What metric is used to measure web accessibility regarding color contrast?

The contrast ratio between foreground and background text is the appropriate metric for measuring web accessibility in terms of color contrast. This ratio assesses how well the text stands out against its background, which is crucial for users with visual impairments, including color blindness. A higher contrast ratio indicates better visibility and readability, making it easier for all users to perceive the content on a web page.

Web accessibility guidelines, such as those outlined by the Web Content Accessibility Guidelines (WCAG), recommend specific contrast ratio thresholds to ensure that text is legible for individuals with varying degrees of vision. This metric directly impacts the overall inclusivity of web content, as it enables a broader audience to access and engage with information effectively.

The other options do not serve as direct measures of color contrast. For instance, visibility scores and readability levels do not specifically address the contrast between text and background, while font size focuses on the dimensions of text rather than its visibility in relation to color. Hence, using the contrast ratio is the most effective method for evaluating and improving accessibility regarding color contrast in web design.