If you're familiar with the CSS precedence rule, these slides might read like blog post; they did for me.
Strangely, I rarely see the precedence rule brought up in CSS documentation or tutorials. Not until I read "The Ultimate CSS Reference", by Tommy Olsson & Paul O'Brien. I highly recommend this book.
"Coding by Firebug" as it's called in the slides, probably refers to what I used to do before reading the above-mentioned book: tweak styles, wonder why they weren't taking effect, then add classes or IDs to the markup until something sticked. This is the "increasing specificity" also noted in the slides.
Once I understood the CSS rule of specificity - inline, id, class, element - I stopped polluting my markup and CSS.
Nowadays, I generate CSS with SASS partials (templates within a Ruby on Rails app), compiled together by Compass. Each partial starts with an ID at the top, with other ids, classes, or elements nested underneath. These are compiled into CSS - and I'm sure at the CSS level there is TONS of duplication - but this doesn't worry me one bit; my source code is DRY.
I suspect this is what Facebook and the other sites mentioned in the first few slides do too, so the rule duplication is probably not such a code smell as one might think.