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Set the rendered browser viewport to 320 CSS pixels, then inspect the page and its controls for clipping, overlap, missing content, and page-level horizontal scrolling. A screenshot can document what rendered or help catch visual regressions, but the image alone cannot establish whether the page meets WCAG’s reflow requirement.
What 320 pixels means for this test
The target is a 320 CSS-pixel viewport, not a phone display that is physically 320 pixels wide. WCAG 2.1 Success Criterion 1.4.10 uses a width equivalent to 320 CSS pixels for vertically scrolling content. It also describes this as equivalent to viewing a page from a 1280 CSS-pixel-wide starting viewport at 400% zoom. Browser interface elements and scrollbars can affect the space available for the page, so verify the actual viewport rather than relying on a device’s nominal screen dimensions. W3C WAI’s Understanding Reflow explains the viewport considerations; the criterion is specified in WCAG 2.1, Success Criterion 1.4.10.
The expected outcome is that ordinary vertically scrolling content remains available without making a user scroll both horizontally and vertically to read or use it. The criterion allows exceptions for parts whose use or meaning genuinely requires a two-dimensional layout.
Test the page manually at 320 CSS pixels
- Set the viewport. Use responsive browser tools or browser automation to set the page viewport to 320 CSS pixels wide. If you instead use the WCAG zoom equivalence, begin at a 1280 CSS-pixel viewport and zoom to 400%. Confirm the resulting page viewport; a window’s outer width is not necessarily the viewport width.
- Inspect the whole page. Scroll from top to bottom. Check text, navigation, forms, buttons, ordinary images, and other content for clipping, overlap, unintended hiding, or controls that cannot be reached or operated.
- Check for page-level horizontal scrolling. Try to read and use the ordinary content without needing to scroll sideways as well as down. Look beyond the first screen: a wide element lower on the page can create overflow even when the top appears to fit.
- Assess exceptions individually. Maps, video, games, presentations, data tables, and interfaces that need a toolbar to remain visible while content is manipulated may need two-dimensional layout. Keep necessary scrolling within the relevant component where possible; do not treat ordinary content as exempt just because it overflows.
- Record the result. Note the viewport and browser context, the affected content, and whether the issue prevents access to information or functionality. Capture a screenshot if you need visual documentation or a baseline for later comparison.
Use Playwright for repeatable viewport checks
Playwright can emulate browser parameters such as viewport, screen size, user agent, and touch capability. Emulation is useful for repeating a check, but it represents the configuration you choose; it is not identical to testing every physical phone. Record the browser and emulation settings used. See Playwright’s emulation guidance.
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A minimal Playwright Test example sets a 320 CSS-pixel viewport, loads a page, checks the document’s scroll width against its client width, and captures a screenshot. Replace the example URL with the page under test:
import { test, expect } from '@playwright/test';
test('page at 320 CSS pixels', async ({ page }) => {
await page.setViewportSize({ width: 320, height: 800 });
await page.goto('https://example.com');
const hasHorizontalOverflow = await page.evaluate(() => {
return document.documentElement.scrollWidth >
document.documentElement.clientWidth;
});
expect(hasHorizontalOverflow).toBe(false);
await expect(page).toHaveScreenshot('page-320.png', {
fullPage: true,
scale: 'css',
});
});The overflow assertion is a useful signal, not a complete WCAG verdict: a page can have no document-level overflow and still hide or impair content, while a legitimate two-dimensional component may overflow locally. Inspect the actual page and functionality as well. Playwright documents screenshot assertions and visual comparisons.
Choose screenshot scale deliberately
With Playwright’s CSS scale, the screenshot uses one image pixel per CSS pixel. Device scale uses device pixels, so the resulting image can be larger on a high-DPI device. For a screenshot intended to represent a 320 CSS-pixel viewport, CSS scale makes the relationship straightforward; do not mistake a device-pixel image’s width for the CSS viewport width. Playwright’s PageAssertions documentation describes the scale options.
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Keep visual comparisons like-for-like
A screenshot diff can expose changes in wrapping, positioning, or visibility, but rendering may vary with operating system, browser version, settings, hardware, and other conditions. Keep those conditions and capture parameters stable when comparing against a baseline. A changed screenshot’s pixel dimensions alone do not show that reflow failed.
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Choose a responsive fix based on the content
When the test reveals a problem, preserve content, functionality, reading order, and the intended relationships between components at the narrow width. Possible implementation approaches include changing columns with media queries, using grid or flexbox to reflow content, and constraining images to fit their available width. W3C describes these as techniques, not mandatory fixes:
- C32: Using media queries and grid CSS to reflow columns
- C37: Using CSS max-width and height to fit images
- C38: Using CSS width, max-width and flexbox to fit labels and inputs
Retest the page at the same viewport after changing the layout. A visual adjustment is not enough if a control becomes inaccessible or information disappears.
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Troubleshoot common test results
The window says 320 pixels wide, but the page does not
Cause: The measurement may include browser chrome, or may refer to the screen rather than the page viewport. Fix: Check the actual viewport width in the responsive tools or automation configuration. Use 320 CSS pixels as the target, not the outer window or physical screen width.
The screenshot is wider than 320 pixels
Cause: It may be captured at device scale, where image pixels reflect device pixels rather than CSS pixels. Fix: Inspect the configured viewport and screenshot scale separately; use CSS scale when you want one screenshot pixel per CSS pixel.
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Cause: Browser rendering can vary with the operating system, browser version, settings, hardware, and other conditions. Fix: Stabilize the capture environment and settings before treating a diff as a page regression.
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The overflow check passes, but content is still hard to use
Cause: A document-width check cannot determine whether information is hidden, controls work, or reading order remains usable. Fix: Inspect and operate the page at 320 CSS pixels, including content below the fold; assess the outcome required by the criterion, not just a single automated assertion.
A table or other component still scrolls sideways
Cause: Some content may require a two-dimensional layout for its use or meaning, but ordinary page content does not qualify merely because it is wide. Fix: Decide whether the exception is genuinely necessary, and contain required two-dimensional scrolling within the component where practical.
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For an API-based capture, ScreenshotNeo can return a screenshot at a specified viewport width; the capture documents the rendering, but you still need to inspect behavior and access to content to assess reflow. Example cURL call, using the target page in place of the URL:
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpSee the ScreenshotNeo documentation for API parameters. Before capture, it accepts cookie and consent banners like a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets; each step can be turned off. Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and responses report page verdict and billing status in headers. Its MCP server offers take_screenshot, get_page_info, and capture_pdf for AI agents. The free plan includes 1,000 shots per month with no card; paid plans start at $5 for 3,000 shots.
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Frequently Asked Questions
Does a 320 CSS-pixel test mean every phone must have a 320-pixel-wide screen?
No. It is a viewport-equivalent test dimension, not a requirement about a phone’s physical display width.
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