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Test a 390-pixel viewport manually in Chrome
- Open the page in Chrome and open DevTools.
- Turn on Device Mode using the device toolbar.
- Choose Responsive viewport mode, then enter 390 for the width. Set a height that matches the portion of the page you want to inspect.
- Review the layout for horizontal overflow, clipped or overlapping content, awkward line breaks, hard-to-reach controls, and whether the intended mobile layout is active.
- Use the DevTools screenshot command to capture the current viewport, or choose the full-size screenshot option to capture the page. See Chrome DevTools Device Mode documentation for custom dimensions, breakpoint inspection, and screenshot capture.
The width is measured in CSS pixels. The resulting image’s physical pixel dimensions may differ if the capture uses device-pixel scaling.
Check the widths around your breakpoints
A screenshot at 390 pixels verifies the page at that viewport, not every nearby size. In DevTools, display the media-query breakpoints and move the viewport across the relevant transitions. Check widths just below and above a breakpoint, especially where navigation, columns, or controls change layout. DevTools can show breakpoints and let you adjust the viewport to trigger them; it cannot establish that the design works at every size without those checks.
Automate repeatable screenshot checks with Playwright
For visual regression checks, set the viewport width in Playwright and use Playwright Test’s toHaveScreenshot() assertion to compare the page with a stored baseline. This runnable example assumes a Playwright Test project with the application available at the example URL; change the URL and the expected screenshot name to match your project.
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import { test, expect } from '@playwright/test';
test('page layout at 390 CSS pixels', async ({ page }) => {
await page.setViewportSize({ width: 390, height: 844 });
await page.goto('https://example.com');
await expect(page).toHaveScreenshot('page-390.png');
});On its first run, Playwright can create the screenshot baseline; subsequent runs compare against it. Review and commit approved baseline changes intentionally rather than accepting every changed image automatically. See Playwright screenshot comparisons and Playwright viewport and device emulation.
Choose the screenshot scale
Playwright screenshot assertions support CSS scale and device scale. CSS scale produces one image pixel per CSS pixel; device scale produces device pixels and can create a larger image at a high device-pixel ratio. For a direct comparison tied to a 390-CSS-pixel viewport, CSS scale is usually easier to interpret. Use device scale when the test specifically needs device-pixel output.
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Choose viewport or full-page capture
A viewport screenshot compares only the visible area at the selected height. A full-page screenshot is useful for checking content farther down the page, but it does not replace inspecting the initial viewport or testing interactive states. Playwright supports full-page screenshots and a configurable perceived color-difference threshold; adjust comparison settings only when the tolerated rendering variation is understood.
Keep baseline rendering consistent
Screenshot output can vary with the operating system, browser version, settings, hardware, and headless mode. Generate and compare baselines in a consistent environment so environment changes do not look like application regressions. Consult Playwright’s snapshot guidance for baseline and comparison details.
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Know what emulation does—and does not—test
Browser emulation is useful for checking responsive layout, but it does not reproduce every physical-device characteristic. If behavior may depend on a device, operating system, hardware, or browser integration, validate on an actual mobile device too. Chrome’s guidance is: “When in doubt, your best bet is to actually run your page on a mobile device.” See Chrome Device Mode documentation.
Troubleshoot common screenshot-test problems
- The layout does not look mobile: Confirm that the viewport is actually set to 390 CSS pixels and that the page has loaded its intended responsive styles.
- The screenshot is wider than expected: Check whether the test is using device scale rather than CSS scale; device-pixel output can be wider in physical image pixels even when the CSS viewport is 390 pixels.
- A comparison fails unexpectedly: Check for changes in operating system, browser version, settings, hardware, or headless mode. Keep the baseline and comparison environment consistent before treating an image difference as a page regression.
- The 390-pixel capture passes but another mobile width breaks: Add checks around the relevant media-query breakpoints; a single viewport is only a point-in-time check.
- Emulation looks correct but a real phone behaves differently: Test on the physical device, particularly for behavior tied to platform or hardware characteristics.
Or skip the browser setup
ScreenshotNeo can capture a page through one API request. Its screenshot API accepts a URL and can return PNG, JPEG, WebP, or PDF output. Set the viewport width to 390 and choose a height for the view you want to inspect. The parameter names used by other screenshot APIs also work, which can make switching easier. See the ScreenshotNeo API documentation.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com --data-urlencode width=390 --data-urlencode height=844 -o shot.webpScreenshotNeo removes supported cookie and consent banners, newsletter popups, and chat widgets before capture; each cleanup step can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers indicate the page verdict and billing status. It also has an MCP server for AI agents, with tools including take_screenshot, get_page_info, and capture_pdf. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000 shots. Sign up for ScreenshotNeo’s free plan.
Frequently Asked Questions
Does a 390-pixel screenshot mean the image is exactly 390 pixels wide?
Not necessarily. The viewport is 390 CSS pixels wide; device-scale output can contain more physical image pixels.
Should I test on a real phone as well as using browser emulation?
Yes, when behavior may depend on physical-device, platform, or hardware characteristics. Emulation is a layout aid, not a complete substitute.
Quick Recap
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