RE: Physical markup concept snag

> I think the main differnce is illustrated as follow.  Here 
> I'm comparing
> <i> elments to <em> elements.
> 
> <i> this is <i>nested italics</i></i>.
> <em> this is <em>nested emphisis</em></em>.
> 
> In the first case, clearly everything should be in italics.
> In the second case, using <em>, the nested <em> data should 
> be rendered in
> some other way, commonly a monospaced different font.

TeX (for \em) toggles between italic and upright.
How does one write a CSS style to get such toggling
behaviour for <em>?

		Kind regards
		/kent k