Contrast is what makes text readable. Low-contrast text — light grey on white is the classic offender — looks elegant to a designer with a good monitor and is invisible to a lot of real users. Accessibility guidelines put an actual number on 'readable enough'.
What the ratios mean
WCAG measures contrast as a ratio from 1:1 (identical) to 21:1 (black on white). For normal text you want at least 4.5:1 to pass level AA; large or bold text can pass at 3:1. The stricter AAA level asks for 7:1. AA is the common legal and practical target.
How to check a pair
- Open the contrast checker.
- Enter your text colour and background colour.
- Read the ratio and whether it passes AA and AAA for normal and large text.
Fixing text that fails
If a pair fails, do not just nudge the colour slightly — darken the text (or lighten the background) until it clears 4.5:1. Grey body text is the most common failure; a surprisingly dark grey is needed on white to pass, which is why 'accessible' text often looks nearly black.
Contrast is about lightness, not hue. Two vivid colours of similar brightness — like red on green — can fail badly and are also hard for colour-blind users. Check the number, don't trust your eye.