Everything in CSS is global. Every line you write has the potential to conflict with every other line. Because of this, CSS is often the hardest part of scaling any Web application or working on a development team.
In my article published on the Adobe Developer Network, I discuss some strategies for dealing with the global nature of CSS. I talk about finding the balance between paranoid over-protection and just asking for trouble. I also talk about some exciting things already making their way into the CSS spec that will provide real encapsulation in the future.
I hope you find it useful, and as always I welcome your feedback.