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Playwright can compare a page or element with a saved screenshot, and it can control individual requests. Its documented offline option simulates no connection—not a slow one—and the sources cited here do not establish a built-in bandwidth-throttling API. For a genuine low-bandwidth test, apply a separately verified network-shaping mechanism in your test environment, record its settings, and use Playwright to capture and compare the result.
Choose the network condition you actually want to test
“Low bandwidth” can mean different things. Keep these cases distinct in your test and in any results you report:
- Bandwidth-shaped: A separate, verified mechanism limits network performance. State its configured throughput, latency, and packet-loss conditions, and whether it applies to the browser process, host, or wider test environment. The Playwright sources cited here do not specify such a mechanism or its configuration.
- Controlled requests: Playwright routes requests so you can provide predictable responses, replay a HAR, or abort selected requests. This tests those specific responses or missing-resource behavior; it does not reproduce a measured slow connection.
- Offline: Playwright’s documented
offlineoption simulates no network connectivity. Use it for an offline scenario, not as a synonym for slow bandwidth.
A screenshot diff measures visual differences between images. It does not verify that a network profile matches a real user’s connection.
Set up a repeatable visual comparison
Playwright Test provides screenshot assertions against stored reference images. On first use, the test runner writes a reference; later runs compare captures against it. Screenshot assertions require the Playwright Test runner.
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- Install Playwright Test in the project if it is not already installed:
npm init playwright@latest. Follow the prompts to configure the project and install the selected browsers. - Create a test such as
tests/slow-network-visual.spec.tsand use a screenshot assertion as shown below. - Generate the baseline by running
npx playwright test tests/slow-network-visual.spec.ts. Review the generated reference image before treating it as the expected appearance. - Compare later runs under the same browser and environment by running the same command again. Review any reported diff rather than assuming every pixel change is a product regression.
Page-level screenshot
import { test, expect } from '@playwright/test';
test('page matches its visual baseline', async ({ page }) => {
await page.goto('https://example.com');
await expect(page).toHaveScreenshot('example-page.png');
});Replace the example URL with the page you control. A page assertion captures the page; choose a locator assertion instead when only one component or region matters. The documented default screenshot format is PNG. You can request WebP by giving the screenshot a .webp name.
Element-level screenshot
import { test, expect } from '@playwright/test';
test('hero component matches its visual baseline', async ({ page }) => {
await page.goto('https://example.com');
const hero = page.locator('[data-testid="hero"]');
await expect(hero).toHaveScreenshot('hero.png');
});Use a stable selector that identifies the intended element. Locator screenshot assertions are also stabilized by waiting for two consecutive screenshots to match before comparison, which helps avoid capturing an intermediate rendering state.
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Control network-dependent content with routes or HAR
Set routes before navigation so the page’s requests can be intercepted as they occur. Use a fulfilled response for a deterministic API fixture, a HAR when you want to replay recorded responses, or abort selected requests to test how the page behaves when a resource is missing.
Fulfill a predictable API response
import { test, expect } from '@playwright/test';
test('page renders with a controlled API response', async ({ page }) => {
await page.route('**/api/status', async route => {
await route.fulfill({
status: 200,
contentType: 'application/json',
body: JSON.stringify({ status: 'ready' }),
});
});
await page.goto('https://example.com');
await expect(page).toHaveScreenshot('status-page.png');
});Change the route pattern and response body to match the page’s actual API contract. A mock makes that response repeatable; it does not model the delay or throughput of a slow connection.
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Abort a selected resource
import { test, expect } from '@playwright/test';
test('page appearance when selected images fail to load', async ({ page }) => {
await page.route('**/*.{png,jpg,jpeg,webp}', route => route.abort());
await page.goto('https://example.com');
await expect(page).toHaveScreenshot('without-images.png');
});This exercises a missing-image condition. It is not bandwidth throttling: requests are aborted rather than delivered slowly. Restrict route patterns carefully so you do not accidentally block resources needed to load the page or test.
Replay recorded responses from a HAR
Playwright’s network tooling supports HAR replay. Record or prepare a HAR for the relevant page, then configure the test to fulfill requests from that archive before navigating. This is useful when the page depends on a known set of recorded responses. HAR replay controls what responses are served; it does not impose a measured throughput cap. If a service worker intercepts traffic, account for that in the test setup because it can affect request handling.
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Use offline emulation only for an offline scenario
Playwright’s configuration supports an offline option. Setting it simulates unavailable connectivity; it does not represent a connection with limited throughput or added latency. Label screenshots from this configuration as offline results so they are not mistaken for low-bandwidth measurements.
Keep the comparison environment consistent
Screenshot output can vary with the operating system, browser version, browser settings, hardware, power source, and headless mode. Keep the baseline and comparison environment as alike as possible, and document any differences that cannot be held constant.
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Dynamic content can also create visual noise. Playwright documents diff controls, including thresholds, and a screenshot stylesheet that can hide volatile elements. Use these deliberately: hiding a changing timestamp may help isolate a layout change, but hiding content relevant to the test can conceal a real regression. Screenshot assertions wait for two consecutive captures to match before comparison, reducing the chance of comparing an intermediate render.
When reporting results, identify the scenario (bandwidth-shaped, mocked/replayed, selected requests aborted, or offline), whether the capture covers a page or locator, and whether the browser and host environment match the baseline. For a shaped test, also report the separately configured network conditions and where shaping was applied.
Troubleshoot common comparison problems
- The first run creates a screenshot instead of reporting a diff: This is the baseline-generation step. Review and retain the reference, then run the test again for comparisons.
- The page looks “slow-network tested,” but no speed limit was configured: A route mock, HAR replay, aborted request, or offline setting does not establish a bandwidth profile. Add a separate verified shaping mechanism or describe the test accurately as request-controlled or offline.
- Screenshots differ across machines: Check operating system, browser version, settings, hardware, power source, and headless mode. Align conditions with the baseline before interpreting pixel differences.
- Only part of the page is relevant: Use a locator screenshot assertion for the component instead of a page-level capture.
- Changing content causes noisy diffs: Stabilize test data where possible, or use the documented screenshot stylesheet and diff controls for elements intentionally outside the test’s scope.
- A route does not appear to control a request: Register the route before navigation, verify the URL pattern matches the request, and consider whether a service worker is handling traffic.
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