Core Web Vitals

The three stable Core Web Vitals, each measured at the 75th percentile of real page loads (segmented by mobile and desktop):

<quick_reference>

A page passes Core Web Vitals only if all three metrics meet "Good" at the 75th percentile.

Supporting metrics (non-Core but diagnostic)

• FCP (First Contentful Paint) — diagnoses render-blocking resources upstream of LCP
• TTFB (Time to First Byte) — server response time; directly affects LCP
• TBT (Total Blocking Time) — lab proxy for INP; identifies long tasks

Tools matrix

*Lighthouse uses Total Blocking Time (TBT) as a lab proxy for INP. TBT correlates with INP but does not replace field measurement.

Metric lifecycle

Metrics progress through: Experimental → Pending → Stable. All three current Core Web Vitals (LCP, CLS, INP) are Stable. INP replaced FID (First Input Delay) in March 2024. Changes to stable metrics follow an annual cadence with advance notice.

</quick_reference>

Quick setup: measure all three in the field

npm install web-vitals

import { onCLS, onINP, onLCP } from 'web-vitals';

function sendToAnalytics(metric) {
  const body = JSON.stringify(metric);
  (navigator.sendBeacon && navigator.sendBeacon('/analytics', body)) ||
    fetch('/analytics', { body, method: 'POST', keepalive: true });
}

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

Each callback receives { name, value, rating, delta, id, navigationType }. rating is "good", "needs-improvement", or "poor".

The web-vitals library handles bfcache restores, prerendered pages, iframe aggregation, and other edge cases that raw PerformanceObserver does not.

<visual_report>

Generate a visual report

When the user provides metric values or a Lighthouse JSON file, generate an interactive HTML report and open it in the browser:

To locate the script, find scripts/visualize.py relative to this skill's directory. The path depends on how ts-dev-kit is installed:

• Project scope: skills/core-web-vitals/scripts/visualize.py or .claude/skills/core-web-vitals/scripts/visualize.py
• Personal scope: ~/.claude/skills/core-web-vitals/scripts/visualize.py
• Plugin scope: resolve via node_modules/@jgamaraalv/ts-dev-kit/skills/core-web-vitals/scripts/visualize.py

Use find or ls to discover the actual path, then run:

# From manual values (replace SCRIPT_PATH with the discovered path)
python3 SCRIPT_PATH/visualize.py \
  --lcp 2.1 --inp 180 --cls 0.05 \
  --url https://example.com

# From a Lighthouse JSON output
python3 SCRIPT_PATH/visualize.py \
  --lighthouse lighthouse-report.json

# Custom output path, no auto-open
python3 SCRIPT_PATH/visualize.py \
  --lcp 3.8 --inp 420 --cls 0.12 \
  --output cwv-report.html --no-open

The script (scripts/visualize.py) requires only Python 3 stdlib — no packages to install. It outputs a self-contained HTML file with color-coded metric cards, a visual progress bar showing where each value falls on the Good/Needs Improvement/Poor scale, and an overall PASS/FAIL/NEEDS IMPROVEMENT verdict.

</visual_report>

When to read reference files