Lets add 20 CSS properties to match our 20 HTML elements. I would suggest the following top 20. Be warned, knowing what these are capable of doing is only part of the CSS story. You need to grasp the concept of selectors and storage.
Typography
Background Styling
Box Model
Page Layout
I think that little lot will get you quite a way into the wonderful world of CSS.
* Bit of a cheat but I have sneaked in background-color, background-attachment, background-position and background-repeat with background as they are all related.