Your <br>'s probably aren't validating because you have generic text that is not surrounded by valid tags.
Change your doctype to HTML 4.01 transitional, or strict if you're feeling brave It will make for a less steep learning curve for you. XHTML has different guidelines and you're probably doing plain old HTML anyway.
The <u> indeed has been long deprecated, use
instead, although it should be rare - underlined text is generally confused with hyperlinks, not great for your users, which is probably why it was deprecated.