Lighthouse HTML Audit

Lighthouse is an automated auditing tool built into Chrome DevTools (Audits tab) that scores a page across Performance, Accessibility, Best Practices, SEO, and PWA. It runs an audited page through the browser, measures, and produces actionable suggestions.

In Chrome DevTools, switch to Lighthouse, pick the categories, choose Desktop or Mobile, click "Analyze page load". The tool reloads the page in a clean environment and produces a report in under a minute.

Performance measures Core Web Vitals and other speed metrics. Accessibility checks WCAG-aligned rules. Best Practices covers security and modern web standards. SEO checks crawlable markup. PWA verifies installability and offline support.

The Accessibility audit catches: missing alt text, missing form labels, low color contrast, missing language attribute, duplicate ids, missing landmarks, focusable elements without keyboard support. Each issue links to detailed remediation guidance.

Largest Contentful Paint (LCP), Cumulative Layout Shift (CLS), Total Blocking Time (TBT), and Speed Index. Each metric is scored and weighted into the overall Performance score. Lighthouse displays which DOM elements contribute most to each metric.

Checks HTTPS usage, console errors, image aspect ratios, deprecated APIs, mixed content. The Best Practices audit catches a wide range of legacy/insecure patterns that should be modernized.

Catches missing meta description, missing or duplicate <title>, robots.txt issues, links without descriptive text, viewport meta tag missing, hreflang issues. Pair Lighthouse SEO with a real SEO audit tool (Ahrefs, Screaming Frog) for content-level analysis.

Checks every requirement to be installable: HTTPS, valid manifest with minimum fields, service worker registered, viewport tag, icons. Fix all PWA audit failures before launching as an installable app.

The Lighthouse CLI and Lighthouse CI tool integrate audits into CI/CD. Each PR runs Lighthouse on the deployed preview; the action fails if scores regress below thresholds. Prevents silent regression in accessibility, performance, or SEO.

Lighthouse reports can be saved as JSON and compared programmatically: track the LCP score over time to detect regressions, compare two PRs to see which changed which metric, alert on threshold crossings. The data is structured for tooling.

Lighthouse is a synthetic test — it runs in a controlled environment, not on real users. Pair with Real User Monitoring (RUM) tools that track actual user experience. Synthetic tests catch regressions early; RUM confirms users actually feel the improvements.

Some audits flag patterns that are unavoidable in certain contexts. Document any deliberate exceptions, add them to the Lighthouse CI ignore list, and re-evaluate periodically as the platform evolves. Avoid blanket "ignore all" — that defeats the purpose of automated auditing.

Treat Lighthouse scores as a continuous improvement target, not a one-time fix. Build a culture of looking at the score on every PR, addressing regressions immediately, and improving the baseline gradually. Most teams see scores climb steadily over months with this discipline.

Why should Lighthouse synthetic audits be paired with Real User Monitoring (RUM)?

Lighthouse audits a page across Performance, Accessibility, Best Practices, SEO and PWA. Run from Chrome DevTools for one-off checks, Lighthouse CI for continuous integration. Address regressions on every PR, build a culture of measurement-driven improvement, and pair synthetic audits with Real User Monitoring to validate that lab improvements translate to real-user benefits.