HTML Knowledge Base
Comparing <strong> and <b> tags
<strong>
The strong element "represents a span of text with strong importance."2
There is semantic meaning of importance here. In fact, a <strong> tag within another <strong> tag has even more importance. There is nested importance. The html5 spec is enlightening on this.1
<b>
The <b> tag is for "offset text conventionally styled in bold".3 If you read deeper into the details you'll see it adds, "without conveying any extra emphasis or importance".1