Преобразование времени в проценты анимации по ключевым кадрам

function animationListener(event) {
  var type = event.type;
  var label = type;
  
  if (type= = "animationiteration") {
    if (app.interval!=null) {
      clearInterval(app.interval);
    }
    app.time = 0;
    app.startTime = new Date().getTime();
    app.interval = setInterval(intervalFunction, 1000);
    intervalFunction();
    label = "iteration";
  }
  else if (type= = "animationstart") {
    label = "start";
  }
  else if (type= = "animationend") {
    label = "end";
  }
  
  app.stateLabel.innerHTML = label;
}

function intervalFunction() {
  var time = new Date().getTime();
  app.timeLabel.innerHTML = Math.round((time - app.startTime)/1000);
  app.keyframeLabel.innerHTML = window.getComputedStyle(app.ellipse).content;
}

function loadHandler() {
  app.ellipse = document.getElementById("Ellipse_49").parentNode;
  app.stateLabel = document.getElementById("stateLabel");
  app.timeLabel = document.getElementById("timeLabel");
  app.keyframeLabel = document.getElementById("keyframeLabel");
  
  app.ellipse.addEventListener("animationiteration", animationListener);
  app.ellipse.addEventListener("animationend", animationListener);
  app.ellipse.addEventListener("animationstart", animationListener);
}

document.addEventListener("DOMContentLoaded", loadHandler);
var app = {};

* {
  font-family: sans-serif;
  font-size: 11px;
  letter-spacing: .6px;
}

#Ellipse_49 {
  opacity: 1;
  fill: rgba(180, 180, 180, 1);
  stroke: rgb(112, 112, 112);
  stroke-width: 1px;
  stroke-linejoin: miter;
  stroke-linecap: butt;
  stroke-miterlimit: 4;
  shape-rendering: auto;
}

.Ellipse_49 {
  position: absolute;
  overflow: visible;
  width: 56px;
  height: 56px;
  left: 72px;
  top: 51px;
  
  /* remove alternate */
  animation: fadein 8s linear 0s infinite;
}

#container {
   top: 130px;
   left: 10px;
   position: relative;
   display: block;
   align-items: center;
   
}

label {
  width: 80px;
  display: inline-block;
}

@keyframes fadein {
  0% {
    opacity: 1;
    content: "show";
  }
  37.5% {
    /* 3 / 8 */
    opacity: 1;
    content: "fade out";
  }
  50% {
    /* (3 + 1) / 8 */
    opacity: 0.0;
    content: "wait";
  }
  87.5% {
    /* (3 + 1 + 3) / 8 */
    opacity: 0.0;
    content: "fade in";
  }
  100% {
    opacity: 1;
    content: "show";
  }
}

<svg class = "Ellipse_49">
		<ellipse id = "Ellipse_49" rx = "28" ry = "28" cx = "28" cy = "28">
		</ellipse>
</svg>


<div id = "container">
  <label>time: </label>
  <span id = "timeLabel"></span>
  <br>
  <label>state: </label>
  <span id = "stateLabel"></span>
  <br>
  <label>key frame: </label>
  <span id = "keyframeLabel"></span>
</div>

function animationListener(event) {
  var type = event.type;
  var label = type;
  
  if (type= = "animationiteration") {
    if (app.interval!=null) {
      clearInterval(app.interval);
    }
    app.time = 0;
    app.startTime = new Date().getTime();
    app.interval = setInterval(intervalFunction, 15);
    intervalFunction();
    label = "iteration";
  }
  else if (type= = "animationstart") {
    label = "start";
  }
  else if (type= = "animationend") {
    label = "end";
  }
  
  app.stateLabel.innerHTML = label;
}

function intervalFunction() {
  var currentTime = new Date().getTime();
  var time = (currentTime - app.startTime)/1000;
  var duration = parseFloat(window.getComputedStyle(app.ellipse).animationDuration);
  var maxValue = 100;
  var position = ((time * maxValue)/duration);
  
  app.timeLabel.innerHTML = Math.round(time);
  app.keyframeLabel.innerHTML = window.getComputedStyle(app.ellipse).content;
  app.timelineRange.value = position;
  app.positionLabel.innerHTML = Math.round(position) + "%";
}

function loadHandler() {
  app.ellipse = document.getElementById("Ellipse_49").parentNode;
  app.stateLabel = document.getElementById("stateLabel");
  app.timeLabel = document.getElementById("timeLabel");
  app.keyframeLabel = document.getElementById("keyframeLabel");
  app.timelineRange = document.getElementById("timelineRange");
  app.positionLabel = document.getElementById("positionLabel");
  
  app.ellipse.addEventListener("animationiteration", animationListener);
  app.ellipse.addEventListener("animationend", animationListener);
  app.ellipse.addEventListener("animationstart", animationListener);
}

document.addEventListener("DOMContentLoaded", loadHandler);
var app = {};

* {
  font-family: sans-serif;
  font-size: 11px;
  letter-spacing: .6px;
}

@keyframes fadein {
  0% {
    opacity: 1;
    content: "show";
  }
  37.5% {
    /* 3 / 8 */
    opacity: 1;
    content: "fade out";
  }
  50% {
    /* (3 + 1) / 8 */
    opacity: 0.0;
    content: "wait";
  }
  87.5% {
    /* (3 + 1 + 3) / 8 */
    opacity: 0.0;
    content: "fade in";
  }
  100% {
    opacity: 1;
    content: "show";
  }
}

#Ellipse_49 {
  opacity: 1;
  fill: rgba(180, 180, 180, 1);
  stroke: rgb(112, 112, 112);
  stroke-width: 1px;
  stroke-linejoin: miter;
  stroke-linecap: butt;
  stroke-miterlimit: 4;
  shape-rendering: auto;
}

.Ellipse_49 {
  position: absolute;
  overflow: visible;
  width: 50px;
  height: 50px;
  left: 20px;
  top: 50px;
  
  /* remove alternate */
  animation: fadein 4s linear 0s infinite;
}

#container {
   top: 130px;
   left: 10px;
   position: relative;
   display: block;
}

label {
  width: 80px;
  display: inline-block;
}

input[type=range] {
  outline: 0px solid red;
  display: block;
  width: 90%;
  margin-left: 0;
}
input[type=range]::-webkit-slider-runnable-track {
  width: 100%;
  height: 8.4px;
  cursor: pointer;
  box-shadow: 0px 0px 0px #000000, 0px 0px 0px #0d0d0d;
  background: rgb(255,255,255);
}

<svg class = "Ellipse_49">
		<ellipse id = "Ellipse_49" rx = "28" ry = "28" cx = "28" cy = "28">
		</ellipse>
</svg>


<div id = "container">
  <input id = "timelineRange" type = "range" value = "0" min = "0" max = "100">
  <br>
  
  <label>time: </label>
  <span id = "timeLabel"></span>
  <br>
  <label>position: </label>
  <span id = "positionLabel"></span>
  <br>
  <label>state: </label>
  <span id = "stateLabel"></span>
  <br>
  <label>key frame: </label>
  <span id = "keyframeLabel"></span>
  <br>
  <br>
</div>