A brief discussion of several browser compatibility issues encountered

<img class="loadingImage" src="url"/>
.loadingImage {
    width: 100%;
    height: 100%;
    transition: all 1s ease;
    object-fit: cover;
  }

<div class="loadingImage"></div>
.loadingImage {
    width: 100%;
    height: 100%;
    background-size: cover;
    background-position: center;
    background-image: url(url);
}

<video class="video"></video>
.video {
  width: 100%;
  height: auto;
  position: relative;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  object-fit: cover;
}

<video class="video"></video>
.video {
  width: 100%;
  height: auto;
  position: relative;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  object-fit: fill;
}

<video id="video"></video>
let video = document.getElementById('video')
if (video.readyState === 4) {
    console.log('finish!')
}

function checkTransitionEvent() {
  var el = document.createElement('div')
  var transitions = {
    'transition':'transitionend',
    'OTransition':'oTransitionEnd',
    'MozTransition':'transitionend',
    'WebkitTransition':'webkitTransitionEnd'
  }

  for(t in transitions){
      if( el.style[t] !== undefined ){
          return transitions[t];
      }
  }
}

support() {
  let support = "onwheel" in document.createElement("div") ? "wheel" : // Modern browsers support "wheel"
    document.onmousewheel !== undefined ? "mousewheel" : // Webkit and IE support at least "mousewheel"
    "DOMMouseScroll";
  return support
},

var wheelDistance = function(evt){
  if (!evt) evt = event;
  var w=evt.wheelDelta, d=evt.detail;
  if (d){
    if (w) return w/d/40*d>0?1:-1; // Opera
    else return -d/3; // Firefox; TODO: do not /3 for OS X
  } else return w/120; // IE/Safari/Chrome TODO: /3 for Chrome OS X
};

var wheelDirection = function(evt){
  if (!evt) evt = event;
  return (evt.detail<0) ? 1 : (evt.wheelDelta>0) ? 1 : -1;
};

let cancelAnimationFrame = window.cancelAnimationFrame 
  || window.mozCancelAnimationFrame 
  || function(id) { clearTimeout(id) };
let requestAnimationFrame = window.requestAnimationFrame 
  || window.mozRequestAnimationFrame || window.webkitRequestAnimationFrame 
  || window.msRequestAnimationFrame
  || function (callback) { return setTimeout(callback, 1000 / 60) };