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WCAG: 1.4.1: Links must be distinguished from surrounding text in a way that does not rely on color
Ensures users who cannot distinguish between colors can tell when text is a link by verifying the link has either a distinct style that does not rely on color or has a contrast difference of greater than 3:1 (which alerts you that manual testing is required).
Some people with low vision experience low contrast, meaning that there aren't very many bright or dark areas. Everything tends to appear about the same brightness, which makes it hard to distinguish outlines, borders, edges, and details. Text that is too close in luminance (brightness) to the background can be hard to read.
There are nearly three times more individuals with low vision than those with total blindness. One person in twelve has a color deficiency - about 8% of men and 0.4% of women in the US. A person with low vision or color blindness is unable to distinguish text against a background without sufficient contrast.
When a sufficient color contrast ratio of 3:1 is not present to distinguish link text color from surrounding text color, users with low vision who experience low contrast cannot detect visually that the text is intended to function as a link.