For paper you'd like the maximum contrast, but screens emit light, and maximum contrast puts additional strain on the eyes. There's a big but, though. This is for people with normal eyesight. As far as I understand, some people require this hurtful (to me) level of contrast just to be able to read. Additionally, you can significantly lower contrast just adjusting your monitor's brightness, but there's a limit on how…
> but screens emit light, and maximum contrast puts additional strain on the eyes Depends on the screen. You don’t know if the viewer is using e-ink or something like that (or even printing to paper—though in that case especially, colours specified in your stylesheet are more likely to be just ignored). If they’re using an LCD or similar, you also have no idea about their brightness settings. They might be ridiculous…
I tend to think the "sensible" setting for the hardware is to afford the maximum dynamic range. This way no loss occurs if the material requires it, but if not, it needs to be limited it in software. This, of course, would lead to "ridiculously overbright" text, and limited contrast solves this problem more or less completely (this is why I override the background colour for my PDF viewer). My opinion is influenced by working with graphics, though.
I also believe the most common display technology used for desktops should be this baseline, and currently it's LCD.
However, this creates problems for people with different hardware and different eyesight and light sensitivity. Perhaps it's the OS that should provide some tooling to display the colours differently for different displays and different modes (image/text/other). The changes must be informed by the software, so for example, you could mark you site's colourscheme as "for reading" and the dynamic range would adapt properly.
The way the OS adapts that I saw is atrocious, perhaps because optimized for a different metric. My laptop changing the display brightness and contrast when I use Windows on battery looks bad regardless of what's on display.