css3中-webkit-animation-fill-mode属性值为both时的使用方法

大家都知道 -webkit-animation-fill-mode 属性是检索或设置对象动画时间之外的状态，但是一直以来我对它的属性值都存在一个疑问，both和forwards的使用到底有什么区别呢？经过不停的测试、搜索，个人认为差别在于：

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both是设置对象状态为动画结束或开始的状态，主要是取决于-webkit-animation-direction这个属性，为什么这么说呢？请看以下：

HTMl：

CSS：
.center{
border: 1px solid #332;


border-radius: 60px;
background: rgba(0,0,0,0.5);
-webkit-animation:ani 1s ease-in 2 alternate both;
}
@-webkit-keyframes ani{
0%{-webkit-transform:translateX(0);}
50%{-webkit-transform:translateX(40px);}
100%{-webkit-transform:translateX(100px);}
}

注释：当设置方向为反方向的时候，根据动画执行的次数判断小球是否处于backwards还是forwards的状态，以上代码是执行偶数次，动画来回运动，最终回到初始状态极为backwards状态，当为奇数次时，则为结束状态forwards状态