Implements the CSS Grid Layout Module Level 2 specification, which introduces the concept of a "subgrid" to nested grid containers.
The problem CSS Subgrid solves is inheritance within nested grid containers: only direct children of a grid container are aware of its parent's grid properties, e.g., the size of its tracks, the names of each grid line. Whenever web authors work with a grid spanning some area within another grid, the children of the innermost grid are unaware of the outermost grid structure, which could potentially be a disadvantage if authors want those items to be aligned to other descendants of the outermost grid. A "subgrid" is a nested grid container whose row and column definitions are deferred to its parent; with such concept in mind, the problem above is mitigated by inheriting the grid properties and alignment from the spanned grid area in its parent grid.
Docs: https://docs.google.com/document/d/1cpsCmzdDlXUKtMxOUKIoJFyLH54mLVhZnam_z0PriO0/edit?usp=sharing