The <strong>/<b> and <em>/<i> tags have exactly the same effect in most web browsers, but there is a fundamental difference between them: <strong> and <em> have a semantic meaning whereas <b> and <i> only convey styling information like CSS.
When <b> can have simply no effect on a Palm device with limited display or when a screen reader software is used by a blind person, <strong> will:
Consequently:
<i>car</i> <!-- Non-compliant --> <b>train</b> <!-- Non-compliant -->
<em>car</em> <!-- Compliant --> <strong>train</strong> <!-- Compliant -->