<style>
width:300px;
height:200px;
background-color:blue;
box-shadow:4px 5px 6px 7px rgba(0,0,0,.5)
}
</style>
<body>
<div class="one"></div>
</body>
box-shadow
它的4个参数分别表示：x轴的偏移量、y轴的偏移量、模糊半径、阴影颜色