Lighthouse

Definition

Lighthouse is an open-source, automated tool from Google that audits web pages across five categories: Performance, Accessibility, Best Practices, SEO, and Progressive Web App (PWA). It runs in a simulated lab environment using a headless Chrome browser, throttled to emulate a mid-tier mobile device on a 4G connection. Lighthouse generates a score from 0–100 in each category, based on weighted component metrics. Performance score components (Lighthouse 10): Largest Contentful Paint 25%, Total Blocking Time 30%, Cumulative Layout Shift 25%, First Contentful Paint 10%, Speed Index 10%. Lighthouse is built into Chrome DevTools (Lighthouse tab), available as a Node.js CLI (`npm install -g lighthouse`), and is the engine behind PageSpeed Insights (PSI) — which also adds real CrUX field data alongside the Lighthouse lab scores. The SEO category in Lighthouse checks basic on-page SEO signals: title tag presence, meta description, viewport tag, link crawlability, font legibility, and robots meta tag — but it's a much shallower check than a full SEO audit.

Why it matters for SEO

Lighthouse is the most widely used web performance diagnostic tool, and its scores are often used as a proxy for site health — but it's important to understand what it measures and what it doesn't. Lighthouse is a **lab** tool: it simulates a specific device and connection in a controlled environment, which doesn't always reflect real user experience. The Core Web Vitals that are actual Google ranking signals (LCP, INP, CLS) are measured from **field data** (CrUX, real Chrome users) — a page can score 90+ in Lighthouse but have Poor field-data INP because of third-party scripts that only execute in real user sessions. Use Lighthouse for diagnosing performance bottlenecks and testing the impact of specific fixes; use CrUX field data (via GSC or the CrUX API) to assess actual ranking impact.

How DeepSEOAnalysis checks this

The audit runs Lighthouse tests against crawled pages and surfaces the scores and component metrics. Performance, accessibility, best practices, and SEO scores are shown per page. Component metrics (FCP, LCP, TBT, CLS, Speed Index) are displayed alongside real CrUX field data (LCP, INP, CLS) to provide both lab and field perspectives. Lighthouse SEO category findings (missing title, no meta description, etc.) are deduplicated with the full audit's on-page checks — the audit's SEO checks are significantly more comprehensive than Lighthouse's built-in SEO category.

Useful tools and resources

• Full performance audit