The Lighthouse tab
The Lighthouse tab runs an automated audit of the current page and produces a report in four categories: Performance, Accessibility, Best Practices and SEO. It is a starting point for analysis, not a verdict on quality.
Running an audit
Select a mode (Navigation for a page load), a device (Mobile or Desktop), the categories, and start the analysis. The run has to be performed in a private window with extensions disabled: extension scripts affect the measurements. Results vary between runs, so single numbers are compared with caution and trends matter more than one figure.
What the Accessibility score covers
The Accessibility category checks a subset of accessibility requirements automatically: colour contrast, alt attributes on images, form field names, heading order, valid ARIA attributes. A score of 100 means the automated checks passed. It does not mean the page is accessible: keyboard navigation, screen reader behaviour, focus order and the meaningfulness of alternative text are verified manually.
The standard behind the checks
The checks are based on WCAG — the accessibility guidelines published by the W3C. Level AA of WCAG 2.1 and 2.2 is the level normally referenced in requirements and contracts. In the European Union, accessibility requirements for a range of digital products and services are set by the European Accessibility Act (Directive (EU) 2019/882), which has applied since 28 June 2025; the corresponding technical standard is EN 301 549. For a tester this means accessibility defects are the same defects as any other, with a documented source of requirements behind them.
Where to read further
- WCAG 2.2 quick reference — the criteria with examples, the most practical entry point.
- WCAG 2.2 specification — the primary source.
- W3C WAI: evaluating accessibility — manual checking procedures.
- European Accessibility Act — scope and dates on the European Commission site.
At the interview
These are about what an automated audit shows and what is left to manual checking. Try answering before opening the answer.
juniorentry-level knowledge
1. What does Lighthouse show?
An automated audit of the current page in four categories: Performance, Accessibility, Best Practices and SEO. It is a starting point for analysis rather than a verdict on quality: results vary between runs, so a trend says more than a single number.
2. Why is the audit run in a private window with extensions disabled?
Extension scripts run on the same page and end up inside the measurements, which distorts the performance score and some of the checks. In a private window extensions are off by default, so the runs stay comparable.
3. How do you run a Lighthouse audit?
In the Lighthouse tab, pick a mode (Navigation for a page load), a device (Mobile or Desktop) and the categories, then start the analysis. The report covers the current page, so the address being tested is the one that has to be open.
middlea more advanced level
4. Does an Accessibility score of 100 mean the page is accessible?
No. The category automatically checks part of the requirements: colour contrast, alt attributes, names on form fields, heading order, valid ARIA. Keyboard navigation, screen reader behaviour, focus order and whether the alternative text is meaningful are all verified by hand.
5. What standard is behind the accessibility checks?
WCAG, the accessibility guidelines published by the W3C; level AA is the one normally written into requirements. In the European Union the requirements for a range of digital products and services are set by the European Accessibility Act, which has applied since 28 June 2025; the corresponding technical standard is EN 301 549.
Questions on the other topics are collected on the QA interview questions on DevTools page.