For a quick mobile-sized screenshot, set Chrome Headless’s window dimensions with --window-size. To test mobile-specific behavior such as touch input, user-agent detection, or device pixel ratio, use device emulation instead: window dimensions alone do not configure those properties.
Take a screenshot at mobile-like dimensions
Chrome’s Headless documentation shows this basic command:
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chrome --headless --screenshot --window-size=412,892 https://example.comReplace the URL with the page you want to capture. The command asks Chrome for a screenshot with a 412-by-892 window. These are example dimensions, not the specifications of a particular phone. Chrome’s documentation also shows a historical example using --window-size=412,732. Check the Headless documentation for the Chrome release you use before relying on command-line details: Chrome Headless documentation.
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This is useful for checking how a responsive layout looks at a chosen size. It does not, by itself, reproduce every condition of a mobile browser.
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Know what a window size does—and does not—emulate
A narrow window sets dimensions for the capture, but mobile rendering may also depend on CSS viewport characteristics, device pixel ratio, user agent, client hints, and touch behavior. If those affect your test, configure device emulation rather than treating --window-size as a complete phone simulation.
Chrome DevTools Device Mode offers presets and custom CSS-pixel dimensions, and can configure properties such as resolution, pixel ratio, touch, and mobile viewport behavior. See Chrome DevTools Device Mode. ChromeDriver also documents mobile emulation settings, including device metrics and user-agent configuration: ChromeDriver mobile emulation.
Choose the right emulation for the screenshot
Use Headless window dimensions for a quick layout check
Choose a width and height that match the responsive case you need to inspect, then capture with --screenshot. This is the simplest option when the question is how the page layout responds to a particular size.
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Use device emulation when browser behavior matters
Use DevTools Device Mode or ChromeDriver mobile emulation when the test depends on more than dimensions. Set the CSS viewport size and, when relevant, device pixel ratio, user agent or client hints, and touch behavior. These settings can change resolution-sensitive CSS, window.devicePixelRatio, image selection, or user-agent-dependent rendering.
Use a real device for hardware-dependent results
Device Mode is an approximation, not a mobile device running the page. Google describes it as a “first-order approximation” and notes that it cannot reproduce some hardware-specific behavior, such as the effects of mobile CPU architecture. If that distinction matters, validate with Remote Debugging or on the actual device. See the Device Mode documentation.
Set up a useful comparison
When comparing mobile cases, record the settings that explain the result rather than labeling a capture only with a phone name:
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- CSS viewport width and height: These determine the responsive layout case.
- Device pixel ratio: Include it when resolution-sensitive CSS,
window.devicePixelRatio, or image selection matters. - Mobile behavior: Note whether you enabled touch and configured user-agent or client-hint behavior.
- Capture scope: Decide whether you need the visible viewport or a full-page image; these are different deliverables.
- Validation method: Use real hardware if the result depends on behavior the emulator cannot reproduce.
There is no single universally correct device profile for every audience. Choose settings to match the behavior you are investigating, and verify the screenshot against that test case.
Keep virtual screens separate from mobile emulation
Headless Chrome’s virtual screen configuration controls display properties such as origin, size, scale factor, orientation, and work area. Those display settings are not a substitute for configuring browser mobile-emulation properties. Consult the Headless documentation and the relevant emulation documentation for the Chrome version in use.
Avoid applying emulation twice with Lighthouse
Lighthouse has its own screen and user-agent emulation controls. If another layer, such as Puppeteer, has already applied emulation, avoid duplicating it in Lighthouse; its documentation describes when external emulation should be taken into account. See Lighthouse emulation documentation.
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- YOUR CONTENT, SUPER SMOOTH: The ultra-clear 6.7" FHD+ Super AMOLED display of Galaxy A17 5G helps bring your content to life, whether you're scrolling through recipes or video chatting with loved ones.¹
- LIVE FAST. CHARGE FASTER: Focus more on the moment and less on your battery percentage with Galaxy A17 5G. Super Fast Charging powers up your battery so you can get back to life sooner.²
- MEMORIES MADE PICTURE PERFECT: Capture every angle in stunning clarity, from wide family photos to close-ups of friends, with the triple-lens camera on Galaxy A17 5G.
- NEED MORE STORAGE? WE HAVE YOU COVERED: With an improved 2TB of expandable storage, Galaxy A17 5G makes it easy to keep cherished photos, videos and important files readily accessible whenever you need them.³
- BUILT TO LAST: With an improved IP54 rating, Galaxy A17 5G is even more durable than before.⁴ It’s built to resist splashes and dust and comes with a stronger yet slimmer Gorilla Glass Victus front and Glass Fiber Reinforced Polymer back.
Troubleshoot common mismatches
The screenshot has the right size, but the page still looks desktop-oriented
--window-size controls dimensions; it does not necessarily apply the mobile behavior your test requires. Configure mobile emulation and check the viewport, user agent, and touch settings relevant to the page.
Images or resolution-sensitive styles differ from the expected result
Check the device pixel ratio as well as CSS viewport dimensions. Resolution-sensitive CSS and image selection can respond to pixel ratio, so matching width and height alone may not reproduce the case.
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Device Mode simulates mobile experience from a desktop or laptop and does not run on mobile hardware. If hardware-specific behavior is in scope, use Remote Debugging or test on the real device.
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Lighthouse results look different from the separately emulated capture
Check whether Lighthouse and the automation layer are both applying emulation. Lighthouse advises against duplicating settings when they have already been applied externally.
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