<U>

Usage Recommendation
Use it, but be aware that many browsers use underline to indicate a link.

<U> underlines the enclosed text.

My favorite book is <U>To Kill a Mockingbird</U>

produces

My favorite book is To Kill a Mockingbird

It's generally best to avoid <U>. Most browsers default to using an underline to indicate a link, so a plain underline that isn't a link is confusing. If you want to designate the name of a book, try italics instead:

My favorite book is To Kill a Mockingbird