INP web.dev CLS Performance skill

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