ASP.net 4 Style Sheet 'filter' is not a known CSS property

In my ASP.net Web application I get the warning 'filter' is not a known CSS property
on my style sheet with the following code. Not sure what to do.
Thanks


body {
}
.Panel1_Gradient
{
       filter:progid:DXImageTransform.Microsoft.Gradient
       (GradientType=1, StartColorStr='#FF0000', EndColorStr='#FFB6F1')
}
 
.Panel2_Gradient
{
       filter:progid:DXImageTransform.Microsoft.Gradient
       (GradientType=0, StartColorStr='#F0F0F0', EndColorStr='#000000')
}