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To test a page at 390 pixels wide, set the browser viewport to 390 CSS pixels, inspect the layout, and capture a screenshot. For a one-off check, use Chrome DevTools Device Mode; for repeatable regression checks, set the viewport in Playwright and compare the screenshot with a stored baseline. A 390-pixel check covers that width only, so also inspect widths around the page’s responsive breakpoints.
What “390 pixels wide” means
Responsive CSS layout responds to the viewport in CSS pixels. The screenshot file’s pixel width can differ if the capture uses device-pixel scaling: a high device-pixel ratio can produce more image pixels for the same 390-CSS-pixel viewport. For a screenshot that is one image pixel per CSS pixel, use CSS scale in Playwright; use device scale when you specifically need device-pixel output.
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Test manually in Chrome DevTools
- Open the page in Chrome and open DevTools. Select the Toggle device toolbar control to enable Device Mode.
- Choose Responsive viewport mode and enter 390 for the width. Set a height appropriate to the view you want to review.
- Inspect the page at that size. Look for horizontal overflow, clipped or overlapping content, awkward line breaks, hard-to-reach controls, and whether the intended mobile layout is active.
- Capture the current viewport screenshot, or use the full-size screenshot option when you need the whole page. Chrome documents custom dimensions, media-query breakpoint inspection, and both capture options in its Device Mode guide.
- Check relevant widths on either side of visible media-query breakpoints. A screenshot at 390 pixels verifies one width, not every nearby layout transition.
Automate the check with Playwright
Playwright Test can set a viewport and use expect(page).toHaveScreenshot() to compare the rendered page with a stored visual baseline. The example below assumes Playwright Test is installed and configured in the project. Save it as a test file, adjust the page URL and readiness condition for your app, and run it with npx playwright test.
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test('page renders at 390 CSS pixels wide', async ({ page }) => {
await page.setViewportSize({ width: 390, height: 844 });
await page.goto('https://example.com');
await expect(page).toHaveScreenshot('page-390.png', {
fullPage: true,
scale: 'css',
threshold: 0.2,
});
});Replace https://example.com with the page under test. The first run creates a baseline; subsequent runs compare captures against it. Review and intentionally update the baseline when a design change is expected, rather than accepting unexplained visual differences.
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Choose the screenshot output
- CSS scale: one screenshot image pixel per CSS pixel, useful when the test should correspond directly to the 390-pixel CSS viewport.
- Device scale: output uses device pixels, which can make the image larger at high device-pixel ratios.
- Viewport or full page: omit
fullPageor set it tofalseto capture only the viewport; set it totrueto capture the full page. - Difference tolerance: Playwright supports a configurable perceived color-difference threshold. Tune it only for known rendering noise; a looser threshold can also hide small visual regressions.
Keep comparisons reproducible
Screenshot output can vary with the operating system, browser version, settings, hardware, and headless mode. Generate and compare baselines in a consistent environment, including the same browser version and operating-system setup where practical. If rendering differs unexpectedly, first check that the environment and page state are stable before changing the comparison threshold. See Playwright’s visual comparisons guidance and viewport and device emulation configuration.
What to inspect at 390 pixels
- Can the page scroll horizontally when it should not?
- Are images, cards, tables, and code blocks clipped or wider than the viewport?
- Do headings and body text wrap into readable lines without collisions?
- Do navigation, forms, and interactive controls fit and remain usable?
- Does the expected mobile layout appear, and does it remain coherent just above and below relevant breakpoints?
These are review checks, not a guarantee that a design will pass on every device. Device Mode is an emulation aid: it does not reproduce every physical-device characteristic. Chrome’s guidance is: “When in doubt, your best bet is to actually run your page on a mobile device.” See Chrome DevTools Device Mode.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
Does a 390-pixel screenshot have to be 390 pixels wide as an image file?
No. The viewport is 390 CSS pixels wide; device-pixel scaling can produce a wider image file.
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Should I use a real phone as well as emulation?
Yes, when platform- or hardware-dependent behavior matters; emulation does not reproduce every physical-device characteristic.
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