A better test for someone using CSS would be activities that CSS is typically used for. Have them make a complex layout that is responsive at multiple display sizes, or have them architect a set of reusable classes for a design library or something. You may think that this sort of cleverness is generalizable: that someone who would come up with this solution would also be good at laying out web components. But there is no relationship between this (very neat) trick and CSS skills, so it would be a terrible test to give people.
