Non-text Contrast extends color contrast rules beyond body copy to the visual components people rely on to operate an interface: button boundaries, input field outlines, focus indicators, toggle states, and icons that carry meaning. Each needs a contrast ratio of at least 3:1 against its immediate background in its default, unfocused state.
This is one of the most commonly failed checks in real audits, especially on flat, minimalist designs that use light gray borders and low-contrast placeholder-style buttons. Low-vision users, anyone using a phone outdoors, and users on older or lower-quality monitors are all affected when a button or checkbox boundary fades into the page around it.