Color contrast
The luminance difference between text (or UI) and its background, which determines readability for low-vision users.
WCAG requires at least 4.5:1 for normal text and 3:1 for large text and UI components. Low-contrast gray text is one of the most common — and most automatable — failures.
Example
Black (#000) on white (#FFF) is 21:1; light gray (#999) on white is about 2.8:1 and fails.