How Much of WCAG an Automated Scanner Actually Catches
A team runs axe and Lighthouse on a new component and reports "zero accessibility errors found."
Per this subject's guidance on automated testing, what does that result actually prove?
The correct answer is "Only that automated checks (roughly 30-40% of WCAG issues) found nothing."
This subject states the coverage figure directly: automated tools like axe, Lighthouse, and WAVE typically catch on the order of 30-40% of WCAG issues — missing alt text, contrast failures, missing labels, invalid ARIA — but they cannot evaluate whether a label is meaningful, whether tab order is logical, or whether an interaction genuinely makes sense with a screen reader running. A clean automated scan is evidence of "nothing a machine could catch," not evidence the component is actually usable — the manual keyboard and screen-reader passes are still required.
The distractors overclaim what a scan proves: "fully accessible, needs no further testing" is the exact overclaim this subject warns against, since roughly 60-70% of issues are outside automated coverage; a zero-error scan says nothing about AAA conformance specifically — conformance level isn't what these tools assess at all; and dismissing automated scanners as invalid goes too far the other way — they're a real, useful first pass, just not a complete one.
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