The most-authoritative page I've seen on this topic is part of a style guide at Western Michigan University. It first points out that most of the recommendations in the guide “are consistent with AP style, which is the standard for the styleguides of most universities and for writing for the Web”, and later lists the following as errors to be avoided, because they create print and electronic publications that are unattractive and difficult to read:
• Using ALL CAPITAL LETTERS
• Capitalizing Common Nouns
• Underlining words to create emphasis
• Placing words within quotation marks “to create emphasis”
• Using bold face or italic type to create emphasis
• Concluding sentences with more than one exclamation point!!!!
A relevant blog entry (part 2 of a “Five Things That Really Annoy Me” article by “divine caroline” gives EMPHASIS!!! pride of place. It says
There is a reason this is number one [on list of annoyances] There is nothing more agonizing than reading something that is overemphasized [...] This is how it looks to the copy editor…
and then presents examples of egregious all-caps, bold, and underlining.