Q:

css slideshow background has to load

I made a simple css slideshow for my background with keyframe and I have resized the pictures to about 60kb each, (they look awfull but I was testing to see if it would work) and to my surprise it didnt work, the pictures still have to load up. 
This is my css for my slideshow, not sure if it can help but I'll put it up anyway. 
body {
  background-size: cover;
  animation: div 20s infinite;
  height: 100vh;
  margin: 0;
  background-position: center;
  background-repeat: no-repeat;
  background-blend-mode: darken;
}

html {
  background: rgba(255, 255, 255, 0.53);
}

@keyframes div {
  0% {
    background-image: url("test.jpg");
  }

  20% {
    background-image: url("test.jpg");
  }

  40% {
    background-image: url("test2.jpg");
  }

  80% {
    background-image: url("test2.jpg");
  }

  100% {
    background-image: url("test.jpg");
  }

A:

Load all of them initially otherwise you need to wait for the loading inside the keyframe even if the size of the image is small. You can then animate the background-size to show one at time

body {
  animation: div 5s infinite;
  height: 100vh;
  margin: 0;
  background-image:
    url(https://i.picsum.photos/id/110/800/800.jpg),
    url(https://i.picsum.photos/id/151/800/800.jpg),
    url(https://i.picsum.photos/id/127/800/800.jpg),
    url(https://i.picsum.photos/id/251/800/800.jpg),
    url(https://i.picsum.photos/id/126/800/800.jpg);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}

html {
  background: rgba(255, 255, 255, 0.53);
}

@keyframes div {
  20% {
    background-size: cover,0 0,0 0,0 0,0 0;
  }

  40% {
    background-size: 0 0,cover,0 0,0 0,0 0;
  }

  60% {
    background-size: 0 0,0 0,cover,0 0,0 0;
  }

  80% {
    background-size: 0 0,0 0,0 0,cover,0 0;
  }

  100% {
    background-size: 0 0,0 0,0 0,0 0,cover;
  }
}

