Is use of underline deprecated and non validated?
It's deprecated in HTML 4 http://www.w3.org/TR/REC-html40/present/graphics.html#edef-U so won't validate.
Use styles instead. Maybe a <span>
tag. Although, if you want the thing you're trying to add an underline to, to be emphasized without styles enabled. Use an <em>
tag and use CSS to give it an underline.