The em element indicates text that has emphasis.
This example uses the em element to emphasize text.
<p>Cats are <em>very</em> cute animals.</p>
Incidentally, the level of stress that a particular piece of content has is given by its number of ancestor em elements.
<p>Chicken Little: <q><em>The sky is falling… <em>the sky is falling… <em>the sky is falling!</em></em></em></q></p>
The em 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 em element in italics, but you can change that in CSS.
If the text is of great importance, seriousness, or urgency the strong element would be a better choice.
If you are looking to indicate text that is in an alternate voice, mood, or language, the i element would be a better choice.
If you wish to italicize the name of a creative work (e.g. a magazine, book, or film title), use the cite element instead.
Microsoft Developer Network: [Windows Internet Explorer API reference Article]