I think he means that even if you only use the "copy" class in a single <div>, you could specify the CSS simply as ".copy" instead of "div.copy", so you don't restrict it only to divs.
Personally, I prefer to be more specific in my CSS, so I know what each class is intended for, and if I later use <p class="copy"> or <em class="copy"> for something different, the generic ".copy" selector won't bite me in back.
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