For testing it would be very helpful if you could force the use of the "prefers-contrast: more" media query via a switch in developer tools. Chrome provides settings for forcing several accessibility options (hidden under the optional "rendering" settings), I'd very much like Firefox to have this as well. Maybe it could even be a user setting, so users don't have to enable high contrast mode in their OS just to tell the browser to use "prefers-contrast: more" stylesheets.