The pseudo-obsolete <b>and <i> elements are classic examples of this. They used to stand for “bold” and “italic”, respectively, but HTML5 attempted to create a clear separation between a document’s structure and its presentation. Thus, <i>was replaced with <em>, since emphasized text can be displayed in all sorts of ways aside from being italicized (e.g., in a different font, a different color, or a bigger size). Same for <b> and <strong>.