Learn CSS: Grid -minmax

In this exercise, in step 2, when we set the second column minmax(50px,300px) , what will be default value of second column? And how is the width of the columns determined because we deleted width of grid?

The second column’s size is determined by the minmax() function. It will be as close to its maximum value that it can be without disrupting the sizing of the other columns.
Since the grid itself has no width specified it will take up as much horizontal space as it can without overflowing its parent container, then the columns with a definite size specified take up their space, and the then the column with its minmax()takes up the remaining space in the grid.

NOTE: A column with minmax() won’t be smaller than its minimum argument and it will overflow if there is not enough space.