Both buttons on this page have a text-decoration: underline; set with CSS. However, only the button that is also floated with CSS seems to display it (tested in Firefox 1.5.0.1). This seems to be the case only when the DOCTYPE is specified.
text-decoration: underline;
This button has text-decoration: underline; but no float set on it:
float
This button has text-decoration: underline; and float: left; set on it:
float: left;