I'm done here, but you're just getting started

The best way to learn how to build shit in CSS is to go forth and build websites.

But if you're looking for more resources, here's some good stuff:

Froggy Flexbox. Flexbox is a powerful but counterintuitive layout option that's better than floats for vertical alignment, and spacing elements out evenly. I considered doing a page on that, but these guys already did a great job.
CSS Grid Garden is a great into the CSS grids, the next big tool for layout.
Ethan Marcotte has written two excellent books on responsive design with A Book Apart: Responsive Web Design and Responsive Patterns and Practices. (And really, most of A Book Apart's catalogue is awesome).
All About Floats on CSS-Tricks
Practical Typgraphy. A short crash course in typography.
- By the way, front end developers should study design. Not only will it help your own apps turn out nicer, it will make it easier to look at a note or comp from a designer, and understand their intent - the problems their trying to solve - instead of just the exact dimensions and colors.

Help make this better

Did I miss a topic? Is there a resource I should add to the list above? Open an Issue on Github.

I'd love to see pull requests from other people! Here's what will get accepted:

Stick to the format. One page per problem. Each problem should be a minimal example that demonstrates how to use a particular feature of CSS or tackle a particular challenge.
Be clear. Be brief. Be accurate. Avoid jargon—don't assume the reader knows common programming terms yet.