$startInterval: 0%;
$endInterval: 100%;
$start: left top;
$end: left bottom;

@mixin linear-gradient($startColor, $startInterval, $endColor, $endInterval, $start, $end) {
    $mozStart: top;
    @if (start == left top) {
        background: $startColor;
        background: -moz-linear-gradient($mozStart, $startColor $startInterval, $endColor $endInterval);
        background: -webkit-gradient(linear, $start, $end, color-stop($startInterval, $startColor), color-stop($endInterval, $endColor))
    }
}

// opacity
@mixin opacity($arguments) {
    opacity: $arguments;
    filter: unquote('progid:DXImageTransform.Microsoft.Alpha(Opacity=' + round($arguments * 100) + ')');
}