Lighthouse

Long definition

Lighthouse is the engine behind most of Google's public web-quality tooling. It loads a page in a controlled Chromium environment with simulated network throttling (slow 4G by default) and a 4× CPU slowdown, then runs roughly 100 audits across its five categories. The output is a JSON report with per-audit pass/fail, aggregated category scores, and prioritized opportunities.

The five categories:

• Performance — loading metrics (FCP, LCP, TBT, CLS, Speed Index) plus optimization opportunities (unused CSS, oversized images, third-party impact).
• Accessibility — automated WCAG checks (color contrast, ARIA roles, alt text, focus order). Catches roughly 30-40% of accessibility issues; manual review still required.
• Best Practices — security (HTTPS, mixed content), modern web standards (browser API usage), and console errors.
• SEO — basic on-page checks: title, meta description, robots.txt validity, mobile-friendliness, structured data sanity.
• PWA — progressive web app criteria (manifest, service worker, installability). Removed from default reports in Lighthouse 12.

Lighthouse's data is lab data: a single synthetic load under controlled conditions. It does not represent what real users experience. For that you need field data from CrUX. PageSpeed Insights shows both side by side — Lighthouse on the right (lab) and CrUX on the left (field) — precisely so you can compare.

Distribution channels: Chrome DevTools (Lighthouse panel), the github.com/GoogleChrome/lighthouse CLI and Node module, the PageSpeed Insights API, and the WebPageTest integration. The CI variant, Lighthouse CI, runs against pull requests and tracks score regressions over time.

Common misconceptions

• "My Lighthouse score is what Google ranks me on." Google ranks on Core Web Vitals from CrUX field data, not on Lighthouse scores. A site can score 100 in Lighthouse and fail Core Web Vitals in field, or score 60 in Lighthouse and pass.
• "Lighthouse scores are deterministic." They have variance — typically ±5 points run-to-run on the same URL — because of network simulation and CPU contention. Average 3-5 runs before drawing conclusions, or use Lighthouse CI's median-of-runs mode.
• "Lighthouse Accessibility 100 means the site is accessible." Lighthouse only catches automated-detectable WCAG issues. Keyboard traps, screen-reader semantics, and form-error messaging often need manual review even at 100.
• "Mobile and desktop scores should match." They shouldn't. Mobile uses 4× CPU throttling and slow 4G; desktop uses no throttling and broadband. The same page legitimately scores 30-40 points lower on mobile.