Displaying Loading Image using CSS

I currently have the following CSS code that I apply to a div element that displays a responsive image.

.my-img-container {
  position: relative;
  &:before {
    display: block;
    content: " ";
    background: url("https://lorempixel.com/300/160/") no-repeat;
    background-size: 100% 100%;
    width: 100% !important;
    padding-top: 56.25%;
  }
  >img {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    width: 100% !important;
    height: 100% !important;
  }
}

<div class="my-img-container">
  <img srcset="https://lorempixel.com/540/304 540w, https://lorempixel.com/960/540 960w" sizes="(min-width: 576px) 540px, 100vw" src="https://lorempixel.com/300/160">
</div>

What I'm trying to do is have an image display in the background while the responsive image is loading, hence the content in the CSS, but it's not working. Any idea why?

Displaying Loading Image using CSS

2 Answers