<div></div>

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

div {
  animation: counter 5s infinite alternate ease-in-out;
  counter-reset: num var(--num);
  font: 800 40px system-ui;
  padding: 2rem;
}
div::after {
  content: counter(num);
}

@keyframes counter {
  from {
    --num: 0;
  }
  to {
    --num: 100;
  }
}