In plain language
Normal text must meet at least 4.5:1 contrast against its background, and large text must meet at least 3:1.
Who it affects
Low-vision and older readers, plus anyone viewing a dim or sunlit screen.
How to check
Use a contrast checker (e.g., Colour Contrast Analyser) on body text, placeholders, links, buttons — in every state: normal, hover, focus, selected. Worked example — A community forum's discussion thread: measure text overlaid on a video background at a representative frame: if it drops below ratio during any part of playback, it fails.
Common misconception
Passing an automated contrast checker on the design file doesn't guarantee compliance. Real states like placeholder text, hover, disabled-looking-but-active buttons, and dark mode are the ones that usually fail. Real example: on a hotel booking site's room-selection page, the site sets a text color without a background color (or vice versa). If a user applies their own color scheme, text can become invisible (e.g., white on white).