#grid-auto-columns
default
grid-auto-columns: auto;
The implicity-created columns have an auto size.
grid-auto-columns: 100px;
Here we combine grid-template-areas: "header header header" "sidebar main
main" with grid-template-columns: 50px 200px.
In this situation, the grid-template-areas defines 3 columns, while the
grid-template-columns only defines 2 column widths.
As a result, the third column width takes its value from the grid-auto-columns property:
100px.