I've never understood this argument. If the display had pure black (depending on the definition of pure black here), it would either be an ideal absorber or reflector across the EM spectrum. My phone is not a theoretically ideal mirror, and it also doesn't accidentally cryogenically freeze surfaces when it's face down and displaying a black screen.

Similarly, it cannot boil water or charge my house's solar batteries if I display an all #fff screen.

(If it could do these things, then I'd argue for clamping contrast in the display driver, not in untrusted CSS code!)

Even in the realm of thermodynamically plausible displays, if it is so bright it's burning stuff into your retina, then turn the brightness down before you permanently have an xkcd burnt on to your retina!

Also, I'm reasonably certain such a monitor would fail safety certifications, so maybe don't buy black / gray market hardware, then ruin it for the rest of us?