That's an awfully big assumption. For starters, you're assuming that the browser window itself is centered on the screen, and that the device is centered in front of the user.
I don't think this is the point of centering an element horizontally at all, and the result you have created here is not at all what I as a user want.
Guess they think like the OP.
The same way the vertical centering counts the viewport height only, not the url bar and the tab bar.
Anyone got a Tampermonkey script or tiny extension to re-override this to make such sites actually center in the viewport, like I prefer?
PS: if you switch from the touchpad to a wireless mouse, we'll move the div accordingly
2. The demo doesn't even work, the div stays centered in the viewport, not the program window when I open a side bar (Firefox).
3. Rarely does my browser even cover the center of my workspace, so it would be impossible to center anything in the center of my view without forcibly moving the program window from within the website.