The ::backdrop CSS pseudo-element is a box the size of the viewport which is rendered immediately beneath any element being presented in fullscreen mode. This includes both elements which have been placed in fullscreen mode using the Fullscreen API and <dialog> elements.

When multiple elements have been placed into fullscreen mode, the backdrop is drawn immediately beneath the frontmost such element, and on top of the older fullscreen elements.

/* Backdrop is only displayed when dialog is opened with dialog.showModal() */
dialog::backdrop {
  background: rgba(255, 0, 0, 0.25);
}

All fullscreen elements are placed in a last-in/first out (LIFO) stack in the top layer, which is a special layer in the viewport which is always rendered last (and therefore on top) before drawing the viewport's contents to the screen. The ::backdrop pseudo-element makes it possible to obscure, style, or completely hide everything located below the element when it's the topmost one in the top layer.

::backdrop neither inherits from nor is inherited by any other elements. No restrictions are made on what properties apply to this pseudo-element.