如何用css做出酷炫的视差滚动效果

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
       div {
        height: 100vh;
        background: rgba(0, 0, 0, .7);
        color: #fff;
        line-height: 100vh;
        text-align: center;
        font-size: 20vh;
        }
        .a-img1 {
        background-image: url(https://images.pexels.com/photos/1097491/pexels-photo-1097491.jpeg);
        background-attachment: fixed;
        background-size: cover;
        background-position: center center;
        }
        .a-img2 {
        background-image: url(https://images.pexels.com/photos/2437299/pexels-photo-2437299.jpeg);
        background-attachment: fixed;
        background-size: cover;
        background-position: center center;
        }
        .a-img3 {
        background-image: url("https://images.pexels.com/photos/1005417/pexels-photo-1005417.jpeg");
        background-attachment: fixed;
        background-size: cover;
        background-position: center center;
        }
       </style>
</head>
<body>
       <div class="a-text">1</div>
       <div class="a-img1">2</div>
       <div class="a-text">3</div>
       <div class="a-img2">4</div>
       <div class="a-text">5</div>
       <div class="a-img3">6</div>
       <div class="a-text">7</div>
</body>
</html>