What is Lighthouse?

Lighthouse is Google's open-source web page audit tool (released 2017). It automatically tests performance, SEO, accessibility, best practices, and PWA. Available in Chrome DevTools, CLI, as a Node module, and integrated in PageSpeed Insights (PSI).

Lighthouse generates a score 0-100 for each category + detailed recommendations on what to improve.

5 audit categories

1. Performance

Core Web Vitals — LCP, INP, CLS
First Contentful Paint (FCP)
Speed Index
Total Blocking Time (TBT)
Time to Interactive (TTI) — deprecated in 2024

2. Accessibility

ARIA roles correctness
Color contrast (WCAG 2.1)
Form labels
Heading hierarchy
Image alt text
Keyboard navigation

3. Best Practices

HTTPS usage
Console errors
JS library vulnerabilities
Image aspect ratio
Browser compatibility

4. SEO

Meta tags (title, description)
Viewport meta
Crawlable links
robots.txt validity
Mobile-friendly

5. PWA (Progressive Web App)

Service worker
Manifest.json
Installable

Scoring

Score	Status
90-100	Good (green)
50-89	Needs Improvement (orange)
0-49	Poor (red)

Performance score is a weighted composition: LCP 25%, TBT 30%, CLS 25%, FCP 10%, Speed Index 10%.

Lighthouse vs PageSpeed Insights vs CrUX

Lighthouse — lab data (controlled environment, single test run)
PageSpeed Insights (PSI) — web version of Lighthouse + CrUX field data (real users)
Chrome User Experience Report (CrUX) — field data from real Chrome users (last 28 days)

Lab vs Field: Lab is simulation in specific throttle setting (Slow 4G, Moto G4 CPU). Field is what users actually experience.

For SEO, Google uses field data (CrUX) as a ranking signal. Lab data (Lighthouse) is for debugging.

How to run Lighthouse

Chrome DevTools

F12 → Lighthouse tab → Analyze page load
Options: Mobile/Desktop, categories, throttling

CLI

npm install -g lighthouse
lighthouse https://example.com --view --preset=desktop

Node module

const lighthouse = require('lighthouse');
const result = await lighthouse(url, { onlyCategories: ['performance'] });

CI/CD

Lighthouse CI (@lhci/cli) — automatic audits in PR
GitHub Action — treosh/lighthouse-ci-action

Bulk audit

PageSpeed Insights API — programmatic CrUX + Lighthouse data
MarketingOS — cli cwv check <company> <url1> <url2> uses PSI API

Performance optimization checklist (Lighthouse)

Image optimization — WebP/AVIF, responsive srcset, lazy loading
Minify CSS/JS — at build (Next.js, Vite, Webpack do auto)
Code splitting — dynamic imports for below-fold
Font optimization — display: swap, preconnect, subset
Critical CSS — inline above-the-fold styles
Defer non-critical JS — <script defer> / <script async>
CDN — global edge (Cloudflare Pages)
HTTP/2 + brotli — server config
Cache headers — long max-age for static, immutable
Reduce 3rd-party — minimize trackers, lazy-load analytics

Common mistakes

Score chasing — 100/100 vs 95/100 doesn't change ranking, but +10 hours of work
Lab vs field disconnect — Lighthouse shows 95, but CrUX shows 70 → your optimization doesn't reflect real users
Single page audit — typically you test home, but each page has different score
No monitoring — one-time audit vs continuous monitoring (PSI weekly)

Related terms

Core Web Vitals — what Lighthouse measures
SEO — Lighthouse SEO category
Technical SEO — context
Cloudflare Pages — tool for score improvement
Astro — framework wins Lighthouse
Conversion rate — performance → CR correlation

Lighthouse — Google's performance and SEO audit tool