Indicating to the user which fields are required is something that increases usuability, helping the user fill out the form more effectively and efficiently. These visual cues have absolutely nothing to do with the structure of the document, and would best be handled by CSS.

All versions of Internet Explorer from 5.5 forward support CSS expressions. If you are not familiar with CSS expressions in IE, they are a powerful and non-standard way commonly used to plug the gaping and vast holes in IE’s CSS support.