Choose a viewport that answers the question you are testing. For responsive debugging, start with the page’s actual CSS media-query breakpoints and capture widths just below, at, and above each important transition. Then choose a fixed height, screenshot scale, and capture mode (viewport-only or full-page) that match your review. Named phone, tablet, and laptop presets are useful examples, but they are not universal standards.
Start with the purpose of the screenshot
The “right” viewport depends on what you need to learn. A single 375px image cannot represent every mobile layout, and a wide desktop preset will not expose a navigation change designed for a 768px breakpoint.
- Layout debugging: test the site’s real breakpoint transitions.
- Design review: choose dimensions that keep the relevant component visible and record them for reviewers.
- Visual regression: keep width, height, screenshot scale, browser context, and capture mode constant between runs.
- Presentation imagery: select a familiar aspect ratio or a custom size that frames the content well.
- Device-specific behavior: configure emulation settings in addition to width and height.
A viewport is a layout coordinate space measured in CSS pixels. It is not automatically the same as the saved image’s pixel dimensions.
Find the breakpoints that matter
Inspect the page’s CSS
Look for @media rules using min-width and max-width. A navigation menu might change at 768px, while a card grid changes at 1024px. Those values, rather than a device name, should drive your test matrix.
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Use Chrome DevTools to locate transitions
- Open the page in Chrome and choose More tools → Developer tools.
- Click Toggle device toolbar (or press
Ctrl+Shift+Mon Windows/Linux orCmd+Shift+Mon macOS). - Choose Responsive, enter an exact width and height, and drag the width across the breakpoint.
- Inspect the breakpoint bars and source declarations shown by Device Mode; capture immediately below, at, and above the transition.
For a breakpoint at 768px, useful samples are 767px, 768px, and 769px. Add a few pixels of tolerance when a component has several interacting rules.
Named presets: useful examples, not standards
Chrome for Developers lists these Device Mode width examples: Mobile S (320px), Mobile M (375px), Mobile L (425px), Tablet (768px), Laptop (1024px), Laptop L (1440px), and 4K (2560px). They are convenient reference sizes in the tool, not a study of visitor traffic and not mandatory breakpoints. A site may use entirely different thresholds.
| Preset example | Width | Best use |
|---|---|---|
| Mobile S | 320px | Check very narrow layouts and wrapping |
| Mobile M | 375px | Common compact-phone example |
| Mobile L | 425px | Check wider phone spacing |
| Tablet | 768px | Tablet-sized example or a breakpoint candidate |
| Laptop | 1024px | Compact desktop layout |
| Laptop L | 1440px | Wide desktop review |
| 4K | 2560px | Very wide-screen behavior |
Choose viewport width and height separately
Width usually controls responsive layout. Height controls how much of the initial screen is visible. For a first-screen review, use a realistic fixed height such as 800px or 900px and keep it unchanged across comparisons. If the question concerns all content, use full-page capture instead of making the viewport extremely tall.
Viewport-only versus full-page
- Viewport capture: records the currently visible area, useful for above-the-fold comparisons and what a user sees on load.
- Full-page capture: captures the complete scrollable document, useful for page-length review, print-like archives, and checking content below the fold.
These modes answer different questions. A full-page image does not show how the first screen appears at a particular height, and a viewport image does not prove that lower sections render correctly.
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CSS pixels, device pixels, and DPR
Device pixel ratio (DPR) describes the relationship between physical screen pixels and logical CSS pixels. Chrome explains it this way: “DPR tells Chrome how many screen pixels to use to draw a CSS pixel.” A 375px CSS viewport can therefore produce an image wider than 375 physical pixels on a high-density emulated device.
| Scale choice | Output | Use it when |
|---|---|---|
| CSS scale | One image pixel per CSS pixel | You need stable dimensions for diffs, documentation, or pipelines |
| Device scale | Uses device pixels; high-DPI output can be larger | You need sharper presentation imagery or device-density representation |
Playwright exposes this choice with scale: "css" or scale: "device". Record the scale with every baseline. “375px viewport” and “375px image” are not interchangeable statements.
When to emulate a named device
Simple responsive resizing changes the viewport dimensions. Device emulation can also set device scale factor, user agent, touch behavior, and mobile settings. Use those controls when the site branches on user agent, detects touch, or serves mobile-specific behavior. Playwright’s emulation guide documents these settings, while Chrome Device Mode lets you configure custom-device fields.
Emulation is a tool approximation, not proof that the page behaves identically on every physical phone. For hardware, browser, sensor, or performance issues, test on the actual target environment as well.
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A reproducible Playwright workflow
Playwright’s browser context uses a default 1280 × 720 viewport for consistent automation, but that is a tool default, not a universal recommendation. Set dimensions explicitly:
import { chromium } from 'playwright';
const browser = await chromium.launch();
const page = await browser.newPage({
viewport: { width: 768, height: 900 },
deviceScaleFactor: 1
});
await page.goto('https://example.com', { waitUntil: 'networkidle' });
await page.screenshot({ path: 'viewport.png', fullPage: false, scale: 'css' });
await browser.close();For a full document, change fullPage to true. To test a transition, run the same script for widths such as 767, 768, and 769, keeping every other setting fixed. If you need high-density output, use scale: 'device' and set the intended deviceScaleFactor.
Practical test matrices
Breakpoint matrix
For each breakpoint under review, capture the width immediately below, at, and above the threshold. Include the height and scale in the filename, for example home-w768-h900-css.png.
Design-review matrix
Choose a small, stable set such as 375×800, 768×900, and 1440×900 only if those dimensions expose the layouts your team discusses. Add 320px or 2560px when narrow wrapping or wide-screen whitespace is a known concern.
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Regression matrix
Freeze browser version, viewport, device scale factor, color scheme, locale, timezone, authentication state, network conditions, and wait strategy. A changed font load or cookie banner can create a difference unrelated to CSS.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common failures and fixes
The screenshot is the wrong width
Cause: the host window or automation default controls the viewport. Fix: set an explicit viewport in DevTools or the browser context and verify the recorded dimensions.
A 375px test produces a wider file
Cause: device scale output or a DPR above 1. Fix: choose CSS scale for one output pixel per CSS pixel, or keep device scale and document the expected physical dimensions.
The breakpoint appears inconsistent
Cause: you tested only a named preset, or another media query, scrollbar, or loaded font affects the transition. Fix: inspect the source rules and test adjacent widths with identical page state.
Full-page capture is cut off or incomplete
Cause: lazy content has not loaded, or the page changes while it is being captured. Fix: wait for the relevant selector or network idle, trigger any required scrolling, and use the automation tool’s full-page option.
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Desktop and mobile screenshots differ for reasons unrelated to width
Cause: user-agent, touch, device scale factor, or mobile emulation settings differ. Fix: configure those fields deliberately and label the run as emulated rather than physical-device evidence.
Or skip the browser setup
ScreenshotNeo provides a website screenshot API and MCP server. Its one-call request can set the target URL and capture options without maintaining a browser script:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo documentation for viewport, device presets, full-page capture, scale, waits, and other options. ScreenshotNeo removes cookie or consent banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, with verdict and billing information in response headers. Its MCP server provides take_screenshot, get_page_info, and capture_pdf for Claude, Cursor, and other MCP clients. The Free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 shots. Create a free ScreenshotNeo account.
Further automation examples
Python
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);Decision checklist
- What behavior am I trying to inspect?
- Which CSS breakpoints control that behavior?
- Have I tested just below, at, and just above each transition?
- Are width and height fixed and recorded?
- Do I need CSS-scale or device-scale output?
- Is viewport-only or full-page capture the correct question?
- Do user agent, DPR, touch, or mobile settings matter?
- Are waits, fonts, consent UI, and lazy content deterministic?
Frequently Asked Questions
Is 375px a standard mobile breakpoint?
No. It is a Chrome Device Mode preset example. Use the breakpoints defined by the site you are testing.
Should every responsive test include 320px, 375px, 768px, and 1440px?
Only if those widths serve your review. A breakpoint-driven matrix is more informative than an arbitrary universal list.
Does full-page capture replace testing viewport height?
No. Full-page capture checks the whole scrollable document; viewport height checks what is visible in the initial window.
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