Remove hint about disabled option, not relevant for an enabled option
Generalize typography to inputs and buttons
Adjust the size of inputs to see full placeholder

I also increased the size of the input for the number of contributions,
which displayed a single digit, not enough for the largest number of
contributions, which have two digits.
First layout styles: one column for authors, one column for categories

The left column contains the form with the filters and the filtered
list of authors in CSV format.

The right column contains the form with the selection of the grouping
criterion and the filtered list of categories in CSV format.