I seems reasonable to provide at least one method of emphasis of either kind -- one for highlighting words and phrases, and one for just distinguishing them.

The actual PresentationDependsOnStyle of the page, but the markup should at least try to reflect the most common looks, in order to make it MostlyWysiwyg.

Common methods of emphasis:

Not highlighting:
* italic text
* slanted text
* sans serif font
* monospace font
* small caps

* bold text
* underline
* text color
* background color
* bullets or other marks
* larger/smaller type
* letter spacing

Note that some of those techniques are generally frowned upon, as they hinder readability. Especially monospace font, letter spacing and varying type size will be hard to read, as they change the letter spacing and thus cheat our sense of "word length". Colored and/or underlined text can be confused with links, so should be only used for them.

Although ''italics'' and __underline__ fall into different categories above, there is a tradition in typography that says [they are the same thing|http://www.wonderfulwritingskillsunhandbook.com/HTML/Information/Punctuation/italicsunderline.html]. 

* Some information available from [wikipedia article about emphasis|http://en.wikipedia.org/wiki/Emphasis_%28typography%29].
* [Emphasis on Web Style Guide|http://www.webstyleguide.com/type/emphasis.html]
* Similar research on meatball: [MB:CharacterFormattingRUles|http://www.usemod.com/cgi-bin/mb.pl?CharacterFormattingRules]