Does this include subgrids? As I understand, these are necessary for handling some basic use cases[1], but browser vendors were refusing to implement them?
[1] https://blogs.igalia.com/mrego/2016/02/12/subgrids-thinking-...
[1] https://blogs.igalia.com/mrego/2016/02/12/subgrids-thinking-...
However `display: contents` may help plaster over the gap.
EDIT, reference: http://gridbyexample.com/video/subgrid-display-contents/