Amplifying Anatoly G's answer: <b>
and <i>
and their ilk were added into HTML by people who didn't understand the whole point of the language and the fact that its tags were for the semantics of your text, not for presentation. Of course to be fair to these people, this is because the people who made HTML at first really had no clue about the uses HTML was going to be put to. As a result a whole bunch of ill-considered tags (<blink>
anyone? <frame>
?) were added that muddied up the HTML scene badly and that we still have to live with to this day.
These days you should eschew the use of the presentation tags in favour of proper use of CSS so that your tags reflect your semantic intent while your CSS controls the presentation of same.