CSS has long been the "language" of terror for more sophisticated developers. CSS can be messy, ugly, illogical, and utterly wonderful to work with sometimes. Recently, tools like SCSS and LESS have emerged which let us write more structured CSS in a DRY way (I've written about it before).
But we still have the problem with the actual "content" of the CSS. All the classes, IDs, and tag names. Hell is other people's CSS it's said, and it's true. Really bloody true. Trying to comprehend a semi-large CSS code base is like putting your feet into wet socks. If it's one person writing the CSS, it's he who's got the structure in his head. He models it according to his views and sense of organization).
Hence, Knyle Style Sheets, is an interesting idea. It's basically like a style guide – documentation – for a site or app's CSS. Go ahead and read about it, and check out the project on GitHub. I really like the thinking behind this, and would consider it a necessity for larger projects.