If you look at some talks on ideas behind oocss you can understand that a lot of elements share similar styles. It's all boxes in the end. There is this one talk about refactoring a huge pile of css into a small number of reusable visual styles, I think it was for facebook. The semantics of an element might be a comment box, but visually it's a box with a shadow and a chat box or sidebar might share the same look, thereby it would make sense to assign them the same class (classes are intended for extracting common visual styles). Doing styles the zen garden way was usually done via long selectors to target the elements and you would end up with css code that is tightly coupled to your markup. Moving html sections would cause the styles to break. Using strictly visual styles (ie functional classes) makes it easier to distinguish the style aspect from the content and HTML is already a presentational medium.