Learning CSS
The items in this first section are visual and interactive demonstrations of concepts in CSS. The resources in the second section, pulled from an up-to-date version of Awesome CSS Learning, are part of a more complete summary of CSS learning resources. Also see this web book's Online Learning Overview and Massive Open Online Courses pages, which emcompass CSS (but also other topics') resources as well.
Select Interactives and Visualizations
HTML & CSS is Hard (But it Doesn’t Have to Be)
Relearn CSS layout: Every Layout (free for students)