A lot of people make games or fun projects for #100DaysOfCode when their job might be updating a CRUD MVC app using Y framework. I think as long as it puts them in the "coding" mindset it's worthwhile.
I agree with that 100% — if it isn't fun, it's work. And if you build enough knowledge about how CSS works in general, when it comes to implementing specific layout or responsive design techniques you have a good understanding of the basics to build upon.
