col-anim
.collapsible-block-content{
  opacity:0;
  max-height:0;
  animation:change 3s linear both;
}
@keyframes change {
20% {
  opacity:0.7;
  max-height: 20rem;
}
100% {
  opacity:1;
  max-height:99999rem;
}
}

http://pseudo-scp-jp.wikidot.com/col-anim-2

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