Why it matters
WCAG 1.4.10 requires content to reflow at 320 CSS px (equivalent to 400% zoom) without horizontal scrolling; overflow also breaks mobile layouts.
How to fix it
Find the element wider than the viewport (fixed widths, long unbroken strings, wide tables/code) and let it shrink: max-width: 100%, min-width: 0 in flex/grid children, overflow-wrap: anywhere, or a scroll container for tables.
Example
img, video, pre, table { max-width: 100%; }
.flex > * { min-width: 0; }References
More accessibility rules
void a11y reports 48 rules in this category. WCAG 2.2 AA through axe-core, plus checks axe can't do alone: visible keyboard focus, 24/44px target size on mobile, 320px reflow and disabled zoom.
link-in-text-blockLinks in text are distinguished only by colorautocomplete-validInvalid autocomplete valuemeta-viewportZooming is disabledbypassNo way to skip repeated contentvideo-captionVideo has no captionsaxe-otherOther axe-core accessibility violationtarget-sizeTouch target is smaller than 24×24 pxfocus-not-visibleKeyboard focus is not visible