过度前
transform: scaleY(0);
transform-origin: top;
过度后
transform: scaleY(1);
过渡元素的max-height
过渡前
max-height: 0;
过渡后
max-height: 500px;
http://stackoverflow.com/questions/3508605/how-can-i-transition-height-0-to-height-auto-using-css