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30 – Accessibility, Best Practice, Performance & SEO (Lighthouse)

The test runs Google Lighthouse, the same tool that's built into Chrome and sits behind PageSpeed Insights.

What is tested?

Lighthouse assesses four areas: performance, accessibility, best practice and search engine optimisation. Among the individual checks are:

  • Largest Contentful Paint: how long before the page's largest content block appears
  • Cumulative Layout Shift: how much content jumps around during loading
  • Unused CSS: stylesheets that are loaded but not used
  • Unused JavaScript: scripts that are loaded but not used
  • Legacy JavaScript: code written for old browsers being sent to modern ones too

How it affects the score

Lighthouse's own scores are converted to Webperf's five-point scale.

Running the test 

python default.py -u https://example.com -t 30 -r

Worth knowing

Same tool, different numbers

Lighthouse measures in a simulated environment, and the result is affected by the machine, the network and the throttling. That's why the numbers differ between Webperf and PageSpeed Insights, often by 10–15 points, even though both run Lighthouse. What's interesting is the trend within one tool, not the absolute figure.

See also