The problems start when you add a class to 10,000 elements on the page. And yes, I've seen pages doing this with jquery where the jquery overhead was what made

The problems start when you add a class to 10,000 elements on the page. And yes, I've seen pages doing this with jquery where the jquery overhead was what made the difference between a 20ms operation (not user-perceptible) and a 1s operation (very much user-perceptible).