If only we had <sizer> (or display:sizer) element that could expand, disappear on wrap and not count as a regular child (sort of like a space in a text node)… But that would be so '90s.
decorative nodes to the DOM doesn’t seem like a good solution
Why? You could add the same constraints to the sizer: min, max, flex shrink/grow, put a line or any other shape in it. It wouldn’t even take a separate “display” mode, just make :row-start, :row-end and :row-wrap pseudoclasses to control collapsibility of regular divs at the sides of a container (or whatever styling you need, anything). Instead they feed us yet another crippled special case with these gaps.