So, when your background is too bright... why not reduce monitor brightness? The letters will stay as black as they are (hey its 2016), your background gets less bright. This has other advantages too. Why must a website creator choose how much (maximum) light my screen emits around the letters?
I do reduce often, but this makes everything else on my desktop too dark so I can't read it. So I have a keyboard shortcuts that run xrandr --brightness <intensity> that I use frequently, but it is not the ideal solution for everyone. Better solution of this problem would be if websites detected brightness+contrast preference set up in browsers, like they detect language or browser window size, and changed their CSS accordingly. It would be great accessibility and UX improvement of the Web. We should make some standard for that and push it up into browser projects. Anybody experienced with such a task?
Just because I want a less bright background for reading text doesn't mean I want my monitor to be incapable of displaying a bright white - it's not only used for reading text.