Resize text and reflow
Your layout still works when someone enlarges text or narrows the window to 320px: it rewraps, nothing clips, no sideways scrolling.
See it
What it is
Two WCAG criteria that travel together. Resize text (1.4.4) says content stays usable when text is scaled to 200%. Reflow (1.4.10) says content rewraps rather than forcing you to scroll in two directions at once to read a sentence. The normative test for reflow is a viewport 320 CSS pixels wide, which is what 400% zoom gets you only if you started from a 1280px window, so quote the width, not the zoom percentage. Note also that WCAG asks you to kill the two-dimensional scrolling, not to produce a single column: a data table or a map is explicitly allowed to keep scrolling both ways because it needs to.
Reach for it whenever you set a size. Sizing type, containers, and spacing in rem instead of px means user font settings actually do something. Fixed pixel heights on cards, buttons, and headers are the usual culprit: the box stays put, the text grows, and the words get clipped or spill over.
Gotchas: an 'overflow: hidden' turns a reflow problem into invisible content, so the bug never shows up in review. A 'user-scalable=no' or 'maximum-scale=1' in your viewport meta tag blocks pinch zoom entirely and is an instant fail. And page zoom is the easy test: text-only zoom (Firefox) and a bumped-up browser default font size are the harder ones, because they scale type without scaling your px-based containers.
Ask AI for it
Test this layout against WCAG resize text (1.4.4) and reflow (1.4.10), then fix what the tests find. Run two passes: text scaled to 200%, and a viewport 320 CSS pixels wide. Report the specific failures (fixed heights clipping text, boxes overlapping, content cut off the right edge, horizontal scrolling on ordinary reading content) and fix those, rather than doing a blanket px to rem conversion, which does not guarantee reflow and usually just inflates the spacing. Replace 'overflow: hidden' on text containers with wrapping, drop fixed heights from anything holding text, and make sure the viewport meta tag has no 'user-scalable=no' or 'maximum-scale'. Content that genuinely needs two-dimensional scrolling, like a data table or a map, is allowed to keep it: a single column is not the requirement.