https://drafts.csswg.org/css-grid/#subgrids

I've re-read the latest spec text and it still looks rather immature to me. I suspect that it needs several rounds of feedback from implementors (while we're actually implementing it!) before it will mature. As you know, there have been many significant improvements in other parts of the Grid/Alignment specs based on our feedback and I suspect that this will be even more true for subgrid given that it's a relatively complex sub-feature of Grid.

@jensimmons, @meyerweb and others have expressed that subgrid is an important feature. I fully agree. This makes me think that we should be extra careful in designing this feature and that the spec for it shouldn't be finalized before everyone can give solid feedback on it, based on experimental implementations (i.e. Nightly/Canary builds).

IIUC, the CSS Grid spec will soon have CR status. Firefox and Chrome will soon release their initial implementations of Grid layout, but both vendors have chosen to exclude subgrid in the first version (there are several good reasons for this, but I won't go in to that here).

In this situation, I believe that it would be bad to finalize the syntax and semantics of the subgrid feature at this point - it's definitely not ready to have CR status IMHO. If the CSSWG intends to move Grid to CR soon, then I think you should exclude the subgrid feature completely and instead move that to the next level of the Grid spec. Otherwise, I think Grid CR status should be gated on solid implementation feedback for subgrid, preferably from two independent implementations.

Thanks for your consideration.