It annoys me slightly when people write in HTML and underline book titles such as, say, Animal Farm.
As far as I know, "proper" typography used italics for book titles, and underlining is merely a necessity forced on users of typewriters who had no separate italics typeface. Well, guess what: in HTML you do have an "italics" tag! So I consider it proper to italicise titles (Animal Farm).
In the spirit of semantic markup, I suppose a separate tag would be better, but I can't think of one whose meaning matches, so I just use the generic
<i> tag. (Which I also use for, say, marking up foreign words that are still perceived to be foreign, when I do so, as in vice versa.)