There should be enough contrast between text and its background that it's readable by people with moderately low vision (who do not use contrast-enhancing assistive technology). For people without color deficiencies, hue and saturation have minimal or no effect on legibility as assessed by reading performance (Knoblauch et al., 1991). Color deficiencies can affect luminescence contrast somewhat. Therefore, in the recommendation, the contrast is calculated in such a way that color is not a key factor so that people who have a color vision deficit will also have adequate contrast between the text and the background.
Large text is easier to read, and therefore has lower contrast requirements. Decorative text that conveys no meaning, and logotypes are excluded.
Official Requirements:
1.4.3 Contrast (Minimum): The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for the following: (Level AA)
- Large Text: Large-scale text and images of large-scale text have a contrast ratio of at least 3:1;
- Incidental: Text or images of text that are part of an inactive user interface component, that are pure decoration, that are not visible to anyone, or that are part of a picture that contains significant other visual content, have no contrast requirement.
- Logotypes: Text that is part of a logo or brand name has no minimum contrast requirement.
How to fix:
- Make sure to specify both text color and background color, and make sure they have enough contrast.
- if using a background image, make sure the part of the image under the text provides sufficient contrast.
- Provide a control so your visitors can switch to a higher contrast presentation.
- Set neither the text color nor the background, and allow the visitor to use his browser defaults.
