I am convinced now that these micro frameworks are not the way to go (in my very humble experience). CSS is super easy and simple. And unless you want to support IE, it's not nearly the main pain point, especially in contrast with JS which has room for much more trouble.
A week of genuine effort in learning the small nits and bits of CSS will go a long way.
What would you recommend as a learning resource? If you had a week, and wanted to dig in and learn CSS in the way you describe, what would your ideal curriculum look like?
A static website I think is a good way. The most important part is layout. Once you master flex and css selectors, everything else is a derivative one way or another.
It's probably not a good idea to focus an entire week on CSS alone. It's more economical and practical to spend a week's worth of hours in an otherwise broader project. That gives you real, practical experience in building real-world UI.
I agree with Achshar's post. I'd also recommend using caniuse.com to get a better understanding of what is and isn't supported in the various browsers.
A week of genuine effort in learning the small nits and bits of CSS will go a long way.