In this internal teach & learn session at Paypal, Hoa Doan Klein goes through the core principles behind OOCSS and SMACSS, and then explains how her team applied those principles to build their project (an online billing form).
She struggles a little bit when asked questions around the concept of “re-paints” and “overriding rules”, but this is still a video you should watch if you want to know what OOCSS and SMACSS are all about.