core-web-vitals

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• SKILL.md
• LCP.md · references/

SKILL.md (first lines)

---
name: core-web-vitals
description: Optimize Core Web Vitals (LCP, INP, CLS) for better page experience and search ranking. Use when asked to "improve Core Web Vitals", "fix LCP", "reduce CLS", "optimize INP", "page experience optimization", or "fix layout shifts".
license: MIT
metadata:
  author: web-quality-skills
  version: "1.0"
---

# 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**
```html
<!-- ❌ 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**
```html
<!-- ❌ 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**
```javascript
// ❌ 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

```markdown
- [ ] 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)
```

### LCP element identification
```javascript
// 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

From Testing-itay/DevTools-Security-Lab, MIT.

More from Testing-itay/DevTools-Security-Lab