Document.visibilityState read-only property returns the visibility of the
document, that is in which context this element is now visible. It is useful to know if the document is in the background or an invisible tab, or only loaded for pre-rendering.
Possible values are:
The page content may be at least partially visible. In practice this means that the page is the foreground tab of a non-minimized window.
The page content is not visible to the user. In practice this means that the document is either a background tab or part of a minimized window, or the OS screen lock is active.
Typical use of this can be to prevent the download of some assets when the document is solely prerendered, or stop some activities when the document is in the background or minimized.