<div></div>

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

div::before {
  counter-reset: my-counter var(--num);
  content: counter(my-counter);
  animation: count 10s ease-in-out infinite alternate;
}

@keyframes count {
  to { --num: 100; }
}

/* everything below is for demo appearances and not important to the concept */

body {
  box-sizing: border-box;
  display: grid;
  place-items: center;
  min-height: 100vh;
  margin: 0;
  padding: 20px;
  color: #556270;
  background-image: linear-gradient(-45deg, #c5e0dc, #dcedc2);
  font: 700 6rem/1 'Signika', sans-serif;
}