Gridview fixedheader problem in a master contentpage concept

Question

Gridview fixedheader problem in a master contentpage concept
<div style="OVERFLOW: auto; HEIGHT:600px" >
<asp:GridView Width="16px" ID="gvCustomers" AutoGenerateColumns="False"
runat="server" BackColor="white" BorderColor="#3366CC"
BorderStyle="None" BorderWidth="1px" CellPadding="4" PageSize="15" >
<HeaderStyle CssClass="gvFixedHeader" />
 
i use css file and div tag style ="overglow:auto;height:600px
css file Gvfixedheader=
.Gvfixedheader
{
font-weight:normal;
position:relative;
background-color;white;
z-index:1000px
}