If you have a CSS class in your skin (This is assuming that CSS compression is still turned on) and you do not want it to be changed into a 'compressed name' (After you have deployed your skin) then you cannot have any corresponding names.
For example, if you want the add a class 'temp' into an element of your site via MDF and need to use this class with JS, then you cannot have a CSS entry matching it, else the class will get compressed, thus changed!
This solves the case of the missing class's!
Pro Tip: Some stuff you need to double class to override default styles eg: .powerClass.styleClass{