Reflow

Someone with low vision zooms the browser to 400 percent. A 1280 pixel window then has 320 CSS pixels of room across, and WCAG’s Reflow criterion (1.4.10) asks for one thing in return: the content still works, in a single column, scrolling one direction only. Nobody should have to scroll sideways to read the end of every line, because at that magnification the sideways trip is enormous and it happens on every line.

The width is the interesting part of the wording. The criterion is written as 320 CSS pixels wide for vertically scrolling content, and 256 pixels tall for horizontally scrolling content, precisely so it can be satisfied either by a responsive layout or by a page that was always narrow. A site that already reflows to a phone has usually already passed, which is why this reads as free to teams with a mobile layout and as a rebuild to teams shipping a fixed-width desktop application.

Zoom is not the same as a small screen, though, and that is where it goes wrong in practice. Browser zoom scales CSS pixels, so media queries do fire and the phone layout does arrive. What the phone never had is a desktop’s furniture: a sticky header that eats half of the remaining height, a side panel positioned in absolute units, a min-width on a wrapper, a modal sized in viewport units that now clips its own buttons. Fixed pixel widths, white-space: nowrap, and horizontally scrolling toolbars are the usual culprits, and none of them show up until somebody actually zooms.

The exceptions are narrow and specific: content that genuinely requires two dimensional layout is exempt, which covers data tables, maps, diagrams, video, and interfaces like a spreadsheet or a code editor where the second dimension carries meaning. Exempt means the table may scroll sideways inside its own region. It does not mean the page around it may. Test it the way a reader would: one window, zoom to 400 percent, and try to get through the task without touching the horizontal scrollbar.