On web pages, most people assume that anything that is underlined is a link, no matter what color it is. So reserve underlining for links. Underlining for emphasis or to indicate a book title is an old-fashioned technique that comes from typewriter days.
Italics have always been the way to show a book title in printed documents. Use that on your web pages. Otherwise, use italics sparingly. Italics work as a light form of emphasis, but you don't want to overemphasize too much at once. An entire paragraph in italics would be very hard to read.
(There's also a difference between a font that was designed to be italic and the italic versions of many typefaces, which just slope the letters of the regular font.)
Don't use italics for headings. First, they do not stand out on the page nearly as well as bold. Second, people do not all agree on how to put italics into a hierarchy in a set of headings. When I show people sets of headings all at the same point size where one is in bold type, one is in italic type, and one is in regular type, people always say the bold one is the highest level - the most important heading. But then I get mixed results as to whether the heading in italic is more important or less important than the heading in regular type. Use bold or color for all your headings. Change the size and possibly the placement to show which is the more important level of heading.
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