The problem with having repetition in your compiled CSS files is that they become bigger as a result. Depending on how much you care about page load speed, this

The problem with having repetition in your compiled CSS files is that they become bigger as a result. Depending on how much you care about page load speed, this can be an issue. I'm not entirely sure what she's suggesting since I'm reading the slides without any context, but it seems, given the thing about cutting down css file size for Facebook, the method she's suggesting reduces duplication in the actual css file.