# Core Web Vitals

> Core Web Vitals reference for measuring, diagnosing, and improving LCP, INP, and CLS page performance against Google's thresholds

- Skill: `jgamaraalv/core-web-vitals` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add jgamaraalv/core-web-vitals`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jgamaraalv/core-web-vitals/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools, Docs & Writing
- Author: jgamaraalv (https://skillmd.com/u/jgamaraalv)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/jgamaraalv/core-web-vitals

---


# Core Web Vitals

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

| Metric                              | Measures         | Good     | Needs Improvement | Poor     |
| ----------------------------------- | ---------------- | -------- | ----------------- | -------- |
| **LCP** — Largest Contentful Paint  | Loading          | ≤ 2.5 s  | 2.5–4.0 s         | > 4.0 s  |
| **INP** — Interaction to Next Paint | Interactivity    | ≤ 200 ms | 200–500 ms        | > 500 ms |
| **CLS** — Cumulative Layout Shift   | Visual stability | ≤ 0.1    | 0.1–0.25          | > 0.25   |

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

| Tool                                 | Type        | LCP | INP   | CLS | Notes                                  |
| ------------------------------------ | ----------- | --- | ----- | --- | -------------------------------------- |
| Chrome User Experience Report (CrUX) | Field       | ✓   | ✓     | ✓   | 28-day rolling window of real users    |
| PageSpeed Insights                   | Field + Lab | ✓   | ✓     | ✓   | Field = CrUX data; Lab = Lighthouse    |
| Search Console CWV report            | Field       | ✓   | ✓     | ✓   | Groups URLs by template                |
| Chrome DevTools Performance panel    | Field + Lab | ✓   | ✓     | ✓   | Local profiling, interaction tracing   |
| Lighthouse                           | Lab         | ✓   | TBT\* | ✓   | CI integration; INP → use TBT as proxy |

\*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 setup: measure all three in the field

```bash
npm install web-vitals
```

```js
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.

## 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:

```bash
# 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.

## When to read reference files

| Reference                                  | Read when…                                                                         |
| ------------------------------------------ | ---------------------------------------------------------------------------------- |
| [references/lcp.md](references/lcp.md)     | LCP > 2.5 s, diagnosing slow image/text load, preload/CDN questions                |
| [references/inp.md](references/inp.md)     | INP > 200 ms, slow click/key/tap response, long task investigations                |
| [references/cls.md](references/cls.md)     | CLS > 0.1, elements jumping on scroll or load, font/image shift                    |
| [references/tools.md](references/tools.md) | Setting up monitoring, using DevTools/Lighthouse/PSI, top-9 optimization checklist |