axe-core is an open-source accessibility testing engine developed by Deque Systems. It analyzes a web page's DOM and reports WCAG violations rule by rule; it can run inside browser extensions, command-line tools and continuous integration (CI) pipelines. Due to the inherent limits of automated testing, it catches roughly 30-40% of violations; the rest require manual testing.
axe-core is a rules engine written in JavaScript: it walks the page's DOM, checks every element against its rule set and returns findings in categories such as violations, passes and needs review. Missing alt text, insufficient contrast, invalid ARIA usage and unlabeled form fields are its most typical findings. The rule set prioritizes avoiding false positives, which makes the engine trustworthy: a violation it reports is very likely a real one. Its strength lies in running at every scale: a developer can test a single page instantly with the axe DevTools browser extension, teams can wire the engine into test frameworks such as Playwright or Cypress to run automated accessibility checks in CI on every code change, and many site-wide accessibility scanners run axe-core underneath. Alternative tools such as WAVE offer similar automated checks with different rule sets and reporting styles.
Knowing its limit matters: automated testing can catch roughly 30-40% of violations. Whether an image has alt text can be measured by a machine, but whether that text is meaningful, whether the keyboard flow proceeds logically and whether screen reader announcements actually make sense can only be seen through manual testing. The right setup combines both: continuous automated scanning plus regular manual audits.
Frequently asked questions
Is axe-core free?
Yes, the axe-core engine is open source and free to use; its source code is public and it can be added to projects as a library. Deque Systems also offers paid products built on top of the engine, such as advanced reporting and team features, but the core engine and the basic version of the axe DevTools extension are free. Many free and commercial scanning tools also run axe-core underneath.
Does axe-core find all accessibility errors?
No. By its nature, automated testing can catch roughly 30-40% of violations; this limit is not specific to axe-core but applies to all automated tools. A machine can measure whether alt text exists but cannot judge whether it is meaningful; the correctness of keyboard flow and the screen reader experience require manual testing. The healthiest approach is to run automated scans continuously, fix findings quickly and carry out manual audits at regular intervals.
This content is for information only and is not legal advice.