Underlined text was used to indicate the final piece should be typeset with italics

Typewriter-written and handwritten text doesn't have italic or bold letters available for emphasis, so a single underline became a standard way to mark up text to indicate it should be italicized in the final typeset edition of a work. Like double spaces between sentences, we're using conventions from older technology that are no longer relevant.

Underlined text on the web signifies the text is a hyperlink. Underlined text that isn't a hyperlink can create confusion for people.

Underlining | Typography for Lawyers