Hover the number.
<div></div>

@property --num {
  syntax: "<integer>";
  initial-value: 0;
  inherits: false;
}

div {
  transition: --num 5s;
  counter-set: num var(--num);
  font: 800 40px system-ui;
}
div::after {
  content: counter(num);
}
div:hover {
  --num: 100;
}

body {
  margin: 2rem;
}