Web development and performance
What is Lighthouse?
Lighthouse is an open-source auditing tool that loads a page under simulated conditions and scores performance, accessibility, best practices and SEO out of 100. It is a diagnostic instrument for finding what to fix, rather than the metric your ranking actually depends on.
Also called: PageSpeed Insights · lab data
What it is genuinely good for
- Identifying which resource sits on the critical path
- Catching accessibility failures that automated testing can detect
- Verifying that a fix actually landed in the build you deployed
- A consistent baseline to compare against after changes
What it is not
It is not what search engines assess. Core Web Vitals are judged on field data from real visitors over a rolling 28-day window. A perfect lab score with failing field data is common, and the field data is the one that counts.
The accessibility caveat worth stating
Automated testing catches roughly a third of accessibility issues. A score of 100 means no detectable failures, not an accessible site — keyboard testing and a screen reader pass are still required, and they find different things.
Where this is covered in depth
A definition can only go so far. Core Web Vitals, and the order that actually reduces them covers this properly — 3 minutes, free, no email required.
Who wrote this
Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan. These definitions come from client work rather than from a content brief — where an entry describes a mistake, it is usually one found on a real site. More about how I work.