Axe Core

axe-core is the open-source accessibility testing engine built by Deque Systems, and is the rules engine underneath most automated a11y tooling in the JavaScript ecosystem, including the axe browser extension, Cypress-axe, and jest-axe. It runs a set of WCAG 2.x rules against rendered DOM and reports specific violations with element references, rather than a generic accessibility score.

Because it is a library, not a hosted service, teams typically wire it into unit tests, end-to-end tests (Cypress, Playwright), or CI pipelines so accessibility regressions get caught the same way a broken test would. It is free and MIT-licensed, with no paid tier — Deque's commercial products (axe DevTools, axe Monitor) build on top of the same engine for teams wanting dashboards, cross-page scanning, and manual-testing workflows.

axe-core catches roughly 30-50% of WCAG issues automatically (industry-wide, not a Deque-specific figure); it is a strong first line of defense but doesn't replace manual accessibility review for things like keyboard flow and screen-reader experience.