I'm the lead front-end developer on a 9000 page website, we're currently looking to refactor our CSS along the lines of SMACSS/BEM in order to make it more maintainable/reusable.

I've read the SMACSS guide book, watched various OOCSS and BEM presentations on youtube and I understand the concepts, however the particular implementation of the principle will vary from site to site - we have 3 front-end developers here, and also changes tend to stick - by which I mean 5 or 10 years later you are working with the consequences of your (or someones) decisions - so I would like to be able to clearly document how we designed the CSS architecture in the same way that we might document how we wrote our web apps.

Has anyone any experience of developing a large modular reusable CSS architecture for a big site that has a team of developers on? What kind of documentation is useful? Is there a kind of diagram to show the relationships between HTML and CSS ?