[csswg-drafts] [css-grid-3] Subgrids and virtual item grouping (#13693)

alisonmaher has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-grid-3] Subgrids and virtual item grouping ==
Per [spec](https://drafts.csswg.org/css-grid-3/#track-sizing-subgrid), in Grid lanes, subgrid items contribute to every possible track they could in the root grid (lanes). We also resolved that items in an auto placed subgrid will have its items contribute to _all_ of the root grid lanes tracks even if that item could never contribute to all tracks (as an optimization).

Given this, I'm wondering if we should update the [virtual item grouping](https://drafts.csswg.org/css-grid-3/#track-sizing-performance) to more explicitly include subgrid placement, too. For example, the item may be auto placed, but its subgrid may have a definite position, so grouping by both would allow us to land on the correct tracks to place the virtual items into.

The current spec does group by "the placement of the item, i.e. which tracks it is allowed to be placed in", which could be interpreted to imply the above, but I think it would be good to be more explicit about subgrid placement contributing to these groupings to make this slightly clearer.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13693 using your GitHub account


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Received on Friday, 20 March 2026 15:07:10 UTC