Reflow
Content must be presentable in a 320 CSS-pixel-wide viewport without two-dimensional scrolling, except for content that genuinely requires it (data tables, maps, complex diagrams).
What it requires
WCAG 2.1 SC 1.4.10 makes mobile-responsive design a normative requirement. At 320 CSS pixels (the equivalent of 1280 px desktop zoomed to 400%), users must not need horizontal scrolling for body content.
Exceptions: data tables that genuinely need their full width to convey relationships; maps; image carousels; toolbars meant to scroll horizontally. Everything else must reflow.
Common Shopify failure
Product grid hard-set to four columns on mobile via `grid-template-columns: repeat(4, 1fr)` — at 320 px viewport, items overflow. Comparison tables with `min-width: 800px`. Footer columns that don't collapse to single-column on narrow screens.
How to fix it
AccessComply's reflow audit detects fixed-width containers via computed-style scan at 320 px viewport, rewrites grid declarations to `repeat(auto-fit, minmax(<min>, 1fr))`, and adds `overflow-x: auto` wrappers around genuine wide-content elements.
Primary source: W3C — WCAG 2.1 Understanding 1.4.10