1.方向 [offsetRight|-offsetLeft] [offsetButtom|-offsetTop] [blur] [color]

box-shadow: 0px -3px 3px rgba(0,0,0,0.5);

box-shadow: 3px -3px 3px rgba(0,0,0,0.5);

box-shadow: -3px 0px 3px rgba(0,0,0,0.5),3px 0px 3px rgba(0,0,0,0.5);

2.距离 [offsetRight|-offsetLeft] [offsetButtom|-offsetTop] [blur] [size] [color]

box-shadow: 0px -3px 3px -3px -3px rgba(0,0,0,0.5);

3.内阴影 [offsetRight|-offsetLeft] [offsetButtom|-offsetTop] [blur] [size] [color] inset

box-shadow: 0px -3px 3px -3px rgba(0,0,0,0.5) inset;

4.三边阴影（非box-shadow）