@property --length {
  syntax: "<length>";
  initial-value: 0px;
  inherits: true;
}

span {
  font-size: calc(1px + 2rem);
}

span::after {
  --length: 1em;
  counter-reset: val calc(tan(atan2(var(--length), 1px)));
  content: counter(val);
  /* se ha reportado un error en Safari, requiere un envoltorio con calc() */
}