Document.hidden
  The Document.hidden read-only property returns a Boolean
  value indicating if the page is considered hidden or not.
    Note: This property is described as "historical" in the Page Visibility Level 2
specification. Consider using the Document.visibilityState
    property instead.
  
Syntax
var boolean = document.hidden
Examples
document.addEventListener("visibilitychange", function() {
  console.log( document.hidden );
  // Modify behavior...
});
Usage notes
    Warning: Due to prerendering, it may happen that document.hidden is
    true, even if the page is actually visible to the user. In such scenario
    the page load starts with
    document.visibilityState = "prerender"
    and transitions to document.visibilityState = "visible" after some delay.
    This scenario can be reproduced by opening a new browser tab in Safari, pasting a URL
    in the URL bar, and navigating to that URL.
  
Specifications
| Specification | 
|---|
| HTML Standard  # dom-document-hidden  | 
Browser compatibility
BCD tables only load in the browser