To define a subgrid on columns only use the value
subgrid on a grid item of the parent as the value of
grid-template-columns. The rows can now be defined with a track listing as normal of
grid-template-rows or an implicit grid can be used as usual. Any grid created for rows will be for the subgrid only, and the parent grid row containing the subgrid will grow to be large enough to contain the content (assuming it has an
auto or other content-based size).
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