CSS Grid Level 2 – subgrid is coming to Firefox

A grid defined as a subgrid is pretty much the same as a regular nested grid with its own track listings. A less obvious use case for subgrid is to help in the situation where you have an unknown amount of repeated content in your layout, and want to be able to place an item from the start to the end of the grid. If you make the repeating section a subgrid for columns, with implicit rows, all of those rows fit into the same grid area of the parent, rather than creating more rows on the parent.

Source: hacks.mozilla.org