css 页面滚动 多背景固定不动

常常看到一些网站，滚动页面可是背景图不会跟着滚动，好像一直固定在浏览器窗口，感受挺酷的，哇哦 ~ ~ 原来都是 background-attachment 这位大兄弟的功劳

background-attachment：须要配合background-image一块儿使用 　　scroll：默认值，背景随内容滚动而滚动 　　fixed：背景固定不动，不受内容滚动影响

直接上代码，哇哈哈哈 ~ ~

<style>
    .txt{
        width: 100%;
        height: 1000px;
        text-align:center;
    }
    .bg1, .bg2, .bg3, .bg4{
        width: 100%;
        height: 400px;
        background-size: cover;
        background-attachment: fixed;
        text-align: center;
        color: #fff;
        background-position: center center;
        background-repeat: no-repeat; 
    }
    .bg1{ background-image: url(https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1536819591022&di=fcf314d0fbb32a96d67dc015ee642734&imgtype=0&src=http%3A%2F%2Fold.bz55.com%2Fuploads%2Fallimg%2F141218%2F139-14121Q05415.jpg); }
    .bg2{ background-image: url(https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1536819612013&di=5c21265ec22ca18b9f97459b0ffb5989&imgtype=0&src=http%3A%2F%2Fpic1.win4000.com%2Fwallpaper%2F6%2F5860a7fd794c2.jpg); }
    .bg3{ background-image: url(https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1536819626165&di=0e6a7e49f95c3030848ba38e6333a983&imgtype=0&src=http%3A%2F%2Fimg5q.duitang.com%2Fuploads%2Fitem%2F201312%2F27%2F20131227233228_tLBH3.jpeg); }
    .bg4{ background-image: url(https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1536819591022&di=fcf314d0fbb32a96d67dc015ee642734&imgtype=0&src=http%3A%2F%2Fold.bz55.com%2Fuploads%2Fallimg%2F141218%2F139-14121Q05415.jpg); }
</style>

<div class="txt">永远相信美好的事情即将发生</div>
<div class="bg1">原来就是这么简单哦</div>
<div class="txt">永远相信美好的事情即将发生</div>
<div class="bg2">原来就是这么简单哦</div>   
<div class="txt">永远相信美好的事情即将发生</div>
<div class="bg3">原来就是这么简单哦</div>
<div class="txt">永远相信美好的事情即将发生</div>
<div class="bg3">原来就是这么简单哦</div>

<a href="https://codepen.io/wsydxiangwang/pen/MqGxyG" target="_blank">穿梭进入演示空间</a>