Is validator having some fault?

I am trying to put semi transparent layer by applying following css
properties:

#semiTransLayer
{
    background-color: #4E4B8F;
    filter: alpha(opacity=40);
    opacity: 0.4;
}

This works correct in both IE and Mozila.
But, while trying to validate through http://jigsaw.w3.org/css-validator/ ,
it gives following error:

Parse Error opacity=40)

While validating, I had selected css level 3 from options.
I am also sure, the same method is given in w3schools at below link:

http://www.w3schools.com/Css/css_image_transparency.asp

Can you please solve my problem. Is there any problem with validator?

Cheers, Mrunal