I understand WCAG is technology agnostic, so it doesn't specify which browsers, devices, and operating systems that a website needs to be accessible on.
However, our site isn't responsive on certain platforms, like Opera or IE6/Windows XP. This affects accessibility, because suddenly content will be broken and inaccessible. For example, in IE6, black text over a white background ends up hovering over a black background, making it unreadable.
But it's impossible to account for every device, browser, and OS ever. So I feel like my only option is to be compliant for major browsers/devices, and then meet the Robust principle. So even if text contrast breaks on IE6/Windows XP, I won't get sued for non-compliance.
Our goal here is to not get sued for WCAG non-compliance. We don't know how likely we'll be sued for not supporting Windows XP, but the site owner is very worried about the legalities.