// 透明度动画工具
.animate-opacity(@animation_name: animation-opacity, @start: 0, @end: 1, @duration: 1s, @easing: linear, @delay: 0s, @loop: infinite, @repeat: alternate) {
    .animation(@animation_name @duration @easing @delay @loop @repeat);

    & when (@prefix = true) {
        @-webkit-keyframes @animation_name { .keyframes; }
        @-moz-keyframes @animation_name { .keyframes; }
    }

    @keyframes @animation_name { .keyframes; }

    .keyframes() {
        from { opacity: @start; }
        to   { opacity: @end;   }
    }
}