/**
 * @method 背景透明层transparent 
 * @description 透明层只处理背景透明
 * @param {string} $color - 背景透明层的颜色
 * @param {number} $alpha - 透明度 0 ~ 1之间
 * @example 调用
 * 
 * ```scss
 * div {
 *    @ include transparent(#000, .5);
 * }
 * ```
 * 
 * ```css
 * div {
 *    	background-color: transparent;
		background-color: (0,0,0,.5);
		filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#7fffffff, endColorstr=#7fffffff,);
		zoom: 1;
 * }
 * ```
 */
@mixin transparent($color, $alpha) {
	$rgba: rgba($color, $alpha);
	$ie-hex-str: ie-hex-str($rgba);
	background-color: transparent;
	background-color: $rgba;
	filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#{$ie-hex-str}, endColorstr=#{$ie-hex-str});
	zoom: 1;
}