The strong element indicates text that is of great importance, seriousness, or urgency.
The relative level of importance of a piece of content is given by its number of ancestor strong elements.
This example uses the strong element to indicate important information.
<p><strong><strong>Warning.</strong> This dungeon is dangerous.</strong></p>
The strong element is a phrasing-level element. It must not contain block-level elements, but it can contain other phrasing-level elements.
By default, most browsers render the strong element with bold text, but you can change that in CSS.
If you are looking to emphasize a word or phrase, the em element would be a better choice.
If you want to bold text, but the text is not important, you should use the CSS rule font-weight: bold on the appropriate element enclosing the text.
Microsoft Developer Network: [Windows Internet Explorer API reference Article]