JSFiddle - React, Tailwind, and code Playground

HTML

<div>

<video id="video1" preload="auto"  loop="loop">
  <source src="http://www.w3schools.com/html/movie.mp4" type="video/mp4">>
  bgvideo
</video>
    
<video id="video2" preload="auto" loop="loop">
  <source src="http://www.w3schools.com/html/movie.mp4" type="video/mp4">>
  bgvideo
</video>
    
<video id="video3" preload="auto"  loop="loop">
  <source src="http://www.w3schools.com/html/movie.mp4" type="video/mp4">>
  bgvideo
</video>
    
<video id="video4" preload="auto"  loop="loop">
  <source src="http://www.w3schools.com/html/movie.mp4" type="video/mp4">>
  bgvideo
</video>

</div>

CSS

div {padding-top:300px; width:320px;}

video {
   
    padding-bottom:300px;    
   
}

JavaScript

var videos = document.getElementsByTagName("video"),
fraction = 0.8;
function checkScroll() {

    for(var i = 0; i < videos.length; i++) {

        var video = videos[i];

        var x = video.offsetLeft, y = video.offsetTop, w = video.offsetWidth, h = video.offsetHeight, r = x + w, //right
            b = y + h, //bottom
            visibleX, visibleY, visible;

            visibleX = Math.max(0, Math.min(w, window.pageXOffset + window.innerWidth - x, r - window.pageXOffset));
            visibleY = Math.max(0, Math.min(h, window.pageYOffset + window.innerHeight - y, b - window.pageYOffset));

            visible = visibleX * visibleY / (w * h);

            if (visible > fraction) {
                video.play();
            } else {
                video.pause();
            }

    }

}

window.addEventListener('scroll', checkScroll, false);
window.addEventListener('resize', checkScroll, false);