Lots of us 'know' CSS but don't use it to the max. More perniciously, despite demos like the famous zen garden, it's easy to forget about some of the more powerful -- but more abstract -- applications of CSS.
The challenge? Really, really don't write more code than you need to. Particularly (slide 32) he suggests:
Don't use ID's in your CSS whatsoever for one small project.
That might be tricky, unless you're already a CSS master. If not, then remember: no shortcuts during creation means no waste during curation. A stitch in time saves nine. Which isolates nicely part of why CSS is so great (besides letting you make stuff look cool).
Click through the slides and exercise your CSS-jitsu: