[platz]

Except here .large doesn't exist on it's own, it's only used inside the .btn selector.

the .large class for headlines would similarly only be used inside the .h1 selector.

So there would be no collision in this case.

[ryanbrunner]

You can't guarantee that someone won't add a 'top-level' class name in the future that matches one of your 'modifier' class names. It's essentially the same argument as saying that global variables are OK because no one will ever have a global variable with the same name as yours.