o account for accumulation of rounding errors, that might make 5*20 taller than 100vh in some browsers*/
}

span {
  background: green;
  display: inline-block; /* This should not change it's size or position, but makes the size of the background predictable*/
  color: transparent;
}

#red { /* Not necessary when when comparing to the reference, but makes human judgement easier */
  position: absolute;
  background: red;
  writing-mode: vertical-rl;
  z-index: -1;
  font-family: monospace;
  font-size: 20px