Glossary · Accessibility term
Reflow
Reflow is content rearranging itself to fit a narrow viewport instead of making you scroll sideways to read every line. The rule behind it is written as a viewport size rather than as a zoom level. What it asks for is a width equivalent to 320 CSS pixels, with no scrolling in two directions at once and nothing lost on the way. Content that disappears at that width fails it just as surely as content you have to chase sideways. The familiar 400% figure is only what that width comes to if you start from a window 1280 CSS pixels wide, which is why narrowing a window is a cleaner test than zooming. It does not require a single column, so a carousel or a side-by-side comparison can pass. And parts of a page that genuinely need two dimensions are excused outright, which covers maps, diagrams, video, games, presentations and data tables.
In practice
The requirement is a viewport size, and the zoom percentage is only a way of getting there. Content has to work at a width equivalent to 320 CSS pixels for vertically scrolling text, and at a height of 256 CSS pixels for content that scrolls horizontally instead. That second half exists for vertically written languages such as traditional Chinese and Japanese. So narrowing the window is the cleaner test. At 400% zoom on a 1280 by 1024 display the real viewport lands near 318 by 236, once the scrollbar and the browser's own bars have taken their share, and the exact figure moves with your browser and your settings. Close enough to test with. Not the number the rule states, and not arithmetic to quote in a report.
Nothing here asks for a single column. That is the common route for an article page and W3C says outright that it is not the requirement, so a horizontally scrolling carousel and a side-by-side comparison view can both pass. What breaks instead is predictable. Fixed-width containers, columns that never collapse, and sticky headers that grow until they cover half the screen. Moving content around is not a loss of information either, so hiding your navigation behind a menu button is a conforming pattern rather than a compromise.
The exception is written for parts of a page, not for pages. A data table is excused because it needs two dimensions to mean anything, and the text inside one cell still has to reflow. The same goes for a map, a diagram, a video, a game, a presentation, and an interface that has to keep a toolbar in view while you work.
One assumption the rule never makes. Reflow imposes no text size at all. W3C even works through a case where an author drops the text from 20px to 10px at high zoom to stop unwanted sideways scrolling, which is legitimate here. Text size is the other rule's job.
Why it matters
When a line of text runs past the edge of the viewport, reading it means scrolling back and forth for every single line, and the cost W3C names is not only the physical effort. It is losing your place. That is why this rule exists for people who enlarge text, and it is also why an automated pass proves so little here. The one published test rule mapped to this criterion checks whether your page blocks zooming at all. A tool can be scripted to narrow a viewport and look for overflow, and some do, and none of them can tell you whether what survived is still usable.
The check that works
Narrow your browser window until the page is about 320 CSS pixels wide, rather than zooming to 400% and trusting the arithmetic. If body text needs horizontal scrolling to read a line, that is a failure. If a map or a data table scrolls in two directions, that is the exception doing its job.
Where this shows up on the site
Related terms
- ZoomZoom means enlarging a page, usually with Ctrl or Cmd and the plus key, and two WCAG rules govern what has to survive it.
- Low visionLow vision is sight loss that ordinary glasses cannot correct, and it covers far more ground than a single blurred-vision picture suggests.
- Screen magnifierA screen magnifier enlarges part of the screen, from the zoom built into every operating system up to dedicated software like ZoomText.
- PerceivablePerceivable is the first of WCAG's four principles, and the standard states it as information and user interface components being presentable in ways people can perceive.
- Target sizeTarget size is how big a clickable or tappable thing is, and two WCAG rules govern it at two different numbers.
Knowing the word is the easy part.
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