Strike out text is often used by editors to show revisions. On a web page, it might be useful to emphasize a URL that has been changed or removed.

The URL <http://www.somesite.com> has been changed to <http://www.newsite.com>

Underlined text should be used sparing if at all. It is too easy to confuse underlined text with a link to underlined text. So easy that some browsers don't recognize underlined text.