diff --git a/src/elements/private/I18N/I18N.test.ts b/src/elements/private/I18N/I18N.test.ts
index 12f55f2cf..8c108a5a5 100644
--- a/src/elements/private/I18N/I18N.test.ts
+++ b/src/elements/private/I18N/I18N.test.ts
@@ -1,5 +1,6 @@
-import { expect, fixture } from '@open-wc/testing';
+import { expect, fixture, waitUntil } from '@open-wc/testing';
 import { createModel } from '@xstate/test';
+import type { i18n } from 'i18next';
 import { createMachine } from 'xstate';
 import { I18N } from './I18N';
 
@@ -68,7 +69,6 @@ function testLang(lang: 'en' | 'fr') {
 
 async function testText(element: I18N) {
   await element.whenReady;
-  await element.requestUpdate();
 
   const lang = element.lang as 'en' | 'fr';
   const ns = element.ns as 'global' | 'custom';
@@ -77,6 +77,17 @@ async function testText(element: I18N) {
   const value = opts?.value ?? '';
   const text = key === '' ? '' : samples.text[lang][ns][key].replace('{{value}}', value);
 
+  // `whenReady` only covers the one-time i18next init. Assigning `ns` or `lang` starts another
+  // load that it does not track, and until that lands `_t` falls back to the global namespace, so
+  // wait for the bundle this state actually needs before reading the rendered text.
+  const i18nInstance = (element as unknown as { _i18n: i18n })._i18n;
+  await waitUntil(
+    () => !!i18nInstance.getResourceBundle(lang, ns),
+    `i18next never loaded the ${ns} namespace for ${lang}`
+  );
+
+  await element.requestUpdate();
+
   expect(element.shadowRoot!.textContent).to.equal(text);
 }
 
diff --git a/src/elements/public/I18n/I18n.test.ts b/src/elements/public/I18n/I18n.test.ts
index ab1876550..662c45ed3 100644
--- a/src/elements/public/I18n/I18n.test.ts
+++ b/src/elements/public/I18n/I18n.test.ts
@@ -1,6 +1,6 @@
 import './index';
 
-import { expect, fixture, html, oneEvent } from '@open-wc/testing';
+import { expect, fixture, html, oneEvent, waitUntil } from '@open-wc/testing';
 
 import { FetchEvent } from '../NucleonElement/FetchEvent';
 import { I18n } from './I18n';
@@ -67,8 +67,13 @@ describe('I18n', () => {
 
     event.preventDefault();
     event.respondWith(Promise.resolve(new Response(JSON.stringify(resource))));
-    await new Promise(resolve => setTimeout(resolve));
-    await element.requestUpdate();
+
+    // The element re-renders itself when i18next loads a bundle, but reading the response body
+    // takes more than one task, so wait for the bundle instead of a fixed number of ticks.
+    await waitUntil(
+      () => !!I18n.i18next.getResourceBundle('en', 'baz'),
+      'i18next never loaded the baz namespace'
+    );
 
     expect(element).shadowDom.to.equal('<span>bar</span>');
   });
@@ -89,8 +94,12 @@ describe('I18n', () => {
 
     event.preventDefault();
     event.respondWith(Promise.resolve(new Response(JSON.stringify(resource))));
-    await new Promise(resolve => setTimeout(resolve));
-    await element.requestUpdate();
+
+    // See the note above: wait for the bundle, not for a fixed number of ticks.
+    await waitUntil(
+      () => !!I18n.i18next.getResourceBundle('es', 'shared'),
+      'i18next never loaded the es translations'
+    );
 
     expect(element).shadowDom.to.equal('<span>bar</span>');
   });
diff --git a/src/elements/public/PaymentCardEmbed/PaymentCardEmbed.test.ts b/src/elements/public/PaymentCardEmbed/PaymentCardEmbed.test.ts
index 63aca9be8..faa9c629c 100644
--- a/src/elements/public/PaymentCardEmbed/PaymentCardEmbed.test.ts
+++ b/src/elements/public/PaymentCardEmbed/PaymentCardEmbed.test.ts
@@ -22,6 +22,22 @@ class TestElement extends PaymentCardEmbed {
 
 customElements.define('test-element', TestElement);
 
+// The Lumo default font stack, verbatim from @vaadin/vaadin-lumo-styles/typography.js. The element
+// forwards `getComputedStyle(...).fontFamily`, and browsers rewrite that stack when serializing it
+// (current Chrome reports BlinkMacSystemFont as "system-ui"), so resolve it here instead of
+// hardcoding one browser's output.
+const LUMO_FONT_FAMILY =
+  '-apple-system, BlinkMacSystemFont, "Roboto", "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"';
+
+function computedFontFamily(fontFamily: string): string {
+  const probe = document.createElement('div');
+  probe.style.fontFamily = fontFamily;
+  document.body.appendChild(probe);
+  const computed = getComputedStyle(probe).fontFamily;
+  probe.remove();
+  return computed;
+}
+
 describe('PaymentCardEmbed', () => {
   it('imports and defines foxy-spinner element', () => {
     expect(customElements.get('foxy-spinner')).to.exist;
@@ -162,8 +178,7 @@ describe('PaymentCardEmbed', () => {
         '--lumo-size-xs': '26px',
         '--lumo-border-radius-m': '4px',
         '--lumo-border-radius-s': '4px',
-        '--lumo-font-family':
-          '-apple-system, BlinkMacSystemFont, Roboto, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"',
+        '--lumo-font-family': computedFontFamily(LUMO_FONT_FAMILY),
         '--lumo-font-size-m': '16px',
         '--lumo-font-size-s': '14px',
         '--lumo-font-size-xs': '13px',
diff --git a/web-test-runner.config.js b/web-test-runner.config.js
index 3cb423860..4a1b1683e 100644
--- a/web-test-runner.config.js
+++ b/web-test-runner.config.js
@@ -8,8 +8,26 @@ export default Object.assign({}, webServerConfig, {
 
   browsers: [
     puppeteerLauncher({
+      // Test pages run as concurrent tabs, and a browser produces no frames for a tab that is not
+      // the visible one. Anything waiting on a frame - `nextFrame()`, `elementUpdated()` on a
+      // non-Lit element, Vaadin's own internals - then never settles and the test times out.
+      // Focus emulation makes every page behave as the focused one, so frames keep coming.
+      createPage: async ({ context }) => {
+        const page = await context.newPage();
+
+        try {
+          const session = await page.target().createCDPSession();
+          await session.send('Emulation.setFocusEmulationEnabled', { enabled: true });
+        } catch {
+          // Older or non-Chromium browsers may not support it. Frame-dependent tests can time out
+          // when that happens, but the run still starts, which beats failing every file.
+        }
+
+        return page;
+      },
+
       launchOptions: {
-        executablePath: '/usr/bin/chromium',
+        executablePath: process.env.CHROME_PATH || '/usr/bin/chromium',
         args: ['--no-sandbox', '--disable-setuid-sandbox'],
       },
     }),
@@ -25,6 +43,40 @@ export default Object.assign({}, webServerConfig, {
     },
   },
 
+  // `ResponsiveMixin` sets breakpoint attributes on the element it observes, so the resize it
+  // reacts to can produce another one in the same delivery cycle. The browser then reports
+  // "ResizeObserver loop completed with undelivered notifications" - a notice, not an application
+  // error, since the pending notification is delivered on the next frame. It arrives at
+  // `window.onerror`, where the test framework counts it against whichever test is running, so
+  // drop it before that handler sees it. This classic script runs before the module below.
+  //
+  // Match the full message, not the "ResizeObserver loop" prefix: "ResizeObserver loop limit
+  // exceeded" starts the same way but means the browser gave up on a runaway loop, which is a
+  // real bug and must still fail the test.
+  testRunnerHtml: testFramework => `
+    <!DOCTYPE html>
+    <html>
+      <head>
+        <script>
+          window.addEventListener(
+            'error',
+            event => {
+              var benign = 'ResizeObserver loop completed with undelivered notifications';
+              if (event.message && event.message.indexOf(benign) === 0) {
+                event.stopImmediatePropagation();
+                event.preventDefault();
+              }
+            },
+            true
+          );
+        </script>
+      </head>
+      <body>
+        <script type="module" src="${testFramework}"></script>
+      </body>
+    </html>
+  `,
+
   middleware: [
     (context, next) => {
       const url = context.url;