filter: progid:DXImageTransform fails W3C CSS validation

I am using the following three lines in CSS to enable smooth transitions between "slides", but line #1 fails W3C CSS validation.

filter: progid:DXImageTransform.Microso开发者_C百科ft.alpha(opacity=100);
-moz-opacity: 1;
opacity: 1;  

Is there a way to reformulate the same intent (smooth transitions through alpha blending) but with methods that won't fail validation?

You can use IE conditional comments to serve a different stylesheet with the filter in it.

Check out CSS Opacity Properties? , they are using " " around progid:DXImageTransform.Microsoft.alpha(opacity=100) which clears validation.

I assume the CSS still works correctly.