SOURCE

var vd = document.querySelector("video");
window.addEventListener('scroll',function(){
    let op = 1 + (window.scrollY / window.innerHeight)*-1;
    vd.style.opacity = op;
})

<link href="//at.alicdn.com/t/font_849071_nephwzbsea.css" rel="stylesheet">

<section>
    <video src="https://klxxcdn.oss-cn-hangzhou.aliyuncs.com/histudy/hrm/media/bg3.mp4" autoplay loop />
</section>
<section>
    <div class="content">
        <h2>此处是标题</h2>
        <p>
            此处省略1000个字...
        </p>
    </div>
</section>

*{
    margin:0;
    padding:0;
}

body{
    position: relative;

}

section{
    min-height: 100vh;
    padding:50px;
    overflow: hidden;
    display: flex;
    justify-content: center;
    align-items: center;
    overflow: hidden;
    position: relative;
}

section video{
    position: absolute;
    top: 0;
    left: 0;
    width:100%;
    height: 100%;
    object-fit: cover;
    pointer-events: none;
    transition: opacity,transform .1s;
}

section{
    background: #242424;
    color: #fff;
}