From the article: Do not confuse the <b> element with the <strong>, <em>, or <mark> elements. The <strong> element represents text of certain importance, <em>

From the article: Do not confuse the <b> element with the <strong>, <em>, or <mark> elements. The <strong> element represents text of certain importance, <em> puts some emphasis on the text and the <mark> element represents text of certain relevance. The <b> element doesn't convey such special semantic information; use it only when no others fit.