CSS3 @keyframes iOS需要添加forwards 否则只会执行一次
如：
.drop-in {
animation: drop-in 1000ms;
animation-fill-mode: forwards; // 保持动画结束后的状态
}
@keyframes drop-in {
from {
transform: rotate(-30deg) translateY(-100%);
opacity: 0;
}
to {
transform: rotate(0deg) translateY(0%);
opacity: 1;
}
}