A Lighthouse accessibility score is not a WCAG audit: how they differ

Lighthouse is Google's free tool built into Chrome. Its accessibility tab gives a score from 0 to 100, and many people treat it like a certificate: "I got 94, I'm fine." It is not one, and it helps to know why before showing that number to anyone.

What Lighthouse measures

Under the hood, Lighthouse uses axe-core, the same rules engine most serious tools use (ours included). It checks the page you give it, at the screen size you have open, and computes the score by weighting the rules that fail. It is an accurate but small snapshot: one page, one size, one moment.

What it misses

• The other pages. A lawsuit does not cite your home page: it cites the cart, the checkout and the contact form. Lighthouse only looks at the URL you give it.
• Phones. Tap targets too small for a finger, text that breaks when zoomed: they only show on a phone-sized screen, and Lighthouse audits at whatever size you run it.
• The score hides severity. An image without a description and a checkout form unusable by keyboard can weigh about the same in the number. For a blind person they do not.
• It does not say how to fix. You get the rule name and a link to English documentation. Writing the correction and checking it works is still your job.
• No history. If you have 94 today and a plugin drops it to 70 tomorrow, nobody tells you.
• And like every automated tool it covers roughly half of the WCAG criteria. Captions, reading order, whether pop-ups close without a mouse: those are checked by hand.

What a WCAG audit is

WCAG (Web Content Accessibility Guidelines) are the technical guidelines the ADA in the US, the European law and most national laws refer to. An audit against WCAG checks the pages that matter, on desktop and phone, lists every failure with its severity, says what to fix, and leaves a dated record. A tool does the automated part; a person with a checklist does the manual part.

So what is Lighthouse for?

For what it is: a fast, free first look. If it says 40, you have a serious problem. If it says 95, you still do not know whether the checkout works with a keyboard. What your developer needs is the list of elements with their corrections; what your lawyer needs is a dated record of what was checked and what was fixed. Neither comes out of a number.