Of all the Web Content Accessibility Guidelines (WCAG), visual designers have the most influence over the readability of text. Contrast between foreground and background colors are important for general readability, and critical for those with low-vision. However, given the popularity of grey body text, many sites display text that isn’t contrasty enough to meet these guidelines. It certainly doesn’t need to be this way though.
Cathy O’ Connor’s Design Accessibly, See Differently: Color Contrast Tips And Tools is a great resource for designers interested in creating accessible sites. Try running some of your recent work through the WebAIM Color Contrast Checker, you might be surprised what fails.