Migrate a11y e2e tests to Playwright (#46038)

* Migrate a11y tests.

* Run a11y tests with firefox and webkit browsers.

* Run the tests only with chromium for now.

* Improve the tests descriptions and comments.

test/e2e/specs/editor/various/a11y.spec.js

@@ -0,0 +1,96 @@
+/**
+ * WordPress dependencies
+ */
+const { test, expect } = require( '@wordpress/e2e-test-utils-playwright' );
+
+test.describe( 'a11y', () => {
+	test.beforeEach( async ( { admin } ) => {
+		await admin.createNewPost();
+	} );
+
+	test( 'navigating through the Editor regions four times should land on the Editor top bar region', async ( {
+		page,
+		pageUtils,
+	} ) => {
+		// On a new post, initial focus is set on the Post title.
+		await expect(
+			page.locator( 'role=textbox[name=/Add title/i]' )
+		).toBeFocused();
+		// Navigate to the 'Editor settings' region.
+		await pageUtils.pressKeyWithModifier( 'ctrl', '`' );
+		// Navigate to the 'Editor publish' region.
+		await pageUtils.pressKeyWithModifier( 'ctrl', '`' );
+		// Navigate to the 'Editor footer' region.
+		await pageUtils.pressKeyWithModifier( 'ctrl', '`' );
+		// Navigate to the 'Editor top bar' region.
+		await pageUtils.pressKeyWithModifier( 'ctrl', '`' );
+
+		// This test assumes the Editor is not in Fullscreen mode. Check the
+		// first tabbable element within the 'Editor top bar' region is the
+		// 'Toggle block inserter' button.
+		await page.keyboard.press( 'Tab' );
+		await expect(
+			page.locator( 'role=button[name=/Toggle block inserter/i]' )
+		).toBeFocused();
+	} );
+
+	test( 'should constrain tabbing within a modal', async ( {
+		page,
+		pageUtils,
+	} ) => {
+		// Open keyboard help modal.
+		await pageUtils.pressKeyWithModifier( 'access', 'h' );
+
+		const closeButton = page.locator(
+			'role=dialog[name="Keyboard shortcuts"i] >> role=button[name="Close"i]'
+		);
+		// The close button should not be focused by default; this is a strange UX
+		// experience.
+		// See: https://github.com/WordPress/gutenberg/issues/9410
+		await expect( closeButton ).not.toBeFocused();
+
+		await page.keyboard.press( 'Tab' );
+
+		// Ensure the Close button of the modal is focused after tabbing.
+		await expect( closeButton ).toBeFocused();
+	} );
+
+	test( 'should return focus to the first tabbable in a modal after blurring a tabbable', async ( {
+		page,
+		pageUtils,
+	} ) => {
+		await pageUtils.pressKeyWithModifier( 'access', 'h' );
+
+		// Click a non-focusable element after the last tabbable within the modal.
+		const last = page
+			.locator( 'role=dialog[name="Keyboard shortcuts"i] >> div' )
+			.last();
+		await last.click();
+
+		await page.keyboard.press( 'Tab' );
+
+		await expect(
+			page.locator(
+				'role=dialog[name="Keyboard shortcuts"i] >> role=button[name="Close"i]'
+			)
+		).toBeFocused();
+	} );
+
+	test( 'should return focus to the last tabbable in a modal after blurring a tabbable and tabbing in reverse direction', async ( {
+		page,
+		pageUtils,
+	} ) => {
+		await pageUtils.pressKeyWithModifier( 'access', 'h' );
+
+		// Click a non-focusable element before the first tabbable within the modal.
+		await page.click( 'role=heading[name="Keyboard shortcuts"i]' );
+
+		await pageUtils.pressKeyWithModifier( 'shift', 'Tab' );
+
+		await expect(
+			page.locator(
+				'role=dialog[name="Keyboard shortcuts"i] >> role=button[name="Close"i]'
+			)
+		).toBeFocused();
+	} );
+} );