The <u> element displays underlined text. This element was deprecated in HTML 4.0 (you're supposed to use styles instead now), so it is not supported in XHTML 1.0 Strict or XHTML 1.1. It is supported in XHTML 1.0 Transitional and XHTML 1.0 Frameset, however. Here are the attributes of this element:
class— Gives the style class of the element. (XHTML 1.0 Transitional, XHTML 1.0 Frameset.)
dir— Sets the direction of text that doesn't have an inherent direction in which you should read it, called directionally neutral text. You can set this attribute to LTR, for left-to-right text, or RTL, for right-to-left text. (XHTML 1.0 Transitional, XHTML 1.0 Frameset.)
id— Refers to the element; set this attribute to a unique identifier. ...