Before this change, parsed grid-template-columns/grid-template-rows
were represented as two lists: line names and track sizes. The problem
with this approach is that it erases the relationship between tracks
and their names, which results in unnecessarily complicated code that
restores this data (incorrectly if repeat() is involved) during layout.
This change solves that by representing line definitions as a list of
sizes and names in the order they were defined.
Visual progression https://genius.com/
