Note that when people speak of colour contrast ratios, they’re looking at a painfully simplified model that is useful, but nowhere near sufficient. The most significant way in which it is insufficient is the consideration of font weights: thinner fonts need higher contrast to be legible. This is particularly seriously demonstrated here by the use of the monospace font-family: some platforms have a default monospaced font that is way thinner than others (like, weight 250–300 instead of 400), and so Solarized, which I find unpleasantly low-contrast even with a fairly heavy font, becomes positively awful.