CSS Literal

If for any reason Stylus cannot accommodate a specific need, you can always resort to literal CSS with @css:

@css {
    .ie-opacity {
        filter: progid:DXImageTransform.Microsoft.Alpha(opacity=25);
        -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(opacity=25)";
    }
}

Compiling to:

.ie-opacity {        
        filter: progid:DXImageTransform.Microsoft.Alpha(opacity=25);
        -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(opacity=25)";
}