They may indeed have reasons, but in the substantial majority of cases they don’t, or their reasons are badly flawed, and they would do well to reduce their brightness.

I will continue to recommend that people reduce their brightness in almost all cases. That’s my job as a friendly person.

As a web developer, though, there is little scope for insisting that they’re wrong, which is why I say we have to take most people’s badly configured monitors into account.

Lot of displays I encountered use PWM when you reduce the brightness. That causes the screen to flicker if it's not on 100% (see it yourself via a slow motion cam on you phone).

Good to know. I admit I haven’t seriously used anything but a good quality monitor for the last few years, and simple research suggests PWM is only likely to cause visible artefacts on LED backlights (TFTs are slow enough that they don’t dim completely when cycling quickly), which only became popular fairly recently, so that might account for my being unfamiliar with the probelm. Certainly there are still some types of patterns or of content which when scrolling look awful on LCDs and I understand that PWM (though I didn’t know that was the specific term) can be part of the problem there. Yet for reference, in my earlier teen years and before I always found it hard to cope with CRTs at 60Hz (it rapidly produced migraines, so I used polarising sunglasses to help; 80Hz was barely OK, 90Hz was acceptable and 120Hz which I could use on one monitor was lovely).

> It’s not anyone’s job as a web designer to insist they're wrong.

But this goes both ways. I've seen developers complain about the W3C accessibility standards on contrast because it looks too severe on their retina display full-brightness Macbook.

But the relative cost of having too much contrast (it's uncomfortable to read, but still perfectly legible) and the relative ease of fixing it (turn down your laptop brightness) is very minor when compared against the many people with crappy, low-light displays that can't adjust their brightness or fix the problem of low contrast text.

Often what people mean when they say to optimize for what people have is that they'd prefer to optimize for the highest quality devices first. But for a nontrivial number of websites and demographics, those devices are the minority, and in those situations high-contrast black-on-white text will look better on a wider variety of the devices that ordinary people are using.