What problem does this solve or what need does it fill?

Subgrid layout is useful when creating nested grids.

What solution would you like?

Implement support for grid-template-columns: subgrid and grid-template-rows: subgrid in Taffy:

• Subgrid layout is defined in the CSS Grid Level 2 specification https://www.w3.org/TR/css-grid-2/

Additional context

Much like block layout, subgrid layout requires access to non-direct descendants of a node, so some design work will be required to create an API that allows for this.