Success Criteria

WCAG 1.4.10 Reflow

Success Criterion 1.4.10 requires that content can be presented without loss of information or functionality, and without requiring scrolling in two dimensions, at a 320 CSS pixel width (for vertically-scrolling content) or 256 CSS pixel height (for horizontally-scrolling content) — equivalent to viewing a 1280px-wide design zoomed to 400%. It's a Level AA criterion under Guideline 1.4 (Distinguishable), added in WCAG 2.1.

How this differs from Resize Text (1.4.4)

1.4.4 requires text to be resizable to 200% without loss of function, but doesn't specify a viewport width — a desktop page zoomed to 200% might still fit fine if the viewport itself is wide. 1.4.10 is stricter and more specific: it simulates someone with low vision using magnification software or extreme browser zoom on a narrow viewport, which is a meaningfully harder constraint to satisfy than 1.4.4 alone.

The "two-dimensional scrolling" rule, precisely

Vertical scrolling to read a long page is always fine — that's normal, expected behavior. What's NOT allowed is being forced to scroll horizontally to read ordinary text content line by line, at the 320px-equivalent width. Content genuinely requiring a wide format — a data table, a complex diagram, a code sample — is exempt from the reflow requirement for that specific piece of content, even though the surrounding page still needs to reflow.

Where this typically fails

• Fixed-pixel-width layouts that don't respond to viewport size at all
• Multi-column designs that don't collapse to a single column on narrow viewports
• Content that overflows its container and gets clipped rather than wrapping or reflowing
• Horizontal navigation bars with too many items to fit that don't collapse into a mobile-style menu

The practical takeaway

This criterion is largely satisfied "for free" by genuinely responsive, mobile-first design — a layout that already adapts cleanly down to a narrow phone-width viewport has, in most cases, already solved the 400%-zoom-on-desktop version of the same underlying problem. Sites that fail 1.4.10 are almost always sites that never built proper responsive layouts in the first place, not sites that got responsive design right but missed one edge case.

Common questions

What is WCAG 1.4.10 Reflow?

A Level AA criterion requiring content to reflow to a 320px-wide viewport (or 400% zoom) without requiring horizontal scrolling to read a block of text.

Why 320 pixels?

It corresponds to a mobile viewport and to zooming a 1280px desktop page to 400%, which low-vision users commonly need.

What is exempt from 1.4.10?

Content that genuinely requires two-dimensional layout, such as data tables, complex images, maps, and toolbars, may retain horizontal scrolling.