Lighthouse vs functional website testing: different questions, different evidence

What Lighthouse is good at

Lighthouse is useful for automated checks around performance, accessibility signals, best practices, and SEO-related implementation. It is fast, repeatable, and excellent for identifying categories of technical improvement.

• Performance metrics and opportunities
• Automated accessibility signals
• Selected best-practice checks
• Selected SEO implementation checks

What Lighthouse does not prove

It does not follow your business logic like a real customer and verify the real outcome of every journey. A page can score well while the conversion path is broken.

• That the contact email actually arrives
• That registration reaches the intended account state
• That a CTA leads to the correct business destination
• That cart quantities and totals behave correctly
• That checkout confirmation and order state are coherent
• That a mobile overlay does not block the one button the user needs

What functional website QA asks instead

Functional QA starts with an intended user action and asks whether the full path works. The evidence is the observed behavior, the expected behavior, the reproduction path, and the resulting state—not a single numeric score.

Use both when the website matters commercially

Performance and accessibility signals affect experience; functional failures stop outcomes. A sensible launch process can use automated audits for broad technical signals and functional QA for the business-critical journeys.

The key is not to treat one tool as proof of something it was never designed to verify.