I took EdX’s W3C CSS & HTML course, which was okay and I learned a few new things. It’s free on the audit track, but you have to sign up for an account. It covers how to use flexbox, but I think the other stuff is in the follow up CSS class. I haven’t taken that one yet.
The W3C site itself (free, no sign in) is a good resource too. You can go through their mini lessons as well and those are fairly straight-forward. They do a few little tests every so often so you make sure you understand what you’ve read.
Going to second Free Code Camp (again, obv. free but requires a sign up), which has a series of lessons that cover CSS and Bootstrap. Their Responsive Web Design Certificate gets into basic grids, flex boxes, and animations. However, their system is kind of buggy and nitpicky compared to the others.
For instance, if you don’t use the exact id tag that they want you to use (image vs img-responsive) or you happen to spell the one they want wrong: game over. Even if the element in question functions exactly like it’s supposed to. There’s also been multiple times where my answer was the exact same as the one in the answer code, but it only works if I paste in the text from the answer code.