/kb-playwright-testing

name: kb-playwright-testing
description: "Reference data, not a reviewer. Browser accessibility testing using Playwright and @axe-core/playwright. Keyboard scans, contrast verification, and accessibility tree snapshots."
license: MIT
disable-model-invocation: true
user-invocable: false
metadata:
  tier: reference
  domain: cross-cutting
  output: none
  effort: low
  title: Playwright Testing

Playwright Accessibility Testing

Reusable knowledge module for browser-based accessibility testing using Playwright and @axe-core/playwright.

MCP Tools Available

Each tool, with its purpose and requires @axe-core or playwright.

| Tool | Purpose | Requires @axe-core/playwright | |------|---------|------------------------------| | `run_playwright_keyboard_scan` | Tab-order traversal, keyboard trap detection | No | | `run_playwright_state_scan` | Click triggers, scan revealed content with axe-core | Yes | | `run_playwright_viewport_scan` | Multi-viewport axe-core + touch target measurement | Yes | | `run_playwright_contrast_scan` | Computed-style contrast ratio after CSS cascade | No | | `run_playwright_a11y_tree` | Browser accessibility tree snapshot | No |

@axe-core/playwright Patterns

Full Page Scan

import AxeBuilder from '@axe-core/playwright';

const results = await new AxeBuilder({ page })
  .withTags(['wcag2a', 'wcag2aa', 'wcag21a', 'wcag21aa', 'wcag22aa'])
  .analyze();

Scoped Element Scan

const results = await new AxeBuilder({ page })
  .include('.modal-content')
  .withTags(['wcag2a', 'wcag2aa'])
  .analyze();

Single Rule Verification

const results = await new AxeBuilder({ page })
  .include('#hero-image')
  .withRules(['image-alt'])
  .analyze();
expect(results.violations).toEqual([]);

Scan After Interaction

await page.click('[aria-expanded="false"]');
await page.waitForSelector('.accordion-content', { state: 'visible' });
const results = await new AxeBuilder({ page })
  .include('.accordion-content')
  .analyze();

Keyboard Traversal Patterns

Record Tab Sequence

const tabStops = [];
for (let i = 0; i < maxTabs; i++) {
  await page.keyboard.press('Tab');
  const info = await page.evaluate(() => {
    const el = document.activeElement;
    return {
      tagName: el?.tagName,
      role: el?.getAttribute('role'),
      name: el?.getAttribute('aria-label') || el?.textContent?.trim().slice(0, 50),
      id: el?.id,
      tabIndex: el?.tabIndex
    };
  });
  tabStops.push(info);
}

Detect Keyboard Traps

A keyboard trap is detected when the same element receives focus after consecutive Tab presses:

let trapCount = 0;
let lastSelector = '';
for (const stop of tabStops) {
  const currentSelector = `${stop.tagName}#${stop.id}`;
  if (currentSelector === lastSelector) {
    trapCount++;
    if (trapCount >= 3) { /* TRAP DETECTED */ }
  } else {
    trapCount = 0;
  }
  lastSelector = currentSelector;
}

Focus Management After Modal Open

await page.click('[data-modal-trigger]');
await page.waitForSelector('[role="dialog"]', { state: 'visible' });
const focusedRole = await page.evaluate(() =>
  document.activeElement?.closest('[role="dialog"]') ? 'inside-dialog' : 'outside-dialog'
);
// focusedRole should be 'inside-dialog'

Focus Management Test Templates

Modal Focus Trap Test

test('modal traps focus correctly', async ({ page }) => {
  await page.goto(url);
  await page.click('[data-open-modal]');
  await page.waitForSelector('[role="dialog"]', { state: 'visible' });

  // Focus should be inside the dialog
  const inDialog = await page.evaluate(() =>
    document.activeElement?.closest('[role="dialog"]') !== null
  );
  expect(inDialog).toBe(true);

  // Tab through dialog — should not escape
  for (let i = 0; i < 20; i++) {
    await page.keyboard.press('Tab');
    const stillInDialog = await page.evaluate(() =>
      document.activeElement?.closest('[role="dialog"]') !== null
    );
    expect(stillInDialog).toBe(true);
  }

  // Escape should close and return focus to trigger
  await page.keyboard.press('Escape');
  const focusedId = await page.evaluate(() => document.activeElement?.id);
  expect(focusedId).toBe('modal-trigger-id');
});

Skip Link Test

test('skip link moves focus to main content', async ({ page }) => {
  await page.goto(url);
  await page.keyboard.press('Tab'); // Focus skip link
  await page.keyboard.press('Enter'); // Activate it
  const focusedId = await page.evaluate(() => document.activeElement?.id);
  expect(focusedId).toBe('main-content');
});

CI Integration

GitHub Actions with Playwright

name: Accessibility Tests
on: [push, pull_request]

jobs:
  a11y:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
      - run: npm ci
      - run: npx playwright install --with-deps chromium
      - name: Start dev server
        run: npm run dev &
        env:
          CI: true
      - name: Wait for server
        run: npx wait-on http://localhost:3000 --timeout 30000
      - name: Run accessibility tests
        run: npx playwright test tests/a11y/
      - uses: actions/upload-artifact@v4
        if: failure()
        with:
          name: a11y-test-results
          path: test-results/

Playwright Config for Accessibility Tests

// playwright.config.js (a11y section)
export default {
  testDir: './tests/a11y',
  use: {
    baseURL: process.env.BASE_URL || 'http://localhost:3000',
    // Use Chromium only — @axe-core/playwright is Chromium-validated
    browserName: 'chromium',
  },
  webServer: {
    command: 'npm run dev',
    url: 'http://localhost:3000',
    reuseExistingServer: !process.env.CI,
  },
};

Graceful Degradation

Detection Pattern

let _playwrightAvailable = null;
as