CSS is hard. It’s a simple language, but getting it right and avoiding specificity hell can be a challenge if you don’t have the right framework to back you up. Especially in large scaling projects, you might start adding ID selectors here and !important properties there and the next thing you know you’ve backed yourself into a corner where even the smallest of UI changes will take hours to work out. Ew.
Video from SEM.js November 2014: http://youtu.be/HoQ-QEusyS0