It's called sRGB and it supposedly has a "black point" which is not entirely black. Nobody uses it though, on photos it would be useless. Also for me black on white is perfectly fine in a well lit room. White on pitch black is indeed problematic. Anyway, it could be a user/browser preference instead of a designer preference.
Lowering brightness shouldn't distort color-calibration too. You are supposed to match your brightness to the surrounding lighting too.
Because then displaying color-calibrated content on your display (such as movies, games, photos) is also completely distorted.
Instead, we need a way to specify CSS colors relative to a colorspace and contrast ratio.