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Axe Core

Axe Core is Deque Systems' open-source accessibility rule engine — a JavaScript library that audits a rendered DOM for WCAG (Web Content Accessibility Guidelines) and best-practice violations. It ships as axe-core on npm and is the substrate under framework bindings like @axe-core/playwright, @axe-core/webdriverjs, @axe-core/cypress, and the axe DevTools browser extension.

What it does

• Runs rules against the current DOM — Axe's rule set maps to WCAG success criteria (Level A, AA, AAA) plus best-practice rules; each rule is a function over the DOM that produces zero or more violations.
• Tags rules for scope-picking: wcag2a, wcag2aa, wcag21a, wcag21aa, wcag22aa, best-practice, EN-301-549, ACT, etc. Callers pick which rule-set runs via tag filters.
• Returns structured violations — each violation has id (rule name), impact (critical / serious / moderate / minor), description, help, helpUrl, plus nodes[] (the actual elements failing the rule with target CSS selectors and failureSummary).
• Minimizes false positives as a design goal. The Axe rules list documents each rule's purpose and WCAG mapping.

Impact severity taxonomy

Axe categorises every violation by impact:

• critical — directly prevents a user from accessing the content.
• serious — high likelihood of blocking access.
• moderate — partial access loss.
• minor — nuisance but not blocking.

Callers filter on impact to control signal volume — see concepts/severity-filtered-violation-reporting and patterns/severity-gated-violation-reporting.

Integration shape

Axe Core is DOM-centric: it runs in whatever environment exposes a DOM. Typical integrations:

• Browser extensions (axe DevTools) — manual audit on the currently loaded page.
• E2E test frameworks — inject Axe into the browser context (@axe-core/playwright, @axe-core/puppeteer, @axe-core/webdriverjs, Cypress via cypress-axe).
• Component-test frameworks — run Axe against a rendered JSDOM / happy-dom (jest-axe).

Seen in

• sources/2025-01-07-slack-automated-accessibility-testing-at-slack — Slack chose Axe Core over alternatives citing "extensive capabilities and compatibility with our current end-to-end (E2E) test frameworks" and its low false-positive rate. Integration via @axe-core/playwright into the existing Playwright E2E suite, scoped to WCAG 2.1 A + AA tags and filtered to critical only at launch.

Related

• systems/axe-core-playwright — the Playwright binding.
• systems/playwright — the E2E framework carrying the Axe integration at Slack.
• concepts/wcag-2-1-a-aa-scope — canonical scope-picker for automated a11y via Axe's tag filter.
• concepts/severity-filtered-violation-reporting — canonical control lever over Axe's signal volume.
• patterns/severity-gated-violation-reporting — how Slack applied severity-filtering.