RenderDetailsMarker should be RenderReplaced instead of RenderBlock It doesn't act like a block, and does not have children, correct? In html.css, we even make it inline! summary::-webkit-details-marker { display: inline-block; width: 0.66em; height: 0.66em; margin-right: 0.4em; }
As per web-spec, it should be list-item for which we have separate bug to track. So I am closing this one.