Perceivable
WCAG 1.4.10
Reflow
Content must reflow to a single column at 320px wide (400% zoom) without horizontal scrolling. Exceptions for data tables, toolbars, and maps.
In plain terms
Pages must rearrange to fit a small or zoomed-in screen without forcing you to scroll sideways to read each line.
Why this matters
People who zoom to 400% — common with low vision — are left with a roughly 320px-wide column. If the page forces two-way scrolling, reading becomes a slow, line-by-line chore.
How to detect
Quick check
Set the browser window to 1280px and zoom to 400% (or use a 320px-wide viewport). Read a few paragraphs: do you have to scroll horizontally? Is anything clipped or overlapping?