There is a special internal position:popup mode that cannot be set in CSS. Only internally. position:popup is accompanied with the pair of anchor-position:1..9

There is a special internal position:popup mode that cannot be set in CSS. Only internally. position:popup is accompanied with the pair of anchor-position:1..9 and popup-position:1..9 props. For the meaning of these numbers see NUMPAD keys on keyboard. They define attachment of popup to the anchor. anchor-position:1; popup-position:7; means border-box bottom-left corner of anchor corresponds to top-left margin box corner of popup. That positioning schema defines position of the popup relative to the anchor.