The Document.exitFullscreen() is a method that takes the document out of full-screen mode; this is used to reverse the effects of a call to make an element in the document full-screen using its Element.requestFullscreen() method.
The :fullscreen CSS pseudo-class applies to any element that's currently being displayed in full-screen mode. It selects not only to the top level element, but to the whole stack of elements that appears.
Each element in the top layer's stack has a ::backdrop pseudo-element. This pseudo-element is a box rendered immediately below the element (and above the element below the element in the stack, if any), within the same top layer.