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                <main class="wrapper">
  <div class="bg-img"></div>


                html {
  box-sizing: border-box;

*, *::before, *::after {
  box-sizing: inherit;
  &:focus {
    outline: none;

* {
  font-family: monaco, courier;

body {
  margin: 0;
  padding: 0;

.wrapper {
  width: 100%;
  min-height: 100vh;
  padding: 10px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  background: #ddd;
  // @media (min-width: 768px) {
  //   background: palegreen;
  // }

.bg-img {
  width: 100%;
  max-width: 600px;
  height: 400px;
  background-image: url('');
	background-size: cover;
  background-position: center;
	background-repeat: no-repeat;
	border: 4px solid blue;


                gsap.fromTo('.bg-img', {
    // make sure it gets converted to pixels so that it interpolates cleanly
		backgroundSize: bgSize(".bg-img", {size: "150% auto"}),
	}, {
    // make sure it gets converted to pixels so that it interpolates cleanly
		backgroundSize: bgSize(".bg-img", {size: "cover"}),
		delay: 0.5,
	  duration: 2,
		ease: "power1.inOut"

config is optional and can have any of the following properties: 
 - size [string] - the size to set and convert into px before it gets returned, like "cover" or "150% auto".
 - nativeWidth [number] - native width of the image (in pixels)
 - nativeHeight [number] - native height of the image (in pixels)
 Note: if you can define the nativeWidth and nativeHeight, it helps becaues it can skip tasks like creating 
 an Image and loading the URL to detect the native size automatically. Sometimes images don't load fast enough,
 so skipping that step avoids the whole issue. 
function bgSize(element, config) {
  config = config || {};
	let e = gsap.utils.toArray(element)[0],
			cs = window.getComputedStyle(e),
			imageUrl = cs.backgroundImage,
      {nativeWidth, nativeHeight} = config,
			size = config.size || cs.backgroundSize,
			image, w, h, ew, eh, ratio;
	if (imageUrl && (!/\d/g.test(size) || size.indexOf("%") > -1)) {
    if (!nativeWidth || !nativeHeight) {
      image = new Image();
      image.setAttribute("src", imageUrl.replace(/(^url\("|^url\('|^url\(|"\)$|'\)$|\)$)/gi, ""));
      nativeWidth = image.naturalWidth;
      nativeHeight = image.naturalHeight;
		ew = e.offsetWidth;
		eh = e.offsetHeight;
		if (!nativeWidth || !nativeHeight) {
      console.log("bgSize() failed;", imageUrl, "hasn't loaded yet.");
      nativeWidth = ew;
      nativeHeight = eh;
		ratio = nativeWidth / nativeHeight;
		if (size === "cover" || size === "contain") {
			if ((size === "cover") === (nativeWidth / ew > nativeHeight / eh)) {
				h = eh;
				w = eh * ratio;
			} else {
				w = ew;
				h = ew / ratio;	
		} else { // "auto" or %
      size = size.split(" ");
			w = ~size[0].indexOf("%") ? ew * parseFloat(size[0]) / 100 : nativeWidth;
			h = ~size[1].indexOf("%") ? eh * parseFloat(size[1]) / 100 : nativeHeight;
		size = Math.ceil(w) + "px " + Math.ceil(h) + "px";
		config.size && ( = size);
	return size;