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

.counter1 {
  animation: counter 5s ease-in-out;
  /* counter-reset: num var(--num); */
  counter-reset: var(--num) num;
  font: 800 40px system-ui;
  padding: 2rem;
}
.counter1::after {
  content: counter(num);
}

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