css 使用expression ie6死机

.submitButton
{
    _star : expression(onmouseover=function(){this.className="submitButtonHover";},         onmouseout=function(){this.className="submitButton";}     );
    border: none;
    background-color: Transparent;
    background-image: url(Images/btn1.gif);
    text-align: center;
    width: 70px;
    height: 26px;
    color: White;
    font-weight: bold;
   
}

.submitButtonHover
{
    border: none;
    background-color: Transparent;
    background-image: url(Images/btn2.gif);
    text-align: center;
    width: 70px;
    height: 26px;
    color: White;
    font-weight: bold;
}