Why is it a good idea to avoid underlining text for emphasis on web pages? Because when users see underlined text on your website, they will most likely think that the underlined text is a hyperlink. Some users will even try to click on the underlined text to see what it links to — for example, try clicking here. See what I mean? See how irritating that is?
For this reason, it is always a good idea to avoid underlining text for emphasis on web pages: if the user attempts to click on the underlined text in order to “see what’s under there,” they will be disappointed — and subconsciously irritated. Every little inconvenience works against you with your audience (it’s kind of like inviting people over to your house and leaving the rug turned up or extension cords out in the open so people can trip on them). Be sure to make your visitors welcome and comfortable in your web space, and they will come back for more (and your rankings will go up).
In lieu of underlining, consider bolding or CAPITALIZING for emphasis.
Remember, underlining on websites should be for links only! See web design dos and don’ts.