I would rather give them practical tasks that ask basics of CSS. Here are 5 CSS tasks that I will give them:
Assume there are two vertical columns with widths 75% and 25% respectively. I tried to place them side by side but the 2nd column still appears below the first column. What might have gone wrong?
Create a masonry layout using pure CSS.
Create a cube in CSS. Also make sure, the sides are transparent and one can see through it.