box-shadow实现单侧阴影效果: 

.top { box-shadow: 0px -10px 10px -10px rgba(18, 21, 38, 0.3); }    

.right { box-shadow: 10px 0 10px -10px rgba(18, 21, 38, 0.3); }    

.bottom { box-shadow: 0 10px 10px -10px rgba(18, 21, 38, 0.3); }  

.left { box-shadow: -10px 0 10px -10px rgba(18, 21, 38, 0.3); }