ASPX Div onscroll

Under vs.2003 and framework 1.1, this HTML worked just fine. Under vs.2008 and framework 3.5 this no longer works. The 2008 version has a tooltip message that states "Validation (XHTML 1.0 Transitional): Attribute 'onscroll' is not a valid attribute of element 'div'." Does anyone know how to get this to work. The 'div' is nested inside of a panel

<divclass="benefits_List"onscroll="javascript:document.getElementById('benefitScrollPos').value=this.scrollTop;">