Selectors 4 says that sub-pseudo-elements (those whose originating element is another pseudo-element) can be selected by chaining them onto their originating elements. It gives ::before::marker and ::after::marker as examples. View Transitions 1 defines a bunch of sub-pseudo-elements, but then says says that "all of [them] are selected from their ultimate originating element, the document element". But it never outright says that the regular way of selecting sub-pseudo-elements doesn't apply to them. It should probably do that if that is the intention. (Which, given that the Chrome implementation disallows this, seems likely)

Furthermore, Selectors 4 defines pseudo-elements with internal structure and says that these can be selected using >. [View Transitions 1] creates a tree of pseudo-elements and calls it "a structure of pseudo-elements" in a note. It is unclear whether this passes as "internal structure" for the purposes of Selectors 4, so clarification in either spec would we welcome. Going off of the Chrome implementation once again makes it seem like this is not meant to be an internal structure.