What is Contrast (contrast ratio)?

The contrast ratio measures the difference in luminance between the text color and the background color, expressed as a ratio (e.g. 4.5:1). WCAG Level AA requires a minimum ratio of 4.5:1 for normal text and 3:1 for large text (24px or 18.5px bold). Insufficient contrast makes content unreadable for people with low vision or in low-light conditions.