Web Design in a Nutshell, 3rd Edition

Indicating emphasis

The em and strong elements are used for indicating emphasis and even stronger emphasis, as demonstrated in this example.

    <p>We<em>really</em> need to leave <strong>right now</strong>!</p>

Although emphasized text renders reliably in italics, it is not always an appropriate substitute for the i element. For example, if you want to italicize the title of a book, the cite element is the better choice. If there is no good match, create your own meaningful element using a generic span element and apply italics with the font-style style property. To use another example, it is a convention to display words from another language in italics, but that doesn’t necessarily mean that those words are emphasized.

A good rule of thumb is to consider how your document would sound if it were read aloud (as it might be). Do you want the italic words to be read louder or at a different pitch from the rest of the sentence? If the answer is no, then it is probably best to find an alternative to em. The same logic applies to the strong element.