Add playwright-axe test suite (#41814)

.github/workflows/headless-tests.yml

@@ -26,6 +26,14 @@ jobs:
   playwright-tests:
     if: github.repository == 'github/docs-internal' || github.repository == 'github/docs'
     runs-on: ${{ fromJSON('["ubuntu-latest", "ubuntu-20.04-xl"]')[github.repository == 'github/docs-internal'] }}
+    strategy:
+      # When we're comfortable a11y tests aren't generating false positives and helping,
+      # let's remove the matrix and just run playwright in a single job.
+      matrix:
+        node:
+          - playwright-rendering
+          - playwright-a11y
+      fail-fast: false
     timeout-minutes: 60
     steps:
       - name: Check out repo
@@ -46,4 +54,7 @@ jobs:
       - name: Run Playwright tests
         env:
           PLAYWRIGHT_WORKERS: ${{ fromJSON('[1, 4]')[github.repository == 'github/docs-internal'] }}
-        run: npm run playwright-test -- --reporter list
+        # Run playwright rendering tests and a11y tests (axe scans) as distinct checks
+        # so that we can run them without blocking merges until we can be confident
+        # results for a11y tests are meaningul and scenarios we're testing are correct.
+        run: npm run playwright-test -- ${{ matrix.node }} --reporter list