WCAG 2.1 Level AA requires a minimum ratio of 4.5:1 for normal text and 3:1 for large text (18pt+, or 14pt+ bold), with the same 3:1 minimum applying to meaningful graphics and interface component boundaries like input borders. AAA raises the bar further to 7:1 and 4.5:1 respectively.
Contrast is one of the easiest WCAG failures to test with free tools, and also one of the most common: low-contrast gray text on white backgrounds is a near-universal finding in accessibility audits, often introduced by design trends that prioritize a minimal look over legibility.