Heck of a CSS trick here from Dongsung Kim.
There are hidden HDR videos playing at the corners of this page. When a HDR-capable browser encounters one, it switches to HDR mode. For some reason, CSS backdrop-filter + brightness >100% combo seems to behave like HDR—reaching beyond the user-controlled display brightness, up to the maximum HDR brightness—while the everything in between follow[s] along. At least that’s the overall idea, but I still don’t know exactly why it works; especially why with those two CSS properties.
As I look at that demo in Chrome, I see an extra-white text-shadow. In Safari, I see extra-white text. In Firefox, the whites match so I see nothing. Probably a bug.
I wouldn’t recommend actually using the trick, as I’d think the extra-whiteness almost certainly takes extra battery power that a user isn’t opting into, even without the video playing—even though it does feel like a bummer that our screens are capable of whiter whites than we normally have access to. The good news is that the gamut of color on the web is expanding, generally.