Flexbox's layout algorithm, in step 11, explicitly treats "stretch" items as being definitely sized in the cross axis (redoing layout for the contents if necessary). This means that % children of the flex item can resolve properly, which is precisely why we added

Grid doesn't appear to have a similar clause; an auto-sized but stretched grid item would, it appears, be treated as indefinite, so %s on its children resolve to auto. Should we copy over this clause to the Grid layout algo, so more %s are valid?