【CSS】テキスト下線の長さを短くさせるアニメーションを実装する方法のデモページ

.block {
  opacity: 0;
  margin: 70px 0 0;
  text-align: center;
  transition: .3s linear;
  font-size: 28px;
}
.block:after {
  display: block;
  width: 240px;
  content: '';
  margin: 10px auto 0;
  transition: .6s ease;
  border-bottom: 1px solid #222626;
}
.block.active {
  opacity: 1;
}
.block.active:after {
  width: 80px;
}

@media screen and (max-width:640px){
  .block {
    width: 100%;
  }
}

$(window).on('load', function(){
  $(".block").addClass("active");
});