网页中顶部banner图自适应css

//test.css
.index-banner-top {
    width: 100%;
    background: url(../imgs/guanyu.png) no-repeat center center;
    height: 210px;
    background-size:cover
}
@media only screen and (max-width: 640px){

    .index-banner-top {
        height: 100px;
    }
}

//test.html
<div class="index-banner-top"></div>

电脑端显示：

手机端显示：