From http://www.w3schools.com (Copyright Refsnes Data)

WML Text Formatting tags

These tags can affect the layout of a text:

<b>	Defines bold text
<big>	Defines big text
<em>	Defines emphasized text
<i>	Defines italic text
<small>	Defines small text
<strong>	Defines strong text
<u>	Defines underlined text

Attributes

Attribute	Value	Description
xml:lang	language_code	Sets the language used in the element
class	cdata	Sets a class name for the element. The class name is case sensitive. An element can be connected to multiple classes. Multiple class names within the class attribute are separated by white space
id	id	Sets a unique name for the element

Examples

<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN"
"http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>
<card title="Formatting"> 
<p>
 <em>emphasized text</em><br/>
 <strong>strong text</strong><br/>
 <b>bold text</b><br/>
 <i>italic text</i><br/>
 <u>underlined text</u><br/>
 <big>big text</big><br/>
 <small>small text</small>
</p>
</card>
</wml>

Complete WML Reference