CSS Guiding Principles & Best Practices

@julienbenatar @mbildner @spucci @samirrayani @karlsluis @abigailricarte @adamhajari @dodeca Please review and comment.
![image](https://cloud.githubusercontent.com/assets/310970/3769811/129fe670-18e4-11e4-8864-4903a6c0aefd.png)