Contrast (Minimum)
The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for large text which requires 3:1.
In plain language
Text must be easy to read against its background. Normal text needs a contrast ratio of at least 4.5:1, and large text (18pt or 14pt bold) needs at least 3:1.
Why it matters
Low contrast text is hard to read for everyone, but especially for people with low vision, color blindness, or anyone using a screen in bright sunlight. If your users have to squint, the contrast is too low.
In practice
Fails this criterion
Light gray text (#999999) on a white background (#FFFFFF). The contrast ratio is 2.85:1 — below the 4.5:1 minimum.
Meets this criterion
Dark gray text (#595959) on a white background (#FFFFFF). The contrast ratio is 7:1 — well above the minimum.
Test it yourself
Use a contrast checker tool (search "contrast checker" online). Enter the text color and background color of any text on your screen. If the ratio is below 4.5:1 for normal text or below 3:1 for large text, this criterion is failing.