Request to add ::backdrop pseudo-element to popup elements to enable modal behavior for popups.

Two quick examples:

• Creating dialogs with light dismiss (and other popup features)
• Popup navigation elements (separating menus like slide-in menus from the rest of the page content)

Similar to the dialog element, this may possibly require a specific way to open the popup as a modal (or not, since popup is always in top layer, and the presence of the ::backdrop would be enough).