<!DOCTYPE html>
<style>
html {
  margin: 0;
  font-size: 20px;
}
body {
  margin: 1em;
  border: 1px solid blue;
}
body > div {
  margin-bottom: 2em;
  border: 1px solid black;
}
</style>
<body>
  <div style="writing-mode: vertical-lr; height: 1em">
    <div><span>a</span></div>
  </div>
  <div style="writing-mode: vertical-lr; height: 1em">
    <div><span>a</span></div>
  </div>
</body>