Content must reflow at 320 CSS pixels without horizontal scrolling

At each scan viewport, by default 1440 or 390 px wide, the check resizes the page to 320 by 480 CSS px, waits two animation frames plus 250 ms, measures and restores the viewport. In the mobile context, a page whose missing or fixed-width viewport meta tag keeps a wider layout gets one finding on html instead.

Otherwise it reports, widest first and at most 10 per page, boxes ending more than 2 px past the viewport’s inline end edge while their ancestors fit, and fitting elements whose own text does not wrap and runs past that edge. It ignores tables, grids, maps, role="application", aria-hidden content and empty boxes.

When the page scrolls sideways and nothing else is reported, html gets the finding, unless a table, grid, map or role="application" region overflowed. Content cut off by overflow-x: hidden on the page is still reported. Content hidden at 320 px, or held in its own scroll or clip container, is skipped.