Screen magnification

A screen magnifier is an electronic magnifying glass held over the display. It enlarges the whole screen or a region of it, and the magnification goes far past anything a browser’s own zoom offers: AbilityNet notes packages that enlarge up to 64 times, with commercial tools such as SuperNova and ZoomText, alongside the magnifiers built into Windows, macOS, iOS, and Android. At 400 percent a reader sees roughly a sixteenth of the screen area, and at 800 percent a sixty-fourth. Everything else is not small. It is off screen.

That is the whole design consequence, and it is not the same problem reflow solves. Reflow, WCAG 1.4.10, is about content surviving a narrow viewport without two-dimensional scrolling, and it helps enormously, because a single column at a readable width is a column a magnifier can follow straight down. But a magnifier user on a wide desktop layout is still moving a keyhole around a page that never got narrower, and their trouble is spatial: the further apart two things are, the less likely both are ever seen in the same view.

So the failures are all about distance. A toast in the bottom-right corner reporting the result of a button pressed at the top left is invisible, which is why an inline validation message beside its field beats a summary far away, and why an error summary has to move the reader to the field rather than just describe it. A table with row labels on the left and the value on the right can be unreadable in the middle. A tooltip that opens off the side of a magnified view is a tooltip nobody sees. A sticky header that eats half the enlarged viewport takes half of what little is left. The rule that follows from all of this is proximity: put the response next to the thing that caused it, keep a label with its control, and never make understanding depend on two far-apart parts of the screen being read together.

Magnification also changes what motion and focus mean. When a magnifier is tracking the caret or the focus ring, moving focus moves the whole view, so a script that steals focus teleports the reader somewhere else entirely, and a focus ring that is thin or low contrast is a reader lost. Testing for this is cheap and nobody does it: zoom your own operating system to 400 percent and try to complete the flow you just designed. Anything you find yourself hunting for is something a magnifier user hunts for every time. A related and easier check is resize text and text spacing, which catch the layouts that break before magnification is even involved.