.slider {  
    overflow-y: hidden;  
    max-height: 500px; /* 预估的最大高度 */  
  
    transition-property: all;  
    transition-duration: .5s;  
    transition-timing-function: cubic-bezier(0, 1, 0.5, 1);  
}  
.slider.closed {  
    max-height: 0;  
}