Designing advanced layouts with CSS Subgrid - LogRocket Blog
Briefly

CSS Subgrid allows nested grid items to participate in the sizing of the tracks defined in their parent grid.
Recognizing this limitation with the traditional nested grid, CSS Grid Level 2 introduced the subgrid feature to overcome this challenge.
CSS Subgrid allows nested grid items to align with the tracks of the parent grid, resulting in a more flexible layout system.
Read at LogRocket Blog
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