.slider {
  position: relative;
  overflow: hidden;
  height: 600px;
}
.slider > * {
  opacity: 0;
  z-index: 0;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 600px;
  object-fit: cover;
}

.current {
  opacity: 1;
  z-index: 2;
}

.previous {
  z-index: 3;
  transition-property: opacity;
  transition-duration: 1s;
  transition-timing-function: ease;
}