Originally Posted by Jellby
That's not for HTML to say. It should only say it is emphasized, or, at most, why it is emphasized (by adding classes, for instance).
The "how" is the CSS job.
It's true to say, though, that <strong> is more emphatic than <em>. In pretty much every browser, <em> is rendered with italics, and <strong> with bold, but a text-to-speech program might do something entirely different with them, for example.