.content-block {
  width:100vw;
  height:100vh;
  opacity: 0;
  visibility: hidden;  
  transition: opacity 1200ms ease-out, transform 600ms ease-out,
  visibility 1200ms ease-out;
  will-change: opacity, visibility;
  background-color: black;
}

.content-block.is-visible {
    opacity: 1;
    transform: none;
    visibility: visible;
    background-color: black;
}