From the CSS Grid Level 2 draft spec:
2. Grid Containers
Subgrids provide the ability to pass grid parameters down through nested elements, and content-based sizing information back up to their parent grid.
If the element is a grid item (i.e. it is in-flow and its parent is a
display: subgridmakes the element a subgrid (which is a
special type of grid container box) and consequently ignores its
grid-*-gapproperties in favor of adopting the
parent grid tracks that it spans.
A grid item can itself be a grid container by giving it
display: grid. In this case the layout of its contents will be independent of the layout of the grid it participates in.
In some cases it might be necessary for the contents of multiple grid items to align to each other. A grid container that is itself a grid item can defer the definition of its rows and columns to its parent grid container by using
display: subgrid, making it a subgrid.
In this case, the grid items of the subgrid participate in sizing the grid of the parent grid container, allowing the contents of both grids to align. Read more.
This feature has not yet been implemented in major browsers. Who knows when it will be.
In Grid, only the in-flow children of the container become grid items and can accept grid properties.
display: subgrid on a grid item, the children of the item respect the lines of the container.
According to the Grid Level 1 spec,
display: subgrid has been deferred to Level 2.
display: grid on grid items (i.e., nested grid containers) may be useful in some cases.
Another possible workaround is
display: contents. The method is explained here:
- What is an alternative to css subgrid?