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Between text and its background, so that it can be read by people with moderately low vision or impaired contrast perception, without the use of contrast-enhancing assistive technology.
Between link text and background color.
Between link text and surrounding body text.
Sufficient contrast must also be provided to non-text content such as keyboard focus indicators for menu items, buttons, links, and form elements
Color Contrast Between Text and Background Color
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Don’t rely on color to convey critical information such as alerts and error messages. See more info about this under the Alert/Error Messaging section.
Contrast and Non-Text Content
A keyboard user typically uses the Tab key to navigate through interactive elements on a web page—links, buttons, fields for inputting text, etc. When an item has keyboard "focus", it can be activated or manipulated with the keyboard.
A sighted keyboard user must be provided with a visual indicator of the element that currently has keyboard focus. For example a border or outline.
Browsers will by default provide a visual border around these elements when they are in focus. Usually a blue border or dotted outline. However the default style may not provide enough color contrast between foreground and background. In other cases the outline may not be visible as the element may have already been styled with a specific border or outline or made invisible.
Apply a keyboard focus outline that has sufficient color contrast against the background by using the following CSS attribute,
A:focus
The contrast ratio must be at least 4:5 against the background and 3:1 against adjacent colors.
WCAG Related References
1.3.3 Sensory Characteristics (Level A)