ID selectors are supposed to be unique, i.e., only one appearance per HTML document. These are the ID selectors found within each of the four theme homepages:

Many of the class selectors in the four home pages appear several times. In the lists below, the duplicates have been removed for readability:

Quiz: Why have the ie6, ie7, and ie8 selectors in the twentyeleven theme changed from ID to CLASS in the next three themes?