First, I'm not convinced if all (r, g, b, 0) should be different colours or not, especially given how CSS lets you have a "transparent" colour -- that implies i

First, I'm not convinced if all (r, g, b, 0) should be different colours or not, especially given how CSS lets you have a "transparent" colour -- that implies it will blend with any colour towards its transparent version. The other issue is that Firefox is doing anti-aliasing incorrectly, no matter what you decide on the above. The colour between the two borders must be halfway between light grey and white, i.e. between the visible colours on the page.