From https://drafts.csswg.org/css-pseudo-4/#marker-pseudo,

UAs must add the following rule to their default style sheet:

::marker {
  unicode-bidi: isolate;
  font-variant-numeric: tabular-nums;
}

But ::marker only selects markers originated by elements, not by other pseudo-elements.

So I guess the selector should be ::marker, ::before::marker, ::after::marker.