CSS Validation error Parse Error mask()

When performing CSS validation I get the error Parse Error mask() which seems to originate from nice_menus.css

ul.nice-menu li.menuparent ul iframe,
/* For Garland header. */
#header-region ul.nice-menu li.menuparent ul iframe {
	display: none;
	display/**/: block;
	position: absolute;
	top: 0;
	left: 0;
	z-index: -1;
	filter: mask();
	width: 20px;
	height: 20px;
}

I don't know why filter: mask(); is needed or if it is necesery