Reflow
Content reflows to a single column at a 320px-equivalent viewport width without requiring two-dimensional scrolling, except for content that is genuinely tabular (e.g. a data table or map).
Why it matters
This is the criterion behind 'is this site actually usable on a phone, zoomed in.' A fixed-width layout that forces horizontal panning at narrow widths is unusable for anyone using zoom plus a small viewport together.
Needs manual testing
Requires rendering at a narrow viewport and checking for horizontal scroll — a layout/viewport check outside what a static DOM rule can determine reliably.
Fails
<div style="width: 1024px; display: flex;">
<!-- three fixed-width columns, never wrap -->
</div>A hard-coded 1024px container forces horizontal scrolling on any viewport narrower than that, including every phone.
Passes
<div style="display: grid; grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));">
<!-- columns collapse to one at narrow widths -->
</div>auto-fit with a minmax track lets columns collapse into a single column instead of forcing horizontal scroll.
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