What are some ways that brought your proficiency of CSS to another level?
My answer to What are some ways that brought your proficiency of CSS to another level? on Quora
An exercise I found useful when I first learned CSS was to implement CSS versions of the designs of popular sites. This was back when most sites still used tables for layout.
If you do this, it’s not necessarily a good idea to publish your work (though it may be tempting). It’s a great learning exercise though.