你需要包括 transition 标题样式中的属性:

#headerTitle {
font-size: 2em;
  -webkit-transition: all 1s ease-in-out;
  -moz-transition: all 1s ease-in-out;
  -o-transition: all 1s ease-in-out;
  transition: all 1s ease-in-out;
}