Zoom testing checks a page’s behavior at high magnification, both through ordinary browser zoom set to 200 percent or more, which relates directly to the Resize Text success criterion, and through dedicated screen magnification software that some low-vision users run at much higher magnification levels, sometimes 400 percent or beyond.
At high zoom, layouts can break in ways invisible at normal size: navigation menus overlap body content, sidebars push essential controls off screen, and horizontal scrolling can appear where none existed before, all of which zoom testing is specifically designed to catch.