„Fraction values are new units applicable
to the ‘grid-definition-rows’ and ‘grid-definition-columns’ properties:

fr

Fraction of available space.

The distribution of fractional space occurs after all ‘length’ or content-based row
and column sizes have reached their maximum. The total size of the rows
or columns is then subtracted from the available space and the remainder
is divided proportionately among the fractional rows and columns.”