core-web-vitals

par addyosmani

Optimiser les Core Web Vitals (LCP, INP, CLS) pour une meilleure expérience de page et un classement dans les résultats de recherche. Utiliser lorsqu'on demande "améliorer les Core Web Vitals", "corriger le LCP", "réduire le CLS", "optimiser l'INP", "optimisation de l'expérience de page" ou "corriger les décalages de mise en page".

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

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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
<svg> element

Common LCP issues

1. Slow server response (TTFB > 800ms)

Fix: CDN, caching, optimized backend, edge rendering

2. Render-blocking resources

<!-- ❌ Blocks rendering -->
<link rel="stylesheet" href="/all-styles.css">

<!-- ✅ Critical CSS inlined, rest deferred -->
<style>/* Critical above-fold CSS */</style>
<link rel="preload" href="/styles.css" as="style" 
      onload="this.onload=null;this.rel='stylesheet'">

3. Slow resource load times

<!-- ❌ No hints, discovered late -->
<img src="/hero.jpg" alt="Hero">

<!-- ✅ Preloaded with high priority -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high">
<img src="/hero.webp" alt="Hero" fetchpriority="high">

4. 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 } };
}

5. Make navigations instant with the Speculation Rules API

For most sites, the LCP a user actually experiences is dominated by the next page they navigate to, not the one they landed on. Telling the browser to prerender likely-next pages on hover collapses that LCP to ~0ms.

<script type="speculationrules">
{
  "prerender": [{
    "where": { "href_matches": "/*" },
    "eagerness": "moderate"
  }]
}
</script>

eagerness settings (cheapest → most aggressive): conservative (start on pointerdown), moderate (start after ~200ms hover), eager (start as soon as the link is in the viewport), immediate (start on page load). Start with moderate — it captures most navigations without prerendering pages users never visit.

Caveats:

Bandwidth/CPU cost. Each prerender is roughly a full page load. Scope where carefully (href_matches patterns, exclude logout/checkout) and avoid immediate outside small sites.
Side effects fire early. Analytics, ads, and any code that runs on load will fire when the prerender starts, not when the user navigates. Gate side effects on the prerenderingchange event or document.prerendering.
Chromium-only. Safari and Firefox ignore the script — it's a progressive enhancement, never a regression.

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 <head>
- [ ] Fonts don't block text rendering (font-display: swap)
- [ ] LCP element in initial HTML (not JS-rendered)
- [ ] Speculation Rules added for likely-next navigations (moderate eagerness)

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 and yield to the scheduler. scheduler.yield() is the
//    recommended modern API — its continuation is queued at a boosted
//    priority so the rest of your work resumes ahead of unrelated tasks,
//    while still letting the browser handle pending input first.
async function processLargeArray(items) {
  const CHUNK_SIZE = 100;
  for (let i = 0; i < items.length; i += CHUNK_SIZE) {
    items.slice(i, i + CHUNK_SIZE).forEach(expensiveOperation);

    if ('scheduler' in window && 'yield' in scheduler) {
      await scheduler.yield();
    } else {
      // Fallback for browsers without scheduler.yield (Safari, older Firefox).
      // setTimeout(0) yields but loses priority — your continuation may run
      // after unrelated tasks the browser picked up in between.
      await new Promise(r => setTimeout(r, 0));
    }
  }
}

2. 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, then yield before doing the heavy work
button.addEventListener('click', async () => {
  // 1. Immediate visual feedback (cheap DOM update)
  button.classList.add('loading');

  // 2. Yield so the browser can paint the loading state before we block
  if ('scheduler' in window && 'yield' in scheduler) {
    await scheduler.yield();
  }

  // 3. Now do the heavy work — the user already saw the click register
  const result = calculateComplexThing();
  updateUI(result);

  // 4. Lowest-priority work last, when the main thread is idle
  if ('requestIdleCallback' in window) {
    requestIdleCallback(() => trackEvent('click'));
  } else {
    setTimeout(() => trackEvent('click'), 0);
  }
});

3. Third-party scripts

// ❌ Eagerly loaded, blocks interactions
<script src="https://heavy-widget.com/widget.js"></script>

// ✅ 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 });

4. Excessive re-renders (React/Vue)

// ❌ Re-renders entire tree
function App() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <Counter count={count} />
      <ExpensiveComponent /> {/* Re-renders on every count change */}
    </div>
  );
}

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

function App() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <Counter count={count} />
      <MemoizedExpensive />
    </div>
  );
}

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. durationThreshold: 40 matches what the
// web-vitals library uses — 16 (one frame) fires on nearly every interaction
// and drowns the console; 40 surfaces interactions that are starting to feel
// sluggish without spamming.
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: 40 });

For field debugging across real users, prefer the web-vitals/attribution build of the web-vitals library — onINP() from that build attaches a LoAF (Long Animation Frame) breakdown identifying the longest script and the input/processing/presentation phase that ate the budget.

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

<!-- ❌ Causes layout shift when loaded -->
<img src="photo.jpg" alt="Photo">

<!-- ✅ Space reserved -->
<img src="photo.jpg" alt="Photo" width="800" height="600">

<!-- ✅ Or use aspect-ratio -->
<img src="photo.jpg" alt="Photo" style="aspect-ratio: 4/3; width: 100%;">

2. Ads, embeds, and iframes

<!-- ❌ Unknown size until loaded -->
<iframe src="https://ad-network.com/ad"></iframe>

<!-- ✅ Reserve space with min-height -->
<div style="min-height: 250px;">
  <iframe src="https://ad-network.com/ad" height="250"></iframe>
</div>

<!-- ✅ Or use aspect-ratio container -->
<div style="aspect-ratio: 16/9;">
  <iframe src="https://youtube.com/embed/..." 
          style="width: 100%; height: 100%;"></iframe>
</div>

3. 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 = '';
  });
}

4. 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%;
}

5. 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 <url>

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';
<Image src="/hero.jpg" priority fill alt="Hero" />

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

// CLS: Image component handles dimensions automatically

React

// LCP: Preload in head
<link rel="preload" href="/hero.jpg" as="image" fetchpriority="high" />

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

// CLS: Always specify dimensions in img tags

Vue/Nuxt

<!-- LCP: Use nuxt/image with preload -->
<NuxtImg src="/hero.jpg" preload loading="eager" />

<!-- INP: Use async components -->
<component :is="() => import('./Heavy.vue')" />

<!-- CLS: Use aspect-ratio CSS -->
<img :style="{ aspectRatio: '16/9' }" />