<section class="g-word">Header</section>
    <section class="g-img1">IMG1</section>
    <section class="g-word">Content1</section>
    <section class="g-img2">IMG2</section>
    <section class="g-word">Content2</section>
    <section class="g-img3">IMG3</section>
    <section class="g-word">Footer</section>
 

 section {
        height: 100vh;
        background: rgba(0, 0, 0, 0.7);
        color: #fff;
        line-height: 100vh;
        text-align: center;
        font-size: 20vh;
    }
    /*background-attachment: fixed;
     此關鍵字表示背景相對於視口固定。
     即使一個元素擁有滾動機制，背景也不會隨著元素的內容滾動。*/
    
    .g-img1 {
        background-image: url("http://pic7.photophoto.cn/20080407/0034034859692813_b.jpg");
        background-attachment: fixed;
        background-size: cover;
        background-position: center center;
    }
    
    .g-img2 {
        background-image: url("http://up.enterdesk.com/edpic_source/21/00/00/210000f8e772d7fc0758e67ae4b48807.jpg");
        background-attachment: fixed;
        background-size: cover;
        background-position: center center;
    }
    
    .g-img3 {
        background-image: url("https://images.unsplash.com/photo-1440688807730-73e4e2169fb8?dpr=1&auto=format&fit=crop&w=1500&h=1001&q=80&cs=tinysrgb&crop=");
        background-attachment: fixed;
        background-size: cover;
        background-position: center center;
    }