<div class="effect">
    <h1>曲线阴影</h1>
</div>

.effect {
        width: 70%;
    height: 200px;
    margin: 50px auto;
    background: #fff;
    position: relative;
    box-shadow: 0px 1px 4px rgba(0, 0, 0, 0.3), 0 0 40px rgba(0, 0, 0, 0.1) inset;
}
.effect h1 {
    font-size: 20px;
    text-align: center;
    line-height: 200px;
}
.effect:after, .effect:before {
    content: '';
    background: #fff;
    position: absolute;
    top: 50%;
    bottom: 0;
    left: 10px;
    right: 10px;
    box-shadow: 0 0 20px rgba(0, 0, 0, 0.6);
    border-radius: 100px/10px;
    z-index: -1;
}