The ::marker pseudo-element represents the automatically generated marker box of a list item. Like ::before and ::after, ::marker is a tree-abiding pseudo-element, so it always fit within the box tree. But unlike ::before and ::after, currently it only accepts a small subset of safe properties, because the marker box layout is mostly undefined. Note this intent only covers ::marker, not ::before::marker nor ::after::marker (see crbug.com/1097992).
Docs: https://drafts.csswg.org/css-pseudo-4/#marker-pseudo https://drafts.csswg.org/css-lists-3/#marker-pseudo
Explainers: https://developer.mozilla.org/en-US/docs/Web/CSS/::marker